Steven R Archer
- Professor, Natural Resources
- Professor, Arid Lands Resources Sciences - GIDP
- Professor, Global Change - GIDP
- Professor, Remote Sensing / Spatial Analysis - GIDP
Grasslands and savannas throughout the world appear to have been replaced by shrub– and woodlands in recent history. This shift in vegetation structure has implications for the sustainability of pastoral and commercial livestock production systems and may influence climate and atmospheric chemistry via impacts on the carbon, nitrogen and water cycles. Documentation of shifts in woody plant abundance is poor and causes are not well understood. As a result, our ability to anticipate the rate, direction and magnitude of future changes is limited. My research has concentrated on interactions between grasses and woody plants in relation to soils, climate and disturbance. Population, transition probability and dynamic ecosystem simulation models are used in conjunction with remote sensing, GIS, dendrochronology and stable isotope chemistry to reconstruct vegetation history and to examine potential, impending changes and the consequences of such changes on sustainability of grazing systems, ecosystem biogeochemistry and land surface-atmosphere interactions. Field and laboratory experiments on the population biology of grasses and shrub growth forms are emphasized in the context of landscape ecology, succession and historical land–use practices.
- Ph.D. Rangeland Ecosystem Science
- Colorado State University, Ft Collins, Colorado
- M.S. Rangeland Ecosystem Science
- Colorado State University, Colorado
- B.A. Bilogy and Liberal Arts
- Augustana College, Sioux Falls, South Dakota
- Visiting Professor, Department of Environmental Science, Policy, & Management University of California (2012)
- Visiting Scientist, USGS Earth Resources Observation and Science (EROS) Center (2011)
- Adjunct Professor, New Mexico State University, Las Cruces, New Mexico (2007 - Ongoing)
- Professor, Unversity of Arizona (2002 - Ongoing)
- Professor, Texas A&M University, College Station, Texas (1993 - 2002)
- Visiting Scientist, National Center for Atmospheric Research (1991 - 1992)
- Adjunct Professor, Institut Agronomique et Veterinaire Hassan II (1988 - 1998)
- Associate Professor, Texas A&M University (1988 - 1993)
- Assistant Professor, Texas A&M University, College Station, Texas (1983 - 1988)
- Invited participant “Eco-Engineering of life in arid landscapes: discovering & constructiong interactions along the plant-mibrobiome-soil continuum” workshop
- Biosphere 2, Spring 2018
- Invitation to Join Editorial Board
- The Open Plant Science Journal, Fall 2017
- Journal of Ecology & Natural Resources, Summer 2017
- Research Journal of Environmental Sciences, Summer 2017
- Invitation to sit on proposal evaluation panel
- National Science Foundation - DEB Ecosystem Science, Fall 2017
- USDA-NIFA Climate, Land Use, and Land Management, Fall 2017
- Proposal evaluation panel invitation
- National Science Foundation - DEB Population & Community Ecology, Fall 2017
- EJ Dyksterhuis Distinguished Lecture
- Texas A&M University, Spring 2016
- Ecological Society of America, Spring 2016
- American Association for the Advancement of Science (AAAS), Spring 2009
- Plenary Speaker
- 2nd National Congress on Silvopastoral Systems, Santiago del Estero, Argentina, Spring 2012
- South America Resilience and Sustainability Studies Institute workshop in Punta del Este, Uruguay, Spring 2012
- Todd Spieker Lecture
- University of California at Los Angeles, Spring 2012
- R.D. Watt Centenary Lecture
- University of Sydney, Spring 2011
- Leaders in Agriculture Lecture
- Sul Ross State University, Spring 2009
- Scholarly Achievement
- School of Natural Resources and the Environment, Spring 2008
- Davidson Lecture
- Baylor University, Spring 2003
Licensure & Certification
- Fellow, Ecological Society of America (2016)
- Fellow, American Association for the Advancement of Science (2009)
- Certified Senior Ecologist, Ecological Society of America (2002)
Terrestrial Ecology, Natural Resources Ecology and Management, Rangeland Ecology, Rangeland Ecosyistem Science
Plant Ecology, Ecosystem Science, Carbon Cycling, Arid land Ecology, grass-shrub interactions, grazing ecology, grassland and savanna ecology
DissertationRNR 920 (Spring 2019)
Conservation of Natural EnvRNR 200 (Fall 2018)
DissertationRNR 920 (Fall 2018)
Phy Aspects Of Arid LandARL 565 (Fall 2018)
ResearchRNR 900 (Fall 2018)
DissertationRNR 920 (Spring 2018)
ResearchRNR 900 (Spring 2018)
Conservation of Natural EnvRNR 200 (Fall 2017)
DissertationARL 920 (Fall 2017)
DissertationRNR 920 (Fall 2017)
Current ResearchARL 595A (Spring 2017)
DissertationARL 920 (Spring 2017)
DissertationRNR 920 (Spring 2017)
Ecol/Savnns,Shrblnd+WdlnGEOG 619 (Spring 2017)
Ecol/Savnns,Shrblnd+WdlnRAM 619 (Spring 2017)
ResearchARL 900 (Spring 2017)
Conservation of Natural EnvRNR 200 (Fall 2016)
DissertationARL 920 (Fall 2016)
DissertationRNR 920 (Fall 2016)
Independent StudyRNR 499 (Fall 2016)
ResearchARL 900 (Fall 2016)
ThesisRNR 910 (Fall 2016)
- Archer, S. R., Andersen, E. M., Predick, K. I., Schwinning, S., Steidl, R. J., & Woods, S. R. (2017). Woody plant encroachment: causes and consequences. In Rangeland Systems: Processes, Management and Challenges(pp 25-84). Springer.
- Peters, D., Archer, S. R., & Bestelmeyer, B. (2013). Desertification of rangelands. In Ecosystem Functions and Services(pp 239-258). Academic Press.More infoEditor(s): Seastedt, TR | Suding, KN
- Ji, W., Hanan, H., Browning, D., Monger, C., Peters, D., Bestelmeyer, B., Archer, S. R., & et al., . (2019). Constraints on shrub cover and shrub-shrub competition in a U.S. Southwest desert. Ecosphere, 10(2), e02590. doi:doi.org/10.1002/ecs2.2590
- Fulbright, T., Davies, K. W., & Archer, S. R. (2018). Wildlife responses to brush management: A contemporary evaluation. Rangeland Ecology & Management, 71(1), 35-44. doi:http://dx.doi.org/10.1016/j.rama.2017.07.001
- Marsh, S. E., Mcclaran, M. P., Archer, S. R., & Choy-ing, H. (2018). Shrub encroachment into grasslands: end of an era?. PeerJ, 6. doi:10.7717/peerj.5474.
- Hewins, D. B., Sinsabaugh, R. L., Archer, S. R., & Throop, H. L. (2017). Soil-litter mixing and microbial activity mediate decomposition and soil aggregate formation in a sandy shrub-invaded Chihuahuan Desert grassland. Plant Ecology, DOI 10.1007/s11258-017-0703-4.
- Barnes, P. W., Throop, H. L., Archer, S. R., Breshears, D. D., McCulley, R. L., Tobler, M. A., Barnes, P. W., Throop, H. L., Archer, S. R., Breshears, D. D., McCulley, R. L., & Tobler, M. A. (2015). Sunlight and soil–litter mixing: Drivers of litter decomposition in drylands. Progress in Botany, 76, 273-302.
- Bestelmeyer, B., Okin, G., Duniway, M., Archer, S. R., Sayre, N., Williamson, J. C., & Herrick, J. (2015). Desertification, land use and the transformation of global drylands. Frontiers in Ecology and the Environment, 123(1), 28-36.
- Peters, D., Havstad, K., Archer, S. R., & Sala, O. (2015). Beyond desertification: new paradigms for dryland landscapes. Frontiers in Ecology and the Environment, 13(1), 4-12.
- Archer, S. R., & Predick, K. I. (2014). Woody plant proliferation and the ecology of "brush management": an ecosystem services perspective. Journal of Ecology, 102, 1394-1407.More infodoi: 10.1111/1365-2745.12314
- Barnes, P. W., Throop, H. L., Archer, S. R., Breshears, D. D., McCulley, R. L., & Tobler, M. A. (2014). Sunlight and soil-litter mixing: drivers of litter decomposition in drylands. Progress in Botany, 76, 273-302.More infoDoi: 10.1007/978-3-319-08807-5_11
- Browning, D. M., Franklin, J., Archer, S. R., Gillian, J. K., & Guertin, D. P. (2014). Spatial patterns of grassland-shrubland state transitions: a 74 year record on grazed and protected areas. Ecological Applications, 24(6), 1421-1433.
- Lee, H., Fitzgerald, J., Hewins, D. B., McCulley, R. L., Archer, S. R., Rahn, T., & Throop, H. L. (2014). Soil moisture and soil-litter mixing effects on surface litter decomposition: A controlled environment assessment. Soil Biology and Biochemistry, 72, 123-132.More infoAbstract: Recent studies suggest the long-standing discrepancy between measured and modeled leaf litter decomposition in drylands is, in part, the result of a unique combination of abiotic drivers that include high soil surface temperature and radiant energy levels and soil-litter mixing. Temperature and radiant energy effects on litter decomposition have been widely documented. However, under field conditions in drylands where soil-litter mixing occurs and accelerates decomposition, the mechanisms involved with soil-litter mixing effects are ambiguous. Potential mechanisms may include some combination of enhanced microbial colonization of litter, physical abrasion of litter surfaces, and buffering of litter and its associated decomposers from high temperatures and low moisture conditions. Here, we tested how soil-litter mixing and soil moisture interact to influence rates of litter decomposition in a controlled environment. Foliar litter of two plant species (a grass [. Eragrostis lehmanniana] and a shrub [. Prosopis velutina]) was incubated for 32 weeks in a factorial combination of soil-litter mixing (none, light, and complete) and soil water content (2, 4, 12% water-filled porosity) treatments. Phospholipid fatty acids (PLFAs) were quantified one week into the experiment to evaluate initial microbial colonization. A complementary incubation experiment with simulated rainfall pulses tested the buffering effects of soil-litter mixing on decomposition.Under the laboratory conditions of our experiments, the influence of soil-litter mixing was minimal and primarily confined to changes in PLFAs during the initial stages of decomposition in the constant soil moisture experiment and the oscillating soil moisture conditions of the rainfall pulse experiment. Soil-litter mixing effects on CO2 production, total phospholipid concentrations, and bacterial to total PLFA ratios were observed within the first week, but responses were fairly weak and varied with litter type and soil moisture treatment. Across the entire 32-week incubation experiment, soil moisture had a significant positive effect on mass loss, but soil-litter mixing did not. The lack of strong soil-litter mixing effects on decomposition under the moderate and relatively constant environmental conditions of this study is in contrast to results from field studies and suggests the importance of soil-litter mixing may be magnified when the fluctuations and extremes in temperature, radiant energy and moisture regimes common dryland field settings are in play. © 2014 Published by Elsevier Ltd.
- Woods, S. R., Archer, S. R., & Schwinning, S. (2014). Seedling responses to water pulses in shrubs with contrasting histories of grassland encroachment. PLoS ONE, 9, e87278.More infodoi:10.1371/journal.pone.0087278
- Bai, E., Boutton, T. W., Liu, F., Wu, X. B., & Archer, S. R. (2013). 15N isoscapes in a subtropical savanna parkland: spatial-temporal perspectives. Ecosphere, 4(1), 4.
- Hewins, D. B., Archer, S. R., Okin, G. S., McCulley, R. L., & Throop, H. L. (2013). Soil-Litter Mixing Accelerates Decomposition in a Chihuahuan Desert Grassland. Ecosystems, 16(2), 183-195.More infoAbstract: Decomposition models typically under-predict decomposition relative to observed rates in drylands. This discrepancy indicates a significant gap in our mechanistic understanding of carbon and nutrient cycling in these systems. Recent research suggests that certain drivers of decomposition that are often not explicitly incorporated into models (for example, photodegradation and soil-litter mixing; SLM) may be important in drylands, and their exclusion may, in part, be responsible for model under-predictions. To assess the role of SLM, litterbags were deployed in the Chihuahuan Desert and interrelationships between vegetation structure, SLM, and rates of decomposition were quantified. Vegetation structure was manipulated to simulate losses of grass cover from livestock grazing and shrub encroachment. We hypothesized that reductions in grass cover would promote SLM and accelerate mass loss by improving conditions for microbial decomposition. Litter mass decreased exponentially, with the greatest losses occurring in concert with summer monsoons. There were no differences in decay constants among grass cover treatments. A significant, positive relationship between mass loss and SLM was observed, but contrary to expectations SLM was independent of grass cover. This suggests that processes operating at finer spatial scales than those in our grass removal treatments were influencing SLM. Shifts in litter lipid composition suggest increased bacterial contribution to decomposition through time. SLM, which is seldom included as a variable controlling decomposition in statistical or mechanistic models, was a strong driver of decomposition. Results are discussed in the context of other known drivers of decomposition in drylands (for example, UV radiation and climate) and more mesic systems. © 2012 Springer Science+Business Media New York.
- Liu, F., Archer, S. R., Gelwick, F., Bai, E., Boutton, T. W., & Wu, X. B. (2013). Woody plant encroachment into grasslands: Spatial patterns of functional group distribution and community development. PLoS ONE, 8(12).More infoAbstract: Woody plant encroachment into grasslands has been globally widespread. The woody species invading grasslands represent a variety of contrasting plant functional groups and growth forms. Are some woody plant functional types (PFTs) better suited to invade grasslands than others? To what extent do local patterns of distribution and abundance of woody PFTs invading grasslands reflect intrinsic topoedaphic properties versus plant-induced changes in soil properties? We addressed these questions in the Southern Great Plains, United States at a subtropical grassland known to have been encroached upon by woody species over the past 50-100 years. A total of 20 woody species (9 tree-statured; 11 shrub-statured) were encountered along a transect extending from an upland into a playa basin. About half of the encroaching woody plants were potential N2-fixers (55% of species), but they contributed only 7% to 16% of the total basal area. Most species and the PFTs they represent were ubiquitously distributed along the topoedaphic gradient, but with varying abundances. Overstory-understory comparisons suggest that while future species composition of these woody communities is likely to change, PFT composition is not. Canonical correspondence analysis (CCA) ordination and variance partitioning (Partial CCA) indicated that woody species and PFT composition in developing woody communities was primarily influenced by intrinsic landscape location variables (e.g., soil texture) and secondarily by plant-induced changes in soil organic carbon and total nitrogen content. The ubiquitous distribution of species and PFTs suggests that woody plants are generally well-suited to a broad range of grassland topoedaphic settings. However, here we only examined categorical and non-quantitative functional traits. Although intrinsic soil properties exerted more control over the floristics of grassland-to-woodland succession did plant modifications of soil carbon and nitrogen concentrations, the latter are likely to influence productivity and nutrient cycling and may, over longer time-frames, feed back to influence PFT distributions. © 2013 Liu et al.
