David D Breshears
- Professor, Natural Resources
- Professor, Global Change - GIDP
- Regents Professor
- Member of the Graduate Faculty
- Highly Cited Scholar
- Clariavate Analytics - Web of science, Fall 2020
- Highly Cited Researcher - Cross Field
- Clarivate Analytics (Web of Science), Fall 2019
- Ecosphere - Most Cited Paper (from all years)
- Ecological Society of America, Fall 2017
- American Geophysical Union, Fall 2017
- Ecological Society of America, Summer 2015
- Mentored Student Award: Mallory Barnes, McGinnies Award
- Spring 2017
- Most Cited Ecosphere paper published in the past two years
- Ecosphere, Ecological Society of America, Winter 2016
- Highly Cited Researcher
- Thomson Reuters, Fall 2016
- Water Resources Editor's Choice Award
- American Geophysical Union, Water Resources Research, Fall 2016
- 2015 Highly Cited Researcher
- Thomson Reuters, Winter 2015
- Most Cited Publication of past 5 Years throughout 2015
- Forest Ecology and Management, Winter 2014
- Sir Walter Murdoch Distinguished Collaborator
- Murdoch University, Perth Australia, Summer 2014
- Citations to all publications in 2013: > 1180 (~30% increase over 2012)
- ISI Web of knowledge, Fall 2013
- Pending Nomination for Fellow
- Ecological Society of America, Fall 2013 (Award Nominee)
- Top 20 cited papers of 2011
- Journal of Ecology, Spring 2013
- Top 3 most cited papers
- Aeolian Research, Spring 2013
- Top cited paper of past 5 years
- Forest Ecology and Management, Spring 2013
- Citations in 2011: for all publications - 750; for 2009-2011 publications (33) - 252
- Web of Knowledge, Winter 2011
- Detailed coverage of die-off and erosion research: "A Great Aridness" (Chapter 2, 2011, Oxford)
- W. deBuys; Book reviewed by NY Times, Nature Climate Change, Kirkus, Booklist, and others, Fall 2011
- Top most cited article (past 5 years): Allen et al. 2010 (Oct 5 2011 - present)
- Forest Ecology and Management, Fall 2011
- Award to Mentored Student: Director's Postdocotral Fellowship to Henry D. Adams
- Los Alamos National Laboratory, Summer 2011
- Most downloaded article (past 90 days): Breshears et al. 2011 (Jul 15 - Oct 9 2011)
- Ambio., Summer 2011
- Award to Mentored Student: Best Presentation to Henry D. Adams
- Insitute of the Environment Grad Student Blitz, Spring 2011
- Award to Mentored Student: McGinnies Award to Henry D. Adams
- University of Arizona McGuinnies Award, Spring 2011
- Conceptual figure highlighted: from Allen et al. 2010; News and Views: ECOLOGY: Drought and dead trees
- Nature Climate Change 1: 444-445., Spring 2011
- Maurice Cremer Memorial Lecture, Water Seminar Series
- University of Nebraska, Spring 2011
- Most cited articles (since 2009), 3rd: Field et al. 2009 (April 5, 2010 - present)
- Aeolian Research, Spring 2011
- Most downloaded articles (past 1 year), 4th: Ravi et al. 2011 (December 2011)
- Reviews of Geophysics, Spring 2011
- Student-selected invitee for seminar and course participation (declined due to scheduling issues)
- University of Idaho Exploring Biogeosciences course, Spring 2011
- Top Topic in Environment/ Ecology - June 2011 (for Mar 2005-Feb 2011 interval; 1 Top Topic selected every 2 mo): die-off from global-change-type drought
- Science Watch Essential Science; http://www.sciencewatch.com/dr/tt/2011/11-juntt/, Spring 2011
No activities entered.
Dryland Ecohydro&Veg DynamicsECOL 452 (Fall 2021)
Dryland Ecohydro&Veg DynamicsECOL 552 (Fall 2021)
Dryland Ecohydro&Veg DynamicsHWRS 552 (Fall 2021)
Dryland Ecohydro&Veg DynamicsRNR 452 (Fall 2021)
Dryland Ecohydro&Veg DynamicsRNR 552 (Fall 2021)
Dryland Ecohydro&Veg DynamicsWSM 452 (Fall 2021)
Dryland Ecohydro&Veg DynamicsWSM 552 (Fall 2021)
Principles Of ResearchRNR 546 (Fall 2021)
ThesisRNR 910 (Spring 2021)
Dryland Ecohydro&Veg DynamicsRNR 452 (Fall 2019)
Dryland Ecohydro&Veg DynamicsRNR 552 (Fall 2019)
Dryland Ecohydro&Veg DynamicsWSM 452 (Fall 2019)
Dryland Ecohydro&Veg DynamicsWSM 552 (Fall 2019)
Independent StudyRNR 399 (Fall 2019)
Principles Of ResearchRNR 546 (Fall 2019)
Climate Change AdaptationRNR 440 (Fall 2018)
Climate Change AdaptationRNR 540 (Fall 2018)
Dryland Ecohydro&Veg DynamicsECOL 452 (Fall 2018)
Dryland Ecohydro&Veg DynamicsHWRS 552 (Fall 2018)
Dryland Ecohydro&Veg DynamicsRNR 452 (Fall 2018)
Dryland Ecohydro&Veg DynamicsRNR 552 (Fall 2018)
Dryland Ecohydro&Veg DynamicsWSM 452 (Fall 2018)
Dryland Ecohydro&Veg DynamicsWSM 552 (Fall 2018)
Principles Of ResearchRNR 546 (Fall 2018)
Watershed ManagementWSM 462 (Spring 2018)
Watershed ManagementWSM 562 (Spring 2018)
Climate Change AdaptationRNR 440 (Fall 2017)
Climate Change AdaptationRNR 540 (Fall 2017)
DissertationRNR 920 (Fall 2017)
Dryland Ecohydro&Veg DynamicsECOL 452 (Fall 2017)
Dryland Ecohydro&Veg DynamicsHWRS 452 (Fall 2017)
Dryland Ecohydro&Veg DynamicsHWRS 552 (Fall 2017)
Dryland Ecohydro&Veg DynamicsRNR 452 (Fall 2017)
Dryland Ecohydro&Veg DynamicsRNR 552 (Fall 2017)
Dryland Ecohydro&Veg DynamicsWSM 452 (Fall 2017)
Dryland Ecohydro&Veg DynamicsWSM 552 (Fall 2017)
Leadership and CommunicationRNR 596L (Fall 2017)
Renewable Nat ResourcesRNR 696A (Fall 2017)
DissertationRNR 920 (Spring 2017)
Climate Change AdaptationRNR 440 (Fall 2016)
Climate Change AdaptationRNR 540 (Fall 2016)
DissertationRNR 920 (Fall 2016)
Dryland Ecohydro&Veg DynamicsECOL 452 (Fall 2016)
Dryland Ecohydro&Veg DynamicsHWRS 552 (Fall 2016)
Dryland Ecohydro&Veg DynamicsRNR 452 (Fall 2016)
Dryland Ecohydro&Veg DynamicsRNR 552 (Fall 2016)
Dryland Ecohydro&Veg DynamicsWSM 452 (Fall 2016)
Dryland Ecohydro&Veg DynamicsWSM 552 (Fall 2016)
Independent StudyRNR 599 (Fall 2016)
Independent StudyRNR 599 (Spring 2016)
- Breshears, D. D., Field, J. P., Law, D. J., Villegas, J. C., Allen, C. D., Cobb, N. S., & Bradford, J. B. (2017). Rapid Broad-scale Ecosystem Changes and Their Consequences for Biodiversity. In Climate Change and Biodiversity, 2nd Ed.
- Breshears, D. D. (2018). Southwest [Chapter 25]. In Vol. 2. Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment(pp 1101-1184). Washington, DC, USA: U.S. Global Change Research Program.More infoP Gonzalez, GM Garfin, DD Breshears, KM Brooks, HE Brown, EH Elias, A Gunasekara, N Huntly, JK Maldonado, NJ Mantua, HG Margolis, S McAfee, BR Middleton, BH Udall. 2018. Southwest [Chapter 25]. Pages 1101-1184 in: Vol. 2. Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA
- Udall, B. H., Middleton, B. R., McAfee, S., Margolis, H. G., Mantua, N. J., Maldonado, J. K., Huntly, N., Gunasekara, A., Elias, E. H., Brooks, K. M., Brown, H. E., Breshears, D. D., Garfin, G. M., & Gonzalez, P. (2018). Southwest.. In Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II(pp 1101-1184). Washington, DC: U.S. Global Change Research Program. doi:10.7930/NCA4.2018.CH25
- Volkmann, T. H., Sengupta, A., Pangle, L. A., Dontsova, K. M., Barron-Gafford, G. A., Harman, C. J., Niu, G., Meredith, L., Abramson, N., Alves Meira Neto, A., Wang, Y., Adams, J. R., Breshears, D. D., Bugaj, A., Chorover, J. D., Cueva, A., DeLong, S. B., Durcik, M., Ferre, P. A., , Huxman, T. E., et al. (2017). Controlled Experiments of Hillslope Coevolution at the Biosphere 2 Landscape Evolution Observatory: Toward Prediction of Coupled Hydrological, Biogeochemical, and Ecological Changes. In Hydrology of Artificial and Controlled Experiments. Rijeka, Croatia: IN TECH d.o.o.
- Wilcox, B. P., Le Maitre, D., Jobaggy, E., Wang, L., & Breshears, D. D. (2017). Ecohydrology: Processes and Implications for Rangelands. In Rangeland Systems.
- Farella, M. M., Breshears, D. D., & Gallery, R. E. (2020). Predicting drivers of collective soil function with woody plant encroachment in complex landscapes.. Journal of Geophysical Research: Biogeosciences, 125, e2020JG005838.
- Field, J. P., Breshears, D. D., & et al., . (2020). Forest management under megadrought: Urgent needs at finer scale and higher intensity.. Frontiers in Forests and Global Change, 3, 140. doi:doi 10.3389/ffgc.2020.502669.
- Stark, S. C., Breshears, D. D., & S Aragón, J Camilo Villegas, DJ Law, MN Smith, DM M, R Leandro de Assis, DRA de Almeida, G de Oliveira, SR Saleska, ALS Swann, J Mauro, S Moura, JL Camargo, R da Silva, LEOC Aragão, R Cosme Oliveira., . (2020). Reframing tropical savannization: linking changes in canopy structure to energy balance alterations that impact climate.. Ecosphere, 11(9), e03231.. doi:10.1002/ec2.3231.
- Law, D. J., Adams, H. D., Breshears, D. D., Cobb, N. S., Bradford, J. B., Zou, C. B., Field, J. P., Gradea, A. A., Williams, A. P., & Huxman, T. E. (2019). Bioclimatic envelopes for individual demographic events driven by extremes: Plant mortality from drought and warming.. International Journal of Plant Sciences, 180(1), 53-62.
- McDowell, N., Grossiord, C., Adams, H., Pinzon Navarro, S., Mackay, S., Breshears, D. D., Allen, C., Borrego, I., Dickman, T., Collins, A., Gaylord, M., McBranch, N., Pockman, W., Vilagrosa, A., Aukema, B., & Xu, C. (2019). Mechanisms of a coniferous woodland persistence under drought and heat.. Environmental Research Letters, 14(4), 045014.
- Redmond, M. D., Law, D. J., Field, J. P., Meneses, N., Carroll, C. J., Wion, A., Breshears, D. D., Cobb, N. S., Dietze, M. C., & Gallery, R. E. (2019). Targeting extreme events: Complementing near-term ecological forecasting with rapid experiments and regional surveys.. Frontiers in Environmental Science, 7(183).
- Trowbridge, A., Stoy, P., Adams, H. D., Helmig, D., Breshears, D. D., & Monson, R. (2019). Drought supersedes warming in determining volatile and tissue defenses of piñon pine (Pinus edulis).. Environmental Research Letters, 14(6), 065006.
- Fandel, C. A., Breshears, D. D., & McMahon, E. E. (2018). Implicit assumptions of conceptual diagrams in environmental science and best practices for their illustration. ECOSPHERE, 9(1).
- Hartmann, H., Moura, C. F., Anderegg, W., Ruehr, N. K., Salmon, Y., Allen, C. D., Arndt, S. K., Breshears, D. D., Davi, H., Galbraith, D., Ruthrof, K. X., Wunder, J., Adams, H. D., Bloemen, J., Cailleret, M., Cobb, R., Gessler, A., Grams, T., Jansen, S., , Kautz, M., et al. (2018). Research frontiers for improving our understanding of drought-induced tree and forest mortality. NEW PHYTOLOGIST, 218(1), 15-28.
- Matusick, G., Ruthrof, K. X., Kala, J., Brouwers, N. C., Breshears, D. D., & Hardy, G. (2018). Chronic historical drought legacy exacerbates tree mortality and crown dieback during acute heatwave-compounded drought. ENVIRONMENTAL RESEARCH LETTERS, 13(9).
- Ruthrof, K. X., Breshears, D. D., Fontaine, J. B., Froend, R. H., Matusick, G., Kala, J., Miller, B. P., Mitchell, P. J., Wilson, S. K., van, K. M., Enright, N. J., Law, D. J., Wernberg, T., & Hardy, G. (2018). Subcontinental heat wave triggers terrestrial and marine, multi-taxa responses. SCIENTIFIC REPORTS, 8.
- Swann, A., Lague, M. M., Garcia, E. S., Field, J. P., Breshears, D. D., Moore, D., Saleska, S. R., Stark, S. C., Camilo, V. J., Law, D. J., & Minor, D. M. (2018). Continental-scale consequences of tree die-offs in North America: identifying where forest loss matters most. ENVIRONMENTAL RESEARCH LETTERS, 13(5).
- Adams, H. D., & Breshears, D. D. (2017). A multi-species synthesis of physiological mechanisms in drought-induced tree mortality. Nature Ecology and Evolution.More infoAdams et al. (with ~60 other co-authors).
- Adams, H. D., Barron-Gafford, G. A., Minor, R. L., Gardea, A. A., Bentley, L. P., Law, D. J., Breshears, D. D., McDowell, N. G., & Huxman, T. E. (2017). Temperature response surfaces for mortality risk of tree species with future drought. ENVIRONMENTAL RESEARCH LETTERS, 12(11).
- Adams, H. D., Zeppel, M., Anderegg, W., Hartmann, H., Landhausser, S. M., Tissue, D. T., Huxman, T. E., Hudson, P. J., Franz, T. E., Allen, C. D., Anderegg, L., Barron-Gafford, G. A., Beerling, D. J., Breshears, D. D., Brodribb, T. J., Bugmann, H., Cobb, R. C., Collins, A. D., Dickman, L. T., , Duan, H., et al. (2017). A multi-species synthesis of physiological mechanisms in drought-induced tree mortality. NATURE ECOLOGY & EVOLUTION, 1(9), 1285-1291.
- Barnes, M. L., Breshears, D. D., Law, D. J., van, L., Monson, R. K., Fojtik, A. C., Barron-Gafford, G. A., & Moore, D. (2017). Beyond greenness: Detecting temporal changes in photosynthetic capacity with hyperspectral reflectance data. PLOS ONE, 12(12).
- Law, D. J., Field, J. P., Walworth, J. L., Breshears, D. D., & Pessarakli, M. (2017). Candidate halophytic grasses for addressing land degradation: Shoot responses of Sporobolus airoides and Paspalum vagenitum to weekly increasing NaCl concentration. Arid Land Research and Management, 31(2), 169-181.
- Pessarakli, M., Breshears, D. D., Walworth, J. L., Field, J. P., & Law, D. J. (2016). Candidate halophytic grasses for addressing land degradation: Shoot responses of Sporobolus airoides and Paspalum vagenitum to weekly increasing NaCl concentration. Arid Land Research and Management.
- Ruiz, J., Pelletier, J. D., Zeng, X., Breshears, D. D., Huxman, T. E., DeLong, S., Saleska, S. R., Chorover, J. D., Troch, P. A., Barron-Gafford, G. A., Dontsova, K. M., & Van Haren, J. L. (2017). CO2 diffusion into pore spaces limits weathering rate of an experimental basalt landscape. Geology, 45(3), 203-206. doi:10.1130/g38569.1
- Van Haren, J. L., Dontsova, K. M., Barron-Gafford, G. A., Troch, P. A., Chorover, J. D., Saleska, S. R., DeLong, S., Huxman, T. E., Breshears, D. D., Zeng, X., Pelletier, J. D., & Ruiz, J. (2016). CO2 diffusion into pore spaces limits weathering rate of an experimental basalt landscape. Geology.
- Breshears, D. D., Knapp, A. K., Law, D. J., Smith, M. D., Tidwell, D., & Wonka, C. L. (2016). Rangeland responses to predicted increases in drought extremity. Rangelands, 34(4), 191-196.
- Field, J. P., Field, J. P., Breshears, D. D., Breshears, D. D., Law, D. J., Law, D. J., Villegas, J. C., Lopez-Hoffman, L., Brooks, P. D., Lopez-Hoffman, L., Brooks, P. D., Chorover, J. D., Chorover, J., Pelletier, J. D., Villegas, J. C., Pelletier, J. D., Field, J. P., Breshears, D. D., Law, D. J., , Lopez-Hoffman, L., et al. (2016). Understanding ecosystem services from a geosciences perspective. Eos, 97. doi:10.1029/2016EO043591
- Field, J. P., Villegas, J. C., Breshears, D. D., Pelletier, J. D., Law, D. J., Chorover, J. D., Brooks, P. D., Lopez-Hoffman, L., Brooks, P. D., Lopez-Hoffman, L., Chorover, J. D., Law, D. J., Pelletier, J. D., Breshears, D. D., Villegas, J. C., & Field, J. P. (2015). Understanding of ecosystem services from a geosciences perspective. EOS, 97. doi:doi:10.1029/2016EO043591.
- Garcia, E. S., Swann, A. L., Villegas, J. C., Breshears, D. D., Law, D. J., Saleska, S. R., & Stark, S. C. (2016). Synergistic ecoclimate teleconnections from forest loss in different regions structure global ecological responses. PLOS ONE, in press.
- McDowell, N. G., Williams et al, A. P., & Breshears, D. D. (2016). Multi-scale predictions of massive conifer mortality due to chronic temperature rise.. Nature Climate Change.
- McDowell, N. G., Williams, A. P., Xu, C., Pockman, W. T., Dickman, L. T., Sevanto, S., Pangle, R., Limousin, J., Plaut, J., Mackay, D. S., Ogee, J., Domec, J. C., Allen, C. D., Fisher, R. A., Jiang, X., Muss, J. D., Breshears, D. D., Rauscher, S. A., & Koven, C. (2016). Addendum: Multi-scale predictions of massive conifer mortality due to chronic temperature rise (vol 6, pg 295, 2016). NATURE CLIMATE CHANGE, 6(11), 1048-1048.
- Ruthrof, K. X., Fontain, J. B., Matusick, G., Breshears, D. D., Law, D. J., Powell, S., & Hardy, G. (2016). How drought-induced forest die-off alters microclimate and increases fuel loadings and fire potentials. International Journal of Wildland Fire, 25(8), 819-830.
- Stark, S. C., Breshears, D. D., Garcia, E. S., Law, D. J., Minor, D. M., Saleska, S. R., Swann, A. L., Camilo Villegas, J., Aragao, L. E., Bella, E. M., Borma, L. S., Cobb, N. S., Litvak, M. E., Magnusson, W. E., Morton, J. M., & Redmond, M. D. (2016). Toward accounting for ecoclimate teleconnections: intra- and inter-continental consequences of altered energy balance after vegetation change. LANDSCAPE ECOLOGY, 31(1), 181-194.
- Allen, C. D., Breshears, D. D., & McDowell, N. G. (2015). On underestimation of global vulnerability to tree mortality and forest die-off from hotter drought in the Anthropocene. ECOSPHERE, 6(8).
- Barnes, P. W., Throop, H. L., Archer, S. R., Breshears, D. D., McCulley, R. L., & Tobler, M. A. (2015). Sunlight and soil–litter mixing: Drivers of litter decomposition in drylands. Progress in Botany, 76, 273-302.
- Biederman, J. A., Somor, A. J., Harpold, A. A., Gutmann, E. D., Breshears, D. D., Troch, P. A., Gochis, D. J., Scott, R. L., Meddens, A. J., & Brooks, P. D. (2015). Recent tree die-off has little effect on streamflow in contrast to expected increases from historical studies. WATER RESOURCES RESEARCH, 51(12), 9775-9789.
- Field, J. P., Breshears, D. D., Law, D. J., Lopez Hoffman, L. -., Brooks, P. D., Chorover, J., Barron-Gafford, G. A., Gallery, R. E., Litvak, M. E., Lybrand, R., Mcintosh, J. C., Meixner, T. -., Niu, Y. -., Papuga, S. A., Pelletier, J. D., Rasmussen, C. -., & Troch, P. A. (2015). Critical zone services: Expanding context, constraints, and curency beyond ecosystem services.. Vadose Zone Journal, 14(1), 1-7.More infoProcesses within the critical zone, such as soil formation, support and/or control many ecosystem processes and consequently the supply of products that benefits society. An expanded perspective of ecosystem services that encompasses the critical zone would enable more effective management and allow a more comprehensive valuation of services.Processes within the critical zone—spanning groundwater to the top of the vegetation canopy—have important societal relevance and operate over broad spatial and temporal scales that often are not included in existing frameworks for ecosystem services evaluation. Here we expand the scope of ecosystem services by specifying how critical zone processes extend context both spatially and temporally, determine constraints that limit provision of services, and offer a potentially powerful currency for evaluation. Context: A critical zone perspective extends the context of ecosystem services by expressly addressing how the physical structure of the terrestrial Earth surface (e.g., parent material, topography, and orography) provides a broader spatial and temporal template determining the coevolution of physical and biological systems that result in societal benefits. Constraints: The rates at which many ecosystem services are provided are fundamentally constrained by rate-limited critical zone processes, a phenomenon that we describe as a conceptual “supply chain” that accounts for rate-limiting soil formation, hydrologic partitioning, and streamflow generation. Currency: One of the major challenges in assessing ecosystem services is the evaluation of their importance by linking ecological processes to societal benefits through market and nonmarket valuation. We propose that critical zone processes be integrated into an evaluation currency, useful for valuation, by quantifying the energy flux available to do thermodynamic work on the critical zone. In short, characterization of critical zone processes expands the scope of ecosystem services by providing context, constraints, and currency that enable more effective management needed to respond to impacts of changing climate and disturbances.
- Field, J. P., Breshears, D. D., Law, D. J., Villegas, J. C., Lopez Hoffman, L. -., Brooks, P. D., Chorover, J., Barron-Gafford, G. A., Gallery, R. E., Litvak, M. E., Lybrand, R., Mcintosh, J. C., Meixner, T. -., Niu, Y. -., Papuga, S. A., Pelletier, J. D., Rasmussen, C. -., & Troch, P. A. (2015). Critical zone services: Expanding context, constraints, and curency beyond ecosystem services.. Vadose Zone Journal, 1-7.
- Field, J. P., Breshears, D. D., Law, D. J., Villegas, J. C., Lopez-Hoffman, L., Brooks, P. D., Chorover, J., & Pelletier, J. D. (2016). Understanding ecosystem services from a geosciences perspective. Eos, 97. doi:10.1029/2016EO043591
- Field, J. P., Breshears, D. D., Law, D. J., Villegas, J. C., Lopez-Hoffman, L., Brooks, P. D., Chorover, J., Barron-Gafford, G. A., Gallery, R. E., Litvak, M. E., Lybrand, R. A., McIntosh, J. C., Meixner, T., Niu, G., Papuga, S. A., Pelletier, J. D., Rasmussen, C. R., & Troch, P. A. (2015). Critical Zone Services: Expanding Context, Constraints, and Currency beyond Ecosystem Services. VADOSE ZONE JOURNAL, 14(1).
- Magliano, P. N., Breshears, D. D., Fernandez, R. J., & Jobbagy, E. G. (2015). Rainfall intensity switches ecohydrological runoff/runon redistribution patterns in dryland vegetation patches. ECOLOGICAL APPLICATIONS, 25(8), 2094-2100.
- Pangle, L. A., Delong, S. B., Abramson, N., Adams, J., Barron-Gafford, G. A., Breshears, D. D., Brooks, P. D., Chorover, J. D., Dietrich, W. E., Dontsova, K. M., Durcik, M., Espleta, J., Ferre, P. A., Ferriere, R. H., Henderson, W., Hunt, E. A., Huxman, T. E., Millar, D., Murphy, B., , Niu, G., et al. (2015). The Landscape Evolution Observatory: A large-scale controllable infrastructure to study Earth-surface processes.. Geomorphology, 244, 190-203. doi:10.1016/j.geomorph.2015.01.020
- Pangle, L., DeLong, S., Abramson, N., Adams, J., Barron-Gafford, G. A., Breshears, D. D., Brooks, P. D., Chorover, J. D., Dietrich, W. E., Dontsova, K. M., Durcik, M., Espeleta, J., Ferre, P. A., Ferriere, R. H., Henderson, W., Hunt, E., Huxman, T. E., Millar, D., Murphy, B., , Niu, Y., et al. (2015). The Landscape Evolution Observatory: A large-scale controllable infrastructure to study coupled Earth-surface processes. Geomorphology, 244, 190-203.
