Greg A Barron-Gafford

Interests

Research

Biogeography, Ecosystem Ecology

Teaching

Interactive and engaged learning; hands-on informal education.

Courses

Scholarly Contributions

Chapters

• Volkmann, T. H., Troch, P. A., Sengupta, A., Zeng, X., Pangle, L. A., Van Haren, J. L., Tuller, M., Dontsova, K. M., Barron-Gafford, G. A., Sibayan, M., Harman, C. J., Schaap, M. G., Saleska, S. R., Niu, G., Ruiz, J., Meredith, L., Abramson, N., Rasmussen, C., Pohlmann, M. A., , Alves Meira Neto, A., et al. (2018). 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.
• Huxman, T. E., Sibayan, M., Abramson, N., Bugaj, A., Barron-Gafford, G. A., Van Haren, J. L., Zeng, X., Chorover, J. D., Hunt, E., Neilson, J. W., Meira, A. A., Troch, P. A., Dontsova, K. M., Volkmann, T., Pangle, L., & Sengupta, A. (2017). Advancing understanding of hydrological and biogeochemical interactions in evolving landscapes through controlled experimentation and monitoring at the Landscape Evolution Observatory. In Terrestrial Ecosystem Research Infrastructures: Challenges, New Developments and Perspectives(pp 83-118). CRC Press.
• Troch, P. A., Zeng, X., Van Haren, J. L., Tuller, M., Sibayan, M., Schaap, M. G., Saleska, S. R., Ruiz, J., Rasmussen, C., Pohlmann, M. A., Pelletier, J. D., Monson, R. K., Maier, R. M., Kim, M., Huxman, T. E., Ferre, P. A., Durcik, M., DeLong, S. B., Cueva, A., , Chorover, J. D., 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.
• Sengupta, A., Pangle, L. A., Volkmann, T., Dontsova, K. M., Troch, P. A., Meira, A. A., Neilson, J. W., Hunt, E. A., Chorover, J. D., Zeng, X., van Haren, J., Barron-Gafford, G. A., Bugai, A., Abramson, N., Sibayan, M., & Huxman, T. E. (2016). Advancing understanding of hydrological and biogeochemical interactions in evolving landscapes through controlled experimentation and monitoring at the Landscape Evolution Observatory. In Terrestrial Ecosysem Research Infrastructrures: Challenges, New Developments and Perspectives(pp 83-118). Taylor and Francis Group, London UK.
• Sengupta, A., Pangle, L., Volkmann, T., Dontsova, K. M., Troch, P. A., Meira, A. A., Neilson, J. W., Hunt, E., Chorover, J. D., Zeng, X., Van Haren, J. L., Barron-Gafford, G. A., Bugaj, A., Abramson, N., Sibayan, M., & Huxman, T. E. (2016). Advancing understanding of hydrological and biogeochemical interactions in evolving landscapes through controlled experimentation and monitoring at the Landscape Evolution Observatory. In Terrestrial Ecosystem Research Infrastructures: Challenges, New developments and Perspectives(pp 179-193). CRC Press.