- Liu, F., Archer, S. R., Gelwick, F., Bai, E., Boutton, T. W., & Wu, X. B. (2013). Woody plant encroachment into grasslands: spatial patterns of functional group distribution and community development. PLoS ONE, 12(8), e84364.doi:10.1371/journal.pone.0084364.
- Mcclaran, M. P., McMurtry, C., & Archer, S. R. (2013). A tool for estimating impacts of woody encroachment in arid grasslands: allometric equations for biomass, carbon and nitrogen content in Prosopis velutina. J Arid Environments, 88, 39-42.
- Mcclaran, M. P., Mcclaran, M. P., McMurtry, C., McMurtry, C., Archer, S. R., & Archer, S. R. (2013). A tool for estimating impacts of woody encroachment in arid grasslands: allometric equations for biomass, carbon and nitrogen content in Prosopis velutina.. Journal of Arid Environments, 88, 39-42.
- Bai, E., Boutton, T. W., Liu, F., Wu, X. B., & Archer, S. R. (2012). Spatial patterns of soil δ 13C reveal grassland-to-woodland successional processes. Organic Geochemistry, 42(12), 1512-1518.More infoAbstract: Many grasslands and savannas around the world have experienced woody plant encroachment in recent history. In the Rio Grande Plains of southern Texas, subtropical woodlands dominated by C 3 trees and shrubs have become significant components of landscapes once dominated almost exclusively by C 4 grasslands. In this study, spatial variation of soil δ 13C to was used to investigate patterns of transformation. Previous research has shown that grassland-to-shrubland transitions are initiated when discrete, multi-species shrub clusters organized around a honey mesquite (Prosopis glandulosa) tree nucleus established in grassland. It is inferred from space-for-time substitution and modeling studies that as new shrub clusters are initiated and existing clusters enlarge, coalescence will occur, leading to the formation of groves; and that groves will eventually merge to form woodlands. The hypothesis that present-day mesquite groves represent areas where individual discrete shrub clusters have proliferated and coalesced was evaluated by comparing patterns of soil δ 13C within isolated shrub clusters (n=6) to those in nearby groves (n=3). Mean soil δ 13C within discrete clusters was lowest in the center (-23.3‰), increased exponentially toward the dripline (-20.1‰), and stabilized at a relatively high value approximately 15cm beyond the dripline (-18.9‰). The spatial structure of soil δ 13C in groves was consistent with that which would be expected to occur if present-day grove communities were a collection of what once were individual discrete clusters that had fused. As such, it provides direct evidence in support of conceptual and mathematical models derived from indirect assessments. However, spatial patterns of soil δ 13C suggest that groves are not simply a collection of clusters with respect to primary production and SOC turnover. This study illustrates how soil δ 13C values can be used to reconstruct successional processes accompanying vegetation compositional change, and its consequences for ecosystem function. © 2010 Elsevier Ltd.
- Bai, E., Boutton, T. W., Liu, F., Wu, X. B., Hallmark, C. T., & Archer, S. R. (2012). Spatial variation of soil δ13C and its relation to carbon input and soil texture in a subtropical lowland woodland. Soil Biology and Biochemistry, 44(1), 102-112.More infoAbstract: Spatial patterns of soil δ13C were quantified in a subtropical C3 woodland in the Rio Grande Plains of southern Texas, USA that developed during the past 100 yrs on a lowland site that was once C4 grassland. A 50 × 30 m plot and two transects were established, and soil cores (0-15 cm, n = 207) were collected, spatially referenced, and analyzed for δ13C, soil organic carbon (SOC), and soil particle size distribution. Cross-variogram analysis indicated that SOC remaining from the past C4 grassland community co-varied with soil texture over a distance of 23.7 m. In contrast, newer SOC derived from C3 woody plants was spatially correlated with root biomass within a range of 7.1 m. Although mesquite trees initiate grassland-to-woodland succession and create well-defined islands of soil modification in adjoining upland areas at this site, direct gradient and proximity analyses accounting for the number, size, and distance of mesquite plants in the vicinity of soil sample points failed to reveal any relationship between mesquite tree abundance and soil properties. Variogram analysis further indicated soil δ13C, texture and organic carbon content were spatially autocorrelated over distances (ranges = 15.6, 16.2 and 18.7 m, respectively) far greater than that of individual tree canopy diameters in these lowland communities. Cross-variogram analysis also revealed that δ13C - SOC and δ13C-texture relationships were spatially structured at distances much greater than that of mesquite canopies (range = 17.6 and 16.5 m, respectively). These results suggest fundamental differences in the functional nature and consequences of shrub encroachment between upland and lowland landscapes and challenge us to identify the earth system processes and ecosystem structures that are driving carbon cycling at these contrasting scales. Improvements in our understanding how controls over soil carbon cycling change with spatial scale will enhance our ability to design vegetation and soil sampling schemes; and to more effectively use soil δ13C as a tool to infer vegetation and soil organic carbon dynamics in ecosystems where C3-C4 transitions and changes in structure and function are occurring. © 2011 Elsevier Ltd.
- Barnes, P. W., Throop, H. L., Hewins, D. B., Abbene, M. L., & Archer, S. R. (2012). Soil Coverage Reduces Photodegradation and Promotes the Development of Soil-Microbial Films on Dryland Leaf Litter. Ecosystems, 15(2), 311-321.More infoAbstract: Litter decomposition is a central focus of ecosystem science because of its importance to biogeochemical pools and cycling, but predicting dryland decomposition dynamics is problematic. Some studies indicate photodegradation by ultraviolet (UV) radiation can be a significant driver of dryland decomposition, whereas others suggest soil-litter mixing controls decomposition. To test the influence of soil coverage on UV photodegradation of litter, we conducted a controlled environment experiment with shrub (Prosopis velutina) leaf litter experiencing two UV levels and three levels of coverage with dry sterile soil. Under these conditions, decomposition over 224 days was enhanced by UV, but increasing soil coverage strongly and linearly diminished these effects. In a complementary study, we placed P. glandulosa leaf litter in different habitats in the field and quantified litter surface coverage by soil films. After 180 days, nearly half of the surface area of litter placed under shrub canopies was covered by a tightly adhering film composed of soil particles and fungal hyphae; coverage was less in grassy zones between shrubs. We propose a conceptual model for the shifting importance of photodegradation and microbial decomposition over time, and conclude that (1) soil deposition can ameliorate the direct effects of UV photodegradation in drylands and (2) predictions of C losses based solely on UV effects will overestimate the importance of this process in the C cycle. An improved understanding of how development of the soil-litter matrix mediates the shift from abiotic (photodegradation) to biotic (microbial) drivers is necessary to predict how ongoing changes in land cover and climate will influence biogeochemistry in globally extensive drylands. © 2011 Springer Science+Business Media, LLC.
- Throop, H. L., Archer, S. R., Monger, H. C., & Waltman, S. (2012). When bulk density methods matter: Implications for estimating soil organic carbon pools in rocky soils. Journal of Arid Environments, 77(1), 66-71.More infoAbstract: Resolving uncertainty in the carbon cycle is paramount to refining climate predictions. Soil organic carbon (SOC) is a major component of terrestrial C pools, and accuracy of SOC estimates are only as good as the measurements and assumptions used to obtain them. Dryland soils account for a substantial portion of global SOC, but the pool dynamics are highly uncertain. One crucial component of accurate estimates of SOC on an areal basis is bulk density (ρb), the mass of soil per unit volume. Here, we review methods used for calculating ρb and assess their prevalence. We show how treatment of coarse fragments (particles >2 mm diameter) influences ρb values and discuss the implications for SOC estimates in drylands. In four dryland examples, methods that varied in their treatment of coarse fragments led to substantial (up to 26%) differences in ρb. Calculated SOC pools responded proportionally, with SOC differing by up to 518 g C m-2. We suggest a revised method for accounting for coarse fractions in ρb calculations. A large portion of the world's soils, particularly in drylands, are fine enough to allow ρb determination with cores, but contain coarse fragments that substantially impact SOC mass estimates if not explicitly considered. © 2011 Elsevier Ltd.
- Throop, H. L., Reichmann, L. G., Sala, O. E., & Archer, S. R. (2012). Response of dominant grass and shrub species to water manipulation: An ecophysiological basis for shrub invasion in a Chihuahuan Desert Grassland. Oecologia, 169(2), 373-383.More infoPMID: 22159870;Abstract: Increases in woody vegetation and declines in grasses in arid and semi-arid ecosystems have occurred globally since the 1800s, but the mechanisms driving this major land-cover change remain uncertain and controversial. Working in a shrub-encroached grassland in the northern Chihuahuan Desert where grasses and shrubs typically differ in leaf-level nitrogen allocation, photosynthetic pathway, and root distribution, we asked if differences in leaf-level ecophysiology could help explain shrub proliferation. We predicted that the relative performance of grasses and shrubs would vary with soil moisture due to the different morphological and physiological characteristics of the two life-forms. In a 2-year experiment with ambient, reduced, and enhanced precipitation during the monsoon season, respectively, the encroaching C3 shrub (honey mesquite Prosopis glandulosa) consistently and substantially outperformed the historically dominant C4 grass (black grama Bouteloua eriopoda) in terms of photosynthetic rates while also maintaining a more favorable leaf water status. These differences persisted across a wide range of soil moisture conditions, across which mesquite photosynthesis was decoupled from leaf water status and moisture in the upper 50 cm of the soil profile. Mesquite's ability to maintain physiologically active leaves for a greater fraction of the growing season than black grama potentially amplifies and extends the importance of physiological differences. These physiological and phenological differences may help account for grass displacement by shrubs in drylands. Furthermore, the greater sensitivity of the grass to low soil moisture suggests that grasslands may be increasingly susceptible to shrub encroachment in the face of the predicted increases in drought intensity and frequency in the desert of the southwestern USA. © 2011 Springer-Verlag.
- Barger, N. N., Archer, S. R., Campbell, J. L., Huang, C., Morton, J. A., & Knapp, A. K. (2011). Woody plant proliferation in North American drylands: A synthesis of impacts on ecosystem carbon balance. Journal of Geophysical Research G: Biogeosciences, 116(3).More infoAbstract: Changes in the magnitude and direction of ecosystem carbon (C) balance accompanying woody plant encroachment are among the largest contributors to the uncertainty in the North American C budget. In this synthesis we identify the important species contributing to woody encroachment, summarize our current knowledge of aboveground and belowground C storage change with woody encroachment, and evaluate the range of human and natural disturbance factors that alter the course of C gains and losses within ecosystems experiencing woody encroachment. Available data indicate that relative to the historic vegetation, aboveground net primary production (ANPP) decreases with woody plant encroachment in arid regions (mean annual precipitation (MAP) < 336 mm), but increases in semiarid and subhumid regions (on the order of 0.7 g C m -2 yr -1 per mm of MAP over 336 mm). Soil organic carbon response to woody plant encroachment ranged from losses of 6200 g C m -2 to gains of 2700 g C m -2 with an average accumulation of 385 g C m -2 across all studies and did not appear to be closely coupled to ANPP. Taken together, in the absence of disturbance, woody encroachment appears to result in a net ecosystem C gain across most species and ecoregions. However, disturbance associated with wildfire, land management practices, and drought may quickly and significantly offset these gains and should be explicitly factored into regional-scale C balance estimates. Our findings may be used to better constrain future estimates of woody plant encroachment influences on the North American C budget. Copyright 2011 by the American Geophysical Union.
- Bestelmeyer, B. T., Goolsby, D. P., & Archer, S. R. (2011). Spatial perspectives in state-and-transition models: A missing link to land management?. Journal of Applied Ecology, 48(3), 746-757.More infoAbstract: 1. State-and-transition models (STMs) synthesize and communicate knowledge about the alternative states of an ecosystem and causes of state transitions. Data supported narrative descriptions within STMs are used to select or justify management actions. State transitions are characteristically heterogeneous in space and time, but spatial heterogeneity is seldom described in STMs, thereby limiting their utility. 2. We conducted a review that indicates how spatially explicit data can be used to improve STMs. We first identified three spatial scales at which spatial patterns and processes are manifest: patches, sites and landscapes. We then identified three classes of spatial processes that govern heterogeneity in state transitions at each scale and that can be considered in empirical studies, STM narratives and management interpretations. 3. First, spatial variations in land-use driver history (e.g. grazing use) can explain differences in the occurrence of state transitions within land areas that are otherwise uniform. Secondly, spatial dependence in response to drivers imposed by variations in soils, landforms and climate can explain how the likelihood of state transition varies along relatively static environmental gradients. Thirdly, state transition processes can be contagious, under control of vegetation-environment feedbacks, such that the spatiotemporal evolution of state transitions is predictable. 4. We suggest a strategy for considering each of the three spatial processes in the development of STM narratives. We illustrate how spatial data can be employed for describing early warning indicators of state transition, identifying areas that are most susceptible to state transitions, and designing and implementing monitoring schemes. 5. Synthesis and applications. State-and-transition models are increasingly important tools for guiding land-management activities. However, failure to adequately represent spatial processes in STMs can limit their ability to identify the initiation, risk and causes of state transitions and, therefore, the appropriate management responses. We suggest that multi-scaled studies targeted to different kinds of ecosystems can be used to uncover evidence of spatial processes. Such evidence should be included in STM narratives and can lead to novel interpretations of land change and improved management. © 2011 British Ecological Society.
- Browning, D. M., & Archer, S. R. (2011). Protection from livestock fails to deter shrub proliferation in a desert landscape with a history of heavy grazing. Ecological Applications, 21(5), 1629-1642.More infoPMID: 21830707;Abstract: Desertification is often characterized by the replacement of mesophytic grasses with xerophytic shrubs. Livestock grazing is considered a key driver of shrub encroachment, although most evidence is anecdotal or confounded by other factors. Mapping of velvet mesquite (Prosopis velutina) shrubs in and out of exclosures in 1932, 1948, and 2006 in semiarid grasslands of southeastern Arizona, USA, afforded the opportunity to quantify livestock grazing effects on mesquite proliferation over 74 years in the absence of fire to test the widespread assumption that livestock grazing promotes shrub proliferation. In 1932, shrub cover, density, and aboveground biomass were compared on grazed (12%, 173 plants/ha, 4182 kg/ha) and newly protected areas (8%, 203 plants/ha, 3119 kg/ha). By 1948, cover on both areas increased to ∼18%; yet, density on the protected area increased 300% (to 620 plants/ha), nearly twice that of the grazed area (325 plants/ha). From 1932 to 1948, differences in recruitment of new plants and growth of existing plants were reflected in biomass, which was higher on the protected area (415 plants/ha, 8788 kg/ha) relative to the grazed area (155 plants/ha, 7085 kg/ha), although mortality was equally low (∼0.06%). In 2006, 42 years after an herbicide application reset mesquite cover to ∼10% on both areas, aboveground mesquite mass was comparable on both areas (∼4700 kg/ha), but cover and density on the protected area (22%, 960 plants/ha) exceeded that on the grazed area (15%, 433 plants/ha). Mesquite mass in 2006 was substantially below 1948 levels, so continued accrual is likely. That shrub recovery from herbicides on a biomass basis was much less than recovery on a cover basis suggests that remotely sensed biomass estimates should integrate land management history. Contrary to widely held assumptions, protection from livestock since 1932 not only failed to deter woody-plant proliferation, but actually promoted it relative to grazed areas. Results suggest (1) that thresholds for grassland resistance to shrub encroachment had been crossed by the 1930s, and (2) fire management rather than grazing management may be key to maintaining grassland physiognomy in this bioclimatic region. © 2011 by the Ecological Society of America.