- Villegas, J. C., Dominguez, F., Barron-Gafford, G. A., Adams, H. D., Guardiola-Claramonte, M., Sommer, E. D., Selvey, A. W., Espeleta, J. F., Zou, C. B., Breshears, D. D., & Huxman, T. E. (2015). Sensitivity of regional evapotranspiration partitioning to variation in woody plant cover: insights from experimental dryland tree mosaics. GLOBAL ECOLOGY AND BIOGEOGRAPHY, 24(9), 1040-1048.
- Barnes, P. W., Throop, H. L., Archer, S. R., Breshears, D. D., McCulley, R. L., & Tobler, M. A. (2014). Sunlight and soil-litter mixing: drivers of litter decomposition in drylands. Progress in Botany.
- Camillo Villegas, J., Espeleta, J. E., Morrison, C. T., Breshears, D. D., & Huxman, T. E. (2014). Factoring in canopy cover heterogeneity on evapotranspiration partitioning: Beyond big-leaf surface homogeneity assumptions. Journal of Soil and Water Conservation, 69(3), 78A-83A.
- Field, J. P., Breshears, D. D., Law, D. J., Villegas, J., Lopez Hoffman, L. -., Brooks, P. D., Chorover, J., Barron-Gafford, G. A., Gallery, R. E., Litvak, M. E., Lybrand, R., Mcintosh, J. C., Meixner, T. -., Niu, Y. -., Papuga, S. A., Pelletier, J. D., Rasmussen, C. -., & Troch, P. A. (2014). Critical Zone Services: Expanding Context, Constraints, and Currency beyond Ecosystem Services. Vadose Zone Journal.
- Merino-Martin, L., Field, J. P., Villegas, J. C., Whicker, J. J., Breshears, D. D., Law, D. J., & Urgeghe, A. M. (2014). Interplay between particle size and temporal scale for predominant background wind conditions: Insights from burned vs. unburned semiarid grassland adjacent to urbanization. Aeolian Research.
- Merino-Martín, L., Field, J. P., Villegas, J. C., Whicker, J. J., Breshears, D. D., Law, D. J., & Urgeghe, A. M. (2014). Aeolian sediment and dust fluxes during predominant "background" wind conditions for unburned and burned semiarid grassland: Interplay between particle size and temporal scale. Aeolian Research.More infoAbstract: Monitoring of aeolian transport is needed for assessment and management of human health risks as well as for soil resources. Human health risks are assessed based on duration of exposure as well as concentration. Many aeolian studies focus on periods of high wind speed when concentrations are greatest but few studies focus on "background" conditions when concentrations are likely lower but which represent the most prevalent conditions. Such "background" conditions might be especially important at sites with recent disturbance such as fire. Exposure assessments also require improved understanding relating longer-term (days to weeks) measurements of saltation of larger particles to shorter-term (minutes to hours) measurements of smaller inhalable dust particles. To address these issues, we employed three commonly used instruments for measuring dust emissions for unburned and recently-burned sites: Big Springs Number Eight (BSNE) samplers for larger saltating soil particles (>50 μm) with weekly to monthly sampling resolution, DustTraks for suspended particles (diameters
- Niu, G., Pasetto, D., Scudeler, C., Paniconi, C., Putti, M., Troch, P. A., DeLong, S. B., Dontsova, K., Pangle, L., Breshears, D. D., Chorover, J., Huxman, T. E., Pelletier, J., Saleska, S. R., & Zeng, X. (2014). Incipient subsurface heterogeneity and its effect on overland flow generation – insight from a modeling study of the first experiment at the Biosphere 2 Landscape Evolution Observatory. Hydrology and Earth System Sciences.
- Niu, Y., Pasetto, D., Scudeler, C., Paniconi, C., Putti, M., Troch, P. A., DeLong, S., Dontsova, K. M., Pangle, L., Breshears, D. D., Chorover, J. D., Huxman, T. E., Pelletier, J. D., Saleska, S. R., & Zeng, X. (2014). Incipient subsurface heterogeneity and its effect on overland flow generation -- insight from a modeling study of the first experiment at the Biosphere 2 Landscape Evolution Observatory. Hydrol. Earth Syst. Sci., 18, 1873-1883.
- Villegas, J. C., Espeleta, J. E., Morrison, C. T., Breshears, D. D., & Huxman, T. E. (2014). Factoring in canopy cover heterogeneity on evapotranspiration partitioning: Beyond the big-leaf surface homogeneity assumptions. Journal of Soil and Water Conservation, 69(3), 78A-83A.
- Whicker, J. J., Breshears, D. D., & Field, J. P. (2014). Progress on relationships between horizontal and vertical dust flux: Mathematical, empirical and risk-based perspectives. Aeolian Research.More infoSpecial Issue:
- Adams, H. D., Germino, M. J., Breshears, D. D., Barron-Gafford, G. A., Guardiola-Claramonte, M., Zou, C. B., & Huxman, T. E. (2013). Non-structural leaf carbohydrate dynamics of Pinus edulis during drought-induced tree mortality reveal role for carbon metabolism in mortality mechanism. New Phytologist, 197(4), 1142-1151.More infoVegetation change is expected with global climate change, potentially altering ecosystem function and climate feedbacks. However, causes of plant mortality, which are central to vegetation change, are understudied, and physiological mechanisms remain unclear, particularly the roles of carbon metabolism and xylem function.We report analysis of foliar nonstructural carbohydrates (NSCs) and associated physiology from a previous experiment where earlier drought-induced mortality of Pinus edulis at elevated temperatures was associated with greater cumulative respiration. Here, we predicted faster NSC decline for warmed trees than for ambient-temperature trees.Foliar NSC in droughted trees declined by 30% through mortality and was lower than in watered controls. NSC decline resulted primarily from decreased sugar concentrations. Starch initially declined, and then increased above pre-drought concentrations before mortality. Although temperature did not affect NSC and sugar, starch concentrations ceased declining and increased earlier with higher temperatures.Reduced foliar NSC during lethal drought indicates a carbon metabolism role in mortality mechanism. Although carbohydrates were not completely exhausted at mortality, temperature differences in starch accumulation timing suggest that carbon metabolism changes are associated with time to death. Drought mortality appears to be related to temperature-dependent carbon dynamics concurrent with increasing hydraulic stress in P. edulis and potentially other similar species.
- Adams, H. D., Germino, M. J., Breshears, D. D., Barron-Gafford, G. A., Guardiola-Claramonte, M., Zou, C. B., & Huxman, T. E. (2013). Nonstructural leaf carbohydrate dynamics of Pinus edulis during drought-induced tree mortality reveal role for carbon metabolism in mortality mechanism. New Phytologist, 197(4), 1142-1151.More infoPMID: 23311898;Abstract: Vegetation change is expected with global climate change, potentially altering ecosystem function and climate feedbacks. However, causes of plant mortality, which are central to vegetation change, are understudied, and physiological mechanisms remain unclear, particularly the roles of carbon metabolism and xylem function. We report analysis of foliar nonstructural carbohydrates (NSCs) and associated physiology from a previous experiment where earlier drought-induced mortality of Pinus edulis at elevated temperatures was associated with greater cumulative respiration. Here, we predicted faster NSC decline for warmed trees than for ambient-temperature trees. Foliar NSC in droughted trees declined by 30% through mortality and was lower than in watered controls. NSC decline resulted primarily from decreased sugar concentrations. Starch initially declined, and then increased above pre-drought concentrations before mortality. Although temperature did not affect NSC and sugar, starch concentrations ceased declining and increased earlier with higher temperatures. Reduced foliar NSC during lethal drought indicates a carbon metabolism role in mortality mechanism. Although carbohydrates were not completely exhausted at mortality, temperature differences in starch accumulation timing suggest that carbon metabolism changes are associated with time to death. Drought mortality appears to be related to temperature-dependent carbon dynamics concurrent with increasing hydraulic stress in P. edulis and potentially other similar species. © 2013 The Authors. New Phytologist © 2013 New Phytologist Trust.
- Breshears, D. D., Adams, H. D., Eamus, D., McDowell, N. G., Law, D. J., Will, R. E., Williams, A. P., & Zou, C. B. (2013). The critical amplifying role of increasing atmospheric demand on tree mortality and associated regional die-off. Frontiers in Plant Science, 4.More infoNew journal - no impact factor yet.
- Clifford, M. J., Royer, P. D., Cobb, N. S., Breshears, D. D., & Ford, P. L. (2013). Precipitation thresholds and drought-induced tree die-off: Insights from patterns of Pinus edulis mortality along an environmental stress gradient. New Phytologist, 200(2), 413-421.More infoPMID: 23772860;Abstract: Summary: Recent regional tree die-off events appear to have been triggered by a combination of drought and heat - referred to as 'global-change-type drought'. To complement experiments focused on resolving mechanisms of drought-induced tree mortality, an evaluation of how patterns of tree die-off relate to highly spatially variable precipitation is needed. Here, we explore precipitation relationships with a die-off event of pinyon pine (Pinus edulis Engelm.) in southwestern North America during the 2002-2003 global-change-type drought. Pinyon die-off and its relationship with precipitation was quantified spatially along a precipitation gradient in north-central New Mexico with standard field plot measurements of die-off combined with canopy cover derived from normalized burn ratio (NBR) from Landsat imagery. Pinyon die-off patterns revealed threshold responses to precipitation (cumulative 2002-2003) and vapor pressure deficit (VPD), with little to no mortality (< 10%) above 600 mm and below warm season VPD of c. 1.7 kPa. [Correction added after online publication 17 June 2013; in the preceding sentence, the word 'below' has been inserted.] Our results refine how precipitation patterns within a region influence pinyon die-off, revealing a precipitation and VPD threshold for tree mortality and its uncertainty band where other factors probably come into play - a response type that influences stand demography and landscape heterogeneity and is of general interest, yet has not been documented. © 2013 New Phytologist Trust.
- Clifford, M. J., Royer, P. D., Cobb, N. S., Breshears, D. D., & Ford, P. L. (2013). Precipitation thresholds and drought-induced tree die-off: insights from patterns of Pinus edulis mortality along an environmental stress gradient. New Phytologist, 200(2), 413-421.More infoRecent regional tree die-off events appear to have been triggered by a combination of drought and heat - referred to as global-change-type drought'. To complement experiments focused on resolving mechanisms of drought-induced tree mortality, an evaluation of how patterns of tree die-off relate to highly spatially variable precipitation is needed. Here, we explore precipitation relationships with a die-off event of pinyon pine (Pinus edulis Engelm.) in southwestern North America during the 2002-2003 global-change-type drought. Pinyon die-off and its relationship with precipitation was quantified spatially along a precipitation gradient in north-central New Mexico with standard field plot measurements of die-off combined with canopy cover derived from normalized burn ratio (NBR) from Landsat imagery. Pinyon die-off patterns revealed threshold responses to precipitation (cumulative 2002-2003) and vapor pressure deficit (VPD), with little to no mortality (
- Eamus, D., Boulain, N., Cleverly, J., & Breshears, D. D. (2013). Global change-type drought-induced tree mortality: Vapor pressure deficit is more important than temperature per se in causing decline in tree health. Ecology and Evolution, 3(8), 2711-2729.More infoAbstract: Drought-induced tree mortality is occurring across all forested continents and is expected to increase worldwide during the coming century. Regional-scale forest die-off influences terrestrial albedo, carbon and water budgets, and landsurface energy partitioning. Although increased temperatures during drought are widely identified as a critical contributor to exacerbated tree mortality associated with "global-change-type drought", corresponding changes in vapor pressure deficit (D) have rarely been considered explicitly and have not been disaggregated from that of temperature per se. Here, we apply a detailed mechanistic soil-plant-atmosphere model to examine the impacts of drought, increased air temperature (+2°C or +5°C), and increased vapor pressure deficit (D; +1 kPa or +2.5 kPa), singly and in combination, on net primary productivity (NPP) and transpiration and forest responses, especially soil moisture content, leaf water potential, and stomatal conductance. We show that increased D exerts a larger detrimental effect on transpiration and NPP, than increased temperaturealone, with or without the imposition of a 3-month drought. Combined with drought, the effect of increased D on NPP was substantially larger than that of drought plus increased temperature. Thus, the number of days when NPP was zero across the 2-year simulation was 13 or 14 days in the control and increased temperature scenarios, but increased to approximately 200 days when D was increased. Drought alone increased the number of days of zero NPP to 88, but drought plus increased temperature did not increase the number of days. In contrast, drought and increased D resulted in the number of days when NPP = 0 increasing to 235 (+1 kPa) or 304 days (+2.5 kPa). We conclude that correct identification of the causes of global change-type mortality events requires explicit consideration of the influence of D as well as its interaction with drought and temperature. © 2013 The Authors.
- Eamus, D., Boulain, N., Cleverly, J., & Breshears, D. D. (2013). Global change-type drought-induced tree mortality: vapor pressure deficit is more important than temperature per se in causing decline in tree health. Ecology and Evolution.More infoDrought-induced tree mortality is occurring across all forested continents andis expected to increase worldwide during the coming century. Regional-scaleforest die-off influences terrestrial albedo, carbon and water budgets, and landsurfaceenergy partitioning. Although increased temperatures during droughtare widely identified as a critical contributor to exacerbated tree mortality associatedwith “global-change-type drought”, corresponding changes in vapor pressuredeficit (D) have rarely been considered explicitly and have not beendisaggregated from that of temperature per se. Here, we apply a detailed mechanisticsoil–plant–atmosphere model to examine the impacts of drought,increased air temperature (+2°C or +5°C), and increased vapor pressure deficit(D; +1 kPa or +2.5 kPa), singly and in combination, on net primary productivity(NPP) and transpiration and forest responses, especially soil moisture content,leaf water potential, and stomatal conductance. We show that increased Dexerts a larger detrimental effect on transpiration and NPP, than increased temperaturealone, with or without the imposition of a 3-month drought. Combinedwith drought, the effect of increased D on NPP was substantially largerthan that of drought plus increased temperature. Thus, the number of dayswhen NPP was zero across the 2-year simulation was 13 or 14 days in thecontrol and increased temperature scenarios, but increased to approximately200 days when D was increased. Drought alone increased the number of daysof zero NPP to 88, but drought plus increased temperature did not increase thenumber of days. In contrast, drought and increased D resulted in the numberof days when NPP = 0 increasing to 235 (+1 kPa) or 304 days (+2.5 kPa). Weconclude that correct identification of the causes of global change-type mortalityevents requires explicit consideration of the influence of D as well as itsinteraction with drought and temperature.
- Lopez Hoffman, L. -., Breshears, D. D., Allen, C. D., & Miller, M. L. (2013). Key landscape ecology metrics for assessing climate change adaptation options: rate of change and patchiness of impacts. Ecosphere, 4(8), Article 101, pages 1-18.More infoSpecial Feature: Sustainability on the Border.New journal - no impact factor yet.
- Michelotti, E. A., Whicker, J. J., Eisele, W. F., Breshears, D. D., & Kirchner, T. B. (2013). Modeling aeolian transport of soil-bound plutonium: Considering infrequent but normal environmental disturbances is critical in estimating future dose. Journal of Environmental Radioactivity, 120, 73-80.More infoPMID: 23455230;Abstract: Dose assessments typically consider environmental systems as static through time, but environmental disturbances such as drought and fire are normal, albeit infrequent, events that can impact dose-influential attributes of many environmental systems. These phenomena occur over time frames of decades or longer, and are likely to be exacerbated under projected warmer, drier climate. As with other types of dose assessment, the impacts of environmental disturbances are often overlooked when evaluating dose from aeolian transport of radionuclides and other contaminants. Especially lacking are predictions that account for potential changing vegetation cover effects on radionuclide transport over the long time frames required by regulations. A recently developed dynamic wind-transport model that included vegetation succession and environmental disturbance provides more realistic long-term predictability. This study utilized the model to estimate emission rates for aeolian transport, and compare atmospheric dispersion and deposition rates of airborne plutonium-contaminated soil into neighboring areas with and without environmental disturbances. Specifically, the objective of this study was to utilize the model results as input for a widely used dose assessment model (CAP-88). Our case study focused on low levels of residual plutonium found in soils from past operations at Los Alamos National Laboratory (LANL), in Los Alamos, NM, located in the semiarid southwestern USA. Calculations were conducted for different disturbance scenarios based on conditions associated with current climate, and a potential future drier and warmer climate. Known soil and sediment concentrations of plutonium were used to model dispersal and deposition of windblown residual plutonium, as a function of distance and direction. Environmental disturbances that affected vegetation cover included ground fire, crown fire, and drought, with reoccurrence rates for current climate based on site historical patterns. Using site-specific meteorology, accumulation rates of plutonium in soil were modeled in a variety of directions and distances from LANL sources. Model results suggest that without disturbances, areas downwind to the contaminated watershed would accumulate LANL-derived plutonium at a relatively slow rate (
- Michelotti, E. A., Whicker, J. J., Eisele, W. F., Breshears, D. D., & Kirchner, T. B. (2013). Modeling aeolian transport of soil-bound plutonium: considering infrequent but normal environmental disturbances is critical in estimating future dose. Journal of Environmental Radioactivity, 120, 73-80.More infoDose assessments typically consider environmental systems as static through time, but environmental disturbances such as drought and fire are normal, albeit infrequent, events that can impact dose-influential attributes of many environmental systems. These phenomena occur over time frames of decades or longer, and are likely to be exacerbated under projected warmer, drier climate. As with other types of dose assessment, the impacts of environmental disturbances are often overlooked when evaluating dose from aeolian transport of radionuclides and other contaminants. Especially lacking are predictions that account for potential changing vegetation cover effects on radionuclide transport over the long time frames required by regulations. A recently developed dynamic wind-transport model that included vegetation succession and environmental disturbance provides more realistic long-term predictability. This study utilized the model to estimate emission rates for aeolian transport, and compare atmospheric dispersion and deposition rates of airborne plutonium-contaminated soil into neighboring areas with and without environmental disturbances. Specifically, the objective of this study was to utilize the model results as input for a widely used dose assessment model (CAP-88). Our case study focused on low levels of residual plutonium found in soils from past operations at Los Alamos National Laboratory (LANL), in Los Alamos, NM, located in the semiarid southwestern USA. Calculations were conducted for different disturbance scenarios based on conditions associated with current climate, and a potential future drier and warmer climate. Known soil and sediment concentrations of plutonium were used to model dispersal and deposition of windblown residual plutonium, as a function of distance and direction. Environmental disturbances that affected vegetation cover included ground fire, crown fire, and drought, with reoccurrence rates for current climate based on site historical patterns. Using site-specific meteorology, accumulation rates of plutonium in soil were modeled in a variety of directions and distances from LANL sources. Model results suggest that without disturbances, areas downwind to the contaminated watershed would accumulate LANL-derived plutonium at a relatively slow rate (
- Pelletier, J. D., Barron-Gafford, G. A., Breshears, D. D., Brooks, P. D., Chorover, J., Durcik, M., Harman, C. J., Huxman, T. E., Lohse, K. A., Lybrand, R., Meixner, T., Mcintosh, J. C., Papuga, S. A., Rasmussen, C., Schaap, M. G., Swetnam, T. W., & Troch, P. A. (2013). Coevolution of nonlinear trends in vegetation, soils, and topography with elevation and slope aspect: A case study in the sky islands of southern Arizona. J. Geophys. Res. - Earth Surf., 118, 741-758.
- Pelletier, J. D., Pelletier, J. D., Breshears, D. D., Breshears, D. D., Barron-Gafford, G. A., Barron-Gafford, G. A., Brooks, P. D., Brooks, P. D., Chorover, J., Chorover, J. D., Durick, M., Durick, M., Harman, C. J., Harman, C. J., Huxman, T. E., Huxman, T. E., Lohse, K. A., Lohse, K. A., Lybrand, R., , Lybrand, R., et al. (2013). Coevolution of nonlinear trends in vegetation, soils, and topography with elevation and slope aspect: A case study in the sky islands of southern Arizona. Journal of Geophysical Research - Earth Surface, 118(2), 1-18.More infoFeedbacks among vegetation dynamics, pedogenesis, and topographic development affect the “critical zone”—the living filter for Earth's hydrologic, biogeochemical, and rock/sediment cycles. Assessing the importance of such feedbacks, which may be particularly pronounced in water-limited systems, remains a fundamental interdisciplinary challenge. The sky islands of southern Arizona offer an unusually well-defined natural experiment involving such feedbacks because mean annual precipitation varies by a factor of five over distances of approximately 10 km in areas of similar rock type (granite) and tectonic history. Here we compile high-resolution, spatially distributed data for Effective Energy and Mass Transfer (EEMT: the energy available to drive bedrock weathering), above-ground biomass, soil thickness, hillslope-scale topographic relief, and drainage density in two such mountain ranges (Santa Catalina: SCM; Pinaleño: PM). Strong correlations exist among vegetation-soil-topography variables, which vary nonlinearly with elevation, such that warm, dry, low-elevation portions of these ranges are characterized by relatively low above-ground biomass, thin soils, minimal soil organic matter, steep slopes, and high drainage densities; conversely, cooler, wetter, higher elevations have systematically higher biomass, thicker organic-rich soils, gentler slopes, and lower drainage densities. To test if eco-pedo-geomorphic feedbacks drive this pattern, we developed a landscape evolution model that couples pedogenesis and topographic development over geologic time scales, with rates explicitly dependent on vegetation density. The model self-organizes into states similar to those observed in SCM and PM. Our results highlight the potential importance of eco-pedo-geomorphic feedbacks, mediated by soil thickness, in water-limited systems.
- Sankey, J. B., Law, D. J., Breshears, D. D., Munson, S. M., & Webb, R. H. (2013). Employing lidar to detail vegetation canopy architecture for prediction of aeolian transport. Geophysical Research Letters, 40(9), 1724-1728.More infoAbstract: The diverse and fundamental effects that aeolian processes have on the biosphere and geosphere are commonly generated by horizontal sediment transport at the land surface. However, predicting horizontal sediment transport depends on vegetation architecture, which is difficult to quantify in a rapid but accurate manner. We demonstrate an approach to measure vegetation canopy architecture at high resolution using lidar along a gradient of dryland sites ranging from 2% to 73% woody plant canopy cover. Lidar-derived canopy height, distance (gaps) between vegetation elements (e.g., trunks, limbs, leaves), and the distribution of gaps scaled by vegetation height were correlated with canopy cover and highlight potentially improved horizontal dust flux estimation than with cover alone. Employing lidar to estimate detailed vegetation canopy architecture offers promise for improved predictions of horizontal sediment transport across heterogeneous plant assemblages. Key Points Aeolian processes in the bio and geosphere are driven by horizontal dust fluxPredicting dust flux depends on detailed vegetation architecture measurementsAccurate and rapid lidar measurements of vegetation enable dust flux estimation ©2013. American Geophysical Union. All Rights Reserved.
- Sankey, J. B., Law, D. J., Breshears, D. D., Munson, S. M., & Webb, R. H. (2013). Employing lidar to detail vegetation canopy architecture for prediction of aeolian transport. Geophysical Research Letters, 40, 1724-1728.More infoThe diverse and fundamental effects that aeolianprocesses have on the biosphere and geosphere arecommonly generated by horizontal sediment transport atthe land surface. However, predicting horizontal sedimenttransport depends on vegetation architecture, which isdifficult to quantify in a rapid but accurate manner. Wedemonstrate an approach to measure vegetation canopyarchitecture at high resolution using lidar along a gradientof dryland sites ranging from 2% to 73% woody plantcanopy cover. Lidar-derived canopy height, distance (gaps)between vegetation elements (e.g., trunks, limbs, leaves),and the distribution of gaps scaled by vegetation heightwere correlated with canopy cover and highlight potentiallyimproved horizontal dust flux estimation than with coveralone. Employing lidar to estimate detailed vegetationcanopy architecture offers promise for improved predictionsof horizontal sediment transport across heterogeneous plantassemblages.