Journals/Publications

• Barron-Gafford, G. A., Niu, G. Y., Scott, R. L., Ma, Z., Xie, Z., & Zhang, X. (2022). A Microbial‐Explicit Soil Organic Carbon Decomposition Model (MESDM): Development and Testing at a Semiarid Grassland Site. Journal of Advances in Modeling Earth Systems. doi:10.1029/2021ms002485
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  Explicit representations of microbial processes in soil organic carbon (SOC) decomposition models have received increasing attention, because soil heterotrophic respiration remains one of the greatest uncertainties in climate-carbon feedbacks projected by Earth system models (ESMs). Microbial-explicit models have been developed and applied in site- and global-scale studies. These models, however, lack the ability to represent microbial respiration responses to drying-wetting cycles, and few of them have been incorporated in land surface models (LSMs) and validated against field observations. In this study, we developed a multi-layer, microbial-explicit soil organic carbon decomposition model (MESDM), based on two main assumptions that (a) extracellular enzymes remain active at dry reaction microsites, and (b) microbes at wet microsites are active or potentially active, while microbes at the dry microsites are dormant, by dividing the soil volume into wet and dry zones. MESDM with O2 and CO2 gas transport models was coupled with Noah-MP LSM and tested against half-hourly field observations at a semiarid grassland site in the southwest US characterized by pulsed precipitation. The results show MESDM can reproduce the observed soil respiration pulses of various sizes in response to discrete precipitation events (Birch effect) and thus improve the simulation of net ecosystem exchange. Here, both microbial accessibility to accumulated dissolved organic carbon and reactivation of dormant microbes at the dry microsites upon rewetting are critical to reproducing the Birch effect. This study improves our understanding of and ability to simulate complex soil carbon dynamics that experience drying-wetting cycle in climate-carbon feedbacks.
• Barron-Gafford, G. A., Smith, W. K., Parton, W. J., Reed, S. C., Kwon, H., Law, B. E., Flerchinger, G. N., Litvak, M. E., Frankenberg, C., Köhler, P., Dannenberg, M. P., Turner, A. J., Scott, R. L., Knowles, J. F., Biederman, J. A., & Wang, X. (2022). Satellite solar-induced chlorophyll fluorescence and near-infrared reflectance capture complementary aspects of dryland vegetation productivity dynamics. Remote Sensing of Environment. doi:10.1016/j.rse.2021.112858
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  Mounting evidence indicates dryland ecosystems play an important role in driving the interannual variability and trend of the terrestrial carbon sink. Nevertheless, our understanding of the seasonal dynamics of dryland ecosystem carbon uptake through photosynthesis [gross primary productivity (GPP)] remains relatively limited due in part to the limited availability of long-term data and unique challenges associated with satellite remote sensing across dryland ecosystems. Here, we comprehensively evaluated longstanding and emerging satellite vegetation proxies in their ability to capture seasonal dryland GPP dynamics. Specifically, we evaluated: 1) reflectance-based proxies normalized difference vegetation index (NDVI), soil adjusted vegetation index (SAVI), near infrared reflectance index (NIR v ), and kernel NDVI (kNDVI) from the MODerate resolution Imaging Spectroradiometer (MODIS); and 2) newly available physiologically-based proxy solar-induced chlorophyll fluorescence (SIF) from the TROPOspheric Monitoring Instrument (TROPOMI). As a performance benchmark, we used GPP estimates from a robust network of 21 western United States eddy covariance tower sites that span representative gradients in dryland ecosystem climate and functional composition. We found that NIR v and SIF were the best performing GPP proxies and captured complementary aspects of seasonal GPP dynamics across dryland ecosystem types. NIR v offered better performance than the other proxies across relatively low-productivity, sparsely non-evergreen vegetated sites (R 2 = 0.59 ± 0.13); whereas SIF best captured seasonal dynamics across relatively high-productivity sites, including evergreen-dominated sites (R 2 = 0.74 ± 0.07). Notably, across grass-dominated sites, all reflectance-based proxies (NDVI, SAVI, NIR v and kNDVI) showed significant seasonal bias (hysteresis) that strengthened with the total fraction of woody vegetation cover, likely due to seasonal patterns in woody vegetation reflectance that are unrelated to or decoupled from GPP. Future efforts to fully integrate the complementary strengths of NIR v and SIF could significantly improve our understanding and representation of dryland GPP dynamics in satellite-based models. • NIR v and SIF best capture GPP seasonality across Western US dryland sites. • NIR v best captures GPP seasonality across low-productivity, sparely-vegetated sites. • SIF best captures GPP seasonality across high-productivity, evergreen sites. • NDVI, NIR v , kNDVI, and SAVI exhibit hysteresis with GPP that worsen with woody cover.
• Javadian, M., Barron-Gafford, G. A., Behrangi, A., Leeuwen, W. J., Moore, D. J., Fisher, J. B., Scott, R. L., Knowles, J. F., Lee, K., & Smith, W. K. (2022). Canopy Temperature Is Regulated by Ecosystem Structural Traits and Captures the Ecohydrologic Dynamics of a Semiarid Mixed Conifer Forest Site. Journal Of Geophysical Research: Biogeosciences. doi:10.1029/2021jg006617
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  Plant canopy temperature (Tc) is partly regulated by evaporation and transpiration from the canopy surface and can be used to infer changes in stomatal regulation and vegetation water stress. In this study, we used a thermal Unmanned Aircraft Systems in conjunction with eddy covariance, sap flow, and spectral reflectance data to assess the diurnal characteristics of Tc and water stress status over a semiarid mixed conifer forest in Arizona, USA. Diurnal Tc dynamics were closely related to tree sap flow and changes in spectral reflectance associated with stomatal regulation. Consistent with previously reported deviations, we found that on average Tc was 1.8°C lower than the above canopy air temperature (Ta). However, the relationship between Tc and Ta varied significantly according to tree density and tree height classes, with taller and denser trees exhibiting relatively low |Tc-Ta| (2.4 and 2.1°C cooler canopies, respectively) compared to shorter and less-dense tree stands (1.7 and 1.5°C cooler canopies, respectively). We used these data to evaluate space-borne diurnal measurements of Tc and water stress from the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) mission. We found that ECOSTRESS observations of Tc accurately tracked seasonal shifts in diurnal surface temperatures and vegetation water stress, and that site-level observations of heterogeneity in forest composition and structure could be applied to separate the processes of canopy transpiration and soil evaporation within the ECOSTRESS footprint. This study demonstrates how proximal and satellite remote sensing approaches can be combined to reveal the diurnal and seasonally dynamic nature of Tc and water stress.
• Macknick, J., Hartmann, H., & Barron-Gafford, G. A. (2022). The 5 Cs of Agrivoltaic Success Factors in the United States: Lessons From the InSPIRE Research Study. National Renewable Energy Lab Technical Reports. doi:https://doi.org/10.2172/1882930
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  The concept of agrivoltaics (combining agriculture and solar photovoltaics technologies on the same land in novel configurations) has emerged as an approach to mitigate conflicts between solar and agricultural activities by providing mutual benefits and added values to each sector. The U.S. Department of Energy has supported agrivoltaics research since 2015 through its Innovative Solar Practices Integrated with Rural Economies and Ecosystems (InSPIRE) research project (National Renewable Energy Laboratory 2022). The InSPIRE project is the most comprehensive coordinated research effort on agrivoltaics in the United States and has examined opportunities and tradeoffs at over 25 sites across the country that span crop production, pollinator habitat, ecosystem services, animal husbandry, and d. Integrating research sites with active commercial agricultural operations can introduce unique challenges for conducting research. This synthesis aims to highlight the technical and non-technical insights from InSPIRE agrivoltaic field research sites from 2015-2021 to support i) appropriate deployment of agrivoltaic projects; ii) more successful research on agrivoltaics; and iii) more effective partnerships on agrivoltaic projects. The synthesized lessons discussed here are focused less on specific case study outcomes (i.e., the percent change in crop yield in an agrivoltaics configuration), and instead more on the elements that enable and facilitate agrivoltaics projects to be installed and operated along with research to be conducted at those sites. We find that there are some insights that are applicable across all types of agrivoltaic projects, while ecosystem service projects and crop production agrivoltaic projects can often have other unique considerations.
• Barron-Gafford, G. A., Castro, C. L., Meixner, T., & Niu, G. (2021). An improved practical approach for estimating catchment‐scale response functions through wavelet analysis. Hydrological Processes.
• Barron-Gafford, G. A., Dontsova, K., Escalante, A. E., Pavao-Zuckerman, M., Benítez, M., Hernández-Terán, A., Barrón-Sandoval, A., & Avitia, M. (2021). Soil microbial composition and carbon mineralization are associated with vegetation type and temperature regime in mesocosms of a semiarid ecosystem. Fems Microbiology Letters. doi:10.1093/femsle/fnab012
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  Transition from historic grasslands to woody plants in semiarid regions has led to questions about impacts on soil functioning, where microorganisms play a primary role. Understanding the relationship between microbes, plant diversity and soil functioning is relevant to assess such impacts. We evaluate the effect that plant type change in semiarid ecosystems has for microbial diversity and composition, and how this is related to carbon mineralization (CMIN) as a proxy for soil functioning. We followed a mesocosm experiment during 2 years within the Biosphere 2 facility in Oracle, AZ, USA. Two temperature regimes were established with two types of plants (grass or mesquite). Soil samples were analyzed for physicochemical and functional parameters, as well as microbial community composition using 16S rRNA amplicon metagenomics (Illumina MiSeq). Our results show the combined role of plant type and temperature regime in CMIN, where CMIN in grass has lower values at elevated temperatures compared with the opposite trend in mesquite. We also found a strong correlation of microbial composition with plant type but not with temperature regime. Overall, we provide evidence of the major effect of plant type in the specific composition of microbial communities as a potential result of the shrub encroachment.
• Escalante, A. E., Escalante, A. E., Pavao-Zuckerman, M. A., Pavao-Zuckerman, M. A., Dontsova, K. M., Dontsova, K. M., Barron-Gafford, G. A., Barron-Gafford, G. A., Benítez, M., Benítez, M., Hernández-Terán, A., Hernández-Terán, A., Barrón-Sandoval, A., Barrón-Sandoval, A., Avitia, M., & Avitia, M. (2021). Soil microbial composition and carbon mineralization are associated with vegetation type and temperature regime in mesocosms of a semiarid ecosystem. FEMS Microbiology Letters. doi:https://doi.org/10.1093/femsle/fnab012
• Huxman, T. E., Breshears, D. D., Okin, G. S., Villegas, J. C., Espeleta, J. F., Dontsova, K. M., Barron-Gafford, G. A., Caplan, J. S., Law, D. J., & Ravi, S. (2021). Biological invasions and climate change amplify each other’s effects on dryland degradation. Global Change Biology.
• Law, D. J., Barron-Gafford, G. A., Breshears, D. D., Huxman, T. E., Okin, G. S., Villegas, J. C., Espeleta, J. F., Dontsova, K., Caplan, J. S., & Ravi, S. (2021). Biological invasions and climate change amplify each other’s effects on dryland degradation. Global Change Biology. doi:10.1111/gcb.15919
• Dwivedi, R., Eastoe, C., Knowles, J. F., Wright, W. E., Hamann, L., Minor, R., Mitra, B., Meixner, T., McIntosh, J., Ferre, P., Castro, C., Niu, G., Barron-Gafford, G. A., Abramson, N., Papuga, S. A., Stanley, M., Hu, J., & Chorover, J. (2020). Vegetation source water identification using isotopic and hydrometric observations from a subhumid mountain catchment. ECOHYDROLOGY, 13(1).
• Minor, J., Pearl, J. K., Barnes, M. L., Colella, T. R., Murphy, P. C., Mann, S., & Barron-Gafford, G. A. (2020). Critical Zone Science in the Anthropocene: Opportunities for biogeographic and ecological theory and praxis to drive earth science integration. PROGRESS IN PHYSICAL GEOGRAPHY-EARTH AND ENVIRONMENT, 44(1), 50-69.
• Troch, P. A., Ruiz, J., Adams, J., Niu, G., Van Haren, J. L., Barron-Gafford, G. A., Kim, M., Ng, W., Bugaj, A., Abramson, N., Pangle, L., Sibayan, M., Durcik, M., Zeng, X., & Arevalo Borquez, J. A. (2020). Highly sampled measurements in a controlled atmosphere at the Biosphere 2 Landscape Evolution Observatory. Scientific Data, 7, 306. doi:10.1038/s41597-020-00645-5
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  Land-atmosphere interactions at different temporal and spatial scales are important for our understanding of the Earth system and its modeling. The Landscape Evolution Observatory (LEO) at Biosphere 2, managed by the University of Arizona, hosts three nearly identical artificial bare-soil hillslopes with dimensions of 11 × 30 m2 (1 m depth) in a controlled and highly monitored environment within three large greenhouses. These facilities provide a unique opportunity to explore these interactions. The dataset presented here is a subset of the measurements in each LEO’s hillslopes, from 1 July 2015 to 30 June 2019 every 15 minutes, consisting of temperature, water content and heat flux of the soil (at 5 cm depth) for 12 co-located points; temperature, relative humidity and wind speed above ground at 5 locations and 5 different heights ranging from 0.25 m to 9–10 m; 3D wind at 1 location; the four components of radiation at 2 locations; spatially aggregated precipitation rates, total subsurface discharge, and relative water storage; and the measurements from a weather station outside the greenhouses.