- Liu, F., Wu, X. B., Bai, E., Boutton, T. W., & Archer, S. R. (2011). Quantifying soil organic carbon in complex landscapes: An example of grassland undergoing encroachment of woody plants. Global Change Biology, 17(2), 1119-1129.More infoAbstract: The invasion of woody plants into grass-dominated ecosystems has occurred worldwide during the past century with potentially significant impacts on soil organic carbon (SOC) storage, ecosystem carbon sequestration and global climate warming. To date, most studies of tree and shrub encroachment impacts on SOC have been conducted at small scales and results are equivocal. To quantify the effects of woody plant proliferation on SOC at broad spatial scales and to potentially resolve inconsistencies reported from studies conducted at fine spatial scales, information regarding spatial variability and uncertainty of SOC is essential. We used sequential indicator simulation (SIS) to quantify spatial uncertainty of SOC in a grassland undergoing shrub encroachment in the Southern Great Plains, USA. Results showed that both SOC pool size and its spatial uncertainty increased with the development of woody communities in grasslands. Higher uncertainty of SOC in new shrub-dominated communities may be the result of their relatively recent development, their more complex above- and belowground architecture, stronger within-community gradients, and a greater degree of faunal disturbance. Simulations of alternative sampling designs demonstrated the effects of spatial uncertainty on the accuracy of SOC estimates and enabled us to evaluate the efficiency of sampling strategies aimed at quantifying landscape-scale SOC pools. An approach combining stratified random sampling with unequal point densities and transect sampling of landscape elements exhibiting strong internal gradients yielded the best estimates. Complete random sampling was less effective and required much higher sampling densities. Results provide novel insights into spatial uncertainty of SOC and its effects on estimates of carbon sequestration in terrestrial ecosystem and suggest effective protocol for the estimating of soil attributes in landscapes with complex vegetation patterns. © 2010 Blackwell Publishing Ltd.
- Woods, S. R., Archer, S. R., & Schwinning, S. (2011). Early taproot development of a xeric shrub (Larrea tridentata) is optimized within a narrow range of soil moisture. Plant Ecology, 212(3), 507-517.More infoAbstract: Effects of watering amount and frequency on root biomass accumulation and taproot elongation were examined 16-17 days post-germination in seedlings of Larrea tridentata, a dominant shrub in North American hot deserts. Two experimental variables manipulated in a full factorial design greenhouse study were (i) number of "triggering" days: consecutive days (2, 3, 4 or 5) at the start of the experiment on which seedlings received 10 mm of water per day; and (ii) "post-trigger" watering frequency: 5 mm of water either daily or every other day. We hypothesized that taproot elongation would increase with greater numbers of triggering days, whereas higher post-trigger watering frequency would enhance root biomass development. Increasing the number of triggering days from two to four promoted taproot extension without affecting root biomass, and higher watering frequency in the post-trigger phase generally increased root biomass, as expected. Contrary to expectations, root biomass and taproot length were significantly reduced when daily watering followed five consecutive triggering days. Taproot length correlated with root biomass, but irrigation regime also had a biomass-independent effect: with either two or five triggering days, taproots were shorter than expected based on root biomass. Thus, both too little and too much water stymied taproot extension. In natural settings, the adverse response of taproots to too little or too much water could reduce seedling survivorship and restrict establishment to a narrow range of environmental conditions. © 2010 Springer Science+Business Media B.V.
- Liu, F., Wu, X. B., Bai, E., Boutton, T. W., & Archer, S. R. (2010). Spatial scaling of ecosystem C and N in a subtropical savanna landscape. Global Change Biology, 16(8), 2213-2223.More infoAbstract: Widely occurred woody encroachment in grass-dominated ecosystems has the potential to influence soil organic carbon (SOC) and total nitrogen (TN) pools at local, regional, and global scales. Evaluation of this potential requires assessment of both pool sizes and their spatial patterns. We quantified SOC and TN, their relationships with soil and vegetation attributes, and their spatial scaling along a catena (hill-slope) gradient in the southern Great Plains, USA where woody cover has increased substantially over the past 100 years. Quadrat variance analysis revealed spatial variation in SOC and TN at two scales. The larger scale variation (40-45 m) was approximately the distance between centers of woody plant communities and their adjoining herbaceous patches. The smaller scale variation (10 m) appeared to reflect the local influence of shrubs on SOC and TN. Litter, root biomass, shrub, and tree basal area (a proxy for plant age) exhibited not only similar spatial scales, but also strong correlations with SOC and TN, suggesting invasive woody plants alter both the storage and spatial scaling of SOC and TN through ecological processes related primarily to root turnover and, to a lesser extent litter production, as mediated by time of occupancy. Forb and grass biomass were not significantly correlated with SOC and TN suggesting that changes in herbaceous vegetation have not been the driving force for the observed changes in SOC and TN. Because SOC and TN varied at two scales, it would be inappropriate to estimate SOC and TN pools at broad scales by extrapolating from point sampling at fine scales. Sampling designs that capture variation at multiple scales are required to estimate SOC and TN pools at broader scales. Knowledge of spatial scaling and correlations will be necessary to design field sampling protocols to quantify the biogeochemical consequences of woody plant encroachment at broad scales. © 2009 Blackwell Publishing Ltd.
- Parker, A. F., Owens, P. R., Libohova, Z., Wu, X. B., Wilding, L. P., & Archer, S. R. (2010). Use of terrain attributes as a tool to explore the interaction of vertic soils and surface hydrology in South Texas playa wetland systems. Journal of Arid Environments, 74(11), 1487-1493.More infoAbstract: The objectives of this study were to assess the unique interface between geomorphology, hydric soils, surface hydrology, and plant ecology in playa landforms by: 1) characterizing playa soil properties; 2) quantifying playa microtopography; and 3) determine how watershed attributes dictate the potential for surface water accumulation following episodic precipitation events (tropical storms, hurricanes). Soils of 9 playa basins in the Rio Grande Plains of Texas, USA were analyzed for physical/chemical properties and their microtopography determined via transects. A DEM was used to calculate topographic wetness index (TWI) and evaluate the sizes of playa basins, the upland draining areas into each playa. There were no significant differences among playa soils. TWI showed the potential areas for surface water accumulation coinciding with playa location. TWI can be used as a tool to identify potential water accumulating areas. The soil, site characteristics, and weather conditions determine the duration of standing surface water. © 2010 Elsevier Ltd.
- Stokes, C. J., & Archer, S. R. (2010). Niche differentiation and neutral theory: An integrated perspective on shrub assemblages in a parkland savanna. Ecology, 91(4), 1152-1162.More infoPMID: 20462129;Abstract: Investigations of structure in ecological communities need to move beyond the dichotomy between niche and neutral theory to broader conceptual frameworks that accommodate both neutral stochastic and biological structuring processes in organizing species assemblages. We propose and test a framework that integrates niche and neutralassembly perspectives and determines their relative contributions in structuring diverse shrub species assemblages in a parkland savanna. Our approach proposes that stochastic dispersal processes initially govern the assemblage of species in discrete shrub clusters developing in grassland, but that community structure subsequently develops through the progressive action of first positive, then negative interactions among species. A comparison of observed patterns of occurrence and niche models for 12 shrub cluster species against neutral predictions revealed that neutral stochastic, island biogeographic processes accounted for most patterns of species occurrence. One species showed strong evidence of successional differentiation, whereas evidence of slight recruitment biases for five others was equivocal. Our results demonstrate the usefulness of an approach that accommodates contributions of both neutral and niche assembly rather than assuming either process alone is sufficient to account for community structure. Further development and testing of robust and falsifiable neutral theory will allow ecologists to critically evaluate the relative roles of niche differentiation and neutral, stochastic processes in structuring communities. © 2010 by the Ecological society of America.
- Bai, E., Boutton, T. W., Liu, F., Wu, X. B., Archer, S. R., & Hallmark, C. T. (2009). Spatial variation of the stable nitrogen isotope ratio of woody plants along a topoedaphic gradient in a subtropical savanna. Oecologia, 159(3), 493-503.More infoPMID: 19085012;Abstract: Variation in the stable N isotope ratio (δ15N) of plants and soils often reflects the influence of environment on the N cycle. We measured leaf δ15N and N concentration ([N]) on all individuals of Prosopis glandulosa (deciduous tree legume), Condalia hookeri (evergreen shrub), and Zanthoxylum fagara (evergreen shrub) present within a belt transect 308 m long × 12 m wide in a subtropical savanna ecosystem in southern Texas, USA in April and August 2005. Soil texture, gravimetric water content (GWC), total N and δ15N were also measured along the transect. At the landscape scale, leaf δ15N was negatively related to elevation for all the three species along this topoedaphic sequence. Changes in soil δ15N, total N, and GWC appeared to contribute to this spatial pattern of leaf δ15N. In lower portions of the landscape, greater soil N availability and GWC are associated with relatively high rates of both N mineralization and nitrification. Both soil δ15N and leaf [N] were positively correlated with leaf δ15N of non-N2 fixing plants. Leaf δ15N of P. glandulosa, an N2-fixing legume, did not correlate with leaf [N]; the δ15N of P. glandulosa's leaves were closer to atmospheric N2 and significantly lower than those of C. hookeri and Z. fagara. Additionally, at smaller spatial scales, a proximity index (which reflected the density and distance of surrounding P. glandulosa trees) was negatively correlated with leaf δ15N of C. hookeri and Z. fagara, indicating the N2-fixing P. glandulosa may be important to the N nutrition of nearby non-N2-fixing species. Our results indicate plant 15N natural abundance can reflect the extent of N retention and help us better understand N dynamics and plant-soil interactions at ecosystem and landscape scales. © 2008 Springer-Verlag.
- Bai, E., Boutton, T. W., Wu, X. B., Liu, F., & Archer, S. R. (2009). Landscape-Scale vegetation dynamics inferred from spatial spatterns of soil δ13C in a subtropical savanna parkland. Journal of Geophysical Research G: Biogeosciences, 114(1).More infoAbstract: Grasslands and savannas around the world have experienced woody plant . encroachment during the past 100 years, but we know little regarding the manner in which woody plants spread across the landscape. We used soil δ13C, aerial photography, and geostatistics to quantify patterns of woody encroachment in a 160 x 100 m georeferenced grid subdivided into 10 x 10 m cells in a savanna parkland landscape in southern Texas. δ13C contour maps revealed that centers of closed contours coincided with centers of woody patches, and that larger woody patches developed from smaller woody plant clusters that spread laterally and coalesced. Areas where woody patches were expanding into grassland were characterized by low densities of soil δ13C contour lines, and indicated the direction and extent of woody encroachment. Conversely, areas with high contour densities represented grassland-woodland boundaries that were temporally stable. Indeed, aerial photos from 1930, 1941, 1982, and 2003 confirmed that woody patches with low spatial variability in δ13C corresponded to areas where woody plants had encroached during the past 30-75 years. While aerial photos can only record vegetation cover at the photo acquisition time, kriged maps of soil δ13C allowed us to accurately reconstruct long-term temporal dynamics of woody plant encroachment into grassland. This approach can reliably reconstruct landscape-scale vegetation changes in areas where historical aerial photography or satellite imagery are unavailable and provides a strong spatial context for studies aimed at understanding the functional consequences of vegetation change. Copyright 2009 by the American Geophysical Union.
- Browning, D. M., Archer, S. R., & Byrne, A. T. (2009). Field validation of 1930s aerial photography: What are we missing?. Journal of Arid Environments, 73(9), 844-853.More infoAbstract: Aerial photography from the 1930s serves as the earliest synoptic depiction of vegetation cover. We generated a spatially explicit database of shrub (Prosopis velutina) stand structure within two 1.8 ha field plots established in 1932 to address two questions: (1) What are the detection limits of panchromatic 1936 aerial photography?, and (2) How do these influence P. velutina biomass estimates? Shrub polygons were manually digitized on 1936 imagery and linked to 1932 field measurements of P. velutina canopy area. Aboveground 1932 P. velutina biomass was estimated using a site-specific allometric relationship for field-measured canopy area. Shrub canopy detection limits on the 1936 imagery were comparable to those reported for contemporary imagery. Based on a conservative shrub size detection threshold of 3.8 m2, 5.8% of P. velutina biomass was missed. Spatial resolution (0.6 vs. 1.0 m) did not influence detection limits, but the overall accuracy of shrub cover estimates was greater on 1.0 m images. Presence of the sub-shrub Isocoma tenuisecta may also have significantly influenced estimates of P. velutina canopy area. These analyses illustrate the importance of standardizing aerial photo interpretation protocols, accounting for uncertainty estimating shrub biomass, and caution species-specific interpretations for historic aerial photography.
- Hatch, S. L., Tomas, M. A., & Archer, S. R. (2009). Bouteloua curtipendula var. tenuis (Poaceae: Cynodonteae), new to Texas and the United States and a key to B. curtipendula varieties. Journal of the Botanical Research Institute of Texas, 3(2), 965-967.More infoAbstract: The occurrence of Bouteloua curtipendula (Michx.) Torr. var. tenuis Gould & Kapadia is reported new to Texas and the United States. A key to B. curtipendula varieties in the U.S.A. is provided, along with digital photos of features distinguishing between them.
- Okin, G. S., D'Odorico, P., & Archer, S. R. (2009). Impact of feedbacks on Chihuahuan desert grasslands: Transience and metastability. Journal of Geophysical Research G: Biogeosciences, 114(1).More infoAbstract: A simplistic model of grass-shrub dynamics was used to investigate the role of grass demographic processes on grassland-shrubland dynamics when grasses are in competitive advantage over shrubs. The model suggests that a feedback between grass biomass and soil erosion may cause an abrupt transition to a shrubland state. The model explains how a simple change in either grass recruitment or grass mortality, presumably linked to climate change or grazing, could produce changes in Holocene flora and the conversion of grasslands to shrublands, which has been observed throughout the southwestern U.S. in the past 150 years. Copyright 2009 by the American Geophysical Union.