- , J., Lloret, F., & Breshears, D. (2012). Drought-induced forest decline: causes, scope and implications. Biology Letters: Meeting Report, 8, 689-691.More infoDOI: 10.1098/rsbl.2011.1059. [2010 ISI Journal Impact Factor 3.821]; updated vers: DOI: 10.1098/rsbl.2011.1059. [2011 ISI Journal Impact Factor 3.762]
- Adams, H. D., Luce, C. H., Breshears, D. D., Allen, C. D., Weiler, M., Hale, V. C., Smith, A. M., & Huxman, T. E. (2012). Ecohydrological consequences of drought- and infestation- triggered tree die-off: Insights and hypotheses. Ecohydrology, 5(2), 145-159.More infoAbstract: Widespread, rapid, drought-, and infestation-triggered tree mortality is emerging as a phenomenon affecting forests globally and may be linked to increasing temperatures and drought frequency and severity. The ecohydrological consequences of forest die-off have been little studied and remain highly uncertain. To explore this knowledge gap, we apply the extensive literature on the ecohydrological effects of tree harvest in combination with the limited existing die-off ecohydrology research to develop new, relevant hypotheses. Tree mortality results in loss of canopy cover, which directly alters evaporation, transpiration, and canopy interception and indirectly alters other watershed hydrologic processes, including infiltration, runoff, groundwater recharge, and streamflow. Both die-off and harvest research suggest that for most forests, water yield can be expected to increase following substantial loss of tree cover by die-off. We hypothesize that where annual precipitation exceeds ∼500 mm or water yield is dominated by snowmelt, watersheds will experience significantly decreased evapotranspiration and increased flows if absolute canopy cover loss from die-off exceeds 20%. However, recent observations suggest that water yield following die-off can potentially decrease rather than increase in drier forests. To reliably predict die-off responses, more research is needed to test these hypotheses, including observations of multiple water budget components and the persistence of ecohydrological effects with the post-die-off successional dynamics of tree recruitment, understorey growth, and interactions with additional disturbances. With die-off, mitigation and restoration options are limited and costly, necessitating societal adaptation; therefore, die-off ecohydrology should be a high priority for future research. © 2011.
- Adams, H., Germino, M., Breshears, D., Barron-Gafford, G., Guardiola-Claramonte, M., Zou, C., & Huxman, T. (2012). Non-structural leaf carbohydrate dynamics of Pinus edulis during drought-induced tree mortality reveal role for C metabolism in mortality mechanism. New Phytologist, 197, 1142-1151.More infodoi:10.1111/nph.12102. [2011 ISI Journal Impact Factor: 6.645].
- Adams, H., Germino, M., Breshears, D., Barron-Gafford, G., Guardiola-Claramonte, M., Zou, C., & Huxman, T. (2012). Non-structural leaf carbohydrate dynamics of Pinus edulis during drought-induced tree mortality reveal role for C metabolism in mortality mechanism. New Phytologist.More infodoi:10.1111/nph.12102. [2011 ISI Journal Impact Factor: 6.645].
- Adams, H., Luce, C., Breshears, D., Allen, C., Weiler, M., Hale, V., Smith, A., & Huxman, T. (2012). Ecohydrological consequences of drought- and infestation-triggered tree die-off: insights and hypotheses. Ecohydrology, 5(2), 145-159.More infoSpecial feature: Ecohydrologic Connections and Complexities in Drylands New Perspectives for Understanding Transformative Landscape Change. DOI: 10.1002/eco.233. [2011 ISI Journal Impact Factor: 2.133].
- Breshears, D. D., Kirchner, T. B., Whicker, J. J., Field, J. P., & Allen, C. D. (2012). Modeling aeolian transport in response to succession, disturbance and future climate: Dynamic long-term risk assessment for contaminant redistribution. Aeolian Research, 3(4), 445-457.More infoAbstract: Aeolian sediment transport is a fundamental process redistributing sediment, nutrients, and contaminants in dryland ecosystems. Over time frames of centuries or longer, horizontal sediment fluxes and associated rates of contaminant transport are likely to be influenced by succession, disturbances, and changes in climate, yet models of horizontal sediment transport that account for these fundamental factors are lacking, precluding in large part accurate assessment of human health risks associated with persistent soil-bound contaminants. We present a simple model based on empirical measurements of horizontal sediment transport (predominantly saltation) to predict potential contaminant transport rates for recently disturbed sites such as a landfill cover. Omnidirectional transport is estimated within vegetation that changes using a simple Markov model that simulates successional trajectory and considers three types of short-term disturbances (surface fire, crown fire, and drought-induced plant mortality) under current and projected climates. The model results highlight that movement of contaminated soil is sensitive to vegetation dynamics and increases substantially (e.g., > fivefold) when disturbance and/or future climate are considered. The time-dependent responses in horizontal sediment fluxes and associated contaminant fluxes were sensitive to variability in the timing of disturbance, with longer intervals between disturbance allowing woody plants to become dominant and crown fire and drought abruptly reducing woody plant cover. Our results, which have direct implications for contaminant transport and landfill management in the specific context of our assessment, also have general relevance because they highlight the need to more fully account for vegetation dynamics, disturbance, and changing climate in aeolian process studies. © 2011.
- Breshears, D., Kirchner, T., Whicker, J., Field, ., & Allen, C. (2012). Modeling aeolian transport in response to succession, disturbance and future climate: Dynamic long-term risk assessment for contaminant redistribution. Aeolian Research, 3, 445-457.More infoSpecial Issue: International Aeolian Research Conference VI. [2011 ISI Journal Impact Factor: 2.179].
- Davison, J. E., Graumlich, L. J., Rowland, E. L., Pederson, G. T., & Breshears, D. D. (2012). Leveraging modern climatology to increase adaptive capacity across protected area networks. Global Environmental Change, 22(1), 268-274.More infoAbstract: Human-driven changes in the global environment pose an increasingly urgent challenge for the management of ecosystems that is made all the more difficult by the uncertain future of both environmental conditions and ecological responses. Land managers need strategies to increase regional adaptive capacity, but relevant and rapid assessment approaches are lacking. To address this need, we developed a method to assess regional protected area networks across biophysically important climatic gradients often linked to biodiversity and ecosystem function. We plot the land of the southwestern United States across axes of historical climate space, and identify landscapes that may serve as strategic additions to current protected area portfolios. Considering climate space is straightforward, and it can be applied using a variety of relevant climate parameters across differing levels of land protection status. The resulting maps identify lands that are climatically distinct from existing protected areas, and may be utilized in combination with other ecological and socio-economic information essential to collaborative landscape-scale decision-making. Alongside other strategies intended to protect species of special concern, natural resources, and other ecosystem services, the methods presented herein provide another important hedging strategy intended to increase the adaptive capacity of protected area networks. © 2011 Elsevier Ltd.
- Davison, J., Davison, J., Graumlich, L., Graumlich, L., Rowland, E., Rowland, E., Pederson, G., Pederson, G., Breshears, D., & Breshears, D. (2012). Leveraging modern climatology to increase adaptive capacity across protected area networks. Global Environmental Change, 22, 268-274.More infoDOI:10.1016/j.gloenvcha.2011.10.002. [2010 ISI Journal Impact Factor: 4.918]
- Field, J. P., Breshears, D. D., Whicker, J. J., & Zou, C. B. (2012). Sediment capture by vegetation patches: Implications for desertification and increased resource redistribution. Journal of Geophysical Research G: Biogeosciences, 117(1).More infoAbstract: Desertification impacts a large proportion of drylands and can be driven by a variety of climate and land use factors. Most conceptual models of desertification include the underlying assumption that when herbaceous cover is reduced, increased erosion from bare patches is redistributed to shrub canopy patches, resulting in self-reinforcing "islands of fertility." Notably, however, this underlying assumption has not been explicitly tested with direct field measurements. Here we provide direct measurements of horizontal sediment flux moving into and out of bare-, herbaceous-, and shrub-dominated patch types in a semiarid ecosystem for both simulated and natural dust events, as well as in response to simulated disturbance. Horizontal sediment flux out of the bare patches was ∼20% greater than the herbaceous patches and ∼50% greater than sediment flux out of the shrub-dominated patches. Differences among vegetation patch types indicate that shrub patches capture more sediment than herbaceous patches and, importantly, that bare patches serve as amplified sediment sources following disturbance. Our results provide explicit support for the pervasive but untested desertification redistribution assumption, highlighting that loss of grass cover is a compounding problem that not only increases dust emissions but also precludes capture, and may have global relevance for coupled human-environmental systems at risk due to current or potential desertification. Copyright 2012 by the American Geophysical Union.
- Field, J., Breshears, D., Whicker, J., & Zou, C. (2012). Sediment capture by vegetation patches: Implications for desertification and increased resource redistribution. Journal of Geophysical Research - Biogeoscience, 117.More infoG01033. DOI:10.1029/2011JG001663. [2011 Impact factor: 3.021].
- Law, D. J., Breshears, D. D., Ebinger, M. H., Meyer, C. W., & Allen, C. D. (2012). Soil C and N patterns in a semiarid piñon-juniper woodland: Topography of slope and ephemeral channels add to canopy-intercanopy heterogeneity. Journal of Arid Environments, 79, 20-24.More infoAbstract: Carbon and nitrogen are crucial to semiarid woodlands, determining decomposition, production and redistribution of water and nutrients. Carbon and nitrogen are often greater beneath canopies than intercanopies. Upslope vs. downslope position and ephemeral channels might also cause variation in C and N. Yet, few studies have simultaneously evaluated spatial variation associated with canopy-intercanopy patches and topography. We estimated C and N upslope and downslope in an eroding piñon-juniper woodland for canopies beneath piñons (Pinus edulis) and junipers, (Juniperus monosperma), intercanopies, and ephemeral channels. Soil C and N in the surface and profile beneath canopies exceeded that of intercanopies and channels. Relative to intercanopies, channels had more profile C upslope but less downslope (profile N was not significant). Relative to upslope, profile C downslope for intercanopies was greater and for channels was less (profile N was not significant). Relative to profile, surface soil C and N exhibited less heterogeneity. Although some topographic heterogeneity was detected, results did not collectively support our redistribution hypotheses, and we are unable to distinguish if this heterogeneity is due to in situ or redistribution effects. Nonetheless, results highlight finer topographical spatial variation in addition to predominant canopy and intercanopy variation that is applicable for semiarid woodland management. © 2011 Elsevier Ltd.
- Law, D., Breshears, D., Ebinger, M., Myer, C., & Allen, C. (2012). Journal of Arid Environments, 79, 204-224.More infoDOI:10.1016/j.jaridenv.2011.11.029. [2010 ISI Journal Impact Factor: 1.535].
- Martínez-Vilalta, J., Lloret, F., & Breshears, D. D. (2012). Drought-induced forest decline: Causes, scope and implications. Biology Letters, 8(5), 689-691.More infoPMID: 22171020;PMCID: PMC3440957;Abstract: A large number of episodes of forest mortality associated with drought and heat stress have been detected worldwide in recent decades, suggesting that some of the world's forested ecosystems may be already responding to climate change. Here, we summarize a special session titled 'Drought-induced forest decline: causes, scope and implications' within the 12th European Ecological Federation Congress, held in Ávila (Spain) from 25 to 29 September 2011. The session focused on the interacting causes and impacts of die-off episodes at the community and ecosystem levels, and highlighted recent events of drought- and heat-related tree decline, advances in understanding mechanisms and in predicting mortality events, and diverse consequences of forest decline. Talks and subsequent discussion noted a potentially important role of carbon that may be interrelated with plant hydraulics in the multi-faceted process leading to drought-induced mortality; a substantial and yet understudied capacity of many forests to cope with extreme climatic events; and the difficulty of separating climate effects from other anthropogenic changes currently shaping forest dynamics in many regions of the Earth. The need for standard protocols and multi-level monitoring programmes to track the spatiotemporal scope of forest decline globally was emphasized as critical for addressing this emerging environmental issue. © 2011 The Royal Society.
- Merino-Martin, L., Merino-Martin, L., Breshears, D., Breshears, D., Moreno-de, l., Moreno-de, l., Villegas, J., Villegas, J., Perez-Domingo, S., Perez-Domingo, S., Espigares, T., Espigares, T., Nicolau, J., & Nicolau, J. (2012). Ecohydrological source-sink interrelationships between vegetation patches and soil hydrological properties ... reveal a restoration threshold. Ecological Restoration, 20.More infoDOI: 10.1111/j.1526-100X.2011.00776.x. [2010 ISI Journal Impact Factor: 1.927].
- Merino-Martín, L., Breshears, D. D., Moreno-de, M., Villegas, J. C., Pérez-Domingo, S., Espigares, T., & Nicolau, J. M. (2012). Ecohydrological Source-Sink Interrelationships between Vegetation Patches and Soil Hydrological Properties along a Disturbance Gradient Reveal a Restoration Threshold. Restoration Ecology, 20(3), 360-368.More infoAbstract: Vegetation, soil, and hydrology in drylands often collectively exhibit strong ecohydrological interrelationships in which vegetation both influences and is influenced by runoff, particularly on sites with more gradual slopes. These two-way relationships have important implications for ecological restoration of disturbed sites, such as those being reclaimed following mining, yet studies from both ecological and hydrological perspectives specifically evaluating how the strength of ecohydrological interrelationships varies for a range of natural and restored conditions are still missing. We assessed two-way relationships between vegetation and soil hydrological properties by evaluating patterns of both plant community structure and soil hydrological characteristics related to runoff for natural sites and restored sites following mining. At the plot scale, we identified eight ecohydrological units based on interrelationships between vegetation communities and hydrological properties associated with runoff along a progression from source to sink patch types. Similarly, at the hillslope scale, which included patches of different types, we found a correspondence between the proportions of source and sink patches and both vegetation community and hydrological properties. The relative strength of ecohydrological interrelationships in hillslope mosaics decreased with decreasing disturbance except for rilled hillslopes, likely because parts of the hillslope become isolated from the others. Our results highlight, in general, how ecohydrological interrelationships are related with degree of disturbance, and in particular, how rilling alters ecohydrological interrelationships, thereby precluding effective restoration. © 2011 Society for Ecological Restoration International.
- Royer, P. D., Breshears, D. D., Zou, C. B., Villegas, J. C., Cobb, N. S., & Kurc, S. A. (2012). Density-dependent ecohydrological effects of pionjuniper woody canopy cover on soil microclimate and potential soil evaporation. Rangeland Ecology and Management, 65(1), 11-20.More infoAbstract: Many rangeland processes are driven by microclimate and associated ecohydrological dynamics. Most rangelands occur in drylands where evapotranspiration normally dominates the water budget. In these water-limited environments plants can influence abiotic and biotic processes by modifying microclimate factors such as soil temperature and potential soil evaporation. Previous studies have assessed spatial variation in microclimate and associated ecohydrological attributes within an ecosystem (e.g., under vs. between woody canopies) or across ecosystems (e.g., with differing amounts of woody canopy cover), but generally lacking are assessments accounting systematically for both, particularly for evergreen woody plants. Building on recently quantified trends in near-ground solar radiation associated with a pionjuniper gradient spanning 5% to 65% woody canopy cover, we evaluated trends in soil temperature and associated estimates of potential soil evaporation as a function of amount of woody canopy cover for sites overall and for associated canopy vs. intercanopy locations. Quantified soil temperature trends decreased linearly with increasing woody canopy cover for intercanopy as well as canopy patches, indicating the coalescing influence of individual canopies on their neighboring areas. Notably, intercanopy locations within high-density (65%) woody canopy cover could be as much as ∼10°C cooler than intercanopy locations within low-density (5%) cover. Corresponding potential soil evaporation rates in intercanopies within high-density woody canopy cover was less than half that for intercanopies within low density. Our results highlight ecohydrological consequences of density-dependent shading by evergreen woody plants on soil temperature and potential soil evaporation and enable managers to rapidly estimate and compare approximate site microclimates after assessing amounts of woody canopy cover. Such predictions of microclimate have general utility for improving management of rangelands because they are a fundamental driver of many key processes, whether related to understory forage and herbaceous species or to wildlife habitat quality for game or nongame species. © Society for Range Management.
- Royer, P. D., Breshears, D. D., Zou, C. B., Villegas, J. C., Cobb, N., & Kurc, S. A. (2012). Density-dependent ecohydrological effects of pinyon-juniper woody canopy cover on soil microclimate and potential soil evaporation. Rangeland Ecology and Management, 65, 11-20.
- Royer, P., Breshears, D., Zou, C., Villegas, J., & Kurc, S. (2012). Density-dependent ecohydrological effects of pinon-juniper woody canopy cover on soil microclimate and potential soil evaporation. Rangeland Ecology and Management, 65(1), 11-20.More infoDOI:10.2111/REM-D-11-00007.1 [2011 ISI Journal Impact Factor: 1.461].
- Royer, P., Breshears, D., Zou, C., Villegas, J., Cobb, N., & Kurc, S. (2012). Rangeland Ecology and Management, 65, 11-20.More info[2010 ISI Journal Impact Factor: 1.438]
- Wilcox, B. P., Seyfried, M. S., Breshears, D. D., & McDonnell, J. J. (2012). Ecohydrologic connections and complexities in drylands: New perspectives for understanding transformative landscape change. Ecohydrology, 5(2), 143-144.
- Wilcox, B., Seyfried, M., Breshears, D., & McDonnell, J. (2012). Ecohydrologic connections and complexities in drylands new perspectives for understanding transformative landscape change. Ecohydrology: Introduction to Special Feature, 5, 143-144.More info[2010 ISI Impact Factor: 1.835]
- Breshears, D. D., Chorover, J., Troch, P., Rasmussen, C., Brooks, P., Pelletier, J., Breshars, D., Huxman, T., Kurc, S., Lohse, K., McIntosh, J., Meixner, T., Schaap, M., Litvak, M., Perdrial, J., Harpold, A., & Durcik, M. (2011). How water, carbon and energy driver Critical Zone evolution: The Jemez-Santa Catalina Critical Zone Observatory. Vadose Zone Journal, 10, 884-899.More infoSpecial Section: Critical Zone Observatories ; DOI: 1 0.2136/vzj2010.0132. [2010 ISI Journal Impact Factor: 2.133]
- Breshears, D. D., López-Hoffman, L., & Graumlich, L. J. (2011). When ecosystem services crash: Preparing for big, fast, patchy climate change. Ambio, 40(3), 256-263.More infoPMID: 21644454;PMCID: PMC3357807;Abstract: Abstract Assessments of adaptation options generally focus on incremental, homogeneous ecosystem responses to climate even though climate change impacts can be big, fast, and patchy across a region. Regional drought-induced tree die-off in semiarid woodlands highlights how an ecosystem crash fundamentally alters most ecosystem services and poses management challenges. Building on previous research showing how choice of location is linked to adaptive capacity and vulnerability, we developed a framework showing how the options for retaining desired ecosystem services in the face of sudden crashes depend on how portable the service is and whether the stakeholder is flexible with regard to the location where they receive their services. Stakeholders using portable services, or stakeholders who can move to other locations to obtain services, may be more resilient to ecosystem crashes. Our framework suggests that entering into cooperative networks with regionally distributed stakeholders is key to building resilience to big, fast, patchy crashes. © Royal Swedish Academy of Sciences 2011.
- Breshears, D., Lopez-Hoffman, L., & Graumlich, L. (2011). When ecosystem services crash: preparing for big, fast patchy climate change. Ambio, 40, 256-263.More infoDOI: 10.1007/s13280-010-0106-4.[2010 ISI Journal Impact Factor: 1.705]. Featured in cover photo.
- Chorover, J., Troch, P. A., Rasmussen, C., Brooks, P. D., Pelletier, J. D., Breshears, D. D., Huxman, T. E., Kurc, S. A., Lohse, K., Mcintosh, J. C., Meixner, T., Schaap, M. G., Litvak, M., Perdrial, J., Harpold, A., & Durcik, M. (2011). How water, carbon, and energy drive critical zone evolution: the Jemez-Santa Catalina Critical Zone Observatory. Vadose Zone Journal, 10, 884-899.
- Davison, J., Breshears, D., Van, L. W., & Cassady, G. (2011). Remotely sensed vegetation phenology and productivity along a climatic gradient: on the value of incorporating the dimension of woody plant cover. Global Ecology and Biogeography, 20, 101-113.More infoDOI: 10.1111/j.1466-8238.2010.00571.x. [2010 ISI Journal Impact Factor: 5.273].
- Field, J. P., Breshears, D. D., Whicker, J. J., & Zou, C. B. (2011). Interactive effects of grazing and burning on wind- and water-driven sediment fluxes: Rangeland management implications. Ecological Applications, 21(1), 22-32.More infoPMID: 21516885;Abstract: Rangelands are globally extensive, provide fundamental ecosystem services, and are tightly coupled human-ecological systems. Rangeland sustainability depends largely on the implementation and utilization of various grazing and burning practices optimized to protect against soil erosion and transport. In many cases, however, land management practices lead to increased soil erosion and sediment fluxes for reasons that are poorly understood. Because few studies have directly measured both wind and water erosion and transport, an assessment of how they may differentially respond to grazing and burning practices is lacking. Here, we report simultaneous, co-located estimates of wind- and water-driven sediment transport in a semiarid grassland in Arizona, USA, over three years for four land management treatments: control, grazed, burned, and burned grazed. For all treatments and most years, annual rates of wind-driven sediment transport exceeded that of water due to a combination of ongoing small but nontrivial wind events and larger, less frequent, wind events that generally preceded the monsoon season. Sediment fluxes by both wind and water differed consistently by treatment: burned + grazed > burned > grazed ≥ control, with effects immediately apparent after burning but delayed after grazing until the following growing season. Notably, the wind : water sediment transport ratio decreased following burning but increased following grazing. Our results show how rangeland practices disproportionally alter sediment fluxes driven by wind and water, differences that could potentially help explain divergence between rangeland sustainability and degradataion. © 2011 by the Ecological Society of America.
- Field, J. P., Breshears, D. D., Whicker, J. J., & Zou, C. B. (2011). On the ratio of wind- to water-driven sediment transport: Conserving soil under global-change-type extreme events. Journal of Soil and Water Conservation, 66(2), 51A-56A.
- Field, J., Breshears, D., Whicker, J., & Zou, C. (2011). Interactive effects of grazing and burning on wind- and water-driven sediment fluxes: Rangeland management implications. Ecological Applications, 21, 22-32.More infoDOI: 10.2136/vzj2004.0038. [2010 ISI Journal Impact Factor: 4.276].
- Field, J., Breshears, D., Whicker, J., & Zou, C. (2011). On the ratio of wind-to-water-driven sediment transport: Conserving soil under global-change-type extreme events. Journal of Soil and Water Conservation, 66, 51A-56A.More info[2010 ISI Journal Impact Factor: 1.407]
- Guardiola-Claramonte, M., Troch, P., Breshears, D., Huxman, T., Switanek, M., Durcik, M., & Cobb, N. (2011). Decreased streamflow in semi-arid basins following drought-induced tree die-off: a counter-intuitive and indirect climate impact on hydrology. Journal of Hydrology, 406, 225-233.More infoDOI: 10.1016/j.jhydrol.2011.06.017.[2010 ISI Journal Impact Factor: 2.514].
- McDowell, N. G., Beerling, D. J., Breshears, D. D., Fisher, R. A., Raffa, K. F., & Stitt, M. (2011). The interdependence of mechanisms underlying climate-driven vegetation mortality. Trends in Ecology and Evolution, 26(10), 523-532.More infoPMID: 21802765;Abstract: Climate-driven vegetation mortality is occurring globally and is predicted to increase in the near future. The expected climate feedbacks of regional-scale mortality events have intensified the need to improve the simple mortality algorithms used for future predictions, but uncertainty regarding mortality processes precludes mechanistic modeling. By integrating new evidence from a wide range of fields, we conclude that hydraulic function and carbohydrate and defense metabolism have numerous potential failure points, and that these processes are strongly interdependent, both with each other and with destructive pathogen and insect populations. Crucially, most of these mechanisms and their interdependencies are likely to become amplified under a warmer, drier climate. Here, we outline the observations and experiments needed to test this interdependence and to improve simulations of this emergent global phenomenon. © 2011 Elsevier Ltd.
- McDowell, N., Beerling, D., Breshears, D., Fisher, R., Raffa, K., & Stitt, M. (2011). The interdependence of mechanisms underlying climate-driven vegetation mortality. Trends in Ecology and Evolution, 26, 523-532.More infoDOI: 10.1016/j.tree.2011.06.003. [2010 ISI Journal Impact Factor: 14.448].