• Troch, P. A., Ruiz, J., Adams, J., Niu, G., Van Haren, J. L., Barron-Gafford, G. A., Ng, W., Bugaj, A., Abramson, N., Pangle, L., Sibayan, M., Durick, M., Zeng, X., & Arevalo, J. (2020). Highly Sampled Measurements in a Controlled Atmosphere at the Biosphere 2 Landscape Evolution Observatory. Scientific Data, 7, 306. doi:10.1038/s41597-020-00645-5
• Barron-Gafford, G. A., Hamerlynck, E., Pavao-Zuckerman, M. A., Troch, P. A., Van Haren, J. L., Dontsova, K. M., Juarez, S., Hansart, A., Massol, F., Chollet, S., Ferriere, R. H., Le Galliard, J., Sanchez-Canete, E. P., & Dusza, Y. (2019). Biotic soil-plant interaction processes explain most of hysteric soil CO2 efflux response to temperature in cross-factorial mesocosm experiment.. Scientific Reports.
• Barron-Gafford, G. A., Pavao-Zuckerman, M. A., Minor, R. L., Sutter, L. F., Barenett-Moreno, I., Blackett, D. T., Thompson, M., Dimond, K., Gerlak, A. K., Nabhan, G. P., & Macknick, J. E. (2019). Agrivoltaics: A novel approach to co-locating food production and solar renewable energy to maximize food production, water savings, and energy generation. Nature Sustainability.
• Barron-Gafford, G. A., Pavao-Zuckerman, M. A., Minor, R. L., Sutter, L. F., Barnett-Moreno, I., Blackett, D. T., Thompson, M., Dimond, K., Gerlak, A. K., Nabhan, G. P., & Macknick, J. E. (2019). Agrivoltaics provide mutual benefits across the food-energy-water nexus in drylands. NATURE SUSTAINABILITY, 2(9), 848-855.
• Chorover, J. D., Barron-Gafford, G. A., Minor, R. L., Niu, G., Eastoe, C. J., Ferre, P. A., Mcintosh, J. C., Meixner, T., & Dwivedi, R. (2019). Hydrologic functioning of the deep Critical Zone and contributions to streamflow in a high elevation catchment: testing of multiple conceptual models. Hydrological Processes. doi:10.1002/hyp.13363
• Chorover, J., Hu, J., Stanley, M., Papuga, S., Abramson, N., Meixner, T., Barron-Gafford, G. A., Nui, G., Castro, C., Ferre, P., Mitra, B., Minor, R., Hammon, L., Wright, W., Knowles, J., Eastoe, C., & Dwivedi, R. (2019). Vegetation source water identification using isotopic and hydrometric observations from a sub-humid mountain Catchment. Ecohydrology, e2167, 17. doi:10.1002/eco.2167
• Dwivedi, R., Meixner, T., McIntosh, J. C., Ferre, P., Eastoe, C. J., Niu, G., Minor, R. L., Barron-Gafford, G. A., & Chorover, J. (2019). Hydrologic functioning of the deep critical zone and contributions to streamflow in a high-elevation catchment: Testing of multiple conceptual models. HYDROLOGICAL PROCESSES, 33(4), 476-494.
• Elshall, A. S., Ye, M., Niu, G., & Barron-Gafford, G. A. (2019). Bayesian inference and predictive performance of soil respiration models in the presence of model discrepancy. GEOSCIENTIFIC MODEL DEVELOPMENT, 12(5), 2009-2032.
• Olshansky, Y., Knowles, J. F., Barron-Gafford, G. A., Rasmussen, C., Abramson, N., & Chorover, J. (2019). Soil Fluid Biogeochemical Response to Climatic Events. JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES, 124(9), 2866-2882.
• Potts, D. L., Barron-Gafford, G. A., & Scott, R. L. (2019). Ecosystem hydrologic and metabolic flashiness are shaped by plant community traits and precipitation. AGRICULTURAL AND FOREST METEOROLOGY, 279.
• Potts, D. L., Barron-Gafford, G. A., Butterfield, B. J., Fay, P. A., & Hultine, K. R. (2019). Bloom and Bust: ecological consequences of precipitation variability in aridlands. PLANT ECOLOGY, 220(2, SI), 135-139.
• Roby, M. C., Scott, R. L., Barron-Gafford, G. A., Hamerlynck, E. P., & Moore, D. (2019). Environmental and Vegetative Controls on Soil CO2 Efflux in Three Semiarid Ecosystems. SOIL SYSTEMS, 3(1).
• Smith, W. K., Dannenberg, M. P., Yan, D., Herrmann, S., Barnes, M. L., Barron-Gafford, G. A., Biederman, J. A., Ferrenberg, S., Fox, A. M., Hudson, A., Knowles, J. F., MacBean, N., Moore, D., Nagler, P. L., Reed, S. C., Rutherford, W. A., Scott, R. L., Wang, X., & Yang, J. (2019). Remote sensing of dryland ecosystem structure and function: Progress, challenges, and opportunities. REMOTE SENSING OF ENVIRONMENT, 233.
• Barron-Gafford, G. A. (2018). A considerable fraction of soil-respired CO2 is not emitted directly to the atmosphere.. Nature: Scientific Reports, Article number: 13518 (2018). doi:s41598-018-29803-x
• Barron-Gafford, G. A. (2018). Comparing ecosystem and soil respiration: Review and key challenges of tower-based and soil measurements. Agricultural and Forest Meteorology, 249, 434-443. doi:doi: 10.1002/esp.4306
• Barron-Gafford, G. A. (2018). Impact of Hydraulic Redistribution on Multispecies Vegetation Water Use in a Semiarid Savanna Ecosystem: An Experimental and Modeling Synthesis. Water Resources Research, 54(5), 4009-4027.. doi:10.1029/2017WR021006
• Barron-Gafford, G. A. (2018). Quantification of uncertainties in conifer sap flow measured with the thermal dissipation method. New Phytologist, 219, 1283–1299. doi:10.1111/nph.15241
• Barron-Gafford, G. A. (2018). Relative model score: a scoring rule for evaluating ensemble simulations with application to microbial soil respiration modeling. Stochastic Environmental Research and Risk Assessment, 32(10), 2809-2819. doi:doi.org/10.1007/s00477-018-1592-3
• Barron-Gafford, G. A. (2018). Which way do you lean? Using slope aspect variations to understand Critical Zone processes and feedbacks. Earth Surface Processes and Landforms, 43, 1133-1154. doi:doi: 10.1002/esp.4306.
• Meixner, T., Barron-Gafford, G. A., Durcik, M., Rasmussen, C., Pelletier, J. D., Niu, G., Fang, Y., Knowles, J. F., Dwivedi, R., & Chang, L. (2018). Why Do Large‐Scale Land Surface Models Produce a Low Ratio of Transpiration to Evapotranspiration?. Journal of Geophysical Research: Atmospheres, 123(17), 9109-9130. doi:10.1029/2018JD029159
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  Most land surface models (LSMs) used in Earth System Models produce a lower ratio of transpiration (T) to evapotranspiration (ET) than field observations, degrading the credibility of Earth System Model‐projected ecosystem responses and feedbacks to climate change. To interpret this model deficiency, we conducted a pair of model experiments using a three‐dimensional, process‐based ecohydrological model in a subhumid, mountainous catchment. One experiment (CTRL) describes lateral water flow, topographic shading, leaf dynamics, and water vapor diffusion in the soil, while the other (LSM like) does not explicitly describe these processes to mimic a conventional LSM using artificially flattened terrain. Averaged over the catchment, CTRL produced a higher T/ET ratio (72%) than LSM like (55%) and agreed better with an independent estimate (79.79 ± 27%) based on rainfall and stream water isotopes. To discern the exact causes, we conducted additional model experiments, each reverting only one process described in CTRL to that of LSM like. These experiments revealed that the enhanced T/ET ratio was mostly caused by lateral water flow and water vapor diffusion within the soil. In particular, terrain‐driven lateral water flows spread out soil moisture to a wider range along hillslopes with an optimum subrange from the middle to upper slopes, where evaporation (E) was more suppressed by the drier surface than T due to plant uptake of deep soil water, thereby enhancing T/ET. A more elaborate representation of water vapor diffusion from a dynamically changing evaporating surface to the height of the surface roughness length reduced E and increased the T/ET ratio.
• 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).
• Barron-Gafford, G. A., Falk, D. A., & Minor, J. J. (2017). Fire severity and regeneration strategy influence shrub patch size and structure following disturbance. Forests, 8, 221-231. doi:http://doi:10.3390/f8070221
• Barron-Gafford, G. A., Sanchez-Canete, E. P., Minor, R. L., Hendryx, S. M., Lee, E., Sutter, L. F., Tran, N., Parra, E., Colella, T., Murphy, P. C., Hamerlynck, E. P., Kumar, P., & Scott, R. L. (2017). Impacts of hydraulic redistribution on grass-tree competition vs facilitation in a semi-arid savanna. NEW PHYTOLOGIST, 215(4), 1451-1461.
• Camilo, V. J., Law, D. J., Stark, S. C., Minor, D. M., Breshears, D. D., Saleska, S. R., Swann, A., Garcia, E. S., Bella, E. M., Morton, J. M., Cobb, N. S., Barron-Gafford, G. A., Litvak, M. E., & Kolb, T. E. (2017). Prototype campaign assessment of disturbance-induced tree loss effects on surface properties for atmospheric modeling. ECOSPHERE, 8(3).
• Chorover, J. D., Barron-Gafford, G. A., Minor, R. L., Niu, G., Eastoe, C. J., Ferre, P. A., Mcintosh, J. C., Meixner, T., & Dwivedi, R. (2020). Hydrologic functioning of the deep Critical Zone and contributions to streamflow in a high elevation catchment: testing of multiple conceptual models. Hydrological Processes. doi:10.1002/hyp.13363
• Potts, D. L., Minor, R. L., Braun, Z., & Barron-Gafford, G. A. (2017). Photosynthetic phenological variation may promote coexistence among co-dominant tree species in a Madrean sky island mixed conifer forest. TREE PHYSIOLOGY, 37(9), 1229-1238.
• Sanchez-Canete, E. P., Scott, R. L., van, H. J., & Barron-Gafford, G. A. (2017). Improving the accuracy of the gradient method for determining soil carbon dioxide efflux. JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES, 122(1), 50-64.
• 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.
• Barron-Gafford, G. A., Minor, R. L., Allen, N. A., Cronin, A. D., Brooks, A. E., & Pavao-Zuckerman, M. A. (2016). The Photovoltaic Heat Island Effect: Larger solar power plants increase local temperatures. Nature: Scientific Reports, 6.
• Sengupta, A., Wang, Y., Meira, A. A., Matos, K., Dontsova, K. M., Root, R., Neilson, J. W., Chorover, J. D., Maier, R. M., & Troch, P. A. (2016). Soil Lysimeter Excavation for Coupled Hydrological, Geochemical, and Microbiological Investigations. JoVE, e54536. doi:10.3791/54536
• 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, 1-7.
• Moore, G., McGuire, K., Troch, P., & Barron-Gafford, G. (2015). Ecohydrology and the Critical Zone: Processes and Patterns Across Scales. Principles and Dynamics of the Critical Zone, 19, 239.
• Ogle, K., Barber, J., Barron-Gafford, G. A., Bentley, L., Young, J., Huxman, T., Loik, M., & Tissue, D. (2015). Quantifying ecological memory in plant and ecosystem processes. Ecology Letters, 18, 221-235. doi:10.1111/ele.12399
• 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.
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• Scott, R. L., Biederman, J. A., Hamerlynck, E. P., & Barron-Gafford, G. A. (2015). The carbon balance pivot point of southwestern US semiarid ecosystems: Insights from the 21st century drought. Journal of Geophysical Research: Biogeosciences, 120, 2612--2624. doi:10.1002/2015JG003181
• Stielstra, C. M., Lohse, K., Chorover, J. D., Mcintosh, J. C., Barron-Gafford, G. A., Perdrial, J. N., Litvak, M., Barnard, H., & Brooks, P. (2015). Climatic, landscape, and edaphic controls on soil carbon efflux in seasonally snow covered forest ecosystems. Biogeochemistry.
• 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., & others, . (2015). Sensitivity of regional evapotranspiration partitioning to variation in woody plant cover: insights from experimental dryland tree mosaics. Global Ecology and Biogeography, 24, 1040--1048. doi:10.1111/geb.12349
• Barron-Gafford, G. A., Cable, J. M., Bentley, L. P., Scott, R. L., Huxman, T. E., Jenerette, G. D., & Ogle, K. (2014). Quantifying the timescales over which exogenous and endogenous conditions affect soil respiration. NEW PHYTOLOGIST, 202(2), 442-454.
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  Understanding how exogenous and endogenous factors and above-ground-below-ground linkages modulate carbon dynamics is difficult because of the influences of antecedent conditions. For example, there are variable lags between above-ground assimilation and below-ground efflux, and the duration of antecedent periods are often arbitrarily assigned. Nonetheless, developing models linking above- and below-ground processes is crucial for estimating current and future carbon dynamics. We collected data on leaf-level photosynthesis (A(sat)) and soil respiration (R-soil) in different microhabitats (under shrubs vs under bunchgrasses) in the Sonoran Desert. We evaluated timescales over which endogenous and exogenous factors control R-soil by analyzing data in the context of a semimechanistic temperature-response model of R-soil that incorporated effects of antecedent exogenous (soil water) and endogenous (A(sat)) conditions. For both microhabitats, antecedent soil water and A(sat) significantly affected R-soil, but R-soil under shrubs was more sensitive to A(sat) than that under bunchgrasses. Photosynthetic rates 1 and 3d before the R-soil measurement were most important in determining current-day R-soil under bunchgrasses and shrubs, respectively, indicating a significant lag effect.
• 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.
• Hamerlynck, E. P., Scott, R. L., Cavanaugh, M. L., & Barron-Gafford, G. A. (2014). Water use efficiency of annual-dominated and bunchgrass-dominated savanna intercanopy space. ECOHYDROLOGY, 7(4), 1208-1215.
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  In semi-arid savannas, dominance of intercanopy space by annual or perennial grasses may alter partitioning of ecosystem water and carbon fluxes and affect ecosystem water use efficiency (WUEe), the ratio of net ecosystem carbon dioxide exchange (NEE) to evapotranspiration (ET). To establish if these contrasting growth habits changed controls to WUEe, we tracked volumetric soil moisture (theta(25cm)), ET and transpiration (T), NEE and its constituent ecosystem respiration (R-eco) and gross ecosystem photosynthesis (GEP) fluxes, and community water use efficiency (WUEc = GEP : T) in annual-dominated and bunchgrass-dominated plots in a southern Arizona, United States, savanna. Annual and bunchgrass plots had similar theta(25cm), ET, and T, suggesting the similarity in ET was due to higher soil evaporation in annual plots. Seasonal NEE was delayed and lower in annual plots compared with that in bunchgrass plots, owing to higher Reco in annual plots. Transpiration, GEP, and R-eco in both vegetation types increased following late-season rain, indicating similar late-season phenological constraint. WUEe was lower in annual plots, but with similar WUEc between plot types. These results suggest that differences in annual plant biomass allocation and plot-level leaf area distribution increased proportional soil evaporation and aboveground R-eco contributions, reducing plot-level WUEe, not lowering plant WUE typical of arid-land annuals. Lower plot-level WUEe suggests that any increase in annual plant dominance would increase interannual variation of productivity in savanna intercanopy spaces, which could enhance the negative effects of predicted higher temperatures, greater aridity, and larger and more widely spaced storms on arid-land watershed processes. Published 2013. This article is a U. S. Government work and is in the public domain in the USA.