- Archer, S. R., & Predick, K. I. (2008). Climate change and ecosystems of the Southwestern United States. Rangelands, 30(3), 23-28.More infoAbstract: The Climate Assessment for the Southwest (CLIMAS) is a program by the University of Arizona that aims to study the effects of climate variability and longer-term climate change on human and natural systems in the Southwest United States. The program has already predicted that the southwestern United States will become drier and the transition to a more arid climate is already taking place. Other projections include fewer frost days, warmer temperatures, greater water demand by plants, animals and people, increased frequency of extreme weather events, warmer nights, declines in snow pack, an earlier spring snow melt, which will then reduce water supply, lengthen the dry season, create conditions for drought and insect outbreaks, and increase the frequency and intensity of wildfires.
- Bai, E., Boutton, T. W., Liu, F., Wu, X. B., & Archer, S. R. (2008). Variation in woody plant δ13C along a topoedaphic gradient in a subtropical savanna parkland. Oecologia, 156(3), 479-489.More infoPMID: 18327619;Abstract: δ13C values of C3 plants are indicators of plant carbon-water relations that integrate plant responses to environmental conditions. However, few studies have quantified spatial variation in plant δ13C at the landscape scale. We determined variation in leaf δ13C, leaf nitrogen per leaf area (Narea), and specific leaf area (SLA) in April and August 2005 for all individuals of three common woody species within a 308 × 12-m belt transect spanning an upland-lowland topoedaphic gradient in a subtropical savanna in southern Texas. Clay content, available soil moisture, and soil total N were all negatively correlated with elevation. The δ13C values of Prosopis glandulosa (deciduous N2-fixing tree legume), Condalia hookeri (evergreen shrub), and Zanthoxylum fagara (evergreen shrub) leaves increased 1-4‰ with decreasing elevation, with the δ13C value of P. glandulosa leaves being 1-3‰ higher than those of the two shrub species. Contrary to theory and results from previous studies, δ13C values were highest where soil water was most available, suggesting that some other variable was overriding or interacting with water availability. Leaf Narea was positively correlated with leaf δ13C of all species (p < 0.01) and appeared to exert the strongest control over δ13C along this topoedaphic gradient. Since leaf Narea is positively related to photosynthetic capacity, plants with high leaf Narea are likely to have low p I/p a ratios and therefore higher δ13C values, assuming stomatal conductance is constant. Specific leaf area was not correlated significantly with leaf δ13C. Following a progressive growing season drought in July/August, leaf δ13C decreased. The lower δ13C in August may reflect the accumulation of 13C-depleted epicuticular leaf wax. We suggest control of leaf δ13C along this topoedaphic gradient is mediated by leaf N area rather than by stomatal conductance limitations associated with water availability. © 2008 Springer-Verlag.
- Knapp, A. K., Briggs, J. M., Collins, S. L., Archer, S. R., Bret-Harte, M. S., Ewers, B. E., Peters, D. P., Young, D. R., Shaver, G. R., Pendall, E., & Cleary, M. B. (2008). Shrub encroachment in North American grasslands: Shifts in growth form dominance rapidly alters control of ecosystem carbon inputs. Global Change Biology, 14(3), 615-623.More infoAbstract: Shrub encroachment into grass-dominated biomes is occurring globally due to a variety of anthropogenic activities, but the consequences for carbon (C) inputs, storage and cycling remain unclear. We studied eight North American graminoid-dominated ecosystems invaded by shrubs, from arctic tundra to Atlantic coastal dunes, to quantify patterns and controls of C inputs via aboveground net primary production (ANPP). Across a fourfold range in mean annual precipitation (MAP), a key regulator of ecosystem C input at the continental scale, shrub invasion decreased ANPP in xeric sites, but dramatically increased ANPP (>1000 gm-2) at high MAP, where shrub patches maintained extraordinarily high leaf area. Concurrently, the relationship between MAP and ANPP shifted from being nonlinear in grasslands to linear in shrublands. Thus, relatively abrupt (
- Orradottir, B., Archer, S. R., Arnalds, O., Wilding, L. P., & Thurow, T. L. (2008). Infiltration in icelandic andisols: The role of vegetation and soil frost. Arctic, Antarctic, and Alpine Research, 40(2), 412-421.More infoAbstract: Soil frost formation, snow distribution, and winter/spring/summer terminal infiltration rates (TIRs) were quantified in Icelandic Andisols with contrasting vegetation cover types (grassland, spruce and birch woodland, lupine, and sparsely vegetated lava site). TIRs (mm h-1; determined with double-ring infiltrometers) were generally higher in unfrozen than in frozen soils (102-369 vs. 9-306, respectively in sandy soils; 28-94 vs. 3-72 in finer-textured soils) and differed between land cover types, being consistently highest in birch woodlands. TIR was an inverse function of soil frost depth. Lowest TIRs were associated with deep and dense soil frost, which formed in spruce woodland and grassland communities where snow depth was shallow. Results suggest conditions conducive to erosion by water are most likely to occur during winter warm spells and in spring in vegetation types where snow cover is low or ephemeral. Threefold increases in TIRs occurred one year after livestock grazing was discontinued, suggesting Andisols are hydrologically resilient where vegetation cover is relatively continuous and soil organic carbon content is high. © 2008 Regents of the University of Colorado.
- Simmons, M. T., Archer, S. R., Teague, W. R., & Ansley, R. J. (2008). Tree (Prosopis glandulosa) effects on grass growth: An experimental assessment of above- and belowground interactions in a temperate savanna. Journal of Arid Environments, 72(4), 314-325.More infoAbstract: Savanna trees can impose above- and belowground effects on the herbaceous layer by changing water, nutrients and microclimate. Proposed mechanisms governing savanna tree-on-grass interactions include: (1) improved fertility and structure of soils below tree crowns; (2) improved water relations of shaded plants; and (3) increased competition for light, soil moisture and nutrients. To assess the relative importance and outcome of these interacting positive and negative factors, we conducted a series of field experiments that altered the presence and absence of tree canopy and tree roots at locations both immediately under trees and in interstitial locations in a mesquite (Prosopis glandulosa) savanna. Basal area, tiller density and production of the dominant C3 grass, Nassella leucotricha, and herbaceous layer annual net primary production (ANPP) were quantified in 1998 and 1999. Annual rainfall during these 2 years was substantially below normal and most responses to treatments were neutral. However, a significant reduction in herbaceous ANPP, largely annual C3 grasses, indicated that belowground competition rather than facilitation was the mechanism controlling tree effects on grass in this savanna. Lower than average rainfall was a potentially overriding factor. Hence, it is possible that other tree-on-grass mechanisms might operate under average or above-average rainfall years. © 2007 Elsevier Ltd. All rights reserved.
- Throop, H. L., & Archer, S. R. (2008). Shrub (Prosopis velutina) encroachment in a semidesert grassland: Spatial-temporal changes in soil organic carbon and nitrogen pools. Global Change Biology, 14(10), 2420-2431.More infoAbstract: Recent trends of increasing woody vegetation in arid and semiarid ecosystems may contribute substantially to the North American C sink. There is considerable uncertainty, however, in the extent to which woody encroachment alters dryland soil organic carbon (SOC) and total nitrogen (TN) pools. To date, studies assessing SOC and TN response to woody plant proliferation have not explicitly assessed the variability caused by shrub age or size and subcanopy spatial gradients. These factors were quantified via spatially intensive soil sampling around Prosopis velutina shrubs in a semidesert grassland, using shrub size as a proxy for age. We found that bulk density increased with distance from the bole (P < 0.005) and decreased with increasing shrub size (P = 0.056), while both SOC and TN increased with shrub size and decreased with distance from the bole (P < 0.001 for both). Significant (and predictable) spatial variation in bulk density suggests that use of generic values would generate unreliable estimates of SOC and TN mass, and subcanopy SOC pools could be overestimated by nearly 30% if intercanopy bulk density values were applied to subcanopy sites. Predictive models based on field-documented spatial patterns were used to generate integrated estimates of under-shrub SOC and TN pools, and these were compared with results obtained by typical area-weighting protocols based on point samples obtained next to the bole or at a specified distance from the bole. Values obtained using traditional area-weighting approaches generally overestimated SOC pools relative to those obtained using the spatially integrated approach, the discrepancy increasing with increasing shrub size and proximity of the point sample to the bole. These discrepancies were observed at the individual plant scale and for landscapes populated by various shrub size classes. Results suggest that sampling aimed at quantifying shrub encroachment impacts on SOC and TN pools will require area-weighting algorithms that simultaneously account for shrub size (age) and subcanopy spatial patterns. © 2008 The Authors Journal compilation © 2008 Blackwell Publishing.
- McCulley, R. L., Boutton, T. W., & Archer, S. R. (2007). Soil respiration in a subtropical savanna parkland: Response to water additions. Soil Science Society of America Journal, 71(3), 820-828.More infoAbstract: Soil respiration (SR) is controlled by abiotic parameters (temperature, water availability) interacting with biotic characteristics of the vegetation (quantity and quality of litter inputs, root respiration) and the soil microbial community. Because SR is a major flux in the C cycle, it is important to understand how vegetation change may interact with predicted climate changes to alter SR and ecosystem C storage. We quantified the SR response to increased soil water availability in a subtropical savanna parkland in the Rio Grande Plains of southern Texas. Diel SR was measured monthly from July 1996 to August 1997 in control and irrigated plots located in grasslands and in three contrasting woody plant communities known to have developed on grassland during the past 100 yr. Irrigation increased SR in all plant community types. Soil respiration in woody communities was higher (12.7 g CO2 m -2 d-1 averaged across months and treatments) and more responsive (+103%) to increased water availability than grasslands (9.8 g CO2 m-2 d_1 and +48%, respectively). This SR pattern is probably the result of woody communities having greater soil microbial biomass, soil C pools, and root biomass than adjacent grasslands. Irrigation increased the sensitivity of SR to temperature (Q10 =1.6 and 2.6 for control and irrigated plots, respectively), but Q10 values were similar in woody communities and grasslands. Results suggest SR is water limited, that sensitivity of SR to soil moisture availability increases with increasing woody plant abundance, and that shifts from grass to shrub dominance may have little impact on SR response to the changes in temperature projected to accompany global warming. © Soil Science Society of America.
- Simmons, M. T., Archer, S. R., Ansley, R. J., & Teague, W. R. (2007). Grass effects on tree (Prosopis glandulosa) growth in a temperate savanna. Journal of Arid Environments, 69(2), 212-227.More infoAbstract: The majority of studies on woody-herbaceous interactions have focused on the effects of trees on grasses; relatively few have looked at grass effects on adult trees. In a two-year study in a temperate savanna in northern Texas, tree (Prosopis glandulosa) basal area increased significantly following removal of associated grasses, the response being highly variable in time and space. Tree response to grass removal was strongest on shallow, clayey soils, but was evident only during the year subsequent to treatment. Low intensity surface fire enhanced growth of adult trees, but the magnitude of the response was inconsistent among sites. Clipping of grasses (proxy for grazing) had no effect on tree growth. Contrasts between the clipping and removal experiments suggest clipping may not have been frequent or intense enough to elicit a tree response; or that it is the presence/absence of grasses rather than their aboveground biomass which impacts trees. Enhanced tree growth and foliar N and P content associated with disturbance to the grass layer on shallow sites where vertical segregation of woody plant and grass roots and soil resources are constrained, may reflect reductions in competition for soil moisture and elevated soil N and P availability following fire. © 2006 Elsevier Ltd. All rights reserved.
- Throop, H. L., & Archer, S. R. (2007). Interrelationships among shrub encroachment, land management, and litter decomposition in a semidesert grassland. Ecological Applications, 17(6), 1809-1823.More infoPMID: 17913142;Abstract: Encroachment of woody plants into grasslands, and subsequent brush management, are among the most prominent changes to occur in arid and semiarid systems over the past century. Despite the resulting widespread changes in landcover, substantial uncertainty about the biogeochemical impacts of woody proliferation and brush management exists. We explored the role of shrub encroachment and brush management on leaf litter decomposition in a semidesert grassland where velvet mesquite (Prosopis velutina) abundance has increased over the past 100 years. This change in physiognomy may affect decomposition directly, through altered litter quality or quantity, and indirectly through altered canopy structure. To assess the direct and indirect impacts of shrubs on decomposition, we quantified changes in mass, nitrogen, and carbon in litterbags deployed under mesquite canopies and in intercanopy zones. Litterbags contained foliage from mesquite and Lehmann lovegrass (Eragrostis lehmanniana), a widespread, nonnative grass in southern Arizona. To explore short- and long-term influences of brush management on the initial stages of decomposition, litterbags were deployed at sites where mesquite canopies were removed three weeks, 45 years, or 70 years prior to study initiation. Mesquite litter decomposed more rapidly than lovegrass, but negative indirect influences of mesquite canopies counteracted positive direct effects. Decomposition was positively correlated with soil infiltration into litterbags, which varied with microsite placement, and was lowest under canopies. Low under-canopy decomposition was ostensibly due to decreased soil movement associated with high under-canopy herbaceous biomass. Decomposition rates where canopies were removed three weeks prior to study initiation were comparable to those beneath intact canopies, suggesting that decomposition was driven by mesquite legacy effects on herbaceous cover-soil movement linkages. Decomposition rates where shrubs were removed 45 and 70 years prior to study initiation were comparable to intercanopy rates, suggesting that legacy effects persist less than 45 years. Accurate decomposition modeling has proved challenging in arid and semiarid systems but is critical to understanding biogeochemical responses to woody encroachment and brush management. Predicting brush-management effects on decomposition will require information on shrub-grass interactions and herbaceous biomass influences on soil movement at decadal timescales. Inclusion of microsite factors controlling soil accumulation on litter would improve the predictive capability of decomposition models. © 2007 by the Ecological Society of America.
- Wheeler, C. W., Archer, S. R., Asner, G. P., & McMurtry, C. R. (2007). Climatic/edaphic controls on soil carbon/nitrogen response to shrub encroachment in desert grassland. Ecological Applications, 17(7), 1911-1928.More infoPMID: 17974331;Abstract: The proliferation of woody plants in grasslands over the past 100+ years can alter carbon, nitrogen, and water cycles and influence land surface-atmosphere interactions. Although the majority of organic carbon in these ecosystems resides belowground, there is no consensus on how this change in land cover has affected soil organic carbon (SOC) and total nitrogen (TN) pools. The degree to which duration of woody plant occupation, climate, and edaphic conditions have mediated SOC and TN responses to changes in life-form composition are poorly understood. We addressed these issues at a desert grassland site in Arizona, USA, where the leguminous shrub velvet mesquite (Prosopis velutina) has proliferated along an elevation/precipitation/ temperature gradient and on contrasting soil morphologic surfaces. On sandy loam complexes of mid-Holocene origin, mean SOC and TN of soils in the grassland matrix increased ∼68% and ∼45%, respectively, with increasing elevation. Soil organic carbon pools were comparable and TN pools were ∼23% higher in Pleistocene-aged clay loam complexes co-occurring with Holocene-aged soils at the upper elevation/climatic zone. Across the site, belowground resources associated with large Prosopis plants were 21-154% (SOC) and 18-127% (TN) higher than those in the grassy matrix. The variance in SOC and TN pools accounted for by Prosopis stem size (a rough surrogate for time of site occupation) was highest at the low- and mid-elevation sites (69-74%) and lowest at the upper elevation site (32-38%). Soil δ15N values ranged from 5.5‰ to 6.7‰ across the soil/elevation zones but were comparable in herbaceous and shrub-impacted soils and exhibited a weak relationship with Prosopis basal stem diameter (r2, 0.1) and TN (r2, 0.08). The SOC δ13C values decreased linearly with increasing Prosopis basal diameter, suggesting that size and isotopic composition of the SOC pool is a function of time of Prosopis site occupation. Isotopic mixture models indicate that encroachment of C3 woody plants has also promoted SOC additions from C4 plant sources, indicative of long-term herbaceous facilitation. Grassy sites in contrasting soil/elevation combinations, initially highly distinctive in their SOC pool size and δ13C, appear to be converging on similar values following ∼100 years of woody plant proliferation. © 2007 by the Ecological Society of America.