- Ravi, S., D'Odorico, P., Breshears, D. D., Field, J. P., Goudie, A. S., Huxman, T. E., Junran, L. i., Okin, G. S., Swap, R. J., Thomas, A. D., Pelt, S. V., Whicker, J. J., & Zobeck, T. M. (2011). Aeolian processes and the biosphere. Reviews of Geophysics, 49(3).More infoAbstract: Aeolian processes affect the biosphere in a wide variety of contexts, including landform evolution, biogeochemical cycles, regional climate, human health, and desertification. Collectively, research on aeolian processes and the biosphere is developing rapidly in many diverse and specialized areas, but integration of these recent advances is needed to better address management issues and to set future research priorities. Here we review recent literature on aeolian processes and their interactions with the biosphere, focusing on (1) geography of dust emissions, (2) impacts, interactions, and feedbacks, (3) drivers of dust emissions, and (4) methodological approaches. Geographically, dust emissions are highly spatially variable but also provide connectivity at global scales between sources and effects, with "hot spots" being of particular concern. Recent research reveals that aeolian processes have impacts, interactions, and feedbacks at a variety of scales, including large-scale dust transport and global biogeochemical cycles, climate mediated interactions between atmospheric dust and ecosystems, impacts on human health, impacts on agriculture, and interactions between aeolian processes and dryland vegetation. Aeolian dust emissions are driven largely by, in addition to climate, a combination of soil properties, soil moisture, vegetation and roughness, biological and physical crusts, and disturbances. Aeolian research methods span laboratory and field techniques, modeling, and remote sensing. Together these integrated perspectives on aeolian processes and the biosphere provide insights into management options and aid in identifying research priorities, both of which are increasingly important given that global climate models predict an increase in aridity in many dryland systems of the world. Copyright 2011 by the American Geophysical Union.
- Royer, P. D., Cobb, N. S., Clifford, M. J., Huang, C., Breshears, D. D., Adams, H. D., & Villegas, J. C. (2011). Extreme climatic event-triggered overstorey vegetation loss increases understorey solar input regionally: Primary and secondary ecological implications. Journal of Ecology, 99(3), 714-723.More infoAbstract: Climate extremes such as drought can trigger large-scale tree die-off, reducing overstorey canopy and thereby increasing near-ground solar radiation. This directly affects biotic and abiotic processes, including plant physiology, reproduction, phenology, soil evaporation and nutrient cycling, which themselves affect understory facilitation, productivity and diversity, and land surface-atmosphere fluxes of energy, carbon and water. Although important, assessing extreme-event solar radiation responses regionally following die-off is complex compared with characterizing patch-scale inputs. Estimating regional-scale changes requires integration of broad-scale downward-looking shading patterns due to canopy and topography with fine-scale upward-looking canopy details (e.g. live vs. dead trees, height, diameter, spatial pattern and foliar diffusivity). We quantified increases in near-ground solar radiation following overstorey loss of piñon pine cover in response to a recent extreme drought event (2002-2003). We evaluated 211km2 in south-western USA seasonally and annually using high-spatial resolution satellite imagery, hemispherical ground photography, GIS (Geographic Information System)-based solar radiation modelling tools, in situ meteorological data and tree measurements. Overstorey loss due to die-off produced increases in near-ground solar radiation regionally each season - up to 28Wm-2, an increase of 9.1%, in summer - while simultaneously decreasing spatial variation. Annually the increase was c. 17Wm-2. Larger increases occurred where initial canopy cover was greater or at higher elevations, by as much as c. 80Wm-2 (a 40% increase). Synthesis. Our results are notable in that they quantify increases regionally in near-ground solar radiation in response to a climate extreme triggering widespread tree die-off. The substantial increases quantified are expected to have primary direct effects on processes such as plant physiology, reproduction, phenology, soil evaporation and nutrient cycling, and secondary effects on understory facilitation, productivity and diversity, and land surface-atmosphere fluxes of energy, carbon and water. Consequently, extreme event-induced changes in near-ground solar radiation need to be considered by both ecologists and physical scientists in assessing global change impacts. More generally, our results highlight an important but sometimes overlooked aspect of plant ecology - that plants not only respond to their physical environment and other plants, but also directly modify their physical environment from individual plant to regional scales. © 2011 The Authors. Journal of Ecology © 2011 British Ecological Society.
- Royer, P., Cobb, N., Clifford, M., Huang, C., Breshears, D., Adams, H., & Villegas, V. (2011). Extreme climatic event-triggered overstorey vegetation loss increases understorey solar input regionally:primary & secondary ecological implications. Journal of Ecology, 99, 714-723.More infoDOI: 10.1111/j.1365-2745.2011.01804.x [2010 ISI Journal Impact Factor: 5.260]; Special Feature: Ecological Consequences of Climate Extremes
- Whicker, J., & Breshears, D. (2011). Dust production following forest disturbances: health risks. Encyclopedia of Environmental Health, 185-195.More infoJ. Nriagu, Ed.-in-Chief; Elsevier, New York ; DOI: 10.1016/B978-0-444-52272-6.00472-4
- Adams, H. D., MacAlady, A. K., Breshears, D. D., Allen, C. D., Stephenson, N. L., Saleska, S. R., Huxman, T. E., & McDowell, N. G. (2010). Climate-induced tree mortality: Earth system consequences. Eos, 91(17), 153-154.
- Allen, C. D., Macalady, A. K., Chenchouni, H., Bachelet, D., McDowell, N., Vennetier, M., Kitzberger, T., Rigling, A., Breshears, D. D., Hogg, E. (., Gonzalez, P., Fensham, R., Zhang, Z., Castro, J., Demidova, N., Lim, J., Allard, G., Running, S. W., Semerci, A., & Cobb, N. (2010). A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecology and Management, 259(4), 660-684.More infoAbstract: Greenhouse gas emissions have significantly altered global climate, and will continue to do so in the future. Increases in the frequency, duration, and/or severity of drought and heat stress associated with climate change could fundamentally alter the composition, structure, and biogeography of forests in many regions. Of particular concern are potential increases in tree mortality associated with climate-induced physiological stress and interactions with other climate-mediated processes such as insect outbreaks and wildfire. Despite this risk, existing projections of tree mortality are based on models that lack functionally realistic mortality mechanisms, and there has been no attempt to track observations of climate-driven tree mortality globally. Here we present the first global assessment of recent tree mortality attributed to drought and heat stress. Although episodic mortality occurs in the absence of climate change, studies compiled here suggest that at least some of the world's forested ecosystems already may be responding to climate change and raise concern that forests may become increasingly vulnerable to higher background tree mortality rates and die-off in response to future warming and drought, even in environments that are not normally considered water-limited. This further suggests risks to ecosystem services, including the loss of sequestered forest carbon and associated atmospheric feedbacks. Our review also identifies key information gaps and scientific uncertainties that currently hinder our ability to predict tree mortality in response to climate change and emphasizes the need for a globally coordinated observation system. Overall, our review reveals the potential for amplified tree mortality due to drought and heat in forests worldwide.
- Breshears, D. D., & Ludwig, J. A. (2010). Near-ground solar radiation along the grassland-forest continuum: Tall-tree canopy architecture imposes only muted trends and heterogeneity. Austral Ecology, 35(1), 31-40.More infoAbstract: Solar radiation directly and indirectly drives a variety of ecosystem processes. Our aim was to evaluate how tree canopy architecture affects near-ground, incoming solar radiation along gradients of increasing tree cover, referred to as the grassland-forest continuum. We evaluated a common type of canopy architecture: tall trees that generally have their lowest level of foliage high above, rather than close to the ground as is often the case for shorter trees. We used hemispherical photographs to estimate near-ground solar radiation using the metric of Direct Site Factor (DSF) on four sites in north Queensland, Australia that formed a grassland-forest continuum with tree canopy cover ranging from 0% to 71%. Three of the four sites had tall Eucalyptus trees with foliage several metres above the ground. We found that: (i) mean DSF exceeded >70% of the potential maximum for all sites, including the site with highest canopy cover; (ii) DSF variance was not highly sensitive to canopy coverage; and (iii) mean DSF for canopy locations beneath trees was not significantly lower than for adjacent intercanopy locations. Simulations that hypothetically placed Australian sites with tall tree canopies at other latitude-longitude locations demonstrated that differences in DSF were mostly due to canopy architecture, not specific site location effects. Our findings suggest that tall trees that have their lowest foliage many metres above the ground and have lower foliar density only weakly affect patterns of near-ground solar radiation along the grassland-forest continuum. This markedly contrasts with the strong effect that shorter trees with foliage near the ground have on near-ground solar radiation patterns along the continuum. This consequence of differential tree canopy architecture will fundamentally affect other ecosystem properties and may explain differential emphases that have been placed on canopy-intercanopy heterogeneity in diverse global ecosystem types that lie within the grassland-forest continuum. © 2010 Ecological Society of Australia.
- Field, J. P., Belnap, J., Breshears, D. D., Neff, J. C., Okin, G. S., Whicker, J. J., Painter, T. H., Ravi, S., Reheis, M. C., & Reynolds, R. L. (2010). The ecology of dust. Frontiers in Ecology and the Environment, 8(8), 423-430.More infoAbstract: Wind erosion and associated dust emissions play a fundamental role in many ecological processes and provide important biogeochemical connectivity at scales ranging from individual plants up to the entire globe. Yet, most ecological studies do not explicitly consider dust-driven processes, perhaps because most relevant research on aeolian (wind-driven) processes has been presented in a geosciences rather than an ecological context. To bridge this disciplinary gap, we provide a general overview of the ecological importance of dust, examine complex interactions between wind erosion and ecosystem dynamics from the scale of plants and surrounding space to regional and global scales, and highlight specific examples of how disturbance affects these interactions and their consequences. It is likely that changes in climate and intensification of land use will lead to increased dust production from many drylands. To address these issues, environmental scientists, land managers, and policy makers need to consider wind erosion and dust emissions more explicitly in resource management decisions. © The Ecological Society of America.
- Newman, B. D., Breshears, D. D., & Gard, M. O. (2010). Evapotranspiration partitioning in a semiarid woodland: Ecohydrologic heterogeneity and connecitvity of vegetation patches. Vadose Zone Journal, 9(3), 561-572.More infoAbstract: Partitioning evapotranspiration into its evaporation and transpiration components is critical for understanding ecohydrologic processes in dry lands. Existing partitioning estimates, however, have not adequately accounted for the heterogeneity associated with woody plant canopy patches and inter canopy patches so characteristic of dry land ecosystems. We measured water contents, stable isotopes (δ 2H and δ 18O), Cl -, and NO 3- from core samples collected during an intense drought in canopy and inter canopy patches in a semiarid, piñon-juniper [Pinus edulis Engelm.- Juniperus monosperma (Engelm.) Sarg.] woodland in northern New Mexico to assess patch-scale heterogeneity and evapotranspiration partitioning. Soil zone residence times based on Cl - ranged from 6 to 37 yr, highlighting the long time scale of percolation in these woodlands. The average NO 3- concentration was nearly seven times lower in canopy patches, indicating substantial biogeochemical heterogeneity. Average δ 2H values from shallow soil (
- Ravi, S., Breshears, D. D., Huxman, T. E., & D'Odorico, P. (2010). Land degradation in drylands: Interactions among hydrologic-aeolian erosion and vegetation dynamics. Geomorphology, 116(3-4), 236-245.More infoAbstract: Land degradation in drylands is one of the major environmental issues of the 21st century particularly due to its impact on world food security and environmental quality. Climate change, shifts in vegetation composition, accelerated soil erosion processes, and disturbances have rendered these landscapes susceptible to rapid degradation that has important feedbacks on regional climate and desertification. Even though the role of hydrologic-aeolian erosion and vegetation dynamic processes in accelerating land degradation is well recognized, most studies have concentrated only on the role of one or two of these components, and not on the interactions among all three. Drawing on relevant published studies, here we review recent contributions to the study of biotic and abiotic drivers of dryland degradation and we propose a more holistic perspective of the interactions between wind and water erosion processes in dryland systems, how these processes affect vegetation patterns and how vegetation patterns, in turn, affect these processes. Notably, changing climate and land use have resulted in rapid vegetation shifts, which alter the rates and patterns of soil erosion in dryland systems. With the predicted increase in aridity and an increase in the frequency of droughts in drylands around the world, there could be an increasing dominance of abiotic controls of land degradation, in particular hydrologic and aeolian soil erosion processes. Further, changes in climate may alter the relative importance of wind versus water erosion in dryland ecosystems. Therefore acquiring a more holistic perspective of the interactions among hydrologic-aeolian erosion and vegetation dynamic processes is fundamental to quantifying and modeling land degradation processes in drylands in changing climate, disturbance regimes and management scenarios. © 2009 Elsevier B.V. All rights reserved.
- Reiley, D. K., Breshears, D. D., Zedler, P. H., Ebinger, M. H., & Meyer, C. W. (2010). Soil carbon heterogeneity in piñon-juniper woodland patches: Effect of woody plant variation on neighboring intercanopies is not detectable. Journal of Arid Environments, 74(2), 239-246.More infoAbstract: Soil carbon often varies significantly among vegetation patch types, but less known is how the size and species of plants in the tree canopy patches and the cover types of the intercanopy patches affect the carbon storage, and whether vegetation characteristics affect storage in adjacent patches. To assess this, we measured fine-fraction soil carbon in a semiarid woodland in New Mexico USA for canopy patches of two co-dominant woody species, Pinus edulis and Juniperus monosperma that were paired with intercanopy patch locations covered by herbaceous grass (Bouteloua gracilis) or bare ground. Soil carbon at shallow depths was greater in canopy than intercanopy patches by a factor of 2 or more, whereas within intercanopy patches soil carbon in grass locations exceeded that in bare locations only after accounting for coarse-fraction carbon. Hypothesized differences among canopy patches associated with species or size were not detected (although some size-depth interactions consistent with expectations were detected), nor, importantly, were effects of species or size of woody plant on intercanopy soil carbon. The results are notable because where applicable they justify estimates of soil carbon inventories based on readily observable heterogeneity in above-ground plant cover without considering the size and species of the woody plants. © 2009 Elsevier Ltd. All rights reserved.
- Royer, P. D., Breshears, D. D., Zou, C. B., Cobb, N. S., & Kurc, S. A. (2010). Ecohydrological energy inputs in semiarid coniferous gradients: Responses to management- and drought-induced tree reductions. Forest Ecology and Management, 260(10), 1646-1655.More infoAbstract: Large-scale, rapid reductions in forest and woodland tree cover caused by fire, drought-induced die-off, or wildfire-mitigating thinning prescriptions, all three of which differentially affect canopy structure, are increasingly altering coniferous-dominated landscapes across extensive regions such as western USA. These types of reductions in canopy cover can result in substantial increases in near-ground solar radiation, which in turn drive numerous ecological processes. However, existing relationships for how reductions in canopy cover translate into changes in incoming near-ground solar radiation do not account for the ways in which fire, die-off, and thinning differentially alter either or both the foliar and woody components of canopy architecture and the degree to which such alterations depend on foliar density. We systematically quantified trends in near-ground solar radiation for a broad range of canopy cover for two of the most extensive semiarid coniferous forest and woodland vegetation types in the western USA: those dominated by a combination of piñon and juniper (using Pinus edulis and Juniperus monosperma as representative species) or by ponderosa pine (Pinus ponderosa). We used hemispherical photography to account for how canopy architecture affected mean and variance in near-ground solar radiation over a broad range of canopy cover (from as low as ∼5% to as high as ∼85%). For both vegetation types, we evaluated four disturbance types: undisturbed, controlled burns, drought- and beetle-induced die-off, and prescriptive thinning treatments. We also assessed near-ground solar radiation for undisturbed vegetation spanning an elevation continuum that included both piñon-juniper and ponderosa pine vegetation types. Our results quantify how near-ground solar radiation varies substantially and systematically among forest gradient types and as a function of forest disturbance type. Trends in near-ground solar radiation differed among gradients associated with fire, die-off, or thinning, dependent on how each affects the foliar and/or woody components of canopy architecture. Deviations from undisturbed conditions for remaining disturbed tree cover were greatest for burned, intermediate for die-off and least for thinned. The differences in microclimate quantified here and how they vary with type of tree reduction are relevant for assessing vegetation responses following reductions in tree cover. In addition, the differences are large enough to require consideration in evaluating land surface interactions of forests and woodlands with the atmosphere (e.g., increases of >40Wm-2 relative to undisturbed conditions). Our results provide a means to enable managers to rapidly relate readily-obtainable field estimates of canopy cover to approximate estimates of near-ground solar radiation. © 2010 Elsevier B.V.
- Royer, P. D., Breshears, D. D., Zou, C. B., Cobb, N. S., & Kurc, S. A. (2010). Ecohydrological energy inputs in semiarid coniferous gradients: Responses to management- and drought-induced tree reductions. Forest Ecology and Management, 260, 1646-1655.
- Urgeghe, A. M., Breshears, D. D., Martens, S. N., & Beeson, P. C. (2010). Redistribution of runoff among vegetation patch types: On ecohydrological optimality of herbaceous capture of run-on. Rangeland Ecology and Management, 63(5), 497-504.More infoAbstract: A central tenant of ecohydrology in drylands is that runoff redistribution from bare to vegetated patches concentrates the key limiting resource of water, which can then enhance vegetation growth and biomass. Conversely, a reduction in vegetation patches, particularly those associated with herbaceous plants, can lead to a threshold-like response in which bare patches become highly interconnected, triggering a large increase in hillslope runoff and associated erosion. However, generally lacking is an assessment of how maximization of run-on to herbaceous patches relates to minimization of hillslope-scale runoff. To illustrate how runoff redistribution potentially changes in response to conversion of herbaceous patches to bare ones, we used a spatially distributed model, SPLASH (Simulator for Processes at the Landscape Surface-Subsurface Hydrology), with an example of a semiarid piñon-juniper woodland hillslope with seven combinations of bare and herbaceous patch cover, culminating in complete loss of herbaceous patches, for a 1-yr design storm. As expected, the amount of hillslope runoff increased curvilinearly with reductions in herbaceous cover as runoff per cell increased from bare patches and run-on per cell increased for herbaceous patches. Notably, the total amount of run-on to all herbaceous patches was greatest when the amount of bare cover was intermediate, highlighting a trade-off between the source area for generating runoff and the sink area for capturing run-on. The specific nature of patch-hillslope runoff redistribution responses certainly depends on several site-specific conditions, but the general nature of the response exhibited in our example simulation may be indicative of a general type of response applicable to many rangelands. We suggest that a more robust suite of such relationships could be valuable for managing rangelands by enabling explicit accounting for optimality and trade-offs in biomass per herbaceous patch, total herbaceous cover, and prevention of hillslope-scale connectivity of bare patches that triggers a large increase in runoff and associated erosion. © 2010 Society for Range Management.
- Villegas, J. C., Breshears, D. D., Zou, C. B., & Law, D. J. (2010). Ecohydrological controls of soil evaporation in deciduous drylands: How the hierarchical effects of litter, patch and vegetation mosaic cover interact with phenology and season. Journal of Arid Environments, 74(5), 595-602.More infoAbstract: Soil evaporation, a critical ecohydrological process in drylands, can exhibit substantial spatio-temporal variation. Spatially, ecohydrological controls of soil evaporation may generally depend on a hierarchical structure spanning from the presence or absence of litter, through canopy patches of woody plants and intercanopy patches separating them, up to the overall vegetation mosaic characterized by density of woody plant cover in the landscape, although assessment of these factors in concert is generally lacking. Temporally, ecohydrological controls can be further complicated by not only seasonal climate, but also phenology, particularly in seasonally deciduous drylands. We experimentally assessed the interactive controls on soil evaporation along a gradient of mesquite cover (Prosopis velutina) within the North American monsoon region, with respect to such hierarchical structure and seasonality/phenology. Our results indicate that presence of litter exerts a dominant control on soil evaporation, independent of seasonality; in absence of litter, both patch and mosaic attributes influence soil evaporation variably with season/phenology. Correlations from related measures of incoming energy suggest energy limits evaporation in many cases, although other factors such as wind may potentially influence hierarchical and seasonal/phenological combinations. Our results highlight the need to account for both hierarchical vegetation structure and seasonal/phenological variability to improve ecohydrological predictions of soil evaporation. © 2009 Elsevier Ltd. All rights reserved.
- Villegas, J. C., Breshears, D. D., Zou, C. B., & Royer, P. D. (2010). Seasonally pulsed heterogeneity in microclimate: Phenology and cover effects along deciduous grassland-forest continuum. Vadose Zone Journal, 9(3), 537-547.More infoAbstract: Much of the terrestrial biosphere can be viewed as part of a gradient, with varying amounts of woody plant cover ranging from grassland to forest-the grassland-forest continuum. Woody plant cover directly impacts the soil microclimate through modificati ons of near-ground solar radiation and soil temperature, and these interactive effects are relevant for key ecohydrological processes such as soil evaporation. Trends in how increasing woody plant cover affect soil surface microclimate have recently been evaluated for gradients of evergreen woody plants, but analogous trends for deciduous plants, where phenology should be influential, are lacking. We evaluated season-dependent changes in soil microclimate along a deciduous grassland-forest continuum of velvet mesquite (Prosopis velutina Wooton) using repeated hemispherical photography and continuous soil temperature measurements at the 5-cm depth. Both near-ground solar radiation and soil temperature decreased with increasing canopy cover, even during the leafless season. The trends varied substantially among seasons, however, with differences between canopy and intercanopy patches readily evident only during the period of full leaf-out, during which the correlation between near-ground solar radiation and soil temperature was strongest. Our results provide a more comprehensive understanding about the interactions of canopy cover, canopy structure attributes, and plant phenology that produce seasonally pulsed heterogeneity in the soil surface microclimate. Notably, our results add a new dimension to the moisture "pulse dynamics" perspective commonly applied to dryland ecohydrology, highlighting seasonally pulsed heterogeneity in soil microclimate that could influence soil moisture dynamics in drylands. © Soil Science Society of America.
- Wang, L., Caylor, K. K., Villegas, J. C., Barron-Gafford, G. A., Breshears, D. D., & Huxman, T. E. (2010). Partitioning evapotranspiration across gradients of woody plant cover: Assessment of a stable isotope technique. Geophysical Research Letters, 37(9).More infoAbstract: In water-limited ecosystems, partitioning ecosystemscale evapotranspiration fluxes between plant transpiration and soil/canopy evaporation remains a theoretical and technical challenge. We used the Biosphere 2 glasshouse to assess partitioning of evapotranspiration across an experimentally manipulated gradient of woody plant cover using continuous measurements of near-surface variations in the stable isotopic composition of water vapor (δ2H). Our technique employs a newly-developed laser-based isotope analyzer and the Keeling plot approach for surface flux partitioning. The applicability of the technique was verified by comparison to separate, simultaneous lysimeter and sap flow estimates of ET partitioning. The results showed an expected increase in fractional contribution of transpiration to evapotranspiration as woody cover increased-from T/ET = 0.61 at 25%woody cover to T/ET = 0.83 at 100% cover. Further development of this technique may enable field characterization of evapotranspiration partitioning across diverse woody cover gradients, a central issue in addressing dryland ecohydrological responses to land use and climate change. Copyright © 2010 by the American Geophysical Union.
- Zou, C. B., Royer, P. D., & Breshears, D. D. (2010). Density-dependent shading patterns by Sonoran saguaros. Journal of Arid Environments, 74(1), 156-158.More infoAbstract: Woody plants produce shading patterns that can alter soil evaporation rates and provide nurse-plant effects for seedling establishment. These effects depend on the density of woody plants and associated characteristics of canopy architecture such as height, and can be particularly important in deserts. The tallest stature woody plants in some desert ecosystems are columnar cacti, which have distinctly different architecture than other woody plants for which shading effects have been systematically assessed. Focusing on columnar cacti of the northern Sonoran Desert, we used hemispherical photography to evaluate the effects of saguaro cacti (Carnegiea gigantea) on microclimate along a gradient of increasing cactus density. Notably, incoming annual near-ground solar radiation was reduced by up to ∼10% for the highest density stand (156 cacti ha-1), with spatial variation in shading patterns peaking at nearly the highest stand density (133 cacti ha-1). The annual near-ground solar radiation reductions were more directly related to cacti density than to density of surrounding shrubs. Our results document that at high density, saguaro columnar cacti can have substantial effects on microclimate and, more generally, our results contribute to the growing library of relationships quantifying how shading patterns vary with woody plant architecture and density. © 2009 Elsevier Ltd. All rights reserved.
- Breshears, D. D., Myers, O. B., & Barnes, F. J. (2009). Horizontal heterogeneity in the frequency of plant-available water with woodland intercanopy-canopy vegetation patch type rivals that occuring vertically by soil depth. Ecohydrology, 2(4), 503-519.More infoAbstract: Soil moisture integrates and drives ecohydrological processes in dryland ecosystems. However, despite the central importance of soil moisture, relevant field studies have not holistically assessed key inter-related aspects of ecohydrological spatiotemporal variation: the threshold-like manner in which soil texture controls the frequency at which soil water is readily available for plants, assessment of horizontal heterogeneity associated with vegetation patches in addition to vertical heterogeneity associated with depth, seasonal variation associated with precipitation type (snow vs rain) and inter-annual variation spanning notably wet and dry periods. We measured soil water content by neutron probe in a semiarid piñon-juniper woodland (Pinus edulis and Juniperus monosperma) in northern New Mexico, USA, over 15 years and evaluated an ecohydrological metric-plant-available water, estimated as the percentage of time that soil water content was sufficiently wet to be generally available to plants. The frequency of plant-available water varied significantly across all variables assessed: precipitation amount (across years or seasons), precipitation type, vertically with soil depth and horizontally with vegetation patch type (canopy patches beneath trees, intercanopy patches between trees and edges between the two patch types). Notably, in many cases, horizontal heterogeneity in plant-available water associated with vegetation patch was as substantial as vertical heterogeneity associated with depth, yet such horizontal heterogeneity is not included in most ecological or hydrological models. Our results highlight spatiotemporal variation in the frequency of plant-available water that is substantial, often overlooked, and may need to be explicitly considered for predicting dryland vegetation responses to land use and climate change. Copyright © 2009 John Wiley & Sons, Ltd.