• Kimball, S., Gremer, J., Barron-Gafford, G. A., Angert, A. L., Huxman, T. E., & Venable, D. L. (2014). High water-use efficiency and growth contribute to success of non-native Erodium cicutarium in a Sonoran Desert winter annual community. Conservation Physiology, 2(1), 1-13.
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  The success of non-native, invasive species may be due to release from natural enemies, superior competitive abilities, or both. In the Sonoran Desert, Erodium cicutarium has increased in abundance over the last 30 years. While native species in this flora exhibit a strong among-species trade-off between relative growth rate and water-use efficiency, E. cicutarium seems to have a higher relative growth rate for its water-use efficiency value relative to the pattern across native species. This novel trait combination could provide the non-native species with a competitive advantage in this water-limited environment. To test the hypothesis that E. cicutarium is able to achieve high growth rates due to release from native herbivores, we compared the effects of herbivory on E. cicutarium and its native congener, Erodium texanum. We also compared these two species across a range of environmental conditions, both in a common garden and in two distinct seasons in the field, using growth analysis, isotopic compositions and leaf-level gas exchange. Additionally, we compared the competitive abilities of the two Erodium species in a greenhouse experiment. We found no evidence of herbivory to either species. Physiological measurements in a common environment revealed that E. cicutarium was able to achieve high growth rates while simultaneously controlling leaf-level water loss. Non-native E. cicutarium responded to favourable conditions in the field with greater specific leaf area and leaf area ratio than native E. texanum. The non-native Erodium was a stronger competitor than its native congener in a greenhouse competition experiment. The ability to maintain relatively higher values of water-use efficiency:relative growth rate in comparison to the native flora may be what enables E. cictarium to outcompete native species in both wet and dry years, resulting in an increase in abundance in the highly variable Sonoran Desert.
• Nelson, K., Kurc, S. A., John, G., Minor, R. L., & Barron-Gafford, G. A. (2014). Influence of snow cover duration on soil evaporation and respiration efflux in mixed-conifer ecosystems. Ecohydrology.
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  Subalpine mixed-conifer ecosystems are dependent on snowfall, which is expected to decrease under projected climate change. Changes in snowpack are likely to have important consequences for water and carbon cycling in these and downstream ecosystems. Particularly within semi-arid environments, snowpack changes will directly influence localized water and carbon dynamics and indirectly influence regional-scale levels of water availability and carbon sequestration. In this study, we monitor soil evaporation (E) and soil respiration (R) and evaluate how snow cover affects these effluxes within a mixed-conifer ecosystem within the Santa Catalina Mountains about 10 km north of Tucson, Arizona. Using time-lapse digital photos, we identified areas of consistent short and long snow duration, and we monitored E and R in these areas every 2 weeks for 15 months. Our primary findings include the following: (1) Dynamics of E are not different between long and short snow season sites, (2) E for both short and long snow seasons has a strong relationship with soil moisture and a poor relationship with soil temperature, (3) dynamics of R vary between long and short snow season sites throughout the year, with short snow season fluxes typically higher than those of long snow season sites, and (4) R for short and long snow seasons has a strong relationship with soil temperature and a poor relationship with soil moisture. Because climate change will only exacerbate both drying–wetting and cooling–warming cycles, detangling these complex relationships becomes increasingly important for understanding shifts in carbon dynamics in these subalpine mixed-conifer ecosystems.
• Nelson, K., Kurc, S. A., John, G., Minor, R., & Barron-Gafford, G. A. (2014). Influence of snow cover duration on soil evaporation and respiration efflux in mixed-conifer ecosystems. Ecohydrology, 7(2), 869-880.
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  Abstract: Subalpine mixed-conifer ecosystems are dependent on snowfall, which is expected to decrease under projected climate change. Changes in snowpack are likely to have important consequences for water and carbon cycling in these and downstream ecosystems. Particularly within semi-arid environments, snowpack changes will directly influence localized water and carbon dynamics and indirectly influence regional-scale levels of water availability and carbon sequestration. In this study, we monitor soil evaporation (E) and soil respiration (R) and evaluate how snow cover affects these effluxes within a mixed-conifer ecosystem within the Santa Catalina Mountains about 10km north of Tucson, Arizona. Using time-lapse digital photos, we identified areas of consistent short and long snow duration, and we monitored E and R in these areas every 2weeks for 15months. Our primary findings include the following: (1) Dynamics of E are not different between long and short snow season sites, (2) E for both short and long snow seasons has a strong relationship with soil moisture and a poor relationship with soil temperature, (3) dynamics of R vary between long and short snow season sites throughout the year, with short snow season fluxes typically higher than those of long snow season sites, and (4) R for short and long snow seasons has a strong relationship with soil temperature and a poor relationship with soil moisture. Because climate change will only exacerbate both drying-wetting and cooling-warming cycles, detangling these complex relationships becomes increasingly important for understanding shifts in carbon dynamics in these subalpine mixed-conifer ecosystems. © 2013 John Wiley & Sons, Ltd.
• Pangle, L., Pangle, L., Pangle, L., DeLong, S., DeLong, S., DeLong, S., Abramson, N., Abramson, N., Abramson, N., Adams, J., Adams, J., Adams, J., Barron-Gafford, G. A., Barron-Gafford, G. A., Barron-Gafford, G. A., Breshears, D. D., Breshears, D. D., Breshears, D. D., Brooks, P. D., , Brooks, P. D., et al. (2015). The Landscape Evolution Observatory: A large-scale controllable infrastructure to study coupled Earth-surface processes. Geomorphology.
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• Pfeiffer, A., Minor, R., Heard, M., & Barron-Gafford, G. (2014). Photosynthetic Response of Poplars (Populus) to Climatic Stressors: Investigating Isoprene's Role in Increasing Tolerance to Temperature and Atmospheric Water Stress in Arizona. AGU Fall Meeting Abstracts, 1, 0711.
• Potts, D. L., Barron-Gafford, G. A., & Jenerette, G. D. (2014). Metabolic acceleration quantifies biological systems' ability to up-regulate metabolism in response to episodic resource availability. JOURNAL OF ARID ENVIRONMENTS, 104, 9-16.
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  Precipitation often arrives discretely in semi-arid ecosystems. Under these conditions, natural selection might favor rapid metabolic responses to the sudden availability of otherwise limiting resources. We introduce and define metabolic acceleration (alpha) as the first derivative of the metabolic rate of a living system with respect to time. As such, alpha describes the capacity of a biological system to up- and down-regulate metabolism and may be applied across scales and processes. To better understand the responses of roots and soil microbes to seasonal patterns of rainfall and plant activity, we compared soil respiratory acceleration (alpha(soil)) derived from soil respiration time-series among three microhabitats (under mesquite, under bunchgrasses, and in intercanopy soils) in a semi-arid shrubland near Tucson, Arizona. Across microhabitats, alpha(soil) was greatest during the warm, wet summer months and lowest during cool winter months. Throughout the year, alpha(soil) beneath mesquite was greater than beneath bunchgrasses or in intercanopy soils. Finally, microhabitat-specific responses of alpha(soil) to spring and monsoonal rainfall events were consistent with seasonal contrasts in the photosynthetic activity of deeply-rooted mesquite shrubs and warm-season bunchgrasses. By quantifying the capacity of living systems to respond to episodic resource availability, metabolic acceleration provides a new perspective and potentially unifying metric for biological responses to environmental heterogeneity. Published by Elsevier Ltd.
• Resco de Dios, V., Goulden, M. L., Ogle, K., Richardson, A. D., Hollinger, D. Y., Davidson, E. A., Alday, J. G., Barron-Gafford, G. A., Carrara, A., Kowalski, A. S., Oechel, W. C., Reverter, B. R., Scott, R. L., Varner, R. K., Diaz-Sierra, R., & Moreno, J. M. (2012). Endogenous circadian regulation of carbon dioxide exchange in terrestrial ecosystems. GLOBAL CHANGE BIOLOGY, 18(6), 1956-1970.
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  It is often assumed that daytime patterns of ecosystem carbon assimilation are mostly driven by direct physiological responses to exogenous environmental cues. Under limited environmental variability, little variation in carbon assimilation should thus be expected unless endogenous plant controls on carbon assimilation, which regulate photosynthesis in time, are active. We evaluated this assumption with eddy flux data, and we selected periods when net ecosystem exchange (NEE) was decoupled from environmental variability in seven sites from highly contrasting biomes across a 74 degrees latitudinal gradient over a total of 36 site-years. Under relatively constant conditions of light, temperature, and other environmental factors, significant diurnal NEE oscillations were observed at six sites, where daily NEE variation was between 20% and 90% of that under variable environmental conditions. These results are consistent with fluctuations driven by the circadian clock and other endogenous processes. Our results open a promising avenue of research for a more complete understanding of ecosystem fluxes that integrates from cellular to ecosystem processes.
• Scott, R. L., Huxman, T. E., Barron-Gafford, G. A., Jenerette, G. D., Young, J. M., & Hamerlynck, E. P. (2014). When vegetation change alters ecosystem water availability. GLOBAL CHANGE BIOLOGY, 20(7), 2198-2210.
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  The combined effects of vegetation and climate change on biosphere-atmosphere water vapor (H2O) and carbon dioxide (CO2) exchanges are expected to vary depending, in part, on how biotic activity is controlled by and alters water availability. This is particularly important when a change in ecosystem composition alters the fractional covers of bare soil, grass, and woody plants so as to influence the accessibility of shallower vs. deeper soil water pools. To study this, we compared 5 years of eddy covariance measurements of H2O and CO2 fluxes over a riparian grassland, shrubland, and woodland. In comparison with the surrounding upland region, groundwater access at the riparian sites increased net carbon uptake (NEP) and evapotranspiration (ET), which were sustained over more of the year. Among the sites, the grassland used less of the stable groundwater resource, and increasing woody plant density decoupled NEP and ET from incident precipitation (P), resulting in greater exchange rates that were less variable year to year. Despite similar gross patterns, how groundwater accessibility affected NEP was more complex than ET. The grassland had higher respiration (R-eco) costs. Thus, while it had similar ET and gross carbon uptake (GEP) to the shrubland, grassland NEP was substantially less. Also, grassland carbon fluxes were more variable due to occasional flooding at the site, which both stimulated and inhibited NEP depending upon phenology. Woodland NEP was large, but surprisingly similar to the less mature, sparse shrubland, even while having much greater GEP. Woodland Reco was greater than the shrubland and responded strongly and positively to P, which resulted in a surprising negative NEP response to P. This is likely due to the large accumulation of carbon aboveground and in the surface soil. These long-term observations support the strong role that water accessibility can play when determining the consequences of ecosystem vegetation change.
• Zhang, X., Niu, G., Elshall, A. S., Ye, M., Barron-Gafford, G. A., & Pavao-Zuckerman, M. (2014). Assessing five evolving microbial enzyme models against field measurements from a semiarid savannah-What are the mechanisms of soil respiration pulses?. GEOPHYSICAL RESEARCH LETTERS, 41(18), 6428-6434.
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  Soil microbial respiration pulses in response to episodic rainfall pulses (the "Birch effect") are poorly understood. We developed and assessed five evolving microbial enzyme models against field measurements from a semiarid savannah characterized by pulsed precipitation to understand the mechanisms to generate the Birch pulses. The five models evolve from an existing four-carbon (C) pool model to models with additional C pools and explicit representations of soil moisture controls on C degradation and microbial uptake rates. Assessing the models using techniques of model selection and model averaging suggests that models with additional C pools for accumulation of degraded C in the dry zone of the soil pore space result in a higher probability of reproducing the observed Birch pulses. Degraded C accumulated in dry soil pores during dry periods becomes immediately accessible to microbes in response to rainstorms, providing a major mechanism to generate respiration pulses. Explicitly representing the transition of degraded C and enzymes between dry and wet soil pores in response to soil moisture changes and soil moisture controls on C degradation and microbial uptake rates improve the models' efficiency and robustness in simulating the Birch effect. Assuming that enzymes in the dry soil pores facilitate degradation of complex C during dry periods (though at a lower rate) results in a greater accumulation of degraded C and thus further improves the models' performance. However, the actual mechanism inducing the greater accumulation of labile C needs further experimental studies.