- Herrick, J. E., Bestelmeyer, B. T., Archer, S., Tugel, A. J., & Brown, J. R. (2006). An integrated framework for science-based arid land management. Journal of Arid Environments, 65(2), 319-335.More infoAbstract: Science is frequently touted as the solution to dryland management problems, yet most management decisions are, by necessity, based primarily on expert knowledge and experience. This paper describes an integrated framework for organizing, synthesizing, and applying our growing understanding of aridland ecosystems using a flexible, multi-objective assessment, monitoring, and management approach. The framework is dual-purpose: (1) to coordinate the use of existing tools, resources, and diffuse knowledge, and (2) to facilitate the integration and application of new knowledge as it is developed. In particular, this framework must facilitate the integration of new knowledge about linkages among landscape units across scales. The framework includes five elements: (1) an ecological site-based approach for categorizing land based on soils and climate, (2) a repository for organizing existing data and knowledge about each ecological site, (3) conceptual models that organize information on the impacts of management and climate variability, and protocols for (4) assessing and (5) monitoring key ecosystem attributes fundamental to a variety of management objectives. Within this framework, basic and applied research are explicitly linked to management of arid and semi-arid ecosystems to more effectively articulate research questions and set research priorities. © 2005 Elsevier Ltd. All rights reserved.
- Hughes, R. F., Archer, S. R., Asner, G. P., Wessman, C. A., McMurtry, C., Nelson, J., & Ansley, R. J. (2006). Changes in aboveground primary production and carbon and nitrogen pools accompanying woody plant encroachment in a temperate savanna. Global Change Biology, 12(9), 1733-1747.More infoAbstract: When woody plant abundance increases in grasslands and savannas, a phenomenon widely observed worldwide, there is considerable uncertainty as to whether aboveground net primary productivity (ANPP) and ecosystem carbon (C) and nitrogen (N) pools increase, decrease, or remain the same. We estimated ANPP and C and N pools in aboveground vegetation and surface soils on shallow clay and clay loam soils undergoing encroachment by Prosopis glandulosa in the Southern Great Plains of the United States. Aboveground Prosopis C and N mass increased linearly, and ANPP increased logarithmically, with stand age on clay loam soils; on shallow clays, Prosopis C and N mass and ANPP all increased linearly with stand age. We found no evidence of an asymptote in trajectories of C and N accumulation or ANPP on either soil type even following 68 years of stand development. Production and accumulation rates were lower on shallow clay sites relative to clay loam sites, suggesting strong edaphic control of C and N accumulation associated with woody plant encroachment. Response of herbaceous C mass to Prosopis stand development also differed between soil types. Herbaceous C declined with increasing aboveground Prosopis C on clay loams, but increased with increasing Prosopis C on shallow clays. Total ANPP (Prosopis + herbaceous) of sites with the highest Prosopis basal area were 1.2 × and 4.0 × greater than those with the lowest Prosopis basal area on clay loam and shallow clay soils, respectively. Prosopis ANPP more than offset declines in herbaceous ANPP on clay loams and added to increased herbaceous ANPP on shallow clays. Although aboveground C and N pools increased substantially with Prosopis stand development, we found no corresponding change in surface soil C and N pools (0-10cm). Overall, our findings indicate that Prosopis stand development significantly increases ecosystem C and N storage/cycling, and the magnitude of these impacts varied with stand age, soil type and functional plant traits. © 2006 Blackwell Publishing Ltd.
- Newman, B. D., Wilcox, B. P., Archer, S. R., Breshears, D. D., Dahm, C. N., Duffy, C. J., McDowell, N. G., Phillips, F. M., Scanlon, B. R., & Vivoni, E. R. (2006). Ecohydrology of water-limited environments: A scientific vision. Water Resources Research, 42(6).More infoAbstract: Water-limited environments occupy about half of the Earth's land surface and contain some of the fastest growing population centers in the world. Scarcity or variable distributions of water and nutrients make these environments highly sensitive to change. Given the importance of water-limited environments and the impacts of increasing demands on water supplies and other natural resources, this paper highlights important societal problems and scientific challenges germane to these environments and presents a vision on how to accelerate progress. We argue that improvements in our fundamental understanding of the links between hydrological, biogeochemical, and ecological processes are needed, and the way to accomplish this is by fostering integrated, interdisciplinary approaches to problem solving and hypothesis testing through place-based science. Such an ecohydrological approach will create opportunities to develop new methodologies and ways of thinking about these complex environmental systems and help us improve forecasts of environmental change. Copyright 2006 by the American Geophysical Union.
- Dale, V., Archer, S., Chang, M., & Ojima, D. (2005). Ecological impacts and mitigation strategies for rural land management. Ecological Applications, 15(6), 1879-1892.More infoAbstract: Land-use change and land-management practices affect a variety of ecological processes. Land-use impacts on ecological processes include local extirpations, introductions of new species, changes in land-cover extent, changes in juxtaposition of landcover types, changes to disturbance regimes, changes in vegetation structure and composition, and effects on air, water, and light quality, and noise pollution. Effects of land-use changes on ecological processes are discussed with special reference to changes in rural environments. Our premise is that better understanding of ecological processes improves land management. Mitigation strategies are presented with respect to management of initial ecological conditions, of the changes themselves, and of the altered system. The paper focuses on proactive environmental management efforts and identifies key research issues as (1) quantifying land-use legacies, (2) determining conditions under which land use modifies impacts of other stressors, (3) identifying conditions under which deleterious impacts can be avoided, (4) understanding cumulative impacts of land-use change, (5) improving our understanding of how land use alters resistance or susceptibility to invasion and impacts of pollutants, (6) crafting socioeconomically reasonable incentives for restoring or reducing effects of land-use practices, and (7) accelerating the integration of social and ecological sciences © 2005 by the Ecological Society of America.
- Fensham, R. J., Fairfax, R. J., & Archer, S. R. (2005). Rainfall, land use and woody vegetation cover change in semi-arid Australian savanna. Journal of Ecology, 93(3), 596-606.More infoAbstract: 1. The relative roles of climate and management for driving changes in woody cover in savannas over the past century are the subject of active debate. Perspectives arising from short-term, small-scale, local experiments are rarely tested over larger scales and longer time frames. 2. Regression analysis and aerial photography were used to assess the relative importance of land-use history (fire and grazing), rainfall and initial woody cover (woody cover at the beginning of a sample period relative to the range of woody cover expressed within a land type) in accounting for rates of change in overstorey and understorey cover between the 1940s and 1990s in central Queensland, Australia. Analyses included 279 site-period combinations representing five semi-arid eucalypt savanna land-types within a 125 755 km2 region. 3. Fire and grazing variables provided no explanatory power. In general, relative rainfall (rainfall for a given period standardized against mean annual rainfall) was positively related and initial woody cover negatively related to rates of change in both the overstorey and the understorey. The interaction between rainfall and initial woody cover was significant, reflecting the fact that increases in cover coincided with low initial cover when rainfall is higher than average, whereas decreases in cover typically occurred with high initial cover, regardless of rainfall. 4. On average, overstorey and understorey cover increased over the second half of the 20th century. This pattern is consistent with the first half of the 20th century having more intense droughts and being drier overall than the relatively wet second half. 5. The findings highlight the primary importance of interactions between rainfall fluctuations and density dependence as determinants of large-scale, long-term woody plant cover dynamics in savannas subject to large rainfall excess and deficit over multiyear time-scales. © 2005 British Ecological Society.
- Jurena, P. N., & Archer, S. (2005). Response of two perennial grasses to root barriers and fissures. Journal of Arid Environments, 61(2), 185-192.More infoAbstract: Above- and belowground biomass of contrasting grass growth forms (Hilaria belangeri - shortgrass vs. Bouteloua curtipendula - mid-height grass) was quantified with respect to partial root barriers (at 35 cm soil depth) with artificial fissures. We hypothesized (a) growth would be greatest in the absence of a barrier; and (b) the shallow-rooted H. belangeri would be relatively less affected by barriers with fissures than B. curtipendula. Alternatively (c) B. curtipendula with its deeper root system would exploit fissures and access the resources below barriers better than H. belangeri. The parameters used to evaluate these hypotheses for plants grown in subirrigated 150 cm pots included above- and belowground biomass after 4 months and monthly gravimetric soil moisture at 20 cm intervals to 150 cm. Root barrier treatments had no affect on either species' above-ground growth. As expected, the mid-height grass produced more root biomass and was more deeply rooted. However, partial root barriers had no effect on total root biomass for either species. Although, some 'perching' of root biomass above the barrier occurred in both species, it had no discernable influence on the pattern of soil moisture depletion with depth. Thus, under the conditions of this experiment, the lack of a significant species×barrier interaction suggests the two growth forms were not differentially affected by partial root barriers. © 2004 Elsevier Ltd. All rights reserved.
- Northup, B. K., Zitzer, S. F., Archer, S., McMurtry, C. R., & Boutton, T. W. (2005). Above-ground biomass and carbon and nitrogen content of woody species in a subtropical thornscrub parkland. Journal of Arid Environments, 62(1), 23-43.More infoAbstract: Regression equations were developed to predict above-ground biomass, carbon and nitrogen content from stem and canopy dimensions for 10 shrub species common to subtropical thorn parklands of southern Texas. Projected canopy area yielded slightly more precise estimates of biomass and nutrient concentrations than the sum of stem basal diameters at the soil surface. All such equations were significant (p
- Wu, X. B., & Archer, S. R. (2005). Scale-dependent influence of topography-based hydrologic features on patterns of woody plant encroachment in savanna landscapes. Landscape Ecology, 20(6), 733-742.More infoAbstract: Rainfall in drylands is erratic. Topographic features of landscapes can dampen or amplify temporal variability by spatially influencing patterns of water loss and accumulation. The extent to which portions of a landscape may differentially capture or retain scarce water and nutrient resources is an important determinant of vegetation patterns, particularly with respect to the distribution of woody plants. We therefore hypothesized that historic changes in woody cover on landscapes experiencing similar climate and disturbance regimes would vary with catena-to-catena (hillslope-to-hillslope) variation in topography-based hydrologic features. We tested this hypothesis by comparing topographic wetness index (TWI) values on replicate landscapes where woody plant abundance has increased over the past 100 yr. These landscapes are characterized by savanna parklands on coarse-textured upland portions of catenas that grade (1-3% slopes) into closed-canopy woodlands on fine-textured (lowland) portions of catenas. TWI values for woody and herbaceous communities were comparable within uplands, suggesting factors unrelated to surface/subsurface hydrology determine patterns of woody cover in these catena locations. TWI values for upland savanna parklands were significantly lower than those of closed-canopy woodlands occupying catena footslopes. Furthermore, uplands adjoining historically static woodland boundaries had lower TWI values than those where woodland boundaries had moved upslope 2.1 m yr-1 from 1976 to 1995. Results suggest runoff-runon relationships influence patterns of woody plant cover and change at the catena scale and may override constraints imposed by soil texture. As a result, changes in woody cover potentially accompanying changes in disturbance regimes, climate or atmospheric chemistry are likely to be constrained by topoedaphic settings. Models of vegetation dynamics may therefore need to explicitly account for rainfall-topography-soil texture relationships and associated scale-dependent mechanisms to accurately predict rates and patterns of change in woody and herbaceous plant abundance. © Springer 2005.
- Zou, C. B., Barnes, P. W., Archer, S., & McMurtry, C. R. (2005). Soil moisture redistribution as a mechanism of facilitation in savanna tree-shrub clusters. Oecologia, 145(1), 32-40.More infoPMID: 15942764;Abstract: Plant-soil water relations were examined in the context of a selective removal study conducted in tree-shrub communities occupying different but contiguous soil types (small discrete clusters on shallow, duplex soils versus larger, extensive groves on deep, sandy soils) in a subtropical savanna parkland. We (1) tested for the occurrence of soil moisture redistribution by hydraulic lift (HL), (2) determined the influence of edaphic factors on HL, and (3) evaluated the significance of HL for overstory tree-understory shrub interactions. Diel cycling and nocturnal increases in soil water potential (Ψsoil), characteristic signatures of HL, occurred intermittently throughout an annual growth cycle in both communities over a range of moisture levels (Ψsoil=-0.5 to -6.0 MPa) but only when soils were distinctly stratified with depth (dry surface/wet deep soil layers). The magnitude of mean (±SE) diel fluctuations in Ψsoil (0.19±0.01 MPa) did not differ on the two community types, though HL occurred more frequently in groves (deep soils) than clusters (shallow soils). Selective removal of either Prosopis glandulosa overstory or mixed-species shrub understory reduced the frequency of HL, indicating that Prosopis and at least one other woody species was conducting HL. For Zanthoxylum fagara, a shallow-rooted understory shrub, Prosopis removal from clusters decreased leaf water potential (Ψleaf) and net CO2 exchange (A) during periods of HL. In contrast, overstory removal had neutral to positive effects on more deeply-rooted shrub species (Berberis trifoliolata and Condalia hookeri). Removal of the shrub understory in groves increased A in the overstory Prosopis. Results indicate the following: (a) HL is common but temporally dynamic in these savanna tree-shrub communities; (b) edaphic factors influencing the degree of overstory/understory development, rooting patterns and soil moisture distribution influence HL; (c) net interactions between overstory and understory elements in these woody patches can be positive, negative and neutral over an annual cycle, and (d) Prosopis-mediated HL is an important mechanism of faciliation for some, but not all, understory shrubs. © Springer-Verlag 2005.