- Breshears, D. D., Myers, O. B., Meyer, C. W., Barnes, F. J., Zou, C. B., Allen, C. D., McDowell, N. G., & Pockman, W. T. (2009). Research communications research communications Tree die-off in response to global change-type drought: Mortality insights from a decade of plant water potential measurements. Frontiers in Ecology and the Environment, 7(4), 185-189.More infoAbstract: Global climate change is projected to produce warmer, longer, and more frequent droughts, referred to here as "global change-type droughts", which have the potential to trigger widespread tree die-off. However, droughtinduced tree mortality cannot be predicted with confidence, because long-term field observations of plant water stress prior to, and culminating in, mortality are rare, precluding the development and testing of mechanisms. Here, we document plant water stress in two widely distributed, co-occurring species, piñon pine (Pinus edulis) and juniper (Juniperus monosperma), over more than a decade, leading up to regional-scale die-off of piñon pine trees in response to global change-related drought. Piñon leaf water potentials remained substantially below their zero carbon assimilation point for at least 10 months prior to dying, in contrast to those of juniper, which rarely dropped below their zero-assimilation point. These data suggest that piñon mortality was driven by protracted water stress, leading to carbon starvation and associated increases in susceptibility to other disturbances (eg bark beetles), a finding that should help to improve predictions of mortality during drought. © The Ecological Society of America.
- Breshears, D. D., Whicker, J. J., Zou, C. B., Field, J. P., & Allen, C. D. (2009). A conceptual framework for dryland aeolian sediment transport along the grassland-forest continuum: Effects of woody plant canopy cover and disturbance. Geomorphology, 105(1-2), 28-38.More infoAbstract: Aeolian processes are of particular importance in dryland ecosystems where ground cover is inherently sparse because of limited precipitation. Dryland ecosystems include grassland, shrubland, savanna, woodland, and forest, and can be viewed collectively as a continuum of woody plant cover spanning from grasslands with no woody plant cover up to forests with nearly complete woody plant cover. Along this continuum, the spacing and shape of woody plants determine the spatial density of roughness elements, which directly affects aeolian sediment transport. Despite the extensiveness of dryland ecosystems, studies of aeolian sediment transport have generally focused on agricultural fields, deserts, or highly disturbed sites where rates of transport are likely to be greatest. Until recently, few measurements have been made of aeolian sediment transport over multiple wind events and across a variety of types of dryland ecosystems. To evaluate potential trends in aeolian sediment transport as a function of woody plant cover, estimates of aeolian sediment transport from recently published studies, in concert with rates from four additional locations (two grassland and two woodland sites), are reported here. The synthesis of these reports leads to the development of a new conceptual framework for aeolian sediment transport in dryland ecosystems along the grassland-forest continuum. The findings suggest that: (1) for relatively undisturbed ecosystems, shrublands have inherently greater aeolian sediment transport because of wake interference flow associated with intermediate levels of density and spacing of woody plants; and (2) for disturbed ecosystems, the upper bound for aeolian sediment transport decreases as a function of increasing amounts of woody plant cover because of the effects of the height and density of the canopy on airflow patterns and ground cover associated with woody plant cover. Consequently, aeolian sediment transport following disturbance spans the largest range of rates in grasslands and associated systems with no woody plants (e.g., agricultural fields), an intermediate range in shrublands, and a relatively small range in woodlands and forests. These trends are consistent with previous observations relating large rates of wind erosion to intermediate values for spatial density of roughness elements. The framework for aeolian sediment transport, which is also relevant to dust fluxes, wind erosion, and related aeolian processes, is applicable to a diverse suite of environmental challenges, including land degradation and desertification, dust storms, contaminant transport, and alterations of the hydrological cycle. © 2008 Elsevier B.V.
- Field, J. P., Breshears, D. D., & Whicker, J. J. (2009). Toward a more holistic perspective of soil erosion: Why aeolian research needs to explicitly consider fluvial processes and interactions. Aeolian Research, 1(1-2), 9-17.More infoAbstract: Soil erosion is driven by not only aeolian but also fluvial transport processes, yet these two types of processes are usually studied independently, thereby precluding effective assessment of overall erosion, potential interactions between the two drivers, and their relative sensitivities to projected changes in climate and land use. Here we provide a perspective that aeolian and fluvial transport processes need to be considered in concert relative to total erosion and to potential interactions, that relative dominance and sensitivity to disturbance vary with mean annual precipitation, and that there are important scale-dependencies associated with aeolian-fluvial interactions. We build on previous literature to present relevant conceptual syntheses highlighting these issues. We then highlight relative investments that have been made in soil erosion and sediment control by comparing the amount of resources allocated to aeolian and fluvial research using readily available metrics. Literature searches suggest that aeolian transport may be somewhat understudied relative to fluvial transport and, most importantly, that only a relatively small number of studies explicitly consider both aeolian and fluvial transport processes. Numerous environmental issues associated with intensification of land use and climate change impacts depend on not only overall erosion rates but also on differences and interactions between aeolian and fluvial processes. Therefore, a more holistic viewpoint of erosional processes that explicitly considers both aeolian and fluvial processes and their interactions is needed to optimize management and deployment of resources to address imminent changes in land use and climate. © 2009 Elsevier B.V.
- Henebry, G. M., Richardson, A. D., Breshears, D. D., Abatzoglou, J., Fisher, J. I., Graham, E. A., Hanes, J. M., Knapp, A., Liang, L., Wilson, B. E., & Morisette, J. T. (2009). Phenological trend estimation: A reply to sagarin. Frontiers in Ecology and the Environment, 7(6), 296-.
- Miao, S., Zou, C. B., & Breshears, D. D. (2009). Vegetation responses to extreme hydrological events: Sequence matters. American Naturalist, 173(1), 113-118.More infoPMID: 19061420;Abstract: Extreme hydrological events such as flood and drought drive vegetation dynamics and are projected to increase in frequency in association with climate change, which could result in sequences of extreme events. However, experimental studies of vegetation responses to climate have largely focused on responses to a trend in climate or to a single extreme event but have largely overlooked the potential for complex responses to specific sequences of extreme events. Here we document, on the basis of an experiment with seedlings of three types of subtropical wetland tree species, that mortality can be amplified and growth can even be stimulated, depending on event sequence. Our findings indicate that the impacts of multiple extreme events cannot be modeled by simply summing the projected effects of individual extreme events but, rather, that models should take into account event sequences. © 2009 by The University of Chicago. All rights reserved.
- Morisette, J. T., Richardson, A. D., Knapp, A. K., Fisher, J. I., Graham, E. A., Abatzoglou, J., Wilson, B. E., Breshears, D. D., Henebry, G. M., Hanes, J. M., & Liang, L. (2009). Tracking the rhythm of the seasons in the face of global change: Phenological research in the 21 st century. Frontiers in Ecology and the Environment, 7(5), 253-260.More infoAbstract: Phenology is the study of recurring life-cycle events, classic examples being the flowering of plants and animal migration. Phenological responses are increasingly relevant for addressing applied environmental issues. Yet, challenges remain with respect to spanning scales of observation, integrating observations across taxa, and modeling phenological sequences to enable ecological forecasts in light of future climate change. Recent advances that are helping to address these questions include refined landscape-scale phenology estimates from satellite data, advanced, instrument-based approaches for field measurements, and new cyberinfrastructure for archiving and distribution of products. These breakthroughs are improving our understanding in diverse areas, including modeling land-surface exchange, evaluating climate-phenology relationships, and making land-management decisions.
- R., K., & Breshears, D. D. (2009). Ecohydrology bearings: Invited commentary to challenge paradigms, question assumptions, prioritize needs and enhance interdisciplinary dialogue. Ecohydrology, 2(3), 381-382.
- Whicker, J. J., III, J. P., & Breshears, D. D. (2009). Corrigendum to "Thinning semiarid forests amplifies wind erosion comparably to wildfire: Implications for restoration and soil stability" [J. Arid Environ. 72 (2008) 494-508] (DOI:10.1016/j.jaridenv.2007.08.006). Journal of Arid Environments, 73(6-7), 691-692.
- Breshears, D. D., Huxman, T. E., Adams, H. D., Zou, C. B., & Davison, J. E. (2008). Vegetation synchronously leans upslope as climate warms. Proceedings of the National Academy of Sciences of the United States of America, 105(33), 11591-11592.More infoPMID: 18697950;PMCID: PMC2575300;
- Breshears, D. D., McDowell, N. G., Goddard, K. L., Dayem, K. E., Martens, S. N., Meyer, C. W., & Brown, K. M. (2008). Foliar absorption of intercepted rainfall improves woody plant water status most during drought. Ecology, 89(1), 41-47.More infoPMID: 18376545;Abstract: A large proportion of rainfall in dryland ecosystems is intercepted by plant foliage and is generally assumed to evaporate to the atmosphere or drip onto the soil surface without being absorbed. We demonstrate foliar absorption of intercepted rainfall in a widely distributed, continental dryland, woody-plant genus: Juniperus. We observed substantial improvement in plant water status, exceeding 1.0 MPa water potential for drought-stressed plants, following precipitation on an experimental plot that excluded soil water infiltration. Experiments that wetted shoots with unlabeled and with isotopically labeled water confirmed that water potential responded substantially to foliar wetting, that these responses were not attributable to re-equilibration with other portions of the xylem, and that magnitude of response increased with water stress. Foliar absorption is not included in most ecological, hydrological, and atmospheric models; has implications for interpreting plant isotopic signatures; and not only supplements water acquisition associated with increases in soil moisture that follow large or repeated precipitation events, but also enables plants to bypass soil water uptake and benefit from the majority of precipitation events, which wet foliage but do not increase soil moisture substantially. Foliar absorption of intercepted water could be more important than previously appreciated, especially during drought when water stress is greatest. © 2008 by the Ecological Society of America.
- Kempes, C. P., Myers, O. B., Breshears, D. D., & Ebersole, J. J. (2008). Comparing response of Pinus edulis tree-ring growth to five alternate moisture indices using historic meteorological data. Journal of Arid Environments, 72(4), 350-357.More infoAbstract: Annual growth of semiarid tree species is generally limited by a period of water deficit and this relationship can be reflected in interannual variation in tree-ring width of semiarid species such as Pinus edulis, a piñon pine that is widely distributed across the southwestern United States. Tree-ring width of P. edulis and other semiarid tree species is most frequently related to annual precipitation amount alone or to the Palmer drought severity index (PDSI). But water deficit and associated variation in moisture can also be described using numerous other indices such as the standardized precipitation index (SPI), and a comparison of the performance of several indices that relate historical climate data to tree-ring variation in P. edulis is lacking. We compared abilities to predict radial tree-ring growth of P. edulis using five metrics of water availability: PDSI, two indices based on precipitation alone (total precipitation and SPI), and two indices that factor in temperature to determine water deficit (based on Walter climate diagrams that use monthly precipitation and temperature). Each metric was evaluated over three commonly used time periods (water year, calendar year, and June-August) using the limited available data from P. edulis sites in the southwestern USA where co-located tree-ring and weather data were available. Our results indicate that PDSI was the best predictor of P. edulis ring widths, regardless of time period, and provide a first comparative test of PDSI with SPI and Walter indices that can be further tested as larger data sets become available. © 2007 Elsevier Ltd. All rights reserved.
- McDowell, N., Pockman, W. T., Allen, C. D., Breshears, D. D., Cobb, N., Kolb, T., Plaut, J., Sperry, J., West, A., Williams, D. G., & Yepez, E. A. (2008). Mechanisms of plant survival and mortality during drought: Why do some plants survive while others succumb to drought?. New Phytologist, 178(4), 719-739.More infoPMID: 18422905;Abstract: Severe droughts have been associated with regional-scale forest mortality worldwide. Climate change is expected to exacerbate regional mortality events; however, prediction remains difficult because the physiological mechanisms underlying drought survival and mortality are poorly understood. We developed a hydraulically based theory considering carbon balance and insect resistance that allowed development and examination of hypotheses regarding survival and mortality. Multiple mechanisms may cause mortality during drought. A common mechanism for plants with isohydric regulation of water status results from avoidance of drought-induced hydraulic failure via stomatal closure, resulting in carbon starvation and a cascade of downstream effects such as reduced resistance to biotic agents. Mortality by hydraulic failure per se may occur for isohydric seedlings or trees near their maximum height. Although anisohydric plants are relatively drought-tolerant, they are predisposed to hydraulic failure because they operate with narrower hydraulic safety margins during drought. Elevated temperatures should exacerbate carbon starvation and hydraulic failure. Biotic agents may amplify and be amplified by drought-induced plant stress. Wet multidecadal climate oscillations may increase plant susceptibility to drought-induced mortality by stimulating shifts in hydraulic architecture, effectively predisposing plants to water stress. Climate warming and increased frequency of extreme events will probably cause increased regional mortality episodes. Isohydric and anisohydric water potential regulation may partition species between survival and mortality, and, as such, incorporating this hydraulic framework may be effective for modeling plant survival and mortality under future climate conditions. © The Authors (2008).
- Rich, P. M., Breshears, D. D., & White, A. B. (2008). Phenology of mixed woody-herbaceous ecosystems following extreme events: Net and differential responses. Ecology, 89(2), 342-352.More infoPMID: 18409424;Abstract: Ecosystem responses to key climate drivers are reflected in phenological dynamics such as the timing and degree of "green-up" that integrate responses over spatial scales from individual plants to ecosystems. This integration is clearest in ecosystems dominated by a single species or life form, such as seasonally dynamic grasslands or more temporally constant evergreen forests. Yet many ecosystems have substantial contribution of cover from both herbaceous and woody evergreen plants. Responses of mixed woody-herbaceous ecosystems to climate are of increasing concern due to their extensive nature, the potential for such systems to yield more complex responses than those dominated by a single life form, and projections that extreme climate and weather events will increase in frequency and intensity with global warming. We present responses of a mixed woody-herbaceous ecosystem type to an extreme event: regional-scale piñon pine mortality following an extended drought and the subsequent herbaceous green-up following the first wet period after the drought. This example highlights how reductions in greenness of the slower, more stable evergreen woody component can rapidly be offset by increases associated with resources made available to the relatively more responsive herbaceous component. We hypothesize that such two-phase phenological responses to extreme events are characteristic of many mixed woody-herbaceous ecosystems. © 2008 by the Ecological Society of America.
- Villegas, J. C., Tobón, C., & Breshears, D. D. (2008). Fog interception by non-vascular epiphytes in tropical montane cloud forests: Dependencies on gauge type and meteorological conditions. Hydrological Processes, 22(14), 2484-2492.More infoAbstract: Precipitation is the most fundamental input of water for terrestrial ecosystems. Most precipitation inputs are vertical, via rain, but can be horizontal, via wind-driven rain and snow, or, in some ecosystems such as tropical montane cloud forests (TMCFs), via fog interception. Fog interception can be particularly important in ecosystems where fog is frequently present and there are seasonal periods of lower rainfall. Epiphytes in trees are a major ecological component of TMCFs and are particularly dependent on fog interception during periods of lower rainfall because they lack access to soil water. But assessing fog interception by epiphytes remains problematic because: (i) a variety of field or laboratory methods have been used, yet comparisons of interception by epiphytes versus interception by various types of fog gauge are lacking; (ii) previous studies have not accounted for potential interactions between meteorological factors. We compared fog interception by epiphytes with two kinds of commonly used fog gauges and developed relations between fog interception and meteorological variables by conducting laboratory experiments that manipulated key fog characteristics and from field measurements of fog interception by epiphytes. Fog interception measured on epiphytes was correlated with that measured from fog gauges but was more than an order of magnitude smaller than the actual measurements from fog gauges, highlighting a key measurement issue. Our laboratory measurements spanned a broad range of liquid water content (LWC) values for fog and indicate how fog interception is sensitive to an interaction between wind speed and LWC. Based on our results, considered in concert with those from other studies, we hypothesize that fog interception is constrained when LWC is low or high, and that fog interception increases with wind speed for intermediate values of LWC - a net result of deposition, impaction, and evaporation processes - until interception begins to decrease with further increases in wind speed. Copyright © 2007 John Wiley & Sons, Ltd.
- Wei, H., Nearing, M. A., Stone, J. J., & Breshears, D. D. (2008). A dual Monte Carlo approach to estimate model uncertainty and its application to the rangeland hydrology and erosion model. Transactions of the ASABE, 51(2), 515-520.More infoAbstract: Natural resources models serve as important tools to support decision making by predicting environmental indicators. All model predictions have uncertainty associated with them. Model predictive uncertainty, often expressed as the confidence interval around a model prediction value, may serve as important supplementary information for assisting decision making processes. In this article, we describe a new method called Dual Monte Carlo (DMC) to calculate model predictive uncertainty based on input parameter uncertainty. DMC uses two Monte Carlo sampling loops, which enable model users to not only calculate the model predictive uncertainty for selected input parameter sets of particular interest, but also to examine the predictive uncertainty as a junction of model inputs across the full range of parameter space. We illustrate the application of DMC to the process-based, rainfall event-driven Rangeland Hydrology and Erosion Model (RHEM). The results demonstrate that DMC effectively generated model predictive uncertainty from input parameter uncertainty and provided information that could be useful for decision making. We found that for the model RHEM, the uncertainty intervals were strongly correlated to specific model input and output parameter values, yielding regression relationships (r 2 > 0.97) that enable accurate estimation of the uncertainty interval for any point in the input parameter space without the need to run the Monte Carlo simulations each time the model is used. Soil loss predictions and their associated uncertainty intervals for three example storms and three site conditions are used to illustrate how DMC can be a useful tool for directing decision making.
- Whicker, J. J., III, J. P., & Breshears, D. D. (2008). Thinning semiarid forests amplifies wind erosion comparably to wildfire: Implications for restoration and soil stability. Journal of Arid Environments, 72(4), 494-508.More infoAbstract: Semiarid forests across the western USA and elsewhere are being thinned to reduce risk from fire, restore previous ecological conditions, and/or salvage trees from recently burned areas. Prescriptions and monitoring for thinning generally focus on biotic characteristics of vegetation, like tree density, rather than abiotic characteristics of soils and their loss, which are usually only considered in association with water erosion. Recent studies indicate that sediment transport by wind in forests is substantial and can exceed water transport, yet forest wind erosion responses to tree thinning and/or burning are unknown. We measured wind-driven horizontal dust flux, a metric related to wind erosion, with respect to presence/absence of fire and/or thinning in a ponderosa pine (Pinus ponderosa) forest in northern New Mexico, USA. Passive dust collectors at several sampling heights documented elevated dust fluxes at sites that were burned and/or thinned. Unexpectedly, thinned sites had erosion rates as large as burned sites, documenting significant restoration impacts on soil stability. Thinning and fire impacts on dust flux were correlated with remaining tree or ground cover. The results highlight that dust fluxes provide a readily measurable metric of soil stability that should be integrated into prescription and monitoring plans for forest restoration and thinning. © 2007 Elsevier Ltd. All rights reserved.
- Allen, C. D., & Breshears, D. D. (2007). Climate-induced forest dieback as an emergent global phenomenon. Eos, 88(47), 504-.
- Betancourt, J. L., Schwartz, M. D., Breshears, D. D., Brewer, C. A., Frazer, G., Gross, J. E., Mazer, S. J., Reed, B. C., & Wilson, B. E. (2007). Evolving plans for the USA National Phenology Network. Eos, 88(19), 211-.
- Michener, W. K., Breshears, D. D., Hunsaker, C. T., & Wickland, D. E. (2007). Professional certification: Increasing ecologists' effectiveness. Frontiers in Ecology and the Environment, 5(8), 399-.
- Whicker, J. J., E., J., Ibrahim, S. A., Stone, J. M., Breshears, D. D., & Baker, K. N. (2007). Uranium partition coefficients (Kd) in forest surface soil reveal long equilibrium times and vary by site and soil size fraction. Health Physics, 93(1), 36-46.More infoPMID: 17563491;Abstract: The environmental mobility of newly deposited radionuclides in surface soil is driven by complex biogeochemical relationships, which have significant impacts on transport pathways. The partition coefficient (Kd) is useful for characterizing the soil-solution exchange kinetics and is an important factor for predicting relative amounts of a radionuclide transported to groundwater compared to that remaining on soil surfaces and thus available for transport through erosion processes. Measurements of Kd for U are particularly useful because of the extensive use of U in military applications and associated testing, such as done at Los Alamos National Laboratory (LANL). Site-specific measurements of Kd for U are needed because Kd is highly dependent on local soil conditions and also on the fine soil fraction because U concentrates onto smaller soil particles, such as clays and soil organic material, which are most susceptible to wind erosion and contribute to inhalation exposure in off-site populations. We measured Kd for uranium in soils from two neighboring semiarid forest sites at LANL using a U.S. Environmental Protection Agency (EPA)-based protocol for both whole soil and the fine soil fraction (diameters
- Zou, C. B., Barron-Gafford, G. A., & Breshears, D. D. (2007). Effects of topography and woody plant canopy cover on near-ground solar radiation: Relevant energy inputs for ecohydrology and hydropedology. Geophysical Research Letters, 34(24).More infoAbstract: The emerging interdisciplinary approaches of ecohydrology and hydropedology are sensitive to variation in soil-surface energy inputs, which are primarily modified by topography and woody plant canopies. Yet a synthesis of the interactive effects of these two modification types is lacking. We systematically estimated near-ground surface solar radiation inputs as modified by key attributes of topography (aspect and slope) and tree cover (degree of openness) using solar radiation modeling based on hemispherical photographs. For south aspects, reductions in annual transmission were dominated by canopy cover rather than topography, even when canopy cover was low, whereas for north aspects, canopy effects dominated the reduction in annual transmission for slopes of up to 10° at low canopy cover and up to 30° at high canopy cover. Our results provide a synthetic perspective of the nonlinear, interactive, and temporally dependent effects of slope, aspect, and amount of canopy cover on near-ground solar radiation. Copyright 2007 by the American Geophysical Union.
- Breshears, D. D. (2006). The grassland-forest continuum: Trends in ecosystem properties for woody plant mosaics?. Frontiers in Ecology and the Environment, 4(2), 96-104.More infoAbstract: Many ecosystems can be viewed as lying within a continuum between grassland and forest, where ground coverage by woody plants (trees and shrubs) ranges from non-existent to complete. Patterns of energy, water, and biogeochemistry are often heterogeneous between canopy patches beneath woody plants and the intercanopy patches that separate them. Notably, connectivity between patch types is produced by processes such as shading, root uptake of resources, and redistribution of runoff. Patch-scale connectivity is hypothesized to influence trends in energy, water, and biogeochemistry as a function of woody plant canopy coverage. When connectivity is strong, the mean for an ecosystem property is expected to change in a more curvilinear than linear fashion along the continuum. Associated variance is expected to be greatest not midway along the continuum, as might be expected, but rather at a site with substantially less than 50% canopy coverage. These hypotheses collectively provide a framework for future research and are directly applicable to numerous, seemingly disparate environmental issues associated with encroachment, xerification (desertification), deforestation, die-off, fire, and restoration. © The Ecological Society of America.
- Whicker, J. J., E., J., & Breshears, D. D. (2006). Increased wind erosion from forest wildfire: Implications for contaminant-related risks. Journal of Environmental Quality, 35(2), 468-478.More infoPMID: 16455847;Abstract: Assessments of contaminant-related human and ecological risk require estimation of transport rates, but few data exist on wind-driven transport rates in nonagricultural systems, particularly in response to ecosystem disturbances such as forest wildfire and also relative to water-driven transport. The Cerro Grande wildfire in May of 2000 burned across ponderosa pine (Pinus ponderosa Douglas ex P.&C. Lawson var. scopulorum Englem.) forest within Los Alamos National Laboratory in northern New Mexico, where contaminant transport and associated post-fire inhalation risks are of concern. In response, the objectives of this study were to measure and compare wind-driven horizontal and vertical dust fluxes, metrics of transport related to wind erosion, for 3 yr for sites differentially affected by the Cerro Grande wildfire: unbnmed, moderately burned (fire mostly confined to ground vegetation), and severely burned (crown fare). Wind-driven dust flux was significantly greater in both types of burned areas relative to unburned areas, by more than one order of magnitude initially and by two to three times 1 yr after the fire. Unexpectedly, the elevated dust fluxes did not decrease during the second and third years in burned areas, apparently because ongoing drought delayed post-fire recovery. Our estimates enable assessment of amplification in contaminant-related risks following a major type of disturbance-wildfire, which is expected to increase in intensity and frequency due to climate change. More generally, our results highlight the importance of considering wind- as wel as water-driven transport and erosion, particularly following disturbance, for ecosystem biogeochemistry in general and human and ecological risk assessment in particular. © ASA, CSSA, SSSA.