• 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.
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  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.
• 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.
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  Summary:* 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.
• Barron-Gafford, G. A., Angert, A. L., Venable, D. L., Tyler, A. P., Gerst, K. L., & Huxman, T. E. (2013). Photosynthetic temperature responses of co-occurring desert winter annuals with contrasting resource-use efficiencies and different temporal patterns of resource utilization may allow for species coexistence. Journal of Arid Environments, 91, 95-103.
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  A mechanistic understanding of population dynamics requires close examination of species' differences in how physiological traits interact with environmental variation and translate into demographic variation. We focused on two co-occurring winter annual species (Pectocarya recurvata and Plantago insularis ) that differ in photosynthetic resource-use efficiency and demographic responses to environmental variation and covariation between temperature and water availability. Previous work showed that Pectocarya has higher water-use efficiency and nitrogen allocation to light-driven dynamics of the Calvin cycle (Jmax:VCmaxVCmax) than Plantago, which is often associated with enhanced electron transport capacity at low temperatures and better light harvesting capacity. These traits could enhance Pectocarya photosynthesis during reliably moist but cool, cloudy periods following precipitation. We acclimated plants to low and high temperatures and then measured gas exchange across a 30 °C temperature range. As predicted, optimal temperatures of photosynthesis were lower for Pectocarya than Plantago. Additionally, Pectocarya experienced greater respiratory carbon loss than Plantago at higher temperatures (every 1 °C increase beyond 24 °C increased the ratio of carbon loss to gain 9% and 27% in cold and warm-acclimated plants, respectively). These differential patterns of photosynthetic optimization and assimilation in response to differing rainfall distributions may have important implications for population dynamic differences and species coexistence.
• Barron-Gafford, G. A., Scott, R. L., Jenerette, G. D., Hamerlynck, E. P., & Huxman, T. E. (2013). Landscape and environmental controls over leaf and ecosystem carbon dioxide fluxes under woody plant expansion. Journal of Ecology, 101(6), 1471--1483.
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  Summary:* Many regions of the globe are experiencing a simultaneous change in the dominant plant functional type and regional climatology. We explored how atmospheric temperature and precipitation control leaf- and ecosystem-scale carbon fluxes within a pair of semi-arid shrublands, one upland and one riparian, that have undergone woody plant expansion.* Through a combination of leaf-level measurements on individual bunchgrasses and mesquites shrubs and ecosystem-scale monitoring using eddy covariance techniques, we sought to quantify rates of net carbon dioxide (CO2) flux, CO2 flux temperature sensitivity and the responsiveness of these parameters to seasonal rains and periods of soil dry-down.* We found significant differences in physiological acclimation between the two plant functional types, in that the shrubs consistently conducted photosynthesis across a broader temperature range than co-occurring grasses during dry periods, yet maximum photosynthetic rates in grasses were twice that of mesquites during the wetter monsoon season. Landscape position modulated these temperature sensitivities, as the range of functional temperatures and maximum rates of photosynthesis were two to three times greater within the riparian shrubland in dry times.* Also, it was unexpected that ecosystem-scale CO2 uptake within both shrublands would become most temperature sensitive within the monsoon, when mesquites and grasses had their broadest range of function. This is probably explained by the changing contributions of component photosynthetic fluxes, in that the more temperature sensitive grasses, which had higher maximal rates of photosynthesis, became a larger component of the ecosystem flux.Synthesis: Given projections of more variable precipitation and increased temperatures, it is important to understand differences in physiological activity between growth forms, as they are likely to drive patterns of ecosystem-scale CO2 flux. As access to stable subsurface water declines with decreased precipitation, these differential patterns of temperature sensitivity among growth forms, which are dependent on connectivity to groundwater, will only become more important in determining ecosystem carbon source/sink status.
• Cable, J. M., Ogle, K., Barron-Gafford, G. A., Bentley, L. P., Cable, W. L., Scott, R. L., Williams, D. G., & Huxman, T. E. (2013). Antecedent conditions influence soil respiration differences in shrub and grass patches. Ecosystems, 16(7), 1230--1247.
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  Quantifying the response of soil respiration to past environmental conditions is critical for predicting how future climate and vegetation change will impact ecosystem carbon balance. Increased shrub dominance in semiarid grasslands has potentially large effects on soil carbon cycling. The goal of this study was to characterize the effect of antecedent moisture and temperature conditions on soil respiration in a grassland now dominated by shrubs. Continuous measurements of soil respiration, soil temperature, and soil moisture were made over the entire summer of 2005 within distinct vegetation microsites in this shrubland community—under grasses, shrubs, and in open spaces. We analyzed these data within a Bayesian framework that allowed us to evaluate the time-scale over which antecedent conditions influence soil respiration. The addition of antecedent conditions explained an additional 16% of the variation in soil respiration. High soil moisture during the preceding month increased respiration rates in both the grass and shrub microsites. However, the time period over which antecedent soil moisture influenced the temperature sensitivity of soil respiration was shorter in the shrub compared to the grass microsites (1 vs. 2 weeks, respectively). The depth of moisture was important; for example, for respiration under shrubs, near-surface moisture was more influential on the day of the respiration measurement but subsurface moisture was more influential on the antecedent time scale. Although more mechanistic studies are required, this study is the first to reveal that shrub encroachment changes the time scales over which soil moisture and temperature affect soil respiration.
• Hamerlynck, E. P., Scott, R. L., & Barron-Gafford, G. A. (2013). Consequences of cool-season drought-induced plant mortality to Chihuahuan Desert grassland ecosystem and soil respiration dynamics. Ecosystems, 16(7), 1178--1191.
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  Predicted reductions of cool-season rainfall may expand and accelerate drought-induced plant mortality currently unfolding across the Southwest US. To assess how repeated plant mortality affects ecosystem functional attributes, we quantified net ecosystem CO2 exchange (NEE), ecosystem respiration (Reco), and gross ecosystem photosynthesis (GEP) responses to precipitation (P) at a semidesert grassland over spring (Feb 1–Apr 30) and summer (June 15–Oct 1) plant-active periods across eight years, including two with distinct patterns of extensive species-specific mortality. In addition, we quantified daily soil respiration (Rsoil) in high- (56–88%) and low-mortality (8–27%) plots the summer following the most recent mortality event. Plant mortality coincided with severely dry cool-season conditions (Dec 1–Apr 30). We found a positive relationship between springtime P and GEP, and that springtime conditions influenced GEP response to summer rainfall. High springtime Reco/GEP ratios followed plant mortality, suggesting increased available carbon after mortality was rapidly decomposed. Rsoil in low-mortality plots exceeded high-mortality plots over drier summer periods, likely from more root respiration. However, total cumulative Rsoil did not differ between plots, as variation in surviving plant conditions resulted in high and low C-yielding plots within both plot types. Vegetation status in high C-yielding Rsoil plots was similar to that across the grassland, suggesting Rsoil from such areas underlay higher Reco. This, coupled to springtime drought constraints to GEP, resulted in positive NEE under summer P accumulations that previously supported C-sink activity. These findings indicate that predicted lower cool-season precipitation may strongly and negatively affect summer season productivity in these semiarid grasslands.
• Hamerlynck, E. P., Scott, R. L., Sanchez-CAnete, E. P., & Barron-Gafford, G. A. (2013). Nocturnal soil CO2 uptake and its relationship to subsurface soil and ecosystem carbon fluxes in a Chihuahuan Desert shrubland. Journal of Geophysical Research: Biogeosciences, 118(4), 1593--1603.
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  Despite their prevalence, little attention has been given to quantifying arid land soil and ecosystem carbon fluxes over prolonged, annually occurring dry periods. We measured soil [CO2] profiles and fluxes (Fs) along with volumetric soil moisture and temperature in bare interplant canopy soils and in soils under plant canopies over a three-month hot and dry period in a Chihuahuan Desert shrubland. Nocturnal Fs was frequently negative (from the atmosphere into the soil), a form of inorganic carbon exchange infrequently observed in other deserts. Negative Fs depended on air-soil temperature gradients and were more frequent and stronger in intercanopy soils. Daily integrated ecosystem-level Fs was always positive despite lower daily Fs in intercanopy soils due to nocturnal uptake and more limited positive response to isolated rains. Subsurface [CO2] profiles associated with negative Fs indicated that sustained carbonate dissolution lowered shallow-soil [CO2] below atmospheric levels. In the morning, positive surface Fs started earlier and increased faster than shallow-soil Fs, which was bidirectional, with upward flux toward the surface and downward flux into deeper soils. These dynamics are consistent with carbonate precipitation in conjunction with convection-assisted CO2 outgassing from warming air and soil temperatures and produced a pronounced diurnal Fs temperature hysteresis. We concluded that abiotic nocturnal soil CO2 uptake, through a small carbon sink, modulates dry season ecosystem-level carbon dynamics. Moreover, these abiotic carbon dynamics may be affected by future higher atmospheric carbon dioxide levels and predictions of more prolonged and regular hot and dry periods.
• Huxman, T. E., Kimball, S., Angert, A. L., Gremer, J. R., Barron-Gafford, G. A., & Venable, D. L. (2013). Understanding past, contemporary, and future dynamics of plants, populations, and communities using Sonoran Desert winter annuals. American Journal of Botany, 100(7), 1369--1380.
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  Global change requires plant ecologists to predict future states of biological diversity to aid the management of natural communities, thus introducing a number of significant challenges. One major challenge is considering how the many interacting features of biological systems, including ecophysiological processes, plant life histories, and species interactions, relate to performance in the face of a changing environment. We have employed a functional trait approach to understand the individual, population, and community dynamics of a model system of Sonoran Desert winter annual plants. We have used a comprehensive approach that connects physiological ecology and comparative biology to population and community dynamics, while emphasizing both ecological and evolutionary processes. This approach has led to a fairly robust understanding of past and contemporary dynamics in response to changes in climate. In this community, there is striking variation in physiological and demographic responses to both precipitation and temperature that is described by a trade-off between water-use efficiency (WUE) and relative growth rate (RGR). This community-wide trade-off predicts both the demographic and life history variation that contribute to species coexistence. Our framework has provided a mechanistic explanation to the recent warming, drying, and climate variability that has driven a surprising shift in these communities: cold-adapted species with more buffered population dynamics have increased in relative abundance. These types of comprehensive approaches that acknowledge the hierarchical nature of biology may be especially useful in aiding prediction. The emerging, novel and nonstationary climate constrains our use of simplistic statistical representations of past plant behavior in predicting the future, without understanding the mechanistic basis of change.
• 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., & others, . (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), 741--758.
• 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. D., Chorover, J., 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.
• Barron-Gafford, G. A., Rascher, U., Bronstein, J. L., Davidowitz, G., Chaszar, B., & Huxman, T. E. (2012). Herbivory of wild Manduca sexta causes fast down-regulation of photosynthetic efficiency in Datura wrightii: an early signaling cascade visualized by chlorophyll fluorescence. PHOTOSYNTHESIS RESEARCH, 113(1-3), 249-260.
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  Plants experiencing herbivory suffer indirect costs beyond direct loss of leaf area, but differentially so based on the herbivore involved. We used a combination of chlorophyll fluorescence imaging and gas exchange techniques to quantify photosynthetic performance, the efficiency of photochemistry, and heat dissipation to examine immediate and longer-term physiological responses in the desert perennial Datura wrightii to herbivory by tobacco hornworm, Manduca sexta. Herbivory by colony-reared larvae yielded no significant reduction in carbon assimilation, whereas herbivory by wild larvae induced a fast and spreading down-regulation of photosynthetic efficiency, resulting in significant losses in carbon assimilation in eaten and uneaten leaves. We found both an 89 % reduction in net photosynthetic rates in herbivore-damaged leaves and a whole-plant response (79 % decrease in undamaged leaves from adjacent branches). Consequently, herbivory costs are higher than previously estimated in this well-studied plant-insect interaction. We used chlorophyll fluorescence imaging to elucidate the mechanisms of this down-regulation. Quantum yield decreased up to 70 % in a small concentric band surrounding the feeding area within minutes of the onset of herbivory. Non-photochemical energy dissipation by the plant to avoid permanent damage was elevated near the wound, and increased systematically in distant areas of the leaf away from the wound over subsequent hours. Together, the results underscore not only potential differences between colony-reared and wild-caught herbivores in experimental studies of herbivory but also the benefits of quantifying physiological responses of plants in unattacked leaves.