- McCulley, R. L., Archer, S. R., Boutton, T. W., Hons, F. M., & Zuberer, D. A. (2004). Soil respiration and nutrient cycling in wooded communities developing in grassland. Ecology, 85(10), 2804-2817.More infoAbstract: Grasslands and savannas worldwide are experiencing increases in woody plant abundance. In the subtropical Rio Grande Plains of southern Texas and northern Mexico, this change in physiognomy typically results in soil C and N accumulation. The extent to which this accumulation is the result of increased C and N inputs vs. decreased losses is not known. To address this issue, we compared soil C and N pools, soil respiration, soil microbial biomass, and potential C and N mineralization and nitrification rates in remnant grassland communities and adjacent woody plant communities known to have developed on grassland within the past 100 years. Mean soil organic C (SOC) and total N pools in the upper 20 cm of the profile were 2X larger in wooded communities (3382 and 273 g/m2 for C and N, respectively) than in remnant grasslands (1737 and 150 g/m2). The larger pool sizes in the wooded communities supported higher annual soil respiration (SR; 745 vs. 611 g C·m -2·yr-1 for woodlands and grasslands, respectively) and greater soil microbial biomass C (444 vs. 311 mg C/kg soil), potential rates of N mineralization (0.9 vs. 0.6 mg N·kg-1·d -1) and nitrification (0.9 vs. 0.4 mg N·kg -1·d-1). However, despite higher SR rates, mean residence time of near-surface SOC in wooded communities (11 years) exceeded that of remnant grassland communities (6 years). The fact that increased fluxes of soil C and N were accompanied by increases in SOC and N pools and total SOC mean residence time suggests that shifts from grass to woody plant dominance have increased both labile and recalcitrant pools of SOC and total N, the latter to a greater extent than the former. Given the widespread increase in woody plant abundance in drylands in recent history, the observed net increase in soil C storage that potentially accompanies this change could have global implications for C and N cycling and the climate system.
- Asner, G. P., Archer, S., Hughes, R. F., Ansley, R. J., & Wessman, C. A. (2003). Net changes in regional woody vegetation cover and carbon storage in Texas drylands, 1937-1999. Global Change Biology, 9(3), 316-335.More infoAbstract: Although local increases in woody plant cover have been documented in arid and semiarid ecosystems worldwide, there have been few long-term, large-scale analyses of changes in woody plant cover and aboveground carbon (C) stocks. We used historical aerial photography, contemporary Landsat satellite data, field observations, and image analysis techniques to assess spatially specific changes in woody vegetation cover and aboveground C stocks between 1937 and 1999 in a 400-km2 region of northern Texas, USA. Changes in land cover were then related to topo-edaphic setting and historical land-use practices. Mechanical or chemical brush management occurred over much of the region in the 1940-1950s. Rangelands not targeted for brush management experienced woody cover increases of up to 500% in 63 years. Areas managed with herbicides, mechanical treatments or fire exhibited a wide range of woody cover changes relative to 1937 (-75% to + 280%), depending on soil type and time since last management action. At the integrated regional scale, there was a net 30% increase in woody plant cover over the 63-year period. Regional increases were greatest in riparian corridors (33%) and shallow clay uplands (26%) and least on upland clay loams (15%). Allometric relationships between canopy cover and aboveground biomass were used to estimate net above-ground C storage changes in upland (nonriparian) portions of regional landscapes. Carbon stocks increased from 380g Cm-2 in 1937 to 500 g Cm-2 in 1999, a 32% net increase across the 400 km2 region over the 63-year period. These plant C storage change estimates are highly conservative in that they did not include the substantial increases in woody plant cover observed within riparian landscape elements. Results are discussed in terms of implications for 'carbon accounting' and the global C cycle.
- Hibbard, K. A., Schimel, D. S., Archer, S., Ojima, D. S., & Parton, W. (2003). Grassland to woodland transitions: Integrating changes in landscape structure and biogeochemistry. Ecological Applications, 13(4), 911-926.More infoAbstract: In many of the world's drylands, human-induced alteration of grazing and fire regimes over the past century has promoted the replacement of grasses by woody vegetation. Here, we evaluate the magnitude of changes in plant and soil carbon and nitrogen pools in a subtropical landscape undergoing succession from grassland to thorn woodland in southern Texas. Our approach involved linking a process-based ecosystem model to a transition matrix model. Grass and forest production submodels of CENTURY were parameterized with field data collected from herbaceous and wooded landscape elements broadly representative of habitats in global savanna systems. The Markov (transition matrix) model simulated the displacement of grassland communities under land use practices typical of many modern grasslands and savannas (heavy livestock grazing; no fire) and climate events. The modeled landscape was initialized for pre-Anglo-European settlement grassland conditions and then subjected to heavy, continuous livestock grazing and elimination of fire beginning in the mid-1800s. Rates of woody plant encroachment were directed by the Markov model, and the consequences for net primary production and plant and soil C and N pools were tracked by CENTURY. Modeled output of plant and soil organic C were in good agreement with those quantified for present-day patch types, suggesting our reconstructions were reasonable. Results indicated that, in the absence of woody plant encroachment, heavy grazing and fire suppression would have reduced soil organic carbon mass in southern Texas grasslands 17% (clay loam lowlands) to 18% (sandy loam uplands) by the 1990s. Soil and plant carbon stocks in current (mid-1900s) Prosopis woodlands are estimated to exceed those of the pristine grasslands they have replaced by 1.3X and 10X, respectively. Our reconstructions thus suggest that an initial degradation phase induced by heavy livestock grazing was followed by a woody-plant-induced aggradation phase that is still in progress. Under climatic/atmospheric conditions of the past 100 years, future landscapes would equilibrate at soil and plant C densities that would be 3 X and 15-24X that of the pristine, presettlement grasslands they have replaced. Replacement of grasslands and savannas by woodlands in this bioclimatic region has thus resulted in significant and ongoing increases in landscape-scale ecosystem carbon stocks in a relatively short (∼100 years) period of time.
- House, J. I., Archer, S., Breshears, D. D., Scholes, R. J., Coughenour, M. B., Dodd, M. B., Gignoux, J., Hall, D. O., Hanan, N. P., Joffre, R., Roux, X. L., Ludwig, J. A., Menaut, J. -., Montes, R., Parton, W. J., Jose, J. S., Scanlan, J. C., Scurlock, J. M., Simioni, G., & Thorrold, B. (2003). Conundrums in mixed woody-herbaceous plant systems. Journal of Biogeography, 30(11), 1763-1777.More infoAbstract: Aims: To identify approaches to improve our understanding of, and predictive capability for, mixed tree-grass systems. Elucidation of the interactions, dynamics and determinants, and identification of robust generalizations that can be broadly applied to tree-grass systems would benefit ecological theory, modelling and land management. Methods: A series of workshops brought together scientific expertise to review theory, data availability, modelling approaches and key questions. Location: Ecosystems characterized by mixtures of herbaceous and woody plant life-forms, often termed 'savannas', range from open grasslands with few woody plants, to woodlands or forests with a grass layer. These ecosystems represent a substantial portion of the terrestrial biosphere, an important wildlife habitat, and a major resource for provision of livestock, fuel wood and other products. Results: Although many concepts and principles developed for grassland and forest systems are relevant to these dual life-form communities, the novel, complex, nonlinear behaviour of mixed tree-grass systems cannot be accounted for by simply studying or modelling woody and herbaceous components independently. A more robust understanding requires addressing three fundamental conundrums: (1) The 'treeness' conundrum. What controls the relative abundance of woody and herbaceous plants for a given set of conditions at given site? (2) The coexistence conundrum. How do the life-forms interact with each other? Is a given woody-herbaceous ratio dynamically stable and persistent under a particular set of conditions? (3) The net primary productivity (NPP) conundrum. How does NPP of the woody vegetation, the herbaceous vegetation, and the total ecosystem (woody + herbaceous) change with changes in the tree-grass ratio? Tests of the theory and conceptual models of determinants of mixed woody-herbaceous systems have been largely site- or region-specific and have seldom been broadly or quantitatively evaluated. Cross-site syntheses based on data and modelling are required to address the conundrums and identify emerging patterns, yet, there are very few data sets for which either biomass or NPP have been quantified for both the woody and the herbaceous components of tree-grass systems. Furthermore, there are few cross-site comparisons spanning the diverse array of woody-herbaceous mixtures. Hence, initial synthesis studies should focus on compiling and standardizing a global data base which could be (1) explored to ascertain if robust generalizations and consistent patterns exist; and (2) used to evaluate the performance of savanna simulation models over a range of woody-herbaceous mixtures. Savanna structure and productivity are the result of complex and dynamic interactions between climate, soils and disturbances, notably fire and herbivory. Such factors are difficult to isolate or experimentally manipulate in order to evaluate their impacts at spatial and temporal scales appropriate for assessing ecosystem dynamics. These factors can, however, be evaluated with simulation models. Existing savanna models vary markedly with respect to their conceptual approach, their data requirements and the extent to which they incorporate mechanistic processes. Model intercomparisons can elucidate those approaches most suitable for various research questions and management applications. Conclusion: Theoretical and conceptual advances could be achieved by considering a broad continuum of grass-shrub-tree combinations using data meta-analysis techniques and modelling.
- Jurena, P. N., & Archer, S. (2003). Woody plant establishment and spatial heterogeneity in grasslands. Ecology, 84(4), 907-919.More infoAbstract: Root exclusion experiments demonstrated the importance of belowground competition between grasses and Prosopis glandulosa (honey mesquite) during the critical seeding establishment phase of the woody plant life cycle. Belowground available volume accounted for 67% and 79% of the variance in first- and second-year Prosopis seedling growth and survival, respectively. Available volume in the vertical dimension was more important than that in the horizontal dimension. Trials spanned years with contrasting annual precipitation, suggesting that root competition occurs in years of near-average as well as below-average annual rainfall. Spatial heterogeneity in canopy gaps and belowground biomass was also quantified in a Schizachayriuim-Paspalum grassland matrix and evaluated with respect to Prosopis seedling establishment. Of the 100 grid points encountered in four grass stands, 62% were unoccupied; and 50% of these exceeded 80 cm2 (∼10 cm diameter). Gaps ≥ 10 cm in diameter were sufficient for successful Prosopis germination and survival after one (40%) and two (15%) growing seasons. Herbaceous root biomass was statistically comparable among stands, but point-specific biomass varied three orders of magnitude (3000 g/m2). In addition, root biomass was temporally variable, ranging from a mean (± 1 SE; g/m2) of 768 (86) in a year of below-average annual rainfall (1996; 721 mm) to 1108 (104) in a year of roughly average annual rainfall (1995; 1032 mm). It is often assumed that grasslands dominated by productive, late seral species will be resistant to woody plant encroachment; and that grazing, by reducing the ability of grasses to competitively exclude woody seedlings, makes grasslands susceptible to tree/shrub invasion. However, given the substantial annual variation in belowground biomass observed in this study, it seems reasonable that ungrazed grasslands may be more susceptible to woody plant encroachment in some years and more resistant in others. Furthermore, given the substantial spatial variation in aboveground gap area and belowground biomass, there may be numerous low-competition microsites for woody plant seedlings within ungrazed grasslands. A high degree of temporal and spatial variability in gap area and belowground biomass may therefore help explain successful establishment of woody seedlings in ungrazed or lightly grazed, late seral grasslands in the absence of fire. We found no correlation between aboveground structure and belowground biomass at scales of 1-10 m2. Thus, readily quantifiable attributes such as grass basal area or gap area could not be used to infer site susceptibility to woody plant seedling establishment.
- Nelson, J. A., Barnes, P. W., & Archer, S. (2002). Leaf demography and growth responses to altered resource availability in woody plants of contrasting leaf habit in a subtropical savanna. Plant Ecology, 160(2), 193-205.More infoAbstract: Leaf demography and growth of six common, co-occurring woody plant species that varied in stature (tree vs. shrub) and leaf texture (sclerophyllous, coriaceous, malacophyllous) were examined in a subtropical savanna parkland in southern Texas, USA. We tested the hypotheses that, (a) leaves of plants with evergreen canopies would have longer life spans than those of deciduous species; (b) supplementation of soil moisture would decrease leaf life span in both evergreen and deciduous species; (c) species responses to increased soil moisture availability would be inversely related to leaf longevity; and (d) deciduous growth forms would exhibit a greater growth response to increased soil moisture availability than their evergreen counterparts. A variety of seasonal leaf habits (evergreen, winter-deciduous and summer-deciduous canopies) and leaf life spans (median = 66 to 283 days) were represented by the targeted species, but there was no clear relationship between seasonal leaf habit and leaf longevity. Among species with evergreen canopies, median leaf longevity ranged from short (Zanthoxylum fagara = 116 days; Condalia hookeri = 158 days) to long (Berberis trifoliolata = 283 days) but did not exceed 1 yr. In fact, leaf longevity in evergreen shrubs was often comparable to, or shorter than, that of species with deciduous canopies (Ziziphus obtusifolia = 66 days; Diospyros texana = 119 days; Prosopis glandulosa = 207 days). Augmentation of surface soil moisture had no detectable effect on median leaf life span in any species and there was no clear relationship between leaf longevity and species growth responses to irrigation. Contrary to expectations, species with evergreen canopies responded to irrigation by producing more leaf biomass, longer shoots and more leaf cohorts/year than did deciduous species. Species differences in the annual cycle of leaf initiation, leaf longevity and canopy development, combined with contrasts in root distributions and a highly variable climate, may allow for spatial and temporal partitioning of resources and hence, woody species coexistence and diversity in this system. However, the lack of expected relationships between leaf longevity, leaf habit and plant responses to resource enhancement suggests that structure-function relationships and functional groupings developed in strongly seasonal environments cannot be applied with confidence to these subtropical savannas and thorn woodlands.
- Hibbard, K. A., Archer, S., Schimel, D. S., & Valentine, D. W. (2001). Biogeochemical changes accompanying woody plant encroachment in a subtropical savanna. Ecology, 82(7), 1999-2011.More infoAbstract: Ecosystem properties of surficial (0-10 cm) soils in remnant herbaceous patches were compared to those of contrasting woody plant patch types (upland discrete cluster, upland grove, and lowland woodland) where shifting land cover is known to have occurred over the past 50-77 yr. The purpose of this study was to evaluate and quantify the biogeochemical consequences and subsequent developmental rates of woody plant formation on sites formerly dominated by grasses. Clay and water content of woodland soil patches was higher than that of soils associated with upland discrete cluster and grove patches. Even so, lowland woody patches were generally comparable to upland grove and discrete shrub cluster patches with respect to soil organic carbon (SOC), soil N, the ratio of annual N mineralization:total N, annual litterfall, and root biomass. The fact that finer soil texture, enhanced soil moisture, and the more advanced age of lowland woody patches did not translate into greater accumulations of SOC and N relative to upland grove and discrete cluster patches suggests that C and N losses might be higher in recently developed lowland woodland, communities. Fluctuations in monthly root biomass standing crop (0-10 cm) far exceeded annual foliar litterfall in upland and lowland woody patch types, suggesting that belowground inputs of organic matter may drive changes in soil physical and chemical properties that occur subsequent to woody plant establishment. The estimated annual mean rates of soil C accretion in the "islands of fertility" that developed subsequent to tree/shrub encroachment were variable and ranged from 8 to 23 g/m2 (in groves and discrete clusters, respectively); N accretion ranged from 0.9 to 2.0 g/m2 (in groves and discrete clusters, respectively), even though mean annual N mineralization rates were three- to fivefold greater than those measured in remnant herbaceous patches. Woody plant proliferation in grasslands and savannas in recent history has been widely reported around the world. The causes for this shift in vegetation are controversial and center around changes in livestock grazing, fire, climate, and atmospheric CO2. Our data, which are conservative in that they examine only the upper 10 cm of the soil profile, indicate that the rate and extent of soil C and N accumulation associated with this phenomenon can be rapid, substantial, and accompanied by increased N turnover. This geographically extensive vegetation change thus has important implications for understanding how the global carbon and nitrogen cycles may have been altered since Anglo-European settlement of arid and semiarid regions.