- Whicker, J. J., E., J., Breshears, D. D., & Eberhart, C. F. (2006). From dust to dose: Effects of forest disturbance on increased inhalation exposure. Science of the Total Environment, 368(2-3), 519-530.More infoPMID: 16618498;Abstract: Ecosystem disturbances that remove vegetation and disturb surface soils are major causes of excessive soil erosion and can result in accelerated transport of soils contaminated with hazardous materials. Accelerated wind erosion in disturbed lands that are contaminated is of particular concern because of potential increased inhalation exposure, yet measurements regarding these relationships are lacking. The importance of this was highlighted when, in May of 2000, the Cerro Grande fire burned over roughly 30% of Los Alamos National Laboratory (LANL), mostly in ponderosa pine (Pinus ponderosa) forest, and through areas with soils containing contaminants, particularly excess depleted and natural uranium. Additionally, post-fire thinning was performed in burned and unburned forests on about 25% of LANL land. The first goal of this study was to assess the potential for increased inhalation dose from uranium contaminated soils via wind-driven resuspension of soil following the Cerro Grande Fire and subsequent forest thinning. This was done through analysis of post-disturbance measurements of uranium air concentrations and their relationships with wind velocity and seasonal vegetation cover. We found a 14% average increase in uranium air concentrations at LANL perimeter locations after the fire, and the greatest air concentrations occurred during the months of April-June when wind velocities are highest, no snow cover, and low vegetation cover. The second goal was to develop a methodology to assess the relative contribution of each disturbance type towards increasing public and worker exposure to these resuspended soils. Measurements of wind-driven dust flux in severely burned, moderately burned, thinned, and unburned/unthinned forest areas were used to assess horizontal dust flux (HDF) in these areas. Using empirically derived relationships between measurements of HDF and respirible dust, coupled with onsite uranium soil concentrations, we estimate relative increases in inhalation doses for workers ranging from 15% to 38%. Despite the potential for increased doses resulting from these forest disturbances, the estimated annual dose rate for the public was < 1 μSv yr- 1, which is far below the dose limits for public exposures, and the upper-bound dose rate for a LANL worker was estimated to be 140 μSv yr- 1, far below the 5 × 104 μSv yr- 1 occupational dose limit. These results show the importance of ecosystem disturbance in increasing mobility of soil-bound contaminants, which can ultimately increase exposure. However, it is important to investigate the magnitude of the increases when deciding appropriate strategies for management and long-term stewardship of contaminated lands.
- Betancourt, J. L., Schwartz, M. D., Breshears, D. D., Cayan, D. R., Dettinger, M. D., Inouye, D. W., Post, E., & Reed, B. C. (2005). Implementing a U.S. National phenology network. Eos, 86(51), 539-.
- Breshears, D. D., Cobb, N. S., Rich, P. M., Price, K. P., Allen, C. D., Balice, R. G., Romme, W. H., Kastens, J. H., Floyd, M. L., Belnap, J., Anderson, J. J., Myers, O. B., & Meyer, C. W. (2005). Regional vegetation die-off in response to global-change-type drought. Proceedings of the National Academy of Sciences of the United States of America, 102(42), 15144-15148.More infoPMID: 16217022;PMCID: PMC1250231;Abstract: Future drought is projected to occur under warmer temperature conditions as climate change progresses, referred to here as global-change-type drought, yet quantitative assessments of the triggers and potential extent of drought-induced vegetation die-off remain pivotal uncertainties in assessing climate-change impacts. Of particular concern is regional-scale mortality of overstory trees, which rapidly alters ecosystem type, associated ecosystem properties, and land surface conditions for decades. Here, we quantify regional-scale vegetation die-off across southwestern North American woodlands in 2002-2003 in response to drought and associated bark beetle infestations. At an intensively studied site within the region, we quantified that after 15 months of depleted soil water content, >90% of the dominant, overstory tree species (Pinus edulis, a piñon) died. The die-off was reflected in changes in a remotely sensed index of vegetation greenness (Normalized Difference Vegetation Index), not only at the intensively studied site but also across the region, extending over 12,000 km2 or more; aerial and field surveys confirmed the general extent of the die-off. Notably, the recent drought was warmer than the previous subcontinental drought of the 1950s. The limited, available observations suggest that die-off from the recent drought was more extensive than that from the previous drought, extending into wetter sites within the tree species' distribution. Our results quantify a trigger leading to rapid, drought-induced die-off of overstory woody plants at subcontinental scale and highlight the potential for such die-off to be more severe and extensive for future global-change-type drought under warmer conditions. © 2005 by The National Academy of Sciences of the USA.
- Breshears, D. D., Nyhan, J. W., & Davenport, D. W. (2005). Ecohydrology monitoring and excavation of semiarid landfill covers a decade after installation. Vadose Zone Journal, 4(3), 798-810.More infoAbstract: Landfill covers are intended to protect buried waste from water seepage and biointrusion for thirty to thousands of years, yet most cover studies are limited to a few years and do not directly investigate net changes in the soil profile that affect changing landfill performance. We evaluated water balances, vegetation cover, rooting patterns, and soil profiles of two landfill-cover designs (two plots each) more than a decade after installation at semiarid Los Alamos National Laboratory, NM, USA: a conventional design of 20 cm of topsoil over compacted crushed-tuff and an integrated design of 71 cm of topsoil over an engineered barrier designed to induce lateral flow (geotextile overlying 46 cm of gravel). Water balances for both designs had ∼3% of precipitation as seepage; the integrated plots lost
- Fair, J. M., & Breshears, D. D. (2005). Drought stress and fluctuating asymmetry in Quercus undulata leaves: Confounding effects of absolute and relative amounts of stress?. Journal of Arid Environments, 62(2), 235-249.More infoAbstract: One of the most general types of stress experienced by plants is water-limitation, which becomes particularly pronounced during periods of drought. We evaluated fluctuating asymmetry (FA) in Quercus undulata leaves for two subsequent dry years: 2001, when precipitation was 25% below average, and 2002, when precipitation was 65% below average, from a plot receiving ambient water and one in which water was excluded. In the first and less severe drought year, ambient-water trees had a slightly higher index of FA than the water-exclusion trees, contrary to expectations. However, in the second and much more extreme drought year, water-exclusion trees exhibited greater FA as expected, but in additional observations water-supplement trees exhibited by far the greatest amount of FA, contrary to expected. Further data on plant water potential confirmed that degree of plant stress corresponded to plot treatments: water exclusion>ambient water>water supplement. Stable carbon isotope ratios indicated that trees on the water-supplement plots were less stressed than ambient-water and water-exclusion trees, and leaf size was much greater for water-supplement trees than ambient-water or water-exclusion trees. We hypothesize that the complexity of the results could be due to the confounding effects of relative vs. absolute stress. © 2005 Elsevier Ltd. All rights reserved.
- Hastings, B. K., Breshears, D. D., & Smith, F. M. (2005). Spatial variability in rainfall erosivity versus rainfall depth: Implications for sediment yield. Vadose Zone Journal, 4(3), 500-504.More infoAbstract: Rainfall depth within small semiarid watersheds can have high spatial variability, but spatial variability in rainfall erosivity, a more direct determinant of sediment yield, has not been quantified. Using 12 tipping-bucket rain gauges within a 40-ha piñ on [Pinus edulis Engelm)-juniper (Juniperus monosperma (Engelm.) Sarg.] woodland in New Mexico, we measured rainfall erosivity (EI 30) and associated precipitation and erosion metrics for 14 convective thunderstorms. Spatial variability in EI 30 had a median CV across storms of 22% (range: 9-73%), exceeded the median CV for rainfall depth (15%, range: 5-26%), and varied by up to a factor of five (5-25 N h -1) within 300 m. EI 30 was better correlated with sediment yield measured in
- Huxman, T. E., Wilcox, B. P., Breshears, D. D., Scott, R. L., Snyder, K. A., Small, E. E., Hultine, K., Pockman, W. T., & Jackson, R. B. (2005). Ecohydrological implications of woody plant encroachment. Ecology, 86(2), 308-319.More infoAbstract: Increases in the abundance or density of woody plants in historically semiarid and arid grassland ecosystems have important ecological, hydrological, and socioeconomic implications. Using a simplified water-balance model, we propose a framework for conceptualizing how woody plant encroachment is likely to affect components of the water cycle within these ecosystems. We focus in particular on streamflow and the partitioning of evapotranspiration into evaporation and transpiration. On the basis of this framework, we suggest that streamflow and evaporation processes are affected by woody plant encroachment in different ways, depending on the degree and seasonality of aridity and the availability of subsurface water. Differences in landscape physiography, climate, and runoff mechanisms mediate the influence of woody plants on hydrological processes. Streamflow is expected to decline as a result of woody plant encroachment in landscapes dominated by subsurface flow regimes. Similarly, encroachment of woody plants can be expected to produce an increase in the fractional contribution of bare soil evaporation to evapotranspiration in semiarid ecosystems, whereas such shifts may be small or negligible in both subhumid and arid ecosystems. This framework for considering the effects of woody plant encroachment highlights important ecological and hydrological interactions that serve as a basis for predicting other ecological aspects of vegetation change - such as potential changes in carbon cycling within an ecosystem. In locations where woody plant encroachment results in increased plant transpiration and concurrently the availability of soil water is reduced, increased accumulation of carbon in soils emerges as one prediction. Thus, explicitly considering the ecohydrological linkages associated with vegetation change provides needed information on the consequences of woody plant encroachment on water yield, carbon cycling, and other processes. © 2005 by the Ecological Society of America.
- Ludwig, J. A., Wilcox, B. P., Breshears, D. D., Tongway, D. J., & Imeson, A. C. (2005). Vegetation patches and runoff-erosion as interacting ecohydrological processes in semiarid landscapes. Ecology, 86(2), 288-297.More infoAbstract: Ecological and hydrological processes can interact strongly in landscapes, yet these processes are often studied separately. One particularly important interaction between these processes in patchy semiarid lands is how vegetation patches serve to obstruct runoff and then how this retained water increases patch growth that, in turn, provides feedbacks to the system. Such ecohydrological interactions have been mostly demonstrated for semiarid landscapes with distinctly banded vegetation patterns. In this paper, we use data from our studies and from the literature to evaluate how strongly four ecohydrological interactions apply across other patchy semiarid vegetations, and how these interactions are affected by disturbances. We specifically address four questions concerning ecohydrological interactions: (1) if vegetation patches obstruct runoff flows during rainfall events, how much more soil water is stored in these patches compared to open interpatch areas; (2) if inputs of water are higher in patches, how much stronger is the pulse of plant growth compared to interpatches; (3) if more soil water in patches promotes greater biological activity by organisms such as earthworms that create macropores, how much does this improve soil infiltrability; and (4) if vegetation patches are damaged on a hillslope, how much does this increase runoff and erosion and decrease biomass production? We used the trigger-transfer-reserve-pulse framework developed for Australian semiarid woodlands to put these four questions into a landscape context. For a variety of patchy semiarid vegetation types in Australia, Europe, and North America, we found that patches significantly stored more soil water, produced more growth and had better infiltrability than interpatches, and that runoff and erosion can markedly increase on disturbed hillslopes. However, these differences varied greatly and appeared to depend on factors such as the intensity and amount of input events (rainstorms) and type of topography, soils, and vegetation. Experimental and modeling studies are needed to better quantify how these factors specifically affect ecohydrological interactions. Our current findings do support the conclusion that vegetation patches and runoff-erosion processes do strongly interact in many semiarid landscapes across the globe, not just banded landscapes. © 2005 by the Ecological Society of America.
- Martens, S. N., & Breshears, D. D. (2005). Assessing contaminant transport vulnerability in complex topography using a distributed hydrologic model. Vadose Zone Journal, 4(3), 811-818.More infoAbstract: Modeling of vadose zone hydrology is required to address a variety of applied problems in general and risk assessments associated with contaminants in particular. Risk assessments increasingly must focus on multisite, multipathway analyses as opposed to single-site, single pathway analyses. Such assessments can be particularly challenging when contaminants are widely dispersed in complex topography. Here we highlight how a set of contaminated sites situated within complex topography can be effectively prioritized relative to vulnerability of contaminant transport from surface and subsurface flows. We used a distributed hydrologic model, SPLASH, to assess the lateral flows of surface and subsurface water following the simulation of a 100-year precipitation event, which could correspond to an intense thunder-storm. Our case study was conducted in the North Ancho watershed of Los Alamos National Laboratory, in northern New Mexico, USA, an area with widely dispersed contaminants and diverse topography. Simulated surface flows generally exceeded subsurface flows by more than four orders of magnitude, indicating the relative importance of potential redistribution of contaminants by surface flows for this type of precipitation event. For the 18 potential contaminant release sites investigated, the maximum surface flow varied by more than an order of magnitude across the sites. Half of the sites had surface flows
- Stimson, H. C., Breshears, D. D., Ustin, S. L., & Kefauver, S. C. (2005). Spectral sensing of foliar water conditions in two co-occurring conifer species: Pinus edulis and Juniperus monosperma. Remote Sensing of Environment, 96(1), 108-118.More infoAbstract: Many fundamental ecosystem properties and dynamics are determined by plant water stress, particularly in dryland ecosystems where water is usually limiting. Indeed, under severe drought, plant water stress and associated insect infestations can produce landscape-scale mortality. Despite the fundamental importance of plant water stress in determining properties and dynamics at ecosystem and landscape scales, approaches for remotely sensing plant water stress are largely lacking, particularly for conifers. We evaluated the remotely sensed detection of foliar drought stress in two conifer species, Pinus edulis and Juniperus monosperma, which are co-dominants of extensive-juniper woodlands in North America, the first of which experienced extensive mortality in association with a recent drought. Needle spectra were made on these species in the field using an integrating sphere and portable spectrometer. Two indices of foliar water condition, plant water content (% of dry mass) and plant water potential, were compared to five spectral analyses: continuum removal of the 970 and 1200 nm water absorption features, the Normalized Difference Water Index (NDWI), the Normalized Difference Vegetation Index (NDVI), and the red edge wavelength position. For P. edulis, plant water content was significantly correlated with four of the five indices: NDVI (R2=0.71) and NDWI (R2=0.68) which exhibited stronger relationships than 970 nm continuum removal (R2=0.57) or red edge position (R 2=0.45). All five indices were significantly correlated with P. edulis water content when trees undergoing mortality were included in analyses (R2=0.60-0.93). Although the correlations were weaker than for plant water content, plant water potential was significantly correlated with NDWI (R2=0.49), 970 nm (R2=0.44), NDVI (R2=0.35), and red edge (R2=0.34); again all five indices had significant relationships when trees undergoing mortality were included (R 2=0.51-0.86). The relationships were weaker for J. monosperma: water content was significantly related to 970 nm (R2=0.50) and 1200 nm (R2=0.37) continuums and NDVI (R2=0.33), while water potential was related only to 1200 nm (R2=0.40). Our results demonstrate a critical link between plant physiological characteristics tied to water stress and associated spectral signatures for two extensive co-occurring conifer species. © 2005 Elsevier Inc. All rights reserved.
- Breshears, D. D. (2004). 2004 Distinguished Scientific Achievement Award. Presented to F. Ward Whicker at the 49th annual meeting of the Health Physics Society, Washington, DC 11-15 July 2004.. Health physics, 87(6), 568-570.More infoPMID: 15545758;
- Loik, M. E., Breshears, D. D., Lauenroth, W. K., & Belnap, J. (2004). A multi-scale perspective of water pulses in dryland ecosystems: Climatology and ecohydrology of the western USA. Oecologia, 141(2), 269-281.More infoPMID: 15138879;Abstract: In dryland ecosystems, the timing and magnitude of precipitation pulses drive many key ecological processes, notably soil water availability for plants and soil microbiota. Plant available water has frequently been viewed simply as incoming precipitation, yet processes at larger scales drive precipitation pulses, and the subsequent transformation of precipitation pulses to plant available water are complex. We provide an overview of the factors that influence the spatial and temporal availability of water to plants and soil biota using examples from western USA drylands. Large spatial- and temporal-scale drivers of regional precipitation patterns include the position of the jet streams and frontal boundaries, the North American Monsoon, El Niño Southern Oscillation events, and the Pacific Decadal Oscillation. Topography and orography modify the patterns set up by the larger-scale drivers, resulting in regional patterns (102-106 km2) of precipitation magnitude, timing, and variation. Together, the large-scale and regional drivers impose important pulsed patterns on long-term precipitation trends at landscape scales, in which most site precipitation is received as small events (
- P., D., Herrick, J. E., Urban, D. L., Gardner, R. H., & Breshears, D. D. (2004). Strategies for ecological extrapolation. Oikos, 106(3), 627-636.
- Breshears, D. D., Whicker, J. J., Johansen, M. P., & E., J. (2003). Wind and water erosion and transport in semi-arid shrubland, grassland and forest ecosystems: Quantifying dominance of horizontal wind-driven transport. Earth Surface Processes and Landforms, 28(11), 1189-1209.More infoAbstract: Soil erosion is an important process in dryland ecosystems, yet measurements and comparisons of wind and water erosion within and among such ecosystems are lacking. Here we compare wind erosion and transport field measurements with water erosion and transport from rainfall-simulation for three different semi-arid ecosystems: A shrubland near Carlsbad, New Mexico; a grassland near Denver, Colorado; and a forest near Los Alamos, New Mexico. In addition to comparing erosion loss from an area, we propose a framework for comparing horizontal mass transport of wind- and water-driven materials as a metric for local soil redistribution. Median erosion rates from wind for vertical mass flux measurements (g m-2 d-1) were 1.5 _xt× 10-2 for the shrubland, 8.3 _xt× 10-3 for the grassland, and 9.1 _xt× 10 3 for the forest. Wind-driven transport from horizontal mass flux measurements was greatest in the shrubland (15.0 g m 2 d-1) followed by the grassland (1.5 g m-2 d-1) and the forest sites (0.17 g m 2 d-1). Annual projections accounting for longer-term site meteorology suggest that wind erosion exceeds water erosion at the shrubland by c. 33 times and by c. five times at the forest, but not the grassland site, where the high clay content of the soils contributed to greater amounts of water erosion: water erosion exceeded wind erosion by about three times. Horizontal transport by wind was greater than that by water for all three systems, overwhelmingly so in the shrubland (factor of c. 2200). Our results, which include some of the only wind erosion measurements to date for semi-arid grasslands and forests, provide a basis for hypothesizing trends in wind and water erosion among ecosystems, highlight the importance of wind erosion and transport in semi-arid ecosystems, and have implications for land surface geomorphology, contaminant transport, and ecosystem biogeochemistry. © 2003 John Wiley & Sons, Ltd.
- Ebinger, M. H., Norfleet, M. L., Breshears, D. D., Cremers, D. A., Ferris, M. J., Unkefer, P. J., Lamb, M. S., Goddard, K. L., & Meyer, C. W. (2003). Extending the applicability of laser-induced breakdown spectroscopy for total soil carbon measurement. Soil Science Society of America Journal, 67(5), 1616-1619.More infoAbstract: Soils are the major pool of terrestrial C globally. Estimating inventories and detecting changes in the soil C pool have remained elusive, largely because the spatial distribution of soil C varies considerably. New approaches are needed that enable more rapid, cost-effective, and sensitive measurements of soil C and that reduce uncertainty in estimates of C pools and changes in those pools. In this note we extend the application of a new method of total soil C measurement: laser-induced breakdown spectroscopy (LIBS). Carbon analysis using LIBS provides data in seconds from soils with little preparation, whereas conventional methods such as dry combustion require lengthy sample preparation and longer analysis times. Laser-induced breakdown spectroscopy instruments, unlike conventional instruments, can be used while in the field. This promising new method, however, has potential drawbacks, namely interference with Fe at approximately 248 nm. Here we report results of using a different C line at 193 nm to solve the interference problem. We also use a two-element standardization factor to demonstrate that the 193-nm line is nearly as sensitive to C concentration as the C line at 247.8 nm, and that calibration curves can be readily replicated. Overall these results indicate that LIBS is a very promising method to estimate soil C pools and dynamics.
- Johansen, M. P., Hakonson, T. E., Whicker, F. W., & Breshears, D. D. (2003). Pulsed Redistribution of a Contaminant Following Forest Fire: Cesium-137 in Runoff. Journal of Environmental Quality, 32(6), 2150-2157.More infoPMID: 14674537;Abstract: Of the natural processes that concentrate dispersed environmental contaminants, landscape fire stands out as having potential to rapidly concentrate contaminants and accelerate their redistribution. This study used rainfall simulation methods to quantify changes in concentration of a widely dispersed environmental contaminant (global fallout 137Cs) in soils and surface water runoff following a major forest fire at Los Alamos, New Mexico, USA. The 137Cs concentrations at the ground surface increased up to 40 times higher in ash deposits and three times higher for the topmost 50 mm of soil compared with pre-fire soils. Average redistribution rates were about one order of magnitude greater for burned plots, 5.96 KBq ha-1 mm-1 rainfall, compared with unburned plots, 0.55 KBq ha-1 mm-1 rainfall. The greatest surface water transport of 137Cs, 11.6 KBq ha-1 mm-1, occurred at the plot with the greatest amount of ground cover removal (80% bare soil) following fire. Concentration increases of 137Cs occurred during surface water erosion, resulting in enrichment of 137Cs levels in sediments by factors of 1.4 to 2.9 compared with parent soils. The elevated concentrations in runoff declined rapidly with time and cumulative precipitation occurrence and approached pre-fire levels after approximately 240 mm of rainfall. Our results provide evidence of order-of-magnitude concentration increases of a fallout radionuclide as a result of forest fire and rapid transport of radionuclides following fire that may have important implications for a wide range of geophysical, ecosystem, fire management, and risk-based issues.
- Wilcox, B. P., Breshears, D. D., & Allen, C. D. (2003). Ecohydrology of a resource-conserving semiarid woodland: Effects of scale and disturbance. Ecological Monographs, 73(2), 223-239.More infoAbstract: In semiarid landscapes, the linkage between runoff and vegetation is a particularly close one. In this paper we report on the results of a long-term and multiple-scale study of interactions between runoff, erosion, and vegetation in a piñon-juniper woodland in New Mexico. We use our results to address three knowledge gaps: (1) the temporal scaling relationships between precipitation and runoff; (2) the effects of spatial scale on runoff and erosion, as influenced by vegetation; and (3) the influence of disturbance on these relationships. On the basis of our results, we tested three assumptions that represent current thinking in these areas (as evidenced, for example, by explicit or implicit assumptions embedded in commonly used models). The first assumption, that aggregated precipitation can be used as a surrogate for total runoff in semiarid environments, was not verified by our findings. We found that when runoff is generated mainly by overland flow in these systems, aggregated precipitation amounts alone (by year, season, or individual event) are a poor predictor of runoff amounts. The second assumption, that at the hillslope and smaller scales runoff and erosion are independent of spatial scale, was likewise not verified. We found that the redistribution of water and sediment within the hillslope was substantial and that there was a strong and nonlinear reduction in unit-area runoff and erosion with increasing scale (our scales were slope lengths ranging from 1 m to 105 m). The third assumption, that disturbance-related increases in runoff and erosion remain constant with time, was partially verified. We found that for low-slope-gradient sites, disturbance led to accelerated runoff and erosion, and these conditions may persist for a decade or longer. On the basis of our findings, we further suggest that (a) disturbance alters the effects of scale on runoff and erosion in a predictable way - scale relationships in degraded areas will be fundamentally different from those in nondegraded areas because more runoff will escape off site and erosion rates will be much higher; and (b) there exists a slope threshold, below which semiarid landscapes will eventually recover following disturbance and above which there will be no recovery without mitigation or remediation.
- Wilcox, B. P., Breshears, D. D., & Turin, H. J. (2003). Hydraulic conductivity in a piñon-juniper woodland: Influence of vegetation. Soil Science Society of America Journal, 67(4), 1243-1249.More infoAbstract: In semiarid environments, vegetation affects surface runoff either by altering surface characteristics (e.g., surface roughness, litter absorption) or subsurface characteristics (e.g., hydraulic conductivity). Previous observations of runoff within a piñon-juniper [Pinus edulis Englem. and Juniperus monosperma (Englem.) Sarg.] woodland led us to hypothesize that hydraulic conductivity differs between vegetation types. Using ponded and tension infiltrometers, we measured saturated (Ks) and unsaturated [K(h)] hydraulic conductivity at three levels of a nested hierarchy: the patch (canopy and intercanopy), the unit (juniper canopy, piñon canopy, vegetated intercanopy, and bare intercanopy), and the intercanopy locus (grass, biological soil crust, bare spot). Differences were smaller than expected and generally not significant. Canopy and intercanopy Ks values were comparable with the exception of a small number of exceedingly high readings under the juniper canopy - a difference we attribute to higher surface macroporosity beneath juniper canopies. The unsaturated hydraulic conductivity, K(h), values were higher for canopy soils than for intercanopy soils, although differences were small. At the unit level, the only significant differences were for K(h) between juniper or piñon canopies vs. bare interspaces. Median K values for vegetated intercanopy areas were intermediate between but not significantly different from those for canopies and bare areas. There were no significant differences between grass, biological soil crust, and bare spots within the herbaceous intercanopy area. Overall, the observed differences in K between canopy and intercanopy patches do not account for differences in runoff observed previously.