• Barron-Gafford, G. A., Scott, R. L., Jenerette, G. D., Hamerlynck, E. P., & Huxman, T. E. (2012). Temperature and precipitation controls over leaf- and ecosystem-level CO2 flux along a woody plant encroachment gradient. GLOBAL CHANGE BIOLOGY, 18(4), 1389-1400.
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  Conversion of grasslands to woodlands may alter the sensitivity of CO2 exchange of individual plants and entire ecosystems to air temperature and precipitation. We combined leaf-level gas exchange and ecosystem-level eddy covariance measurements to quantify the effects of plant temperature sensitivity and ecosystem temperature responses within a grassland and mesquite woodland across seasonal precipitation periods. In so doing, we were able to estimate the role of moisture availability on ecosystem temperature sensitivity under large-scale vegetative shifts. Optimum temperatures (Topt) for net photosynthetic assimilation (A) and net ecosystem productivity (NEP) were estimated from a function fitted to A and NEP plotted against air temperature. The convexities of these temperature responses were quantified by the range of temperatures over which a leaf or an ecosystem assimilated 50% of maximum NEP (O50). Under dry pre- and postmonsoon conditions, leaf-level O50 in C3 shrubs were two-to-three times that of C4 grasses, but under moist monsoon conditions, leaf-level O50 was similar between growth forms. At the ecosystems-scale, grassland NEP was more sensitive to precipitation, as evidenced by a 104% increase in maximum NEP at monsoon onset, compared to a 57% increase in the woodland. Also, woodland NEP was greater across all temperatures experienced by both ecosystems in all seasons. By maintaining physiological function across a wider temperature range during water-limited periods, woody plants assimilated larger amounts of carbon. This higher carbon-assimilation capacity may have significant implications for ecosystem responses to projected climate change scenarios of higher temperatures and more variable precipitation, particularly as semiarid regions experience conversions from C4 grasses to C3 shrubs. As regional carbon models, CLM 4.0, are now able to incorporate functional type and photosynthetic pathway differences, this work highlights the need for a better integration of the interactive effects of growth form/functional type and photosynthetic pathway on water resource acquisition and temperature sensitivity.
• Cable, J. M., Barron-Gafford, G. A., Ogle, K., Pavao-Zuckerman, M., Scott, R. L., Williams, D. G., & Huxman, T. E. (2012). Shrub encroachment alters sensitivity of soil respiration to temperature and moisture. JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES, 117.
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  A greater abundance of shrubs in semiarid grasslands affects the spatial patterns of soil temperature, moisture, and litter, resulting in fertile islands with potentially enhanced soil metabolic activity. The goal of this study was to quantify the microsite specificity of soil respiration in a semiarid riparian ecosystem experiencing shrub encroachment. We quantified the response of soil respiration to different microsite conditions created by big mesquite shrubs (near the trunk and the canopy edge), medium-sized mesquite, sacaton bunchgrasses, and open spaces. We hypothesized that soil respiration would be more temperature sensitive and less moisture sensitive and have a greater magnitude in shrub microsites compared with grass and open microsites. Field and incubation soil respiration data were simultaneously analyzed in a Bayesian framework to quantify the microsite-specific temperature and moisture sensitivities and magnitude of respiration. The analysis showed that shrub expansion increases the heterogeneity of respiration. Respiration has greater temperature sensitivity near the shrub canopy edge, and respiration rates are higher overall under big mesquite compared with those of the other microsites. Respiration in the microsites beneath medium-sized mesquites does not behave like a downscaled version of big mesquite microsites. The grass microsites show more similarity to big mesquite microsites than medium-sized shrubs. This study shows there can be a great deal of fine-scale spatial heterogeneity that accompanies shifts in vegetation structure. Such complexity presents a challenge in scaling soil respiration fluxes to the landscape for systems experiencing shrub encroachment, but quantifying this complexity is significantly important in determining overall ecosystem metabolic behavior.
• Hamerlynck, E. P., Scott, R. L., Barron-Gafford, G. A., Cavanaugh, M. L., Susan Moran, M., & Huxman, T. E. (2012). Cool-season whole-plant gas exchange of exotic and native semiarid bunchgrasses. PLANT ECOLOGY, 213(8), 1229-1239.
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  The success of invasive aridland plants may depend on their utilization of precipitation not fully exploited by native species, which could lead to seasonally altered ecosystem carbon and water fluxes. We measured volumetric soil water across 25-cm profiles (theta(25cm)) and springtime whole-plant water- and carbon-fluxes of the exotic Lehmann lovegrass (Eragrostis lehmanniana) and a native bunchgrass, bush muhly (Muhlenbergia porteri), following typical (55 mm in 2009) and El Nio-enhanced accumulations (154 mm in 2010) in a SE Arizona savanna. Across both years, theta(25cm) was higher under lovegrass plots, with similar evapotranspiration (ET) between lovegrass and bush muhly plots. However, in 2010 transpiration (T) was higher in bush muhly than lovegrass, implying higher soil evaporation in lovegrass plots maintained similar ET. Net ecosystem carbon dioxide exchange (NEE) was similar between lovegrass and bush muhly plots in 2009, but was more negative in bush muhly plots following El Nio, indicating greater CO2 assimilation. Ecosystem respiration (R (eco)) and gross ecosystem photosynthesis (GEP) were similar between lovegrass and bush muhly plots in 2009, but were higher in bush muhly plots in 2010. As a result, lovegrass plots reduced ecosystem water-use efficiency (WUEe = NEE/ET), while bush muhly WUEe remained constant between 2009 and 2010. Concurrent whole-plant WUE (WUEp = GEP/T) did not change in lovegrass plots, but increased in bush muhly plots between these years. We concluded that cool-season precipitation use is not a component of Lehmann lovegrass invasive success, but that the change in ET partitioning and attendant shifts in cool-season WUEe may increase interannual variation in ecosystem water- and carbon-exchange dynamics in the water-limited systems it dominates.
• Jardine, K., Barron-Gafford, G. A., Norman, J. P., Abrell, L., Monson, R. K., Meyers, K. T., Pavao-Zuckerman, M., Dontsova, K., Kleist, E., Werner, C., & Huxman, T. E. (2012). Green leaf volatiles and oxygenated metabolite emission bursts from mesquite branches following light-dark transitions. PHOTOSYNTHESIS RESEARCH, 113(1-3), 321-333.
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  Green leaf volatiles (GLVs) are a diverse group of fatty acid-derived compounds emitted by all plants and are involved in a wide variety of developmental and stress-related biological functions. Recently, GLV emission bursts from leaves were reported following light-dark transitions and hypothesized to be related to the stress response while acetaldehyde bursts were hypothesized to be due to the 'pyruvate overflow' mechanism. In this study, branch emissions of GLVs and a group of oxygenated metabolites (acetaldehyde, ethanol, acetic acid, and acetone) derived from the pyruvate dehydrogenase (PDH) bypass pathway were quantified from mesquite plants following light-dark transitions using a coupled GC-MS, PTR-MS, and photosynthesis system. Within the first minute after darkening following a light period, large emission bursts of both C-5 and C-6 GLVs dominated by (Z)-3-hexen-1-yl acetate together with the PDH bypass metabolites are reported for the first time. We found that branches exposed to CO2-free air lacked significant GLV and PDH bypass bursts while O-2-free atmospheres eliminated the GLV burst but stimulated the PDH bypass burst. A positive relationship was observed between photosynthetic activity prior to darkening and the magnitude of the GLV and PDH bursts. Photosynthesis under (CO2)-C-13 resulted in bursts with extensive labeling of acetaldehyde, ethanol, and the acetate but not the C-6-alcohol moiety of (Z)-3-hexen-1-yl acetate. Our observations are consistent with (1) the "pyruvate overflow" mechanism with a fast turnover time (< 1 h) as part of the PDH bypass pathway, which may contribute to the acetyl-CoA used for the acetate moiety of (Z)-3-hexen-1-yl acetate, and (2) a pool of fatty acids with a slow turnover time (> 3 h) responsible for the C-6 alcohol moiety of (Z)-3-hexen-1-yl acetate via the 13-lipoxygenase pathway. We conclude that our non-invasive method may provide a new valuable in vivo tool for studies of acetyl-CoA and fatty acid metabolism in plants at a variety of spatial scales.
• Jenerette, G. D., Barron-Gafford, G. A., Guswa, A. J., McDonnell, J. J., & Camilo Villegas, J. (2012). Organization of complexity in water limited ecohydrology. Ecohydrology, 184-199.
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  Water limited ecohydrological systems (WLES), with their broad extent, large stores of global terrestrial carbon, potential for large instantaneous fluxes of carbon and water, sensitivity to environmental changes, and likely global expansion, are particularly important ecohydrological systems. Strong nonlinear responses to environmental variability characterize WLES, and the resulting complexity of system dynamics has challenged research focussed on general understanding and site specific predictions. To address this challenge our synthesis brings together current views of complexity from ecological and hydrological sciences to look towards a framework for understanding ecohydrological systems (in particular WLES) as complex adaptive systems (CAS). This synthesis suggests that WLES have many properties similar to CAS. In addition to exhibiting feedbacks, thresholds, and hysteresis, the functioning of WLES is strongly affected by self-organization of both vertical and horizontal structure across multiple scales. As a CAS, key variables for understanding WLES dynamics are related to their potential for adaptation, resistance to variability, and resilience to state changes. Several essential components of CAS, including potential for adaptation and rapid changes between states, pose challenges for modelling and generating predictions of WLES. Model evaluation and predictable quantities may need to focus more directly on temporal or spatial variance in contrast to mean state values for success at understanding system-level characteristics. How coupled climate and vegetation changes will alter available soil, surface and groundwater supplies, and overall biogeochemistry will reflect how self-organizational ecohydrological processes differentially partition precipitation and overall net metabolic functioning. Copyright (C) 2011 John Wiley & Sons, Ltd.
• Ogle, K., Lucas, R. W., Bentley, L. P., Cable, J. M., Barron-Gafford, G. A., Griffith, A., Ignace, D., Jenerette, G. D., Tyler, A., Huxman, T. E., Loik, M. E., Smith, S. D., & Tissue, D. T. (2012). Differential daytime and night-time stomatal behavior in plants from North American deserts. NEW PHYTOLOGIST, 194(2), 464-476.
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  Night-time stomatal conductance (g(night)) occurs in many ecosystems, but the g(night) response to environmental drivers is relatively unknown, especially in deserts.
• Abrell, L., Grieve, K. A., Haren, J. V., Lin, G., Malhi, Y., Murthy, R., Pegoraro, E., & Barron-gafford, G. A. (2005). The effect of elevated atmospheric CO2 and drought on sources and sinks of isoprene in a temperate and tropical rainforest mesocosm. Global Change Biology, 11(8), 1234-1246. doi:10.1111/j.1365-2486.2005.00986.x
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  Isoprene is the most abundant volatile hydrocarbon emitted by many tree species and has a major impact on tropospheric chemistry, leading to formation of pollutants and enhancing the lifetime of methane, a powerful greenhouse gas. Reliable estimates of global isoprene emission from different ecosystems demand a clear understanding of the processes of both production and consumption. Although the biochemistry of isoprene production has been studied extensively and environmental controls over its emission are relatively well known, the study of isoprene consumption in soil has been largely neglected. Here, we present results on the production and consumption of isoprene studied by measuring the following different components: (1) leaf and soil and (2) at the whole ecosystem level in two distinct enclosed ultraviolet light-depleted mesocosms at the Biosphere 2 facility: a cottonwood plantation with trees grown at ambient and elevated atmospheric CO2 concentrations and a tropical rainforest, under well watered and drought conditions. Consumption of isoprene by soil was observed in both systems. The isoprene sink capacity of litter-free soil of the agriforest stands showed no significant response to different CO2 treatments, while isoprene production was strongly depressed by elevated atmospheric CO2 concentrations. In both mesocosms, drought suppressed the sink capacity, but the full sink capacity of dry soil was recovered within a few hours upon rewetting. We conclude that soil uptake of atmospheric isoprene is likely to be modest but significant and needs to be taken into account for a comprehensive estimate of the global isoprene budget. More studies investigating the capacity of soils to uptake isoprene in natural conditions are clearly needed.