- Miller, D., Archer, S. R., Zitzer, S. F., & Longnecker, M. T. (2001). Annual rainfall, topoedaphic heterogeneity and growth of an arid land tree (Prosopis glandulosa). Journal of Arid Environments, 48(1), 23-33.More infoAbstract: Basal area growth rates for Prosopis glandulosa were quantified for contrasting landscape elements in a subtropical savanna. We hypothesized that growth rates of P. glandulosa plants (1) are a function of seasonal or annual rainfall; and (2) vary with patch type and in the rank order observed for large trees. Prosopis glandulosa growth was responsive to changes in rainfall (-0.974 cm2 year-1 in a dry year; 9.108 in a wet year), but correlations with rainfall were low. Growth of trees in contrasting patches was not consistently different, suggesting that contrasts in tree sizes across the landscape reflect differences in establishment dates rather than differences in growth rates. © 2001 Acadamic Press.
- Stroh, J. C., Archer, S., Doolittle, J. A., & Wilding, L. (2001). Detection of edaphic discontinuities with ground-penetrating radar and electromagnetic induction. Landscape Ecology, 16(5), 377-390.More infoAbstract: Quantification of edaphic properties which may regulate the spatial distribution of vegetation is often limited by the expense and labor associated with collecting and analyzing soil samples. Here we evaluate the utility of two technologies, ground-penetrating radar (GPR) and electromagnetic induction (EMI), for rapid, extensive and non-destructive mapping of diagnostic subsurface features and soil series map unit boundaries. Strong reflectance from fine-textured, near-surface soils obscured radar signal reflectance from deeper horizons at our field test site in the Rio Grande Plains of southern Texas, USA. As a result, ground-penetrating radar did not delineate known edaphic contrasts along catena gradients. In contrast, EMI consistently distinguished boundaries of soil map units. In several instances, gradients or contrasting inclusions within map units were also identified. In addition, the location and boundary of calcic or cambic-horizon inclusions embedded within a laterally coextensive and well-developed argillic horizon were consistently predicted. Correlations between EMI assessments of apparent conductivity (ECa) and soil properties such as CEC, pH, particle size distribution and extractable bases were low (i.e., explained
- Barnes, P. W., & Archer, S. (1999). Tree-shrub interactions in a subtropical savanna parkland: Competition or facilitation?. Journal of Vegetation Science, 10(4), 525-536.More infoAbstract: Prosopis glandulosa var. glandulosa has played a central role in the encroachment of woody plants in southern Texas, grasslands and savannas by acting as a nurse plant for various shrubs that establish in its understory. To test for continued facilitation of established understory shrubs by Prosopis and to determine if established shrubs compete with the Prosopis nucleus, selective removal experiments were conducted and monitored over a 2 - 5 yr period. Short-term (1 - 3 days) and long-term (2 yr) growth and physiological activities (midday net photosynthesis and leaf/shoot water potential) of two common understory shrubs, Zanthoxylum fagara and Berberis trifoliolata, growing with Prosopis, were generally comparable to those of individuals occurring in clusters where Prosopis was removed. Shrubs growing with an intact Prosopis occasionally showed significantly higher leaf-[N] and pre-dawn water potentials than those in clusters lacking a live Prosopis, especially under drought conditions; however, these differences did not translate into greater midday leaf gas exchange or shoot growth. By comparison, removal of understory shrubs elicited large increases in Prosopis net photosynthesis, annual trunk growth in each of the 5 yr monitored, and seed pod production in three of the four years monitored. Seven of 26 Prosopis plants in experimental clusters with an intact understory died over a 5-yr period, compared to only two of the 26 plants in clusters with the cleared understory. Results indicate that (1) the founding overstory Prosopis plant may continue to facilitate understory shrubs following their establishment, but these beneficial effects appear to be small and transitory, and (2) the understory shrubs have a pronounced negative effect on Prosopis, such that competition between overstory and understory woody plants is strongly asymmetrical. These findings suggest that understory shrubs will likely persist despite changes in microclimate and soils (potentially) that occur after the Prosopis plant, which facilitated their ingress or establishment, has died. Soil resource depletion by shallow-rooted understory shrubs appears to be a primary factor contributing to the demise of the deeply rooted, overstory Prosopis plants, especially on upland sites with duplex soils where below-ground competition is accentuated.
- Boutton, T. W., Archer, S. R., & Midwood, A. J. (1999). Stable isotopes in ecosystem science: Structure, function and dynamics of a subtropical savanna. Rapid Communications in Mass Spectrometry, 13(13), 1263-1277.More infoPMID: 10407309;Abstract: Stable isotopes are often utilized as intrinsic tracers to study the effects of human land uses on the structural and functional characteristics of ecosystems. Here, we illustrate how stable isotopes of H, C, and O have been utilized to document changes in ecosystem structure and function using a case study from a subtropical savanna ecosystem. Specifically, we demonstrate that: (1) δ13C values of soil organic carbon record a vegetation change in this ecosystem from C4 grassland to C3 woodland during the past 40-120 years, and (2) δ2H and δ18O of plant and soil water reveal changes in ecosystem hydrology that accompanied this grassland-to-woodland transition. In the Rio Grande Plains of North America, δ13C values of plants and soils indicate that areas now dominated by C3 subtropical thorn woodland were once C4 grasslands. δ13C values of current organic matter inputs from wooded landscape elements in this region are characteristic of C3 plants (-28 to -25‰, while those of the associated soil organic carbon are higher and range from -20 to -15‰. Approximately 50-90% of soil carbon beneath the present C3 woodlands is derived from C4 grasses. A strong memory of the C4 grasslands that once dominated this region is retained by δ13C values of organic carbon associated with fine and coarse clay fractions. When δ13C values are evaluated in conjunction with 14C measurements of that same soil carbon, it appears that grassland-to-woodland conversion occurred largely within the past 40-120 years, coincident with the intensification of livestock grazing and reductions in fire frequency. These conclusions substantiate those based on demographic characteristics of the dominant tree species, historical aerial photography, and accounts of early settlers and explorers. Concurrent changes in soil δ13C values and organic carbon content over the past 90 years also indicate that wooded landscape elements are behaving as sinks for atmospheric CO2 by sequestering carbon derived from both the previous C4 grassland and the present C3 woody vegetation. Present day woodlands have hydrologic characteristics fundamentally different from those of the original grasslands. Compared to plants in remnant grasslands, tree and shrub species in the woodlands are rooted more deeply and have significantly greater root biomass and density than grasslands. δ18O and δ2H values of plant and soil water confirm that grassland species acquire soil water primarily from the upper 0.5 m of the soil profile. In contrast, trees and shrubs utilize soil water from throughout the upper 4 m of the profile. Thus, soil water that formerly may have infiltrated beyond the reach of the grassland roots and contributed to local groundwater recharge or other hydrologic fluxes may now be captured and transpired by the recently formed woodland plant communities. The natural abundances of stable isotopes revealed fundamental information regarding the impacts of human land use activities on the structure and function of this subtropical savanna. Stable isotopes provided direct, spatially explicit evidence for dramatic changes in ecosystem physiognomy and demonstrated some functional consequences for the hydrologic cycle. Furthermore, grassland-to-woodland conversion has been geographically extensive in the worlds' drylands, suggesting that these ecosystem-level changes in vegetation structure, carbon cycling, and hydrology may have implications for regional/global biogeochemistry and climate.
- Brown, J. R., & Archer, S. (1999). Shrub invasion of grassland: Recruitment is continuous and not regulated by herbaceous biomass or density. Ecology, 80(7), 2385-2396.More infoAbstract: Proliferation of woody plants in grasslands and savannas since 1800s has been widely documented. In the southwestern United States, increased abundance of honey mesquite (Prosopis glandulosa var. glandulosa) has been attributed to heavy grazing by livestock. Here, we test the hypothesis that P. glandulosa invasion of grasslands requires, first, reductions in herbaceous biomass and density such as those that accompany livestock grazing and, second, episodes of high soil moisture availability. No combination of grass density (nonmanipulated or reduced 50%) or defoliation (none, moderate, heavy) significantly affected P. glandulosa seedling emergence within a watering regime (natural and supplemented) at our field site in semiarid southern Texas. Seedling emergence on plots receiving only natural rainfall was high (42%), despite the fact that precipitation was substantially below normal. Supplemental watering, to generate moisture levels approximating years of unusually high annual rainfall, increased emergence to 59%. Seedling survival after 2 yr was high (62-77%) and statistically comparable across the density, defoliation, and watering treatments. Net photosynthesis (A(n)) of 1-yr-old seedlings was enhanced by supplemental watering, but reductions in grass density or biomass had little effect on seedling A(n) or xylem water potential. Height, aboveground biomass, and leaf area were comparable among 1- and 2-yr-old seedings across all density, defoliation, and watering combinations. High seedling Emergence and survival on unwatered plots, even during a 'drought year,' suggests that Prosopis recruitment is not contingent upon unusual or episodic rainfall. Reductions in biomass and density of herbaceous vegetation had no influence on seedling emergence, growth, or survival, suggesting that Prosopis invasion is minimally influenced by grass competition. Historic grazing at this site appears to have altered herbaceous composition and reduced above- and belowground biomass production below the threshold level required for competitive exclusion of woody vegetation. Such data suggest that rates and patterns of seed dispersal may be the primary determinants of P. glandulosa encroachment on present-day landscapes in semiarid regions. Minimizing livestock dispersal of seed (in the case of leguminous shrubs) and maintenance of an effective fire regime (through production of fine fuels) may be crucial for sustaining herbaceous composition and production in grazed systems prone to invasion by unpalatable woody plants.
- Guenther, A., Archer, S., Greenberg, J., Harley, P., Helmig, D., Klinger, L., Vierling, L., Wildermuth, M., Zimmerman, P., & Zitzer, S. (1999). Biogenic hydrocarbon emissions and landcover/climate change in a subtropical savanna. Physics and Chemistry of the Earth, Part B: Hydrology, Oceans and Atmosphere, 24(6), 659-667.More infoAbstract: Biogenic non-methane hydrocarbon (NMHC) emissions strongly influence the chemical composition of the troposphere. Thus, variations in emissions of these compounds are expected to cause changes in concentrations of important atmospheric trace gases. Here, we assess the relative magnitude of potential changes in NMHC (e.g., isoprene and monoterpene) emissions using field flux measurements from a subtropical savanna parkland/thorn woodland site in conjunction with model predictions of climate and landcover change. NMHC emissions of about 40 plant species were characterized. Grasses, as a group, had low emission rates. Several common woody species had high emission rates. However, there was little evidence of emissions being consistently related to woody plant taxonomy, growthform or functional groups. Above-canopy measurements were used to validate modeled isoprene flux predictions of about 2 mg C m-2 h-1 for savanna parkland/thorn woodland and ca. 0.7 mg C m-2 h-1 for the regional landscape, which is a mixture of savanna parkland/thorn woodland and cropland. Linkage of the biogenic emissions model with a plant succession model indicated that landcover change since the early 1800s has elicited a 3-fold increase in total NMHC emissions. This increase reflected changes in vegetation species composition (from domination by grasses which were typically 'low emitters', to shrubs and trees, many of which were 'high emitters') and increases in foliar density. Field measurements on two common shrub species indicated that isoprene emission increased exponentially with increases in leaf temperature from 20 to 40°C and were not suppressed by drought stress. Accordingly, our model predicted that projected increases in ambient temperature (3 to 6°C) emissions could produce a 2-fold increase in biogenic NMHC emissions. Cloud cover, precipitation, relative humidity, and winds also exerted some control over NMHC emissions, but their influence was highly variable and difficult to estimate. Although our results are specific to southern Texas USA, they indicate the magnitude of change in NMHC emissions that could occur at other locations when climate and vegetation composition are altered.
- Asner, G. P., Wessman, C. A., & Archer, S. (1998). Scale dependence of absorption of photosynthetically active radiation in terrestrial ecosystems. Ecological Applications, 8(4), 1003-1021.More infoAbstract: The fraction of photosynthetically active radiation absorbed by plant canopies (fAPAR) is a critical biophysical variable for extrapolating ecophysiological measurements from the leaf to landscape scale. Quantification of fAPAR determinants at the landscape level is needed to improve the interpretation of remote sensing data, to facilitate its use in constraining ecosystem process models, and to improve synoptic-scale links between carbon and nutrient cycles. Most canopy radiation budget studies have focused on light attenuation in plant canopies, with little regard for the importance of the scale-dependent biophysical and structural factors (e.g., leaf and stem optical properties, leaf and stem area, and extent of vegetation structural types) that ultimately determine fAPAR at canopy and landscape scales. Most studies have also assumed that nonphotosynthetic vegetation (litter and stems) contributes little to fAPAR. Using a combined field measurement and radiative transfer modeling approach, we quantified (a) the relative role of the leaf-, canopy-, and landscape-level factors that determine fAPAR in terrestrial ecosystems and (b) the magnitude of PAR absorption by grass litter and woody plant stems. Variability in full spectral-range (400-2500 nm) reflectance/transmittance and PAR (400-700 nm) absorption at the level of individual leaf, stem, and litter samples was quantified for a wide array of broadleaf arborescent and grass species along a 900-km north-south Texas savanna transect. Among woody growth forms, leaf reflectance and transmittance spectra were statistically comparable between populations, species within a genus, and functional types (deciduous vs. evergreen, legume vs. nonlegume). Within the grass life-form, spectral properties were statistically comparable between species and C3/C4 physiologies. We found that tissue-level PAR absorption among species, genera, functional groups, and growth forms and between climatologically diverse regions was statistically similar, and for fresh leaves, it represented the most spectrally similar region of the shortwave spectrum. Subsequent modeling analyses indicated that the measured range of leaf, woody stem, and litter optical properties explained only a small proportion of the variance in tree and grass canopy fAPAR. However, the presence of nonphotosynthetic vegetation (e.g., stem and litter) had a significant effect on canopy fAPAR. In trees with a leaf area index (LAI)
- Asner, G. P., Wessman, C. A., Schimel, D. S., & Archer, S. (1998). Variability in leaf and litter optical properties: Implications for BRDF model inversions using AVHRR, MODIS, and MISR. Remote Sensing of Environment, 63(3), 243-257.More infoAbstract: Canopy radiative transfer models simulate the bidirectional reflectance distribution function (BRDF) of vegetation covers with differing leaf and soil spectral and canopy structural characteristics. Numerical inversion of these models has provided estimates of vegetation structural and biophysical characteristics from multiangle, remotely sensed optical data. The number of angularly unique observations compared to BRDF model parameters largely determines the accuracy of retrievals. To increase this ratio, additional observations of a target must be acquired and the BRDF models and inversions must be simplified. The former will occur when the EOS instruments become available. Previous studies suggest that simplification of BRDF model inversions may best be accomplished by constraining the leaf optical parameters. This study focused on full-range (400-2500 nm) leaf and litter spectral properties convolved to AVHRR, MODIS, and MISR optical channels. Using a diverse array of woody plant and grass species, we found robust and readily usable interrelationships among spectra through correlation, regression, and principal components analyses. Significant differences between green leaf and litter optical properties and their sensor-specific interrelationships indicate that green leaf optical constraints may be useful with BRDF retrievals to detect the onset of canopy senescence. These findings will provide increased efficiency in canopy BRDF model inversions by decreasing the number of observations required to retrieve canopy structural and biophysical information from multiangle remotely sensed data.