- Breshears, D. D., & Allen, C. D. (2002). The importance of rapid, disturbance-induced losses in carbon management and sequestration. Global Ecology and Biogeography, 11(1), 1-5.More infoAbstract: Management of terrestrial carbon fluxes is being proposed as a means of increasing the amount of carbon sequestered in the terrestrial biosphere. This approach is generally viewed only as an interim strategy for the coming decades while other longer-term strategies are developed and implemented - the most important being the direct reduction of carbon emissions. We are concerned that the potential for rapid, disturbance-induced losses may be much greater than is currently appreciated, especially by the decision-making community. Here we wish to: (1) highlight the complex and threshold-like nature of disturbances - such as fire and drought, as well as the erosion associated with each - that could lead to carbon losses; (2) note the global extent of ecosystems that are at risk of such disturbance-induced carbon losses; and (3) call for increased consideration of and research on the mechanisms by which large, rapid disturbance-induced losses of terrestrial carbon could occur. Our lack of ability as a scientific community to predict such ecosystem dynamics is precluding the effective consideration of these processes into strategies and policies related to carbon management and sequestration. Consequently, scientists need to do more to improve quantification of these potential losses and to integrate them into sound, sustainable policy options.
- Whicker, J. J., Breshears, D. D., Wasiolek, P. T., Kirchner, T. B., Tavani, R. A., Schoep, D. A., & Rodgers, J. C. (2002). Temporal and spatial variation of episodic wind erosion in unburned and burned semiarid shrubland. Journal of Environmental Quality, 31(2), 599-612.More infoPMID: 11931452;Abstract: Redistribution of soil, nutrients, and contaminants is often driven by wind erosion in semiarid shrublands. Wind erosion depends on wind velocity (particularly during episodic, high-velocity winds) and on vegetation, which is generally sparse and spatially heterogeneous in semiarid ecosystems. Further, the vegetation cover can be rapidly and greatly altered due to disturbances, particularly fire. Few studies, however, have evaluated key temporal and spatial components of wind erosion with respect to (i) erosion rates on the scale of weeks as a function of episodic high-velocity winds, (ii) rates at unburned and burned sites, and (iii) within-site spatial heterogeneity in erosion. Measuring wind erosion in unburned and recently burned Chihuahuan desert shrubland, we found (i) weekly wind erosion was related more to daily peak wind velocities than to daily average velocities as consistent with our findings of a threshold wind velocity at approximately 7 m s-1; (ii) greater erodibility in burned vs. unburned shrubland as indicated by erosion thresholds, aerodynamic roughness, and nearground soil movement; and (iii) burned shrubland lost soil from intercanopy and especially canopy patches in contrast to unburned shrubland, where soil accumulated in canopy patches. Our results are among the first to quantify post-fire wind erosion and highlight the importance of accounting for finer temporal and spatial variation in shrubland wind erosion. This finer-scale variation relates to semiarid land degradation, and is particularly relevant for predictions of contaminant resuspension and redistribution, both of which historically ignore finer-scale temporal and spatial variation in wind erosion.
- Beeson, P. C., Martens, S. N., & Breshears, D. D. (2001). Simulating overland flow following wildfire: Mapping vulnerability to landscape disturbance. Hydrological Processes, 15(15), 2917-2930.More infoAbstract: The probability of landscape-scale disturbances such as fire are expected to increase in the future due to anticipated climate changes and past land management practices. These disturbances can produce dramatic changes in hydrologic responses (e.g. overland flow) that can pose risks to human life, infrastructure, and the environment. Assessing these risks and associated remediation strategies requires spatially explicit evaluation of upland hydrology. However, most current evaluation methods focus on a specified location within a watershed, precluding estimation of spatially distributed, upland, hydrological response; and those that do consider spatial variability usually do not account for redistribution of overland flow among adjacent subunits. Here we highlight the use of a spatially distributed model for assessing spatial changes in upland hydrologic response following landscape-scale disturbance. Using a distributed model called SPLASH (Simulator for Processes of Landscapes: Surface/Subsurface Hydrology), we simulated pre- and post-fire scenarios based on the Cerro Grande fire (Los Alamos, NM, USA; May 2000) over 17 300 ha (resolution of 30 m × 30 m) for 2 year and 100 year design storms. For the 2 year storm, maximum overland flow rates for burned cells in the post-fire scenario greatly exceeded those for pre-fire conditions (modes: pre-fire, 3·25 × 10-10 m3 s-1; post-fire, 7·0 × 10-10 m3 s-1). For the 100 year storm, maximum overland flow was much greater than for the 2 year storm (modal pre-fire: 31·8 × 10-10 m3 s-1), with the difference between pre- and post-fire simulations being less dramatic (modal post-fire: 48·6 × 10-10 m3 s-1). Mapped differences between pre- and post-fire provide a means for prioritizing upland areas for remediation using an approach that accounts not only for topography, soils, and plant cover, but also for the redistribution of overland flow. More generally, our results highlight the potential utility of spatially distributed models to focus and prioritize rehabilitation efforts for future assessments of risk following landscape-scale disturbance. Copyright © 2001 John Wiley and Sons, Ltd.
- Cremers, D. A., Ebinger, M. H., Breshears, D. D., Unkefer, P. J., Kammerdiener, S. A., Ferris, M. J., Catlett, K. M., & Brown, J. R. (2001). Measuring total soil carbon with laser-induced breakdown spectroscopy (LIBS). Journal of Environmental Quality, 30(6), 2202-2206.More infoPMID: 11790033;Abstract: Improving estimates of carbon inventories in soils is currently hindered by lack of a rapid analysis method for total soil carbon. A rapid, accurate, and precise method that could be used in the field would be a significant benefit to researchers investigating carbon cycling in soils and dynamics of soil carbon in global change processes. We tested a new analysis method for predicting total soil carbon using laser-induced breakdown spectroscopy (LIBS). We determined appropriate spectral signatures and calibrated the method using measurements from dry combustion of a Mollisol from a cultivated plot. From this calibration curve we predicted carbon concentrations in additional samples from the same soil and from an Alfisol collected in a semiarid woodland and compared these predictions with additional dry combustion measurements. Our initial tests suggest that the LIBS method rapidly and efficiently measures soil carbon with excellent detection limits (∼300 mg/kg), precision (4-5%), and accuracy (3-14%). Initial testing shows that LIBS measurements and dry combustion analyses are highly correlated (adjusted r2 = 0.96) for soils of distinct morphology, and that a sample can be analyzed by LIBS in less than one minute. The LIBS method is readily adaptable to a field-portable instrument, and this attribute-in combination with rapid and accurate sample analysis-suggests that this new method offers promise for improving measurement of total soil carbon. Additional testing of LIBS is required to understand the effects of soil properties such as texture, moisture content, and mineralogical composition (i.e., silicon content) on LIBS measurements.
- Johansen, M. P., Hakonson, T. E., & Breshears, D. D. (2001). Post-fire runoff and erosion from rainfall simulation: Contrasting forests with shrublands and grasslands. Hydrological Processes, 15(15), 2953-2965.More infoAbstract: Rainfall simulations allow for controlled comparisons of runoff and erosion among ecosystems and land cover conditions. Runoff and erosion can increase greatly following fire, yet there are few rainfall simulation studies for post-fire plots, particularly after severe fire in semiarid forest. We conducted rainfall simulations shortly after a severe fire (Cerro Grande) in ponderosa pine forest near Los Alamos, New Mexico, USA, which completely burned organic ground cover and exposed unprotected soil. Measurements on burned plots showed 74% of mineral soil was exposed compared with an estimated 3% exposed prior to the fire. Most of the remaining 26% surface area was covered by easily moveable ash. Rainfall was applied at 60 mm h-1 in three repeated tests over 2 days. Runoff from burned plots was about 45% of the total 120 mm of applied precipitation, but only 23% on the unburned plots. The most striking difference between the response of burned and unburned plots was the amount of sediment production; burned plots generated 25 times more sediment than unburned plots (76 kg ha-1 and 3 kg ha-1 respectively per millimetre of rain). Sediment yields were well correlated with percentage bare soil (r = 0.84). These sediment yields were more than an order of magnitude greater than nearly all comparable rainfall simulation studies conducted on burned plots in the USA, most of which have been in grasslands or shrublands. A synthesis of comparable studies suggests that an erosion threshold is reached as the amount of soil exposed by fire increases to 60-70%. Our results provide sediment yield and runoff data from severely burned surfaces, a condition for which little rainfall simulation data exist. Further, our results contrast post-fire hydrologic responses in forests with those in grasslands and shrublands. These results can be applied to problems concerning post-fire erosion, flooding, contaminant transport, and development of associated remediation strategies. Copyright © 2001 John Wiley and Sons, Ltd.
- Martens, S. N., Breshears, D. D., & Barnes, F. J. (2001). Development of species dominance along an elevational gradient: Population dynamics of Pinus edulis and Juniperus monosperma. International Journal of Plant Sciences, 162(4), 777-783.More infoAbstract: We evaluated species-environment relationships within piñon-juniper woodlands in northern New Mexico (United States) using canonical correspondence analysis (CCA). The first CCA axis was associated primarily with elevation. Our results showed separation between piñon and juniper along the elevation gradient, as expected: piñon is relatively more dominant at higher sites, whereas juniper is relatively more dominant at lower sites. To examine how this pattern of dominance might emerge with time, we plotted the position of centroids for three piñon and juniper size classes along the first CCA axis. We found that small piñons and junipers were distributed relatively uniformly across the gradient, whereas large piñons and junipers were strongly segregated along the gradient, with intermediate-sized piñons and junipers intermediate on the CCA axis between small and large. This produced a pattern of increased divergence between the two species that increased with size. We suggest that this pattern emerges as a result of differential mortality between the species rather than as a result of differences in seedling establishment along the gradient. These differences between the species could result from differences in resource use (i.e., physiology) and resource acquisition (i.e., rooting patterns relative to plant available water). We present a conceptual model of how differences between the species in resource acquisition increase with size (age). We suggest that differences in resource acquisition between species, which increase as individuals mature, may play a greater role in determining species dominance along resource gradients than has been previously appreciated.
- Martens, S. N., Breshears, D. D., & Meyer, C. W. (2000). Spatial distributions of understory light along the grassland/forest continuum: Effects of cover, height, and spatial pattern of tree canopies. Ecological Modelling, 126(1), 79-93.More infoAbstract: The understory light environment is a key determinant of vegetation pattern and ecosystem processes, and varies spatially perhaps more than any resource used by plants. Understory light varies along gradients of vegetation structure that range from grassland with no woody canopy cover to forest with nearly complete woody canopy cover. Spatial variability in understory light is largely determined by several characteristics of overstory plants - spatial pattern, height, and cover - which vary concurrently along the grassland/forest continuum. Using a spatially-explicit ray-tracing model, we quantified trends in mean and variance of understory light along the continuum. We modeled understory light over a growing season for two types of plots: (1) generated plots in which cover, spatial pattern, and height of trees were varied systematically, and (2) three actual plots using stand data from pinon-juniper woodland sites for which cover, spatial pattern and height varied concurrently. Mean understory light decreased with increasing canopy cover and was sensitive to changes in height, as expected, but was not sensitive to spatial pattern. Variance in understory light was maximum at an intermediate value of cover that was dependent on both spatial pattern and cover - maximum variance occurred at lower values of cover as height increased and as spatial pattern progressed from regular to random to aggregated. These trends in the overall patterns of understory light were also examined with respect to changes in understory light in canopy and intercanopy locations. Variance in understory light for intercanopy locations was less than that for canopy locations at low canopy cover, but exceeded that for canopy locations as canopy cover increased. The value of canopy cover at which variance in intercanopy locations exceeded that in canopy locations was sensitive to variation in height but not in spatial pattern. The distributions of understory light for the actual plots were generally similar to those for corresponding generated plots, with dissimilarities attributable to differences in cover and height. The general trends highlighted by our simulations are broadly applicable to sites along the grassland/forest continuum. (C) 2000 Elsevier Science B.V.
- Breshears, D. D., & Barnes, F. J. (1999). Interrelationships between plant functional types and soil moisture heterogeneity for semiarid landscapes within the grassland/forest continuum: A unified conceptual model. Landscape Ecology, 14(5), 465-478.More infoAbstract: In semiarid landscapes, the ratio of herbaceous to woody plant biomass is a major determinant of ecosystem properties. This ratio depends to a large extent on the amount and spatial distribution of soil moisture that is available to plants, and these variables, in turn, are determined primarily by climate and land use. Current conceptual models for determining the ratio of herbaceous to woody plant biomass in semiarid plant communities are based either on differences in soil moisture with depth (vertical heterogeneity) from one site to another (Walter's two-layer model) or on differences in soil moisture between canopy and intercanopy patches at the same site (horizontal heterogeneity) that result from disturbances associated with land use (Schlesinger et al.'s model of desertification). We developed a model that unifies these two perspectives by relaxing two assumptions of Walter's two-layer model. First, our model recognizes that soil moisture varies horizontally between canopy and intercanopy patches, not only due to land-use disturbance, a general assumption of the Schlesinger et al. model, but also due to the physical nature of the canopy itself. Second, while retaining the general assumption of Walter that woody plants obtain moisture from deeper soil layers than do herbaceous plants, our model recognizes the existence of two types of woody plants: those that extract a substantial proportion of their moisture from deeper layers and those that extract mainly from shallower layers. By modifying the two-layer hypothesis to include four soil compartments and distinguishing between shallow and deeper-rooted woody species, our model integrates three key concepts in semiarid ecology: (1) the proportion of woody cover increases as moisture in the deeper soil layers increases (Walter's two-layer hypothesis for coexistence of herbaceous and woody plants); (2) land use practices that cause a reduction in herbaceous vegetation and compaction of intercanopy soils lead to a long-term increase in the proportion of woody plants (Schlesinger et al.'s concept, or more generally, that at a given site multiple variations in the proportions of herbaceous and woody plant biomass are possible); and (3) changes in the ratios of herbaceous to woody plant biomass exhibit complex behavior (changes can happen quickly and are not directly reversible without intensive management). This integration of concepts results because rather than assuming a simple, one-way dependence of plant functional types on soil moisture heterogeneity, our model assumes an interdependence between the two: soil moisture heterogeneity constrains the composition of the plant community, which in turn modifies soil moisture heterogeneity. The four-compartment model that we propose enables, for the first time, an integrated picture of both dimensions of soil moisture heterogeneity - horizontal and vertical - and of the interdependence between soil moisture heterogeneity and the proportions of the plant functional types that make up a given plant community. This unified conceptual model can be applied to provide insight into the individual and the combined effects of climate and land use on semiarid plant communities within the grassland/forest continuum, which vary in the proportions of canopy and intercanopy patches.
- Reid, K. D., Wilcox, B. P., Breshears, D. D., & MacDonald, L. (1999). Runoff and erosion in a pinon-juniper woodland: Influence of vegetation patches. Soil Science Society of America Journal, 63(6), 1869-1879.More infoAbstract: In many semiarid regions, runoff and erosion differ according to vegetation patch type. These differences, although hypothesized to fundamentally affect ecological processes, have been poorly quantified. In a semiarid pinon-juniper woodland [Pinus edulis Engelm. and Juniperus monosperma (Engelm.) Sarg.] in northern New Mexico, we measured runoff and erosion from the three patch types that compose these woodlands: Canopy patches (those beneath woody plants), vegetated patches in intercanopy areas, and bare patches in intercanopy areas. The bare intercanopy patches exhibited the highest rates, followed by vegetated intercanopy patches and then by canopy patches. Large convective summer storms, though relatively infrequent, generated much of the runoff and most of the sediment; prolonged frontal storms were capable of generating considerable runoff but little sediment. A portion of the runoff and most of the sediment generated from bare intercanopy patches was redistributed down-slope, probably to adjacent vegetated intercanopy patches, demonstrating connectivity between these two patch types. Our results indicate that there are significant and important differences in runoff and sediment production from the three patch types; that bare intercanopy patches act as sources of both water and sediment for the vegetated intercanopy patches; and that the transfer of water and sediment at small scales is both frequent enough and substantial enough to be considered ecologically significant.
- Allen, C. D., & Breshears, D. D. (1998). Drought-induced shift of a forest-woodland ecotone: Rapid landscape response to climate variation. Proceedings of the National Academy of Sciences of the United States of America, 95(25), 14839-14842.More infoPMID: 9843976;PMCID: PMC24536;Abstract: In coming decades, global climate changes are expected to produce large shifts in vegetation distributions at unprecedented rates. These shifts are expected to be most rapid and extreme at ecotones, the boundaries between ecosystems, particularly those in semiarid landscapes. However, current models do not adequately provide for such rapid effects - particularly those caused by mortality - largely because of the lack of data from field studies. Here we report the most rapid landscape-scale shift of a woody ecotone ever documented: in northern New Mexico in the 1950s, the ecotone between semiarid ponderosa pine forest and pinon-juniper woodland shifted extensively (2 km or more) and rapidly (
- Breshears, D. D., Nyhan, J. W., Heil, C. E., & Wilcox, B. P. (1998). Effects of woody plants on microclimate in a semiarid woodland: Soil temperature and evaporation in canopy and intercanopy patches. International Journal of Plant Sciences, 159(6), 1010-1017.More infoAbstract: The canopies of woody plants in semiarid ecosystems modify the microclimate beneath and around them, with canopy patches usually having lower soil temperatures than intercanopy patches. However, lacking are studies that have evaluated how heterogeneity in soil temperature, induced by woody plant canopies, influences soil evaporation rates and the consequent effects on plant-available water. Soil temperatures were measured and soil evaporation rates were estimated for canopy and intercanopy patches in a semiarid pinon-juniper woodland (Pinus edulis and Juniperus monosperma) in northern New Mexico. Soil temperature was measured at 2-cm depths in four canopy and four intercanopy locations during 1994. Maximum soil temperature in intercanopy patches was greater than in canopy patches between May and September, by as much as 10°C, while soil temperatures in intercanopy patches were lower than in canopy patches during colder parts of the day in the fall and winter months. Equations for soil drying rates for sandy loam soil samples were determined in laboratory experiments over a range of temperatures and soil water contents. Drying rates were disproportionately greater at high soil moisture and high soil temperature. Intercanopy patches were predicted to dry more than canopy patches for days in April through September by as much as 2% volumetric soil water content per day. The difference between patches was amplified at lower soil water contents when expressed as soil water potential, which more directly determines plant-available water. Our results quantify the effects of woody plants on the microclimate with respect to soil temperature and evaporation, which in turn affect herbaceous and woody plants by modifying factors such as germination, the potential for facilitation, and the amount of plant-available water.
- Davenport, D. W., Breshears, D. D., Wilcox, B. P., & Allen, C. D. (1998). Viewpoint: Sustainability of pinon-juniper ecosystems - A unifying perspective of soil erosion thresholds. Journal of Range Management, 51(2), 231-240.More infoAbstract: Many pinon-juniper ecosystem in the western U.S. are subject to accelerated erosion while others are undergoing little or no erosion. Controversy has developed over whether invading or encroaching pinon and juniper species are inherently harmful to rangeland ecosystems. We developed a conceptual model of soil erosion in pinon-jumper ecosystems that is consistent with both sides of the controversy and suggests that the diverse perspectives on this issue arise from threshold effects operating under very different site conditions. Soil erosion rate can be viewed as a function of (1) site erosion potential (SEP), determined by climate, geomorphology and soil erodibility; and (2) ground cover. Site erosion potential and cove act synergistically to determine soil erosion rates, as evident even from simple USLE predictions of erosion. In pinon-juniper ecosystem with high SEP, the erosion rate is highly sensitive to ground cover and can cross a threshold so that erosion increases dramatically in response to a small decrease in cover. The sensitivity of erosion rate to SEP and cover can be visualized as a cusp catastrophe surface on which changes may occur rapidly and irreversibly. The mechanisms associated with a rapid shift from low to high erosion rate can be illustrated using percolation theory to incorporate spatial, temporal, and scale-dependent patterns of water storage capacity on a hillslope. Percolation theory demonstrates how hillslope runoff can undergo a threshold response to a minor change in storage capacity. Our conceptual model suggests that pinion and juniper contribute to accelerated erosion only under a limited range of site conditions which, however, may exist over large areas.
- Breshears, D. D., Myers, O. B., Johnson, S. R., Meyer, C. W., & Martens, S. N. (1997). Differential use of spatially heterogeneous soil moisture by two semiarid woody species: Pinus edulis and Juniperus monosperma. Journal of Ecology, 85(3), 289-299.More infoAbstract: 1. Soil moisture in semiarid woodlands varies both vertically with depth and horizontally between canopy patches beneath woody plants and the intercanopy patches that separate them, such that shallow soil layers in intercanopy locations are wettest, yet few studies have considered both dimensions of spatial variability in testing for acquisition of resources by plants. 2. Three hypotheses were tested relative to the rise of shallow water in intercanopy locations by two coexisting semiarid-woodland tree species, Pinus edulis (a pinon) and Juniperus monosperma (a juniper): (i) both P. edulis and J. monosperma can use shallow water from intercanopy locations; (ii) J. monosperma is able to obtain more shallow water from intercanopy locations than P. edulis, and (iii) the spatial arrangement of the trees influences the amount of water they obtain. Soil moisture and plant water potential (i.e. plant water stress) were measured before and after the addition of water to shallow depths (0.30 cm) of intercanopy locations for trees of both species in two spatial arrangements: isolated and paired with a contiguous tree of the other species. 3. Both species responded to the addition of shallow water in intercanopy locations, as measured by plant water potential. The response of J. monosperma was significantly greater than that of P. edulis, as measured by depletion of shallow soil moisture in intercanopy locations and by change in plant water potential per unit change in soil water potential (the difference was not detectable on the basis of plant water potential alone): in addition, the amount of depletion was correlated with basal area of J. monosperma but not of P. edulis. The responses were not influenced by spatial arrangement (isolated vs. paired with a contiguous tree of the other species). 4. The results of this study are consistent with differences in the relative abundances of the two species across locations, suggesting that species differences in ability to use shallow water in intercanopy locations is important in structuring semiarid woodlands. Further, the results suggest that current theoretical concepts for semiarid ecosystems, which ignore either vertical or horizontal variability in soil moisture, may be inadequate for predicting changes in the ratio of woody to herbaceous plant biomass, particularly for plant communities with co-dominant woody species that differ in ability to acquire spatially heterogeneous resources.
- Breshears, D. D., Rich, P. M., Barnes, F. J., & Campbell, K. (1997). Overstory-imposed heterogeneity in solar radiation and soil moisture in a semiarid woodland. Ecological Applications, 7(4), 1201-1215.More infoAbstract: Degradation of semiarid ecosystems is a major environmental problem worldwide, characterized by a reduction in the ratio of herbaceous to woody plant biomass. These ecosystems can be described as a set of canopy patches comprising woody plants and the intercanopy patches that separate them, yielding an overstory with intermediate closure. Field measurements of microclimate at the scale of canopy patches, particularly for near-ground solar radiation and soil moisture, are largely lacking from both nondegraded and degraded ecosystems. We tested for relationships among spatial patterns of the over-story, near-ground solar radiation, and soil moisture in a semiarid piñon-juniper woodland in northern New Mexico that had a highly heterogeneous overstory (≈50% canopy cover) and was not degraded with respect to ground cover and erosion rates. We used measurements taken every 1 m along a 102-m transect - solar radiation indices were estimated monthly and annually using hemispherical photographs, and soil moisture was measured over 4 yr using time-domain reflectometry (TDR) - and analyzed the data using general least squares linear models that accounted for spatial autocorrelation and temporal heteroscedasticity. Time-averages of solar radiation and of soil moisture both were spatially autocorrelated at scales of up to 4 m (P < 0.05), corresponding approximately to the average lengths of both canopy and intercanopy patches and to the scale of spatial autocorrelation in the canopy/intercanopy pattern of the overstory (3 m; P < 0.05). For near-ground solar radiation, we found expected spatial variation between patches (canopy < intercanopy; P < 0.0001) and within patches for centers vs. edges (canopy center < canopy edge and intercanopy center > intercanopy edge; P < 0.0001) and for north vs. south edges (canopy north edge < canopy south edge and intercanopy south edge < intercanopy north edge; P < 0.0001). For soil moisture, canopy locations were significantly drier than intercanopy locations (P < 0.0001), and edge locations were significantly wetter than center locations both overall and within both patch types (P < 0.0001). Spatial heterogeneity in soil mositure was attributed primarily to canopy interception and drip on the basis of large differences in snow cover between canopy and intercanopy locations. Spatial autocorrelation in the residuals for soil moisture of up to 7 m was attributed to transpiration by woody plants at scales corresponding to belowground root distributions. The spatial heterogeneities in near-ground solar radiation and soil moisture are of sufficient magnitude to affect biotic processes of woody and herbaceous plants, such as growth and seedling establishment. Because land degradation problems in semiarid shrublands and woodlands appear to result from differential impacts to intercanopy vs. canopy patches, our results can be used to help design effective mitigation and remediation strategies. More generally, our results demonstrate how the physical presence of woody canopies reinforces spatial heterogeneity in microclimate and, because our site has intermediate closure of the overstory, bridge the gap along a grassland-forest continuum between related studies in relatively open savannas and in forests with nearly closed canopies.