Proceedings Publications

• Barron-Gafford, G., Honsberg, C. B., Bowden, S. G., & Sampson, R. (2022). Agrivoltaic Modules Optimizing Light for Crops in Dryland Regions. In 2022 IEEE 49th Photovoltaics Specialists Conference (PVSC).
• Barron-Gafford, G. A., & Minor, R. L. (2017, August). Dryland Agrivoltaics: A Novel Ecosystems Approach to the Food-Water-Energy Nexus. In Ecological Society of America Meeting Presentation and Proceedings.
• Barron-Gafford, G. A., & Minor, R. L. (2017, August). Hydraulic redistribution as a driver of tree and grass competition versus facilitation in a semiarid savanna. In Ecological Society of America Meeting Presentation and Proceedings.
• Dwivedi, R., Meixner, T., Mcintosh, J. C., Ferre, P. A., Eastoe, C. J., Minor, R., Barron-Gafford, G. A., & Chorover, J. D. (2017, December). Hydrologic functioning of the deep Critical Zone and contributions to streamflow in a high elevation catchment: testing of multiple conceptual models. In AGU Fall Meeting.

Presentations

• Barron-Gafford, G. A. (2019, 2019). Contributed Presentations to Professional Meetings, Symposia, Colloquia (12 Presentations in 2019). Contributed Presentations to Professional Meetings, Symposia, Colloquia.
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  Submitted presentations (limited to period in rank)Barron-Gafford GA, Thompson M, Baldwin B, Rack J, Larson T, Barnett-Moreno I, Hurley B, Marston S. Solar shaded and kid powered: Boosting climate literacy through experiential learning in a shared agrivoltaics experiment. American Geophysical Union’s Annual Fall Meeting. Dec. 9-13, 2019, San Francisco, CA. Phelps-Garcia L, Barron-Gafford GA, Barnett-Moreno I, Salazar A, Lopez L, Cooke W, Murphy P, Thompson MS. Creating sustainable agriculture through water-smart design. American Geophysical Union’s Annual Fall Meeting. Dec. 9-13, 2019, San Francisco, CA. Knowles JF, Barron-Gafford GA, Blanken P, Dore S, Kolb T, Litvak ME, Scott RL. Mountain forest productivity across a regional climatic gradient in the southwestern USA. American Geophysical Union’s Annual Fall Meeting. Dec. 9-13, 2019, San Francisco, CA. Yang, J, Magney TS, Yan D, Knowles JF, Smith WK, Barron-Gafford GA. Photochemical Reflectance Index (PRI) is sensitive to the ecohydrology of a semi-arid mixed conifer forest. American Geophysical Union’s Annual Fall Meeting. Dec. 9-13, 2019, San Francisco, CA. Lee E, Kumar P, Scott RL, Barron-Gafford GA. Role of groundwater in hydraulic redistribution and inter-annual water use dynamics in semi-arid upland and riparian sites. American Geophysical Union’s Annual Fall Meeting. Dec. 9-13, 2019, San Francisco, CA. Macknick J, Barron-Gafford GA. Combining agriculture with solar energy for food, energy, and water benefits: Lessons learned from 25 field studies of the DOE InSPIRE Project. American Geophysical Union’s Annual Fall Meeting. Dec. 9-13, 2019, San Francisco, CA. Adams H, Hammond W, Dickman LT, Barron-Gafford GA, Law D, Anderegg W, Anderegg LDL, Sevanto S, Pockman W, McDowell N, Breshears DD, Huxman TE. The effect of temperature and ontogeny on the physiological mechanism of drought-induced tree mortality in pines. American Geophysical Union’s Annual Fall Meeting. Dec. 9-13, 2019, San Francisco, CA. Strange BM, Hu J, Barron-Gafford GA, Knowles JF. Increased spatiotemporal resolution of whole-tree carbon assimilation estimated with stable isotopes from tree-ring cellulose. American Geophysical Union’s Annual Fall Meeting. Dec. 9-13, 2019, San Francisco, CA. Dwivedi R, Eastoe CJ, Knowles JF, Mcintosh J, Meixner T, Ferre TPA, Minor RL, Barron-Gafford GA, Abramson N, Stanley M, Chorover J. A comparison of transit time distribution vs. fraction of young water to characterize storage in a mountain headwater catchment: does the tail matter? American Geophysical Union’s Annual Fall Meeting. Dec. 9-13, 2019, San Francisco, CA. Smith WK, Yan D, Yang JL, Wang X, Scott RL, Biederman JA, Dannenberg MP, Barron-Gafford GA, Moore DJP, Knowles JF. Monitoring diurnal to seasonal vegetation photosynthesis across key dryland ecosystem types using near-surface remote sensing techniques. Annual Meeting of the Ecological Society of America. August 11-16, 2019, Louisville, KY. Winkler C, Barron-Gafford GA, Barnett-Moreno I, Ferrante S. Agrivoltaics: Co-locating photovoltaics and winter crops to enhance survivability. UA Honors College Annual Poster Session. April, 2019, Tucson, AZ. Ferrante S, Barron-Gafford GA, Barnett-Moreno I, Winkler C. The Bright Side: Co-locating photovoltaics and winter crops boost renewable energy production. UA Honors College Annual Poster Session. April, 2019, Tucson, AZ.
• Barron-Gafford, G. A. (2019, 2019). Invited Speaker to Professional Meetings, Symposia, Colloquia (7 Presentations in 2019). Invited Speaker to Professional Meetings, Symposia, Colloquia.
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  Invited Symposia (limited to period in rank)Barron-Gafford GA (Invited speaker). Co-locating agriculture and solar power renewables (agrivoltaics) to create a more sustainable energy, food, and water future. US Department of Energy Solar Energy Technologies Office Colloquium. February, 2020, Washington, D.C.Barron-Gafford GA (Invited speaker). Agrivoltaics in drylands: How co-locating renewable energy and food production can create a sustainable southwest. US House of Representatives. February, 2020, Washington, D.C. Barron-Gafford GA (Invited speaker). Salazar A, Phelps-Garcia L, Lopez L, Barnett-Moreno I, Cooke W, Thompson MS, Minor RL, Murphy P, Pavao-Zuckerman M, Macknick J. Co-locating agriculture and solar power renewables (agrivoltaics) to create a more sustainable food, energy, and water future. American Geophysical Union’s Annual Fall Meeting. Dec. 9-13, 2019, San Francisco, CA. Barron-Gafford GA (Invited speaker). Agrivoltaics: Co-location has food, water, and renewable energy benefits. Enel Green Power and Powerhouse Utility Meeting. September, 2019, San Francisco, California. Barron-Gafford GA (Invited speaker). Agrivoltaics in the US: Results, Outreach, and Lessons Learned. World Agroforestry Centre. March, 2019, Nairobi, Kenya. Barron-Gafford GA (Invited speaker). Pavao-Zuckerman M, Minor RL, Barnett-Moreno I, Dimond K, Gerlak A, Murphy P, Thompson MS, Winkler C, Marston S, Macknick J. Agrivoltaics in drylands: Co-location has food, water, and renewable energy benefits. Annual Meeting of the Ecological Society of America. August 11-16, 2019, Louisville, KY. Barron-Gafford GA (Invited speaker). Solar Energy + Agriculture: Co-locating for a sustainable future. Institute for Energy Solutions’ Energy Talks Series at Sky Bar. February 7, 2019, Tucson, AZ.
• Livingston, M., Barron-Gafford, G. A., & Dimond, K. (2019, March). Maker Space for Environmental Monitoring. CELA 2019. Sacramento, CA.
• Macknick, J., Marston, S., Winkler, C., Thompson, M. S., Murphy, P., Gerlak, A. K., Dimond, K., Barnett-Moreno, I., Minor, R. L., Pavao-Zuckerman, M., & Barron-Gafford, G. A. (2019, August). Agrivoltaics in drylands: Co-location has food, water, and renewable energy benefits. 2019 ESA Annual Meeting. Louisville, KY: Eoclogical Society of America (ESA).
• Barron-Gafford, G. A. (2018, Fall). Comparing the Impact of Hydraulic Redistribution on Co-existing Vegetation Interaction in Semi-arid Upland and Riparian Sites. American Geophysical Union Fall Meeting. Washington DC.
• Barron-Gafford, G. A. (2018, Fall). Desertification pathways induced by grass invasion into shrublands: The ecophysiological context. American Geophysical Union Fall Meeting. Washington DC.
• Barron-Gafford, G. A. (2018, Fall). Overwhelming Heat Waves: Climate Envelope Development for Pinus edulis Seedlings. American Geophysical Union Fall Meeting. Washington DC.
• Barron-Gafford, G. A. (2018, Fall). Quantifying plant physiological response to water stress with high-frequency, near-surface observations of chlorophyll fluorescence and photochemical reflectivity. American Geophysical Union Fall Meeting. Washington DC.
• Barron-Gafford, G. A. (2018, Fall). The pulse of a semi-arid forest: intra-seasonal progression of water and carbon exchange.. American Geophysical Union Fall Meeting. Washington DC.
• Barron-Gafford, G. A. (2018, November). Dryland agrivoltaics: Co-locating photovoltaics and agriculture or green infrastructure to increase module performance, food production, and water savings. Human Challenges in Extreme Environments. University of Arizona, Marshall Building: UMI iGlobes, CEMCA, and the Udall Center for Studies in Public Policy, University of Arizona.
• Barron-Gafford, G. A., & Dusza, Y. (2018, April). Quantifying the importance of biotic and abiotic drivers in creating lags in soil CO2 efflux. European Geophysical Union Annual Meeting.
• Barron-Gafford, G. A., & Pérez Sánchez-Cañete, E. (2018, April). Quantification of uncertainties in conifer sap flow measured with the thermal dissipation method. European Geophysical Union Annual Meeting.
• Barron-Gafford, G. A., Minor, R. L., Jojola, J., Barnett-Moreno, I., Martinez, C., Thompson, M., Gerlak, A. K., Nabhan, G. P., Dimond, K., Pavao-Zuckerman, M., & Macknick, J. (2018, December). Dryland Agrivoltacs: A novel approach to collocating food production and solar renewable energy to maximize food production, water savings, and energy generation. AGU Fall Meeting. Washington D.C.: American Geophysical Union.
• Chorover, J. D., Stanley, M., Mitra, B., Abramson, N., Barron-Gafford, G. A., Knowles, J., Minor, R., Niu, G., Wright, W., Castro, C. L., Eastoe, C., Ferre, P. A., Mcintosh, J. C., Meixner, T., & Dwivedi, R. (2018, Fall). An improved and practical approach for estimating catchment-scale response functions through power spectral analysis. American Geophysical Union Fall Meeting. Washington DC.
• Barron-Gafford, G. A., & Minor, R. L. (2017, August). Dryland Agrivoltaics: A Novel Ecosystems Approach to the Food-Water-Energy Nexus. Ecological Society of America Meeting Presentation and Proceedings.
• Barron-Gafford, G. A., & Thompson, M. (2017, October). Agrivoltaics: The Next Frontier of Food Innovation. Ecological Society of America Meeting Presentation and Proceedings. Tucson, AZ.
• Ferriere, R. H., Troch, P. A., Van Haren, J. L., Barron-Gafford, G. A., Gelabert, A., Daval, D., Barre, P., Hunt, E., Massol, F., Llavata, M., Chollet, S., Le Galliard, J., Villasenor, E., Juarez, S., & Dontsova, K. M. (2019, January). Influence of Climate on Plant-Rock Interactions during Early Soil Development.. Science Society of America (SSSA) International Soils Meeting “Soils Across Latitudes”. San Diego, CA: Science Society of America.
• Naito, A., Archer, S. R., Barron-Gafford, G. A., Heilman, P., Katharine, P. I., & Throop, H. (2017, January). Ecosystem services on managed rangelands: a watershed-scale evaluation of trade-offs. American Association of Geographers Annual Meetings. Boston, MA.
• Throop, H., Katharine, P. I., Heilman, P., Barron-Gafford, G. A., Archer, S. R., & Naito, A. (2017, January). Brush management on the Santa Rita Experimental Range, Arizona: a preliminary evaluation of ecosystem services responses. International Biogeography Society Annual Meetings. Tucson, AZ.
• Throop, H., Predick, K. I., Heilman, P., Barron-Gafford, G. A., Archer, S. R., & Naito, A. (2017, January). Evaluating ecosystem services and trade-offs in the context of brush management. Society for Range Management Annual Meeting. St. George, UT.
• Villasenor, E., Dontsova, K. M., Juarez, S., Le Galliard, J., Chollet, S., Llavata, M., Massol, F., Barré, P., Gelabert, A., Daval, D., Troch, P. A., Barron-Gafford, G. A., Van Haren, J. L., & Ferriere, R. H. (2017, December 2017). The effect of elevated CO2 and temperature on nutrient uptake by plants grown in basaltic soil. American Geophysical Union Fall Meeting. San Francisco, CA: American Geophysical Union.
• et al., ., Heilman, P., Barron-Gafford, G. A., Archer, S. R., & Naito, A. T. (2017, June). Brush management: an ecosystem services perspective. Science in the Sonoita Plain Symposium. Appleton-Whittell Research Ranch, Elgin, AZ.
• Barron-Gafford, G. A. (2016, December). Impacts of hydraulic redistribution on overstory-understory interactions in a semiarid savanna. American Geophysical Union Annual Meeting. San Francisco, CA.
• Barron-Gafford, G. A. (2016, December). The Necessity of Determining the Gas Transfer Coefficient In-situ to Obtain More Accurate Soil Carbon Dioxide Effluxes Through the Gradient Method. American Geophysical Union Annual Meeting. San Francisco, CA.
• Barron-Gafford, G. A. (2016, December). The Photovoltaic Heat Island Effect: Larger solar power plants increase local temperatures. American Geophysical Union Annual Meeting. San Francisco, CA.
• Elshal, A. S., & Barron-Gafford, G. A. (2016, December). Numerical Demons in Monte Carlo Estimation of Bayesian Model Evidence with Application to Soil Respiration Models. American Geophysical Union Annual Meeting. San Francisco, CA.
• Ferriere, R. H., Van Haren, J. L., Barron-Gafford, G. A., Troch, P. A., Daval, D., Gelabert, A., Massol, F., Barre, P., Llavata, M., Cros, A., Chollet, S., Le Galliard, J., Dontsova, K. M., & Juarez, S. (2016, April 2016). Effect of elevated CO2 and temperature on abiotic and biologically-driven basalt weathering and C sequestration. 2016 EGU General Assembly. Vienna, Austria: European Geosciences Union.
• Ferriere, R. H., Van Haren, J. L., Barron-Gafford, G. A., Troch, P. A., Daval, D., Gelabert, A., Massol, F., Barre, P., Llavata, M., Cros, A., Chollet, S., Le Galliard, J., Dontsova, K. M., & Juarez, S. (2016, October 2016). Effect of CO2 and temperature on basalt weathering and microbial activity.. International Conference on Ecological Sciences sfecologie2016. Marseille, France.
• Hingley, R., & Barron-Gafford, G. A. (2016, December). Effects of Climate Change and Vegetation Type on Carbon and Nitrogen Accumulation during Incipient Soil Formation. American Geophysical Union Annual Meeting. San Francisco, CA.
• Scott, R. L., & Barron-Gafford, G. A. (2016, December). Insights from a network of long-term measurements of biosphere-atmospheric exchanges of water vapor and carbon dioxide in southern Arizona. American Geophysical Union Annual Meeting. San Francisco, CA.
• Sutter, L. F., & Barron-Gafford, G. A. (2016, December). An Experimental and Modeling Synthesis to Determine Seasonality of Hydraulic Redistribution in Semi-arid Region with Multispecies Vegetation Interaction. American Geophysical Union Annual Meeting. San Francisco, CA.
• Barron-Gafford, G. A. (2015, April). Capturing heterogeneity in carbon fluxes in space and time across a semiarid montane forest. Association of American Geographers Annual Meeting. Chicago, IL.
• Barron-Gafford, G. A. (2015, August, 2015). Examining ecosystem function in space and time within the critical zone through the lenses of ecology and biogeography. Ecological Society of America, Annual Meeting. Baltimore, MD.
• Barron-Gafford, G. A., Minor, R., Heard, M., Sutter, L., Yang, J., & Potts, D. (2015, December). Complex terrain in the Critical Zone: How topography drives ecohydrological patterns of soil and plant carbon exchange in a semiarid mountainous system. American Geophysical Union Annual Meeting. San Francisco, CA.
• Elshall, A., Ye, M., Niu, G., & Barron-Gafford, G. A. (2015, December). Bayesian multimodel inference of soil microbial respiration models: Theory, application and future prospective. American Geophysical Union Annual Meeting. San Francisco, CA.
• Sanchez-Cañete, E., Barron-Gafford, G. A., van Haren, J., & Scott, R. (2015, December). Improving soil CO2 efflux estimates from in-situ soil CO2 sensors with gas transport measurements. American Geophysical Union Annual Meeting. San Francisco, CA.
• Scott, R., Biedermann, J., Barron-Gafford, G. A., & Hamerlynck, E. (2015, December). The Carbon Balance Pivot Point of Southwestern U.S. Semiarid Ecosystems: Insights From the 21st Century Drought. American Geophysical Union Annual Meeting. San Francisco, CA.
• Barron-Gafford, G. A. (2014, April). Heat islanding around solar energy installations ~ Valid concern or unnecessary worry about renewable energy production.. Association of American Geographers Annual Meeting.
• Barron-Gafford, G. A. (2014, November). Mesquites in the Grasslands ~ Environmental and Human Drivers of Landscape Change.. Living Gently on the Land Potluck and Presentation Educational Series,. Appleton-Whittell Research Ranch: National Audubon Society.
• Barron-Gafford, G. A. (2014, September). Aspect as a source of heterogeneity in carbon & water fluxes in space and time. National Critical Zone Observatory All-Hands Meeting.
• Barron-Gafford, G. A. (2014, September). Using Water Smart Design and an Ecosystem Services Approach to Fight Solar Heat Islanding and Enhance Renewable Energy Production. Association of Pacific Coast Geographers Annual Meeting.
• 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.
• Elshall, A. S., Ye, M., & Barron-Gafford, G. A. (2014, December). Quantification of Model Uncertainty in Modeling Mechanisms of Soil Microbial Respiration Pulses to Simulate Birch Effect. American Geophysical Union’s Annual Fall Meeting.
• Ibsen, P. A., Van Leeuwen, W. J., McCorkel, J., Barron-Gafford, G. A., & Moore, D. J. (2014, December). Physiology and thermal imaging of Poplar hybrids with varying temperature tolerance. American Geophysical Union’s Annual Fall Meeting.
• Pavao-Zuckerman, M., Knerl, A., & Barron-Gafford, G. A. (2014, December). Ecohydrology frameworks for green infrastructure design and ecosystem service provision.. American Geophysical Union’s Annual Fall Meeting.
• Ruiz, J., Barron-Gafford, G. A., Van Haren, J. L., Dontsova, K. M., Troch, P. A., & Chorover, J. D. (2014, December). Rapid CO2 consumption during incipient weathering of a granular basaltic hillslope in the Landscape Evolution Observatory, Biosphere 2.. American Geophysical Union’s Annual Fall Meeting.
• Scott, R. L., Biederman, J. A., & Barron-Gafford, G. A. (2014, December). The coupling of ecosystem productivity and water availability in dryland regions.. American Geophysical Union’s Annual Fall Meeting.
• Troch, P. A., Barron-Gafford, G. A., Dontsova, K. M., Fang, Y., Niu, Y., Pangle, L. A., Tuller, M., & Van Haren, J. L. (2014, December). Monitoring and Modeling Water, Energy and Carbon Fluxes at the Hillslope Scale in the Landscape Evolution Observatory.. AGU International Annual Meeting. San Francisco, CA: American Geophysical Union.
• Zaharescu, D., Dontsova, K. M., Burghelea, C., Maier, R. M., Huxman, T. E., & Chorover, J. D. (2014, December). Cracking the Code of Soil Genesis. The Early Role of Rare Earth Elements. American Geophysical Union (AGU) Fall Meeting. San Francisco, CA.
• Scott, R. L., Huxman, T. E., Barron-Gafford, G. A., Jenerette, D., & Young, J. (2013, December). The Ecohydrological Consequences of Woody Plant Encroachment: How Accessibility to Deep Soil Water Resources Affects Ecosystem Carbon and Water Exchange. Annual Meeting of the American Geophysical Union. San Francisco, CA: American Geophysical Union.
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  Woody plant encroachment into grassland systems, a process that has increased rapidly over the last century, has potentially broad ecohydrological consequences by affecting the way ecosystems use water and cycle carbon. This study examines the influence of precipitation- and groundwater-derived water availability by comparing eddy covariance measurements of water vapor and carbon dioxide fluxes over a riparian grassland, shrubland, and woodland, and an upland grassland site in southeastern Arizona USA. Compared to the upland grassland, the riparian sites exhibited greater net carbon uptake (NEP) and higher evapotranspiration (ET) across a longer portion of the year. Among the riparian sites, however, the grassland was less able to take advantage of the stable groundwater supply. Increasing woody plant density facilitated greater water and carbon exchange that became increasingly decoupled from incident precipitation (P). How groundwater accessibility affected NEP was more complex than ET. Respiration (Reco) costs were higher for the riparian grassland so, while it had a similar ET and gross carbon uptake (GEP) to the shrubland, its NEP was substantially less. Also, riparian grassland fluxes were much more variable due to flooding that occurred at the site, which could stimulate or inhibit NEP. Woodland NEP was largest but surprisingly similar to the less mature and dense shrubland even while having much greater GEP. Woodland NEP responded negatively to P, due to the stimulation of Reco likely due to greater amounts of aboveground and soil carbon. With many areas of the world experiencing woody plants encroachment, encroachment into areas where there are additional deep soil water sources, such as in riparian settings or in areas of deep soil moisture recharge, will likely increase carbon sequestration but at the expense of higher water use.

Poster Presentations

• Hu, J., Barron-Gafford, G. A., Knowles, J., & Strange, B. (2019, December). Increased spatiotemporal resolution of whole-tree carbon assimilation estimated with stable isotopes from tree-ring cellulose. American Geophysical Union annual meeting. San Francisco, CA: American Geophysical Union.
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  This poster was presented by my PhD student, Brandon Strange at the annual AGU meeting.
• Naito, A. T., Heilman, P., Guertin, D. P., Barron-Gafford, G. A., Biederman, J. A., & Archer, S. R. (2017, October). Brush management and ecosytem services: quantification of trade-offs on western rangelands. Project Director's Meeting. Tampa, FL: USDA NIFA Agroecosystems Program.
• Troch, P. A., Zeng, X., Wang, Y., Van Haren, J. L., Tuller, M., Sibayan, M., Schaap, M. G., Saleska, S. R., Ruiz, J., Rasmussen, C., Pelletier, J. D., Niu, G., Monson, R. K., Meredith, L., Alves Meira Neto, A., Matos, K. A., Maier, R. M., Kim, M., Hunt, E. A., , Harman, C. J., et al. (2017, December). Controlled Experiments of Hillslope Co-evolution at the Biosphere 2 Landscape Evolution Observatory: Toward Prediction of Coupled Hydrological, Biogeochemical, and Ecological change. 2017 AGU Fall Meeting, Abstract B43A-2105. New Orleans, LA: American Geophysical Union (AGU).
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  Understanding the process interactions and feedbacks among water, microbes, plants, and porous geological media is crucial for improving predictions of the response of Earth’s critical zone to future climatic conditions. However, the integrated co-evolution of landscapes under change is notoriously difficult to investigate. Laboratory studies are typically limited in spatial and temporal scale, while field studies lack observational density and control. To bridge the gap between controlled lab and uncontrolled field studies, the University of Arizona – Biosphere 2 built a macrocosm experiment of unprecedented scale: the Landscape Evolution Observatory (LEO). LEO consists of three replicated, 330-m2 hillslope landscapes inside a 5000-m2 environmentally controlled facility. The engineered landscapes contain 1-m depth of basaltic tephra ground to homogenous loamy sand that will undergo physical, chemical, and mineralogical changes over many years. Each landscape contains a dense sensor network capable of resolving water, carbon, and energy cycling processes at sub-meter to whole-landscape scale. Embedded sampling devices allow for quantification of biogeochemical processes, and facilitate the use of chemical tracers applied with the artificial rainfall. LEO is now fully operational and intensive forcing experiments have been launched. While operating the massive infrastructure poses significant challenges, LEO has demonstrated the capacity of tracking multi-scale matter and energy fluxes at a level of detail impossible in field experiments. Initial sensor, sampler, and restricted soil coring data are already providing insights into the tight linkages between water flow, weathering, and (micro-) biological community development during incipient landscape evolution. Over the years to come, these interacting processes are anticipated to drive the model systems to increasingly complex states, potentially perturbed by changes in climatic forcing. By intensively monitoring the evolutionary trajectory, integrating data with models, and fostering community-wide collaborations, we envision that emergent landscape structures and functions can be linked and significant progress can be made toward predicting the coupled hydro-biogeochemical and ecological responses to global change.
• Troch, P. A., Zeng, X., Wang, Y., Van Haren, J. L., Tuller, M., Sibayan, M., Schaap, M. G., Saleska, S. R., Ruiz, J., Rasmussen, C., Pelletier, J. D., Niu, G., Monson, R. K., Meredith, L., Alves Meira Neto, A., Matos, K. A., Maier, R. M., Kim, M., Hunt, E. A., , Harman, C. J., et al. (2017, December). Controlled Experiments of Hillslope Co-evolution at the Biosphere 2 Landscape Evolution Observatory: Toward Prediction of Coupled Hydrological, Biogeochemical, and Ecological change. AGU International Annual Meeting. New Orleans, LA: American Geophysical Union (AGU).
• Barron-Gafford, G. A. (2016, December). Seasonal and Topographic Variation in Net Primary Productivity and Water Use Efficiency in a Southwest Sky Island Forest. American Geophysical Union Annual Meeting. San Francisco, CA.
• Barron-Gafford, G. A., Van Haren, J. L., Scott, R. L., & Sanchez-Canete, E. (2016, 12). The Necessity of Determining the Gas Transfer Coefficient In-situ to Obtain More Accurate Soil Carbon Dioxide Effluxes Through the Gradient Method. AGU Fall Meeting. San Francisco: AGU.
• Ferriere, R. H., Van Haren, J. L., Barron-Gafford, G. A., Troch, P. A., Daval, D., Gelabert, A., Massol, F., Barre, P., Llavata, M., Cros, A., Chollet, S., Le Galliard, J., Hunt, E., Juarez, S., Dontsova, K. M., & Hingley, R. (2016, December 2016). Effects of Climate Change and Vegetation Type on Carbon and Nitrogen Accumulation during Incipient Soil Formation. American Geophysical Union Fall Meeting. San Francisco, CA: American Geophysical Union.
• Ferriere, R. H., Van Haren, J. L., Barron-Gafford, G. A., Troch, P. A., Daval, D., Gelabert, A., Massol, F., Barre, P., Llavata, M., Cros, A., Chollet, S., Le Galliard, J., Juarez, S., & Dontsova, K. M. (2016, June 2016). Basalt Weathering under Controlled Conditions as Influenced by Biota, Temperature, and CO2. The 26th Goldschmidt Conference. Yokohama, Japan: The European Association for Geochemistry and Geochemical Society.
• Juarez, S., Dontsova, K. M., Le Galliard, J., Chollet, S., Cros, A., Llavata, M., Barre, P., Massol, F., Gelabert, A., Daval, D., Troch, P. A., Barron-Gafford, G. A., Van Haren, J. L., & Ferriere, R. H. (2015, 20-24 Sep 2015). Abiotic and biologically-­‐driven basalt weathering and carbon sequestration under changing climate. 5th International Symposium on Soil Organic Matter. Göttingen (DE).
• Lee, E., Kumar, P., Barron-Gafford, G. A., Scott, R., Hendryx, S., & Sanchez-Cañete, E. (2015, December). Determining the Role of Hydraulic Redistribution Regimes in the Critical Zone. American Geophysical Union Annual Meeting. San Francisco, CA.
• McFarland, E. J., Parra, E. A., Larkin Minor, R. L., Heard, M., & Barron-Gafford, G. A. (2015, 2015). Does the Production of Isoprene Affect the Productivity of Poplars?. Research Experiences for Undergraduates Symposium.
• McFarland, E. J., Parra, E. A., Larkin Minor, R. L., Heard, M., & Barron-Gafford, G. A. (2015, 2015). Does the Production of Isoprene Affect the Productivity of Poplars?. STAR (STEM Teacher and Researcher) Program Posters.
• Parra, E., McFarland, E., Heard, M., Minor, R., & Barron-Gafford, G. A. (2015, December). Effects of isoprene production on the photosynthetic performance of Poplars (Populus sp.) under thermal and moisture stress. American Geophysical Union Annual Meeting. San Francisco, CA.
• Ibsen, P., Barron-Gafford, G. A., Van Leeuwen, W., McCorkel, J., & Moore, D. J. (2014, Dec). Physiology and Thermal Imaging of Poplar Hybrids with Varying Temperature Tolerance. American Geophysical Union Fall Meeting. San Francisco: American Geophysical Union.
• Pfeiffer, A. W., Minor, R. L., Heard, M. M., & Barron-Gafford, G. A. (2014, December). Photosynthetic response of Poplars (Populus) to climatic stressors: Investigating isoprene's role in increasing tolerance to temperature and atmospheric water stress in Arizona. American Geophysical Union’s Annual Fall Meeting.
• Troch, P. A., Barron-Gafford, G. A., Dontsova, K. M., Fang, Y., Niu, Y., Pangle, L., Tuller, M., Van Haren, J. L., Troch, P. A., Barron-Gafford, G. A., Dontsova, K. M., Fang, Y., Niu, Y., Pangle, L., Tuller, M., & Van Haren, J. L. (2014, December). Monitoring and Modeling Water, Energy and Carbon Fluxes at the Hillslope Scale in the Landscape Evolution Observatory. 2014 American Geophysical Union (AGU) Fall Meeting. San Francisco, CA.
• Van Haren, J. L., Tuller, M., Pangle, L., Niu, Y., Fang, Y., Dontsova, K. M., Barron-Gafford, G. A., & Troch, P. A. (2014, December). Monitoring and Modeling Water, Energy and Carbon Fluxes at the Hillslope Scale in the Landscape Evolution Observatory. American Geophysical Union (AGU) Fall Meeting. San Francisco, CA.
• Barron-Gafford, G. A., Minor, R. L., van Haren, J., Dontsova, K. M., & Troch, P. A. (2013, December). Precipitation pulse dynamics of carbon sequestration and efflux in highly weatherable soils. Annual Meeting of the American Geophysical Union. San Francisco, CA: American Geophysical Union.
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  Soils are the primary pool for terrestrial carbon on Earth, and loss of that carbon to the atmosphere or hydrosphere represents a significant efflux that can impact a host of other downstream processes. Soil respiration (Rsoil), the efflux of CO2 to the atmosphere, represents the major pathway by which carbon is lost from the soil system in more weathered soils. However, in newly formed soils, chemical weathering can significantly deplete soil CO2 concentrations. As vegetation colonizes these soils, multiple interacting and contradictory pathways evolve such that soil CO2 concentrations increase in response to plant inputs but are decreased through chemical reactions. Furthermore, abiotic drivers of soil temperature and moisture likely differentially affect these processes. Understanding the bio-geo-chemical drivers and feedbacks associated with soil CO2 production and efflux in the critical zone necessitates an integrated science approach, drawing on input from plant physiologists, bio- and geochemists, and hydrologists. Here, we created a series of 1-meter deep mesocosms filled with granular basalt that supported either a woody mesquite shrub, a bunchgrass, or was left as bare soil. Use of multiple plant functional types allowed us to explore the impacts of plant structure (primarily rooting profiles) on critical zone function in terms of water and carbon exchange surrounding precipitation pulse dynamics. Each mesocosm was outfitted with an array of soil moisture, temperature, water potential, and CO2 concentration sensors at the near-surface, 30, 55, and 80cm depths to quantify patterns of soil moisture and respiratory CO2 efflux in response to rainfall events of varying magnitude and intervening periods of drought. Five replicates of each were maintained under current ambient or projected (+4oC) air temperatures. In addition, we used minirhizotrons to quantify the response of roots to episodic rainfall and confirm differences among plant types and collected soils solution samples to quantify dissolved inorganic carbon (DIC), pH, and other solute concentrations.Importantly, we found Rsoil dynamics to be nearly in direct contrast to our classic understanding of patterns of soil CO2 efflux after rain events. Rsoil rates declined immediately upon wetting and gradually increased to pre-rain rates as the soils dried. Investigation into soil CO2 profile data showed that CO2 concentrations just below the surface declined significantly from near-ambient levels to near ~50ppm, which would directly impact rates of Rsoil. We detected differences among plant functional types in terms of rooting depth, water use, photosynthetic uptake, base rates of Rsoil, the time required to return to pre-rain rates of Rsoil, and the rates of soil weathering. Combining aboveground measurements of carbon uptake with these belowground estimates of carbon pools and efflux will allow us to make much more informed projections of carbon dynamics within highly weatherable soils across a range of global climate change projections and plant functional types.Experiment webpage: http://www.u.arizona.edu/~gregbg/Research__Biosphere_2_Mesocosm.htmlParent project webpage: http://leo.b2science.org/
• 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.
• DeMets, C., Pavao-Zuckerman, M. -., & Barron-Gafford, G. A. (2013, December). Strategies for cooler cities? Ecophysiological responses of semi-arid street trees to storm water harvesting. Annual Meeting of the American Geophysical Union. San Francisco, CA: American Geophysical Union.
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  REU Student Presentation:As the southwestern U.S. moves into an uncertain future in terms of water supply and climate, communities are seeking creative ways to harvest urban runoff. One such solution is to implement water-sensitive urban design features such as rain basins, which are designed to capture and facilitate infiltration of precipitation and storm water as it runs off impermeable surfaces like streets and sidewalks. Rain basins essentially act as temporary cisterns, allowing a given rain event to have a much larger impact in recharging soil water profiles. In this sense, even a “small” rain may yield a more saturated soil profile and stimulate plant physiological activity well beyond plants that lack this additional moisture input. However, the impacts of rain basins on plant function remain unquantified. Therefore, the purpose of our research is to characterize the performance of native mesquite trees in basins relative to non-basin native mesquites. To answer our question, we randomly sampled basin and non-basin native mesquites in two different neighborhoods in Tucson, AZ, and characterized their response to precipitation events. We measured stomatal conductance, a proxy for transpiration, on the first and third days following rain events in 2013. Numerous environmental factors, such as photosynthetically available radiation (PAR), temperature, relative humidity, and soil moisture, were also measured in order to explore relationships with conductance. These measurements were conducted before and during monsoon season in order to determine the significance of water in basin performance, enabling us to better characterize plant response to medium (6 to 12 mm) rain events.Findings from this study indicate that basin and non-basin mesquites have similar pre-monsoon conductance rates, with a mean basin value of 70 +/-10 mmol/(m2*s) and a mean non-basin value of 57 +/-6 mmol/(m2*s) at peak conductance. In contrast, during the monsoon, basin mesquites showed significantly higher peak conductance rates (179 +/-22 mmol/(m2*s)) than non-basin trees (126 +/-9 mmol/(m2*s)). Perhaps more importantly, basin mesquite conductance remained elevated for an extended period of time into the afternoon as compared to non-basin mesquites. While this difference was negligible before the monsoon, it was significant during the monsoon. The day immediately after a medium rainfall event, non-basin mesquites shut down around 13:00, while basin mesquites never shut down completely before the end of the measurement period around 17:30. Soil moisture levels were elevated in the rain basins relative to the non-basin soils, suggesting that basins impact plant functioning through enhanced soil water availability. These preliminary results demonstrate that basins are an effective means of capturing water and irrigating plants. Here we have demonstrated how an appreciation of wildland plant ecophysiology can be applied to an urban setting in support of a suite of ecosystem services. Notably, there is a potential for enhanced urban heat island mitigation in semi-arid cities through the application of water-sensitive urban design features such as rain basins, due to their supporting a longer duration of latent heat flux cooling (i.e., transpiration) into the afternoon.
• Law, D. J., Ravi, S., Breshears, D. D., Barron-Gafford, G. A., & 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.
• Niu, Y. -., & Barron-Gafford, G. A. (2013, October). A microbial enzyme based Soil Organic Carbon (SOC) decomposition model for use in climate change models.. AGU Chapman Conference: Soil-mediated drivers of coupled biogeochemical and hydrological processes across scales. Oracle, AZ: AGU.
• Niu, Y. -., Zhang, X., Barron-Gafford, G. A., & Pavao-Zuckerman, M. -. (2013, December). Modeling the "Birch Effect"; Using a Microbial Enzyme Based Soil Organic Carbon Decomposition and Gas Transport Model. Annual Meeting of the American Geophysical Union. San Francisco, CA: American Geophysical Union.
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  Soil respiration pulses in response to pulsed wetting (&#8220;Birch effect&#8221;; Birch 1958) have long been observed from laboratory and field experiments. The Birch effect produces more CO2 efflux and sustains greater microbial biomass than constantly moist soils. Various mechanisms causing the effect have been proposed. However, the exact mechanism underlying the Birch effect is not clear, and thus most models are not able to simulate this effect.We have recently developed a microbial enzyme based decomposition and gas transport model. The model integrates the most recent advances in the understanding of critical processes, including enzyme-catalyzed degradation of soil organic carbon (SOC) to dissolved organic carbon (DOC), acclimation of carbon use efficiency (CUE) for the uptake of DOC by microbes, and diffusive and convective transport of O2 and CO2 in the soil. The model has four kinds of carbon pools including SOC, DOC, microbial biomass (MIC), and extracellular enzyme (ENZ). However, the model coupled with a land surface model, which accurately simulates soil moisture and temperature, failed to simulate the Birch effect observed at a natural savannah ecosystem site in the southwest US monsoon region.We further divided the DOC and ENZ pools into two sub-pools, one for a wet zone and the other for a dry zone, respectively. We assume that in the dry zone, DOC can be produced through enzyme catalysis, although at a lower rate due to enzyme immobilization, and only in the wet zone can microbes take up DOC. Thus, the modeled DOC accumulates during dry periods and is quickly transitioned into DOC in the wet zone (proportional to saturation) in response to pulsed wetting during a storm, and becomes available for microbial use. In such a way, the model successfully simulates the Birch effect with the Nash&#8211;Sutcliffe model efficiency being ~ 0.75 (correlation coefficient ~ 0.88) at a half-hourly time step. We will also present the effect of gas transport on the Birch effect and the Birch effect on long-term soil carbon dynamics.
• Yang, J., Barron-Gafford, G. A., Minor, R. L., & Heard, M. (2013, December). Examining the Physical Drivers of Photosynthetic Temperature Sensitivity Within a Sub-alpine Mixed Conifer Forest. Annual Meeting of the American Geophysical Union. San Francisco, CA: American Geophysical Union.
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  REU Student Presentation:Current projections of climate change in the southwestern U.S. suggest increasing temperatures and reduced summer precipitation. High temperature and water deficits have major influence on ecosystem functioning by restricting plant growth and productivity. However, there are limited data on what influences plant sensitivity to temperature, and these dynamics are not often captured in ecosystem models. Understanding the sensitivities, linkages, and feedbacks among biotic processes and abiotic forces is especially important within Critical Zone Sciences, which seeks to integrate among disciplines. Here, we analyzed several potential drivers of photosynthetic temperature sensitivity, including differences in soil parent material, aspect, and seasonality within a suite of species. Each of these variables captures a different physical driver: (i) soil parent material influences water holding capacity of the soil; (ii) aspect influences how incoming energy drives evaporative loss of soil water, creating warmer and drier environments on south/east faces; and (iii) seasonality captures temporal patterns of soil moisture recharge. Our research was conducted within two V shaped zero-order catchment basins of the Santa Catalina Critical Zone Observatory in southern Arizona, one with schist bedrock and the other with granite. We used leaf-level gas exchange measurements on 24 trees across a range of temperatures to quantify this plant temperature sensitivity during the dry pre-monsoon and wet monsoon seasons. Preliminary results show that maximum photosynthetic rate was 51% higher during the monsoon than pre-monsoon season. Optimal photosynthetic temperature decreased 25% while the span of functional temperatures (Ω50) was 21% higher following the onset of monsoon rains. During the rainy season, soil parent material became an important factor. The greater water holding capacity of schist soils yielded greater maximum photosynthesis and reduced tree sensitivity to higher temperatures. This variability in the temperature sensitivity and the responsiveness of that sensitivity to increases in available soil moisture is important as we consider the structure and composition of our future forests. More detailed analysis will allow us to examine the relative influence of each abiotic variable in driving this photosynthetic response, which impacts a suite of downstream critical zone processes.
• van Haren, J., Barron-Gafford, G. A., & Dontsova, K. M. (2013, December). CO2 sequestration through weathering of basalt tephra in the Landscape Evolution Observatory (LEO). Annual Meeting of the American Geophysical Union. San Francisco, CA: American Geophysical Union.
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  Weathering of primary silicates is one of the mechanisms involved in carbon removal from the atmosphere, affecting the carbon cycle at geologic timescales. Basalt is one of the most reactive rocks and thus a strong contributor to geologic weathering fluxes. The Landscape Evolution Observatory (LEO), an Earth science research facility at Biosphere 2, Tucson, AZ, consisting of three identical 350m2 and 1m deep slopes, allows conducting controlled experiments investigating the interactions between atmosphere, hydrosphere, lithosphere, and biosphere at an unprecedented scale. This study presents results of the initial experiments where granular basalt that serves as a soil medium in LEO was exposed to rainfall. Soil solution and drainage were collected and analyzed to determine changes in solution composition. Gas-phase CO2 concentrations in the soil were monitored using custom gas samplers and Vaisala CO2 probes and CO2 gas fluxes on the surface was determined using soil chambers. The goal of the study was to determine the impact of precipitation on incipient CO2 driven weathering reactions and inorganic C sequestration in the basalt and how these reactions were distributed along hillslope flow paths. Results indicate a very strong relationship between water inputs and soil CO2 concentrations and fluxes. Within hours of a rainfall event, the surface CO2flux increased three-fold, while soil CO2 concentrations were reduced from near atmospheric to

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• Archer, S. R., Guertin, D. P., Scott, R., Vivoni, E., & Barron-Gafford, G. A. (2021, May). Final Report, Brush management and ecosystem services: a quantification of trade-offs on Western rangelands.. USDA NIFA REEPort.
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  Final Report, 7 pp
• Barron-Gafford, G. A., Vivoni, E., Scott, R., Guertin, D. P., & Archer, S. R. (2020, June). Annual Report, Brush management and ecosystem services: a quantification of trade-offs on Western rangelands. Annual Report, USDA-NIFA Program A1451, ARZT-3013240-G12-517.
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  Annual Report, 7 pp.
• Barron-Gafford, G. A., Vivoni, E., Scott, R., Guertin, D. P., & Archer, S. R. (2020, June). Brush management and ecosystem services: a quantification of trade-offs on Western rangelands. Annual Report, USDA-NIFA Program A1451, ARZT-3013240-G12-517.
  More info

  Annual Report, 7 pp.