- Martens, S. N., Breshears, D. D., Meyer, C. W., & Barnes, F. J. (1997). Scales of above-ground and below-ground competition in a semi-arid woodland detected from spatial pattern. Journal of Vegetation Science, 8(5), 655-664.More infoAbstract: Semi-arid woodlands are two-phase mosaics of canopy and inter-canopy patches. We hypothesized that both above-ground competition (within canopy patches), and below-ground competition (between canopy patches), would be important structuring processes in these communities. We investigated the spatial pattern of trees in a Pinus edulis-Juniperus monosperma woodland in New Mexico using Ripley's K-function. We found strong aggregation of trees at scales of 2 to 4 m, which indicates the scale of canopy patches. Canopy patches were composed of individuals of both species. Crown centers of both species were always less aggregated than stem centers at scales less than canopy patch size, indicating morphological plasticity of competing crowns. In the smallest size classes of both species, aggregation was most intense, and occurred over a larger range of scales; aggregation decreased with increasing size as is consistent with density-dependent mortality from intraspecific competition. Within canopy patches, younger trees were associated with older trees of the other species. At scales larger than canopy patches, younger trees showed repulsion from older conspecifics, indicating below-ground competition. Hence, intraspecific competition was stronger than interspecific competition, probably because the species differ in rooting depth. Woodland dynamics depend on the scale and composition of canopy patches, aggregated seed deposition and facilitation, above- and below-ground competition, and temporal changes in the spatial scale of interactions. This woodland is intermediate in a grassland-forest continuum (a gradient of increasing woody canopy cover) and hence we expected, and were able to detect, the effects of both above- and below-ground competition.
- Wilcox, B. P., & Breshears, D. D. (1997). Interflow in semiarid environments: An overlooked process in risk assessment. Human and Ecological Risk Assessment (HERA), 3(2), 187-203.More infoAbstract: Risk assessment, both human and ecological, embodies fundamental assumptions about hydrological processes, especially how they affect the movement of contaminants in the environment. The lateral movement of water through the soil, or interflow, is frequently a component of risk assessments for humid environments, but not of those for semiarid environments. Our research has shown that, contrary to what was previously thought, interflow can be important in semiarid landscapes and is, therefore, an essential consideration for risk assessment in these regions. To illustrate and assess the effect of interflow on estimates of risk, we (1) developed a simple conceptual model to describe the role that interflow may have in the redistribution of surface and near-surface contamination, and (2) used RESRAD, an exposure model for assessing radionuclide doses to humans, to evaluate the effectiveness of landfill covers in mitigating doses of three contaminants ( 3H, 238U, and 239/240pu) at a site in northern New Mexico at which interflow is known to be occurring. Only those calculations of the model that took interflow into account yielded the result that the radionuclides would contaminate groundwater - underscoring the potential importance of interflow as a mechanism for the transport of contaminants. We conclude that failure to take interflow into account can render risk assessments inaccurate and remediation ineffective. Further, our work demonstrates that a general understanding of hydrological processes is essential for accurate risk assessment, ecological as well as human. © 1997 by ASP.
- Davenport, D. W., Wilcox, B. P., & Breshears, D. D. (1996). Soil morphology of canopy and intercanopy sites in a pinon-juniper woodland. Soil Science Society of America Journal, 60(6), 1881-1887.More infoAbstract: Pinon-juniper woodlands in the semiarid western USA have expanded as much as fivefold during the last 150 yr, often accompanied by losses of understory vegetation and increasing soil erosion. We conducted this study to determine the differences in soil morphology between canopy and intercanopy locations within a pinon (Pinus edulis Engelm.)-juniper [Juniperus monosperma (Engelm.) Sarg.] woodland with uniform parent material, topography, and climate. The woodland studied, located near Los Alamos, NM, has a mean tree age of 135 yr. We examined soil morphology by augering 135 profiles in a square grid pattern and comparing soils under pinon and juniper canopies with intercanopy soils. Only two of the 17 morphological properties compared showed significant differences. The B horizons make up a slightly greater proportion of total profile thickness in intercanopy soils, and there are higher percentages of coarse fragments in the lower portions of canopy soil profiles. Canopy soils have lower mean pH and higher mean organic C than intercanopy soils. Regression analysis showed that most soil properties did not closely correspond with tree size, but total soil thickness and B horizon thickness are significantly greater under the largest pinon trees, and soil reaction is lower under the largest juniper trees. Our findings suggest that during the period in which pinon-juniper woodlands have been expanding, the trees have had only minor effects on soil morphology.
- Yeakley, J., Moen, R. A., Breshears, D. D., & Nungesser, M. K. (1994). Response of North American ecosystem models to multi-annual periodicities in temperature and precipitation. Landscape Ecology, 9(4), 249-260.More infoAbstract: Ecosystem models typically use input temperature and precipitation data generated stochastically from weather station means and variances. Although the weather station data are based on measurements taken over a few decades, model simulations are usually on the order of centuries. Consequently, observed periodicities in temperature and precipitation at the continental scale that have been correlated with largscale forcings, such as ocean-atmosphere dynamics and lunar and sunspot cycles, are ignored. We investigated how these natural climatic fluctuations affect aboveground biomass in ecosystem models by incorporating some of the more pronounced continental-scale cycles in temperature (4, 11, 80, 180 year periods) and precipitation (11 and 19 year periods) into models of three North American forests (using LINKAGES) and one North American grassland (using STEPPE). Even without inclusion of periodicities in climate, long-term dynamics of these models were characterized by internal frequencies resulting from vegetation birth, growth and death processes. Our results indicate that long-term temperature cycles result in significantly lower predictions of forest biomass than observed in the control case for a forest on a biome transition (northern hardwoods/boreal forest). Lower-frequency, higher-amplitude temperature oscillation caused amplification of forest biomass response in forests containing hardwood species. Shortgrass prairie and boreal ecosystems, dominated by species with broad stress tolerance ranges, were relatively insensitive to climatic oscillations. Our results suggest periodicities in climate should be incorporated within long-term simulations of ecosystems with strong internal frequencies, particularly for systems on biome transitions. © 1994 SPB Academic Publishing bv.
- Breshears, D. D., Whicker, F. W., & Hakonson, T. E. (1993). Orchestrating environmental research and assessment for remediation. Ecological Applications, 3(4), 590-594.
- Whicker, F. W., Kirchner, T. B., Breshears, D. D., & Otis, M. D. (1990). Estimation of radionuclide ingestion: The 'PATHWAY' food-chain model. Health Physics, 59(5), 645-657.More infoPMID: 2211122;Abstract: This paper describes the structure of the dynamic food-chain model PATHWAY and its utility for estimating radionuclide ingestion after fallout deposition from nuclear testing in Nevada. Model input requirements are described and output examples are provided. The basic output of PATHWAY is the time-integrated radionuclide ingestion by humans per unit fallout deposition (Bq per Bq m-2). Output specific to sex, age, life-style (diet), location (agricultural practice), event (calender date), and radionuclide may be generated. Uncertainties of model predictions, based on 'Monte Carlo' simulations using parameter value distributions, are described. Results of a sensitivity analysis, based on a ranking of partial correlation coefficients, are reviewed to illustrate the relative importance of parameters and associated transport pathways. Output data for 131I and 137Cs in milk are compared with predictions from several well known food-chain models. Preliminary efforts to validate PATHWAY results with real data sets are described.
- Breshears, D. D., Kirchner, T. B., Otis, M. D., & Whicker, F. W. (1989). Uncertainty in predictions of fallout radionuclides in foods and of subsequent ingestion. Health Physics, 57(6), 943-953.More infoPMID: 2584029;
- Breshears, D. D. (2018, Fall). Co-authored AGU presentations. American Geophyscial Union Annual Meeting 2018. Washington DC: American Geophyscial Union.More infoco-authored presentations listed in attached
- Breshears, D. D. (2018, Summer). Presentations co-authored. Ecological Society of America Annual Meeting 2018. New Orleans, Louisiana: Ecological Society of America.More infoco-authored presentations listed in attached
- Barnes, M. L., Moore, D. J., Breshears, D. D., Law, D. J., & Fojtik, A. C. (2017, August). Beyond greenness: Potential for detecting temporal changes in photosynthetic capacity with hyperspectral imaging. 102nd Annual Meeting of the Ecological Society of America. Portland, OR: ESA.
- Lopez Hoffman, L., Swann, A., Garcia, D., Breshears, D. D., Huang, T., Merideth, R., Semmens, D., & Diffendorfer, J. (2017, August). An approach to understand how ecoclimate teleconnections create telecoupling in ecosystem services across space. Ecological Society of America Annual Meeting. Portland, Oregon.
- Breshears, D. D. (2016, Apr). Tree Die-off from Hot Drought: Reinforcing and Resolving the Risks.. istinguished Collaborator Program Seminar, Murdoch University.. Perth, Australia.: Murdoch University..More infoBreshears, D.D. Tree Die-off from Hot Drought: Reinforcing and Resolving the Risks. Distinguished Collaborator Program Seminar, Murdoch University. 21 April 2016. Perth, Australia.
- Breshears, D. D. (2016, Jan). On Underestimation of Global Vulnerability to Tree Mortality and Forest die‐Off From Hotter Drought in the Anthropocene.. School of Natural Resources and the Environment Seminar Series, University of Arizona.. Tucson, Arizona.: School of Natural Resources and the Environment.More infoBreshears, D.D. On Underestimation of Global Vulnerability to Tree Mortality and Forest die‐Off From Hotter Drought in the Anthropocene. 13 January 2016. Tucson, Arizona.
- Breshears, D. D. (2016, Oct). Connectivities and drivers associated with global vegetation mortality. Ecology and Evolutionary Biology Seminar Series, University of Arizona. Ecology and Evolutionary Biology Seminar Series, University of Arizona: Ecology and Evolutionary Biology, University of Arizona.More infoBreshears, D.D. Connectivities and drivers associated with global vegetation mortality. Ecology and Evolutionary Biology Seminar Series, University of Arizona. 24 October 2016. Tucson, Arizona.
- Breshears, D. D. (2016, Sep). Prototyping Assessment of Ecoclimate Teleconnections Associated with Tree Die-off. Annual NSF Macrosystems Meeting. Washington D.C.: National Science Foundation.More infoBreshears, D.D., A.L.S. Swann, S.C. Stark, S.R. Saleska, E.S. Garcia, J.C. Villegas, D.J. Law, D.M. Minor, J.P. Field, D.J.P. Moore. Prototyping Assessment of Ecoclimate Teleconnections Associated with Tree Die-off. Annual NSF Macrosystems Meeting. 29-30 September 2016. Washington D.C.
- Saleska, S. R., Garcia, E., Stark, S. C., Villegas, J. C., Breshears, D. D., Swann, A. L., Moore, D. J., Law, D. J., Minor, D. M., & Field, J. P. (2016, Dec). Accounting for "Ecoclimate Teleconnections" that structure ecological responses to disturbance and climate, from local to global scales. American Geophysical Union Fall 2016 Annual Meeting. San Francisco: American Geophysical Union.
- Stark, S. C., Stark, S. C., Longo, M., Longo, M., Saleska, S. R., Saleska, S. R., Swann, A. L., Swann, A. L., Breshears, D. D., Breshears, D. D., Villegas, J. C., Villegas, J. C., Wu, J., Wu, J., Schietti, J., & Schietti, J. (2016, Aug). Towards remotely sensed forest dynamics to understand forest change and its consequences for the atmosphere.. 101st Ecological Society of America Annual Meeting. Fort Lauderdale, Florida: Ecological Society of America.
- Breshears, D. D. (2014, April). Tree Die-off Update. University of Colorado. Boulder: Ecology and Evolutionary Biology.More infoInvited seminar
- Breshears, D. D. (2014, Fall). UA-UNAM Dryland Consortiuum Project Annual Report - Dust Emissions and Soil Crusts. UA-UNAM Dryland Consortiuum Project Annual Meeting. Hermosillo.
- Breshears, D. D. (2014, Jun). Ecoclimatic Teleconnections Macrosystems Biology Project Overview. NSF Macrosystems Biology Meeting. Washington DC.
- Breshears, D. D. (2014, March). Ecoclimatic Teleconnections Macrosystems Biology Project Overview. Amazon Projects coordination meeting. Manaus.
- Chorover, J. D., Chorover, J. D., Pelletier, J. D., Pelletier, J. D., Breshears, D. D., Breshears, D. D., Mcintosh, J. C., Mcintosh, J. C., Rasmussen, C., Rasmussen, C., Brooks, P. D., Brooks, P. D., Barron-Gafford, G. A., Barron-Gafford, G. A., Gallery, R. E., Gallery, R. E., Ferre, P. A., Ferre, P. A., Meixner, T., , Meixner, T., et al. (2014, September). The Catalina-Jemez CZO: Transformative Behavior of Energy, Water and Carbon in the Critical Zone II. Interactions between Long and Short Term Processes that Control Delivery of Critical Zone Services.. National Critical Zone Observatory All-Hands Meeting.
- Jacobs, K. L., Buizer, J. l., Garfin, G. M., Breshears, D. D., & Liverman, D. M. (2014, January). Living in Our Future Climate: Adapting to Climate Change. Moderate Panel and Present at CCASS convened conference. Student Union Memorial Center, Kiva Room, UA: UA Institute of the Environment.
- Breshears, D. D. (2013, April). Drought-induced tree mortality: An overview and causes and consequences of an emerging phenomenon. Invited Seminar: Oklahoma State University. Stillwater, OK: Oklahoma State Univeristy.
- Breshears, D. D., Adams, H. D., Eamus, D., McDowell, N. G., Law, D. J., Will, R. E., Williams, A. P., & Zou, C. (2013, December). Large-scale drought-induced vegetation die-off: expanding the ecohydrological emphasis more explicitly on atmospheric demand.. American Geophysical Union. H11N. Eco-hydrology in a Changing Environment. San Francisco.
- Breshears, D. D., Breshears, D. D., Archer, S. R., Archer, S. R., Bojóquerez Ochoa, M., Bojóquerez Ochoa, M., Field, J. P., Field, J. P., Huxman, T. E., Huxman, T. E., Law, D. J., Law, D. J., Logie, C., Logie, C., Reynoso, E. D., Reynoso, E. D., Villegas, J. C., Villegas, J. C., Whicker, J. J., & Whicker, J. J. (2013, December). Consequences in change in vegetation cover for the Critical Zone: Example implications for hydropedology. American Geophysical Union. H32D. Hydropedology: Synergistic Integration of Soil Science and Hydrology in the Critical Zone. San Franciso.
- Breshears, D. D., Villegas, J. C., Espleeta, J. E., Morrison, C. T., & Huxman, T. E. (2013, October). Beyond the big-leaf homogeneity assumption in evapotranspiration partitioning: Factoring in canopy cover heterogeneity. AGU Chapman Conference: Soil-mediated drivers of coupled biogeochemical and hydrological processes across scales. Tucson.
- Ochoa, M. B., Logie, C., Reynoso, E. D., Law, D. J., Field, J. P., Breshears, D. D., & Archer, S. R. (2013, August). Determining a new dryland decomposition input term: small simulated fluvial redistribution of sediment to litter surfaces. Research Experience for Undergraduates Workshop. Biosphere 2: B2 REU Program.
- Lopez Hoffman, L., Soto, J. R., Nuñez-Regueiro, M. M., Laguë, M. M., Baldwin, E., Derbridge, J. J., Breshears, D. D., Swann, A. L., Huang, T., Lien, A., & Feng, X. (2018, August). How does forest die-off affect agriculture thousands of miles away? Revealing the hidden teleconnected network of NEON domains. Ecological Society of America 2018. New Orleans, LA: Ecological Society of America.
- Fojtik, A. C., Barnes, M. L., Breshears, D. D., Law, D. J., & Moore, D. J. (2016, Aug). Water stress reduces evaporative cooling in hybrid poplars during hot drought: genotype influences degree of coupling between thermal stress and atmosphere. Undergraduate Research Opportunities Consortium (UROC) Poster Session (in conjunction with the Biosphere 2 Research Experience for Undergraduates [REU] Program). Tucson, Arizona..
- Fojtik, A. C., Barnes, M. L., Breshears, D. D., Law, D. J., & Moore, D. J. (2016, Dec). Water stress reduces evaporative cooling in hybrid poplars during hot drought: genotype influences degree of coupling between thermal stress and atmosphere. American Geophysical Union Fall 2016 Annual Meeting. San Francisco: American Geophysical Union.
- Garcia, E. S., Swann, A. L., Field, J. P., Breshears, D. D., Law, D. J., Minor, D. M., Moore, D. J., Saleska, S. R., Stark, S. C., & Villegas, J. C. (2016, Oct). Exploring ecoclimate teleconnections: Would large-scale tree die-off elsewhere impact the desert southwest NEON domain?. 13th Annual Research Insights in Semiarid Ecosystems Symposium. Tucson, Arizona.: University of Arizona and Agricultural Research Service.
- Saleska, S. R., Breshears, D. D., Moore, D. J., & Law, D. J. (2016, Fall). Accounting for "Ecoclimate Teleconnections" that structure ecological responses to disturbance and climate, from local to global scales. American Geophysical Union. San Francisco: AGU.More infoScott R Saleska, Elizabeth Garcia, Scott C Stark, Juan Camilo Villegas, David D Breshears, Abigail L. S. Swann, David J Moore, Darin Law, David Minor, Jason P Field Accounting for "Ecoclimate Teleconnections" that structure ecological responses to disturbance and climate, from local to global scales B12C-04
- Logie, C., Ochoa, E., Reynoso, E., Breshears, D. D., Law, D. J., Field, J., & Archer, S. R. (2014, November). Determining a new dryland decomposition input term: sediment redistribution to litter surfaces via wind and rain. UA Grad Blitz. University of Arizona.
- Logie, C., Ochoa, M., Reynoso, E., Law, D. J., Field, J., Breshears, D. D., & Archer, S. R. (2014, October). Determining a new dryland decomposition input term: Sediment redistribution to litter surfaces during fluvial event simulation. 11th Annual Research Insights in Semi-Arid Ecosystems (RISE) Symposium. University of Arizona: SNRE & ARS Southwest Watershed Research Center.
- Breshears, D. D., Breshears, D. D., Field, J. P., Field, J. P., Law, D. J., Law, D. J., Brooks, P. D., Brooks, P. D., Chorover, J., Chorover, J. D., Pelletier, J. D., Pelletier, J. D., Troch, P. A., Troch, P. A., Lopez Hoffman, L., Lopez Hoffman, L. -., Rasmussen, C., Rasmussen, C. -., Papuga, S. A., , Papuga, S. A., et al. (2013, October 2013). Bridging from soil to ecosystem goods and services provided by the Critical Zone. AGU Chapman Conference: Soil-mediated drivers of coupled biogeochemical and hydrological processes across scales. Tucson.
- Kidder, A. L., Breshears, D. D., Papuga, S. A., & Law, D. J. (2013, Fall). Towards understanding the ecohydrologic controls on the phenology of Pima pineapple cactus. Research Insights from Semiarid Ecosystems (RISE). Tucson: UA and ARS.
- Kidder, A. L., Papuga, S. A., Breshears, D. D., & Law, D. J. (2013, Fall 2013). Towards understanding the ecohydrologic controls on the phenology of Pima pineapple cactus. University of Arizona Grad Blitz. Tucson: UA and ARS.
- Law, D. J., Law, D. J., Ravi, S., Ravi, S., Breshears, D. D., Breshears, D. D., Barron-Gafford, G. A., Barron-Gafford, G. A., Huxman, T. E., & Huxman, T. E. (2013, October). Evapotranspiration Partitioning: Competition between abiotic and biotic components of the water budget. AGU Chapman Conference: Soil-mediated drivers of coupled biogeochemical and hydrological processes across scales. Tucson.
- Logie, C. J., Bojórquez Ochoa, M., Reynoso, E. D., Law, D. J., Field, J. P., Breshears, D. D., & Archer, S. R. (2013, November). Determining a new dryland decomposition input term: Sediment redistribution to litter surfaces via wind and rain. University of Arizona Grad Blitz. Tucson: Institute of the Environment.
- Logie, C. J., Bojórquez Ochoa, M., Reynoso, E. D., Law, D. J., Fiield, J. P., Breshears, D. D., & Archer, S. R. (2013, October). Determining a new dryland decomposition input term: Sediment redistribution to litter surfaces during fluvial event simulation. Research Insights in Semi-Arid Ecosystems (RISE). Tucson: University of Arizona and Agricultural Research Service.
- Logie, C. J., Bojórquez Ochoa, M., Reynoso, E. D., Law, D. J., Fiield, J. P., Breshears, D. D., Archer, S. R., Logie, C. J., Bojórquez Ochoa, M., Reynoso, E. D., Law, D. J., Fiield, J. P., Breshears, D. D., & Archer, S. R. (2013, October). Determining a new dryland decomposition input term: Sediment redistribution to litter surfaces during fluvial event simulation. Research Insights in Semi-Arid Ecosystems (RISE) Symposium. Tucson: University of Arizona and Agricultural Research Service.
- Logie, C. J., Field, J. P., Breshears, D. D., & Law, D. J. (2013, May). Affects of vegetation cover on aeolian sediment transport and dust emission in semiarid environments: Relationships between horizontal and vertical dust flux. Airborne Mineral Dust Contaminants: Impacts on Human Health and the Environment. Tucson.
- Logie, C., Ochoa, M. B., Reynoso, E. D., Breshears, D. D., Law, D. J., Field, J. P., & Archer, S. R. (2013, November). Determining a new dryland decomposition input term: sediment redistribution to litter surfaces via wind and rain. UA Grad Blitz. University of Arizona: Office of Graduate Studies.
- Ochoa, M. B., Logie, C., Reynoso, E. D., Breshears, D. D., Law, D. J., Field, J. P., Archer, S. R., Ochoa, M. B., Logie, C., Reynoso, E. D., Breshears, D. D., Law, D. J., Field, J. P., & Archer, S. R. (2013, August/summer). Determining a new dryland decomposition input term: small simulated fluvial redistribution of sediment to litter surfaces. Research Experience for Undergraduates Workshop. Biosphere 2: Biosphere 2 REU Program.
- Reynoso, D. D., Logie, C., Ochoa, M. B., Law, D. J., Field, J. P., Breshears, D. D., & Archer, S. R. (2013, August). Determining a new dryland decomposition input term: natural events of aeolian and fluvial redistribution of sediment to litter surfaces. Research Experience for Undergraduates Workshop. Biosphere 2: B2 REU Program.
- Pelletier, J. D., Murray, A. B., Pierce, J. L., Bierman, P. R., Breshears, D. D., Crosby, B. T., Ellis, M., Foufoula-Georgiou, E., Heimsath, A. M., Houser, C., Lancaster, N., Marani, M., Merritts, D. J., Moore, L. J., Pederson, J. L., Poulos, M. J., Rittenour, T. M., Rowland, J. C., Ruggiero, P., , Ward, D. J., et al. (2015. Forecasting the response of Earth's surface to future climatic and land use changes: A review of methods and research needs(pp 220-251).
- Garfin, G. M., Ferguson, D. B., Jacobs, K. L., & Breshears, D. D. (2014, May). Ask Me Anything. The NEW REDDIT JOURNAL of SCIENCE. http://www.reddit.com/r/science/comments/254diy/science_ama_series_we_helped_create_the_third/More infoOnline, real-time, question and answer chat session about the National Climate Assessment
- Archer, S. R., & Breshears, D. D. (2013, December). Collaborative research: decomposition in drylands: soil erosion and UV interactions. Final Report, NSF Ecosystems Program, DEB 0816162. 17 pp..
- Breshears, D. D. (2010, Fall). Terrestrial Ecology Lab. http://www.cals.arizona.edu/research/breshears/publications.htm