- Boutton, T. W., Archer, S. R., Midwood, A. J., Zitzer, S. F., & Bol, R. (1998). δ13C values of soil organic carbon and their use in documenting vegetation change in a subtropical savanna ecosystem. Geoderma, 82(1-3), 5-41.More infoAbstract: Plants with C3, C,4, and CAM photosynthesis have unique δ13C values which are not altered significantly during decomposition and soil organic matter formation. Consequently, δ13C values of soil organic carbon reflect the relative contribution of plant species with C3, C4, and CAM photosynthetic pathways to community net primary productivity, and have been utilized to document vegetation change, to quantify soil organic matter turnover, and to refine our understanding of earth-atmosphere-biosphere interactions. Here, we review the basis of this methodology, and illustrate its use as a tool for studying grass-woody plant dynamics in a savanna ecosystem. In the Rio Grande Plains of southern Texas, C4 grasslands and savannas have been largely replaced by C3 subtropical thorn woodlands dominated by Prosopis glandulosa. We used δ13C values of soil organic matter, above- and belowground plant biomass, and litter in conjunction with radiocarbon dating and dendrochronology to test the hypotheses that: (1) C3 Prosopis groves in uplands and C3 Prosopis woodlands in low-lying drainages have been long-term components of the landscape; and (2) Prosopis woodlands of low-lying drainages have expanded up-slope since Anglo-European settlement. Current organic matter inputs were not in isotopic equilibrium with soil organic carbon in any of the patch types sampled. In upland grasslands, δ13C values of vegetation (-20‰) were lower than those of soil organic matter (-17‰), suggesting increased C3 forb abundance in response to long-term, heavy grazing (herbaceous retrogression). In wooded landscape elements, δ13C values of current organic matter inputs were characteristic of C3 plants (- 28 to - 25‰), while those of the associated soil organic matter were typically -20 to -15‰. These δ13C values indicate that woodlands, groves, and shrub clusters dominated almost exclusively by C3 plants now occupy sites once dominated by C4 grasses. A particularly strong memory of the C4 grasslands that once occupied these sites was recorded in the δ13C values of organic carbon associated with fine and coarse clay fractions (- 18 to -14‰), probably a consequence of the slow organic carbon turnover rates in those soil fractions. When δ13C values of soil organic carbon were evaluated in conjunction with radiocarbon measurements of that same carbon, it appeared that herbaceous retrogression and a shift from C4 grassland to C3 woodland occurred recently, probably within the last 50-100 years. Demographic characteristics of the dominant tree species corroborated the δ13C and 14C evidence, and indicated widespread establishment of P. glandulosa and associated shrubs over the past 100 years. Together, these data provide direct, spatially explicit evidence that vegetation change has occurred recently across the entire landscape at this site. Environmental conditions where C3, C4, and CAM plants coexist (e.g., dry, alkaline soils) generally do not favor the preservation of pollen and phytoliths, and these same areas usually lack historical records of vegetation change. Consequently, vegetation dynamics have been difficult to quantify in grasslands, savannas, and woodlands. However, our results demonstrate clearly that δ13C values of soil organic matter afford a direct and powerful technique for reconstructing vegetation change in these areas.
- Midwood, A. J., Boutton, T. W., Archer, S. R., & Watts, S. E. (1998). Water use by woody plants on contrasting soils in a savanna parkland: Assessment with δ2H and δ18O. Plant and Soil, 205(1), 13-24.More infoAbstract: In savanna parklands of southern Texas, patches of grassland and 'discrete clusters' of small trees and shrubs occur on sandy loam surface soils underlain by an argillic horizon (claypan) at 40 cm. Large trees and shrubs in 'groves' occur on deep (2 m) sandy loam soils without an argillic horizon. δ2H and δ18O of rainfall, groundwater, and soil and plant water were measured to: (1) determine if coexistence in woody patches occurs via vertical stratification of soil water uptake; (2) document differences in plant water acquisition on contrasting soil types; and (3) evaluate recharge and evaporative losses of soil moisture from grassland vs. wooded landscape elements. Groundwater was isotopically similar to weighted rainfall, suggesting local recharge at this site. Linear regressions of soil water δ2H on δ18O yielded slopes less than the meteoric water line, indicating significant evaporative losses of soil moisture in all landscape elements. Interspecific differences in root density distribution were significant; some woody species had roots well below 1.6 m, while others had few roots below 0.8 m. δ2H and δ18O values of stem water from all plants in groves were lower than those of soil water in the upper 1.5 m of the profile, suggesting all species obtained their water from depths >1.5 m. Deep roots of trees and shrubs at this savanna parkland site thus appeared to have a functional significance that was not revealed by biomass or density determinations. Root densities of species in discrete clusters (claypan present) were typically greater than those of the same species in groves (claypan absent), especially in the upper 80 cm of the soil profile. Consistent with rooting profiles, δ2H and δ18O values of plant water indicated that trees and shrubs in discrete clusters with fine-textured subsoils obtained most of their water at depths
- Weltzin, J. F., Archer, S. R., & Heitschmidt, R. K. (1998). Defoliation and woody plant (Prosopis glandulosa) seedling regeneration: Potential vs realized herbivory tolerance. Plant Ecology, 138(2), 127-135.More infoAbstract: Herbivory by rodents, lagomorphs and insects may locally constrain woody plant seedling establishment and stand development. Recruitment may therefore depend either upon plant tolerance of herbivory, or low herbivore abundance, during seedling establishment. We tested potential herbivory tolerance by quantifying growth, biomass allocation, and survival of defoliated Prosopis glandulosa seedlings under optimal abiotic conditions in the absence of competition. Realized tolerance was assessed by clipping seedlings of known age grown in the field with and without herbaceous competition. At 18-d (= 'young') or 33-d (= 'old') of age, seedlings in the growth chamber were clipped just above the first (cotyledonary) node, above the fourth node, or were retained as non-clipped controls. Potential tolerance to defoliation was high and neither cohort showed evidence of meristematic limitations to regeneration. Clipping markedly reduced biomass production relative to controls, especially belowground, but survival of seedlings defoliated 5 times was still ≥75%. Contrary to expectations, survival of seedlings defoliated above the cotyledonary node 10 times was greater (P < 0.10) for 'young' (75%) than 'old' (38%) seedlings. Under field conditions, survival of defoliated 11-month-old P glandulosa seedlings was ≤59% after one defoliation and only ≤ 13% after six defoliations. Results indicate P. glandulosa is potentially tolerant of repeated shoot removal early in its life cycle. Seedling tolerance to defoliation under field conditions therefore appears dependent upon abiotic stresses or resource limitations rather than a lack of intrinsic adaptations for shoot replacement or a depletion of the seedlings' bud bank. Curtailment of root growth, a consequence of top removal observed in the growth chamber experiment, may reduce the capacity of P. glandulosa seedlings to acquire soil resources needed for meristem activation and shoot growth under field conditions. The importance of resource availability to post-defoliation regeneration was implicated in the field experiment, where survival, shoot elongation, and aboveground productivity of clipped seedlings was greatest in plots without herbaceous interference. In light of the marked increases in P glandulosa abundance in grasslands in recent history, our results suggest that (1) utilization of Prosopis seedlings by herbivores may be infrequent or sporadic, (2) Periodic episodes of seed production and germination may satiate herbivore populations, and/or (3) Prosopis seedling establishment occurs during periods of low herbivore density.
- Li, B., & Archer, S. (1997). Weighted mean patch size: A robust index for quantifying landscape structure. Ecological Modelling, 102(2-3), 353-361.More infoAbstract: Variables such as number of patches and mean patch size have been widely used to describe landscape structure. However, these simple, independent measurements often fail to adequately represent or track offsetting or reinforcing changes that occur from interactions between patch number, size and shape. Here, we present a synthetic 'weighted mean patch size' (WMPS) index which combines information represented in patch size and number. The utility of the WMPS index, which was based on percolation theory, is demonstrated using two data sets. The first data set consisted of output from a cellular automata-based simulation of landscape response to disturbance that varied in intensity and scale. Unlike simple estimates of mean patch size, the WMPS index was highly sensitive to disturbance and varied in a consistent fashion with both intensity and scale of disturbance. The second application involved the use of historical aerial photography to compare the dynamics of tree/shrub clusters (= patches) in a subtropical savanna parkland during a period of drought (1941-1960) and a period of normal to above-normal annual rainfall (1960-1983). Contrary to expectations, mean cluster (= patch) size increased in both periods. In addition, greatest increases in mean patch size occurred during the drought period, apparently the result of higher mortality among small clusters relative to large clusters. In contrast, climate-induced vegetation dynamics indicated by the WMPS index (decrease during drought period; increase in subsequent pluvial period) tracked those of other structural measurements (cluster density, total cover, cluster growth rate) and appeared to reflect dynamic, rainfall-induced changes in patch structure associated with changes in tree and shrub canopy area and plant numbers. These examples suggest the WMPS index, by combining information on patch size and number, can preserve process-level ecological information and provide robust, functionally relevant numerical representations of landscape structure over time and in response to disturbance.
- Scholes, R. J., & Archer, S. R. (1997). Tree-grass interactions in Savannas. Annual Review of Ecology and Systematics, 28, 517-544.More infoAbstract: Savannas occur where trees and grasses interact to create a biome that is neither grassland nor forest. Woody and gramineous plants interact by many mechanisms, some negative (competition) and some positive (facilitation). The strength and sign of the interaction varies in both time and space, allowing a rich array of possible outcomes but no universal predictive model. Simple models of coexistence of trees and grasses, based on separation in rooting depth, are theoretically and experimentally inadequate. Explanation of the widely observed increase in tree biomass following introduction of commercial ranching into savannas requires inclusion of interactions among browsers, grazers, and fires, and their effects on tree recruitment. Prediction of the consequences of manipulating tree biomass through clearing further requires an understanding of how trees modify light, water, and nutrient environments of grasses. Understanding the nature of coexistence between trees and grass, which under other circumstances are mutually exclusive or unequal partners, yields theoretical insights and has practical implications.
- Weltzin, J. F., Archer, S., & Heitschmidt, R. K. (1997). Small-mammal regulation of vegetation structure in a temperate savanna. Ecology, 78(3), 751-763.More infoAbstract: Explanations for documented increases in woody plant dominance in grasslands and savannas of North America include atmospheric CO2 enrichment and changes in climate, livestock grazing, and fire regimes. However, tree/shrub encroachment has also coincided with the eradication of a once widespread native herbivore, the black-tailed prairie dog (Cynomys ludovicianus). We used field experiments and repeat aerial photography to demonstrate that prairie dogs, and the herbivores and granivores associated with their colonies, probably maintained grassland and savanna by preventing woody species such as Prosopis glandulosa (honey mesquite) from establishing or attaining dominance. Prosopis seed and pod disappearance was 3-99 times greater within prairie dog colonies. Ants were the primary agent of seed removal, whereas prairie dogs and associated vertebrates were the primary agents of pod removal. Survival of Prosopis Seedlings protected from vertebrate herbivory was similar on and off prairie dog colonies (≃60%), whereas survival of unprotected seedlings was 3 times greater off- than on-colony. On-colony, prairie dogs and associated herbivores girdled and destroyed all Prosopis saplings within 2 d of planting; survival of 1-yr-old seedlings was reduced by 50% after 3 mo of exposure to on-colony herbivores. Despite high levels of woody plant seed disappearance and seedling herbivory, on-colony 'seedling' reserves were substantial (950 plants/ha). Thus, prairie dogs and the fauna that occur on their colonies suppressed rather than eliminated Prosopis from the colony site. Removal of prairie dogs led to rapid development of Prosopis stands. Repeat aerial photography showed that Prosopis canopy cover on a colony eradicated in 1950 (27%) increased to a level (61%) comparable to that of off-colony Prosopis stands (65%) within 23 yr. These data illustrate how transitions from grassland to woodland vegetation can be mediated by a rodent herbivore. They further demonstrate how purposeful or inadvertent removal of native herbivores can have unforeseen effects on plant species composition and landscape physiognomy. Investigations of environmental constraints on vegetation distribution and abundance should take into account the historical role of herbivores in shaping the present system. Inconsistencies among historic accounts of woody plant distribution and abundance in semiarid western North America may be resolved by considering population dynamics of prairie dogs. Widespread eradication of this formerly abundant rodent has eliminated a significant constraint to woody plant establishment on many semiarid grassland and savanna landscapes and has thereby facilitated transitions to shrubland and woodland states. Past land management designed to remove one perceived impediment to livestock production appears to have contributed significantly to development of another management problem that is now a major detriment to sustainable livestock production.
- Barnes, P. W., & Archer, S. (1996). Influence of an overstorey tree (Prosopis glandulosa) on associated shrubs in a savanna parkland: Implications for patch dynamics. Oecologia, 105(4), 493-500.More infoAbstract: The arborescent legume, honey mesquite (Prosopis glandulosa), appears to play a central role in patch dynamics of southern Texas savannas by modifying soils and microclimate and by facilitating the ingress, establishment and/or growth of shrubs in its understorey. As an indirect test for the occurrence and persistence of facilitation in mature shrub clusters (patches), we examined the gas exchange, water relations and production of associated shrubs growing in patches where a Prosopis overstorey was present and in patches where Prosopis had succumbed to natural mortality. Surface (0-10 cm) soils associated with shrub patches were enriched in total [N] and [C] compared to soils of neighboring herbaceous zones. However, there were no detectable differences in soil [N] or [C] in patches with and without Prosopis. Foliar [N] and biomass of various shrub species were also statistically comparable for patches with and without Prosopis. These results are in accordance with other studies that indicate the nutrient legacy associated with Prosopis occupation of a patch may persist for decades after its demise. In comparison to plants growing in the absence of Prosopis, leaf water potentials (predawn and midday), and net photosynthesis and water vapor conductance (morning and midday) of outer-canopy sunlit leaves over an annual growth cycle were comparable for two common evergreen shrubs. Zanthoxylum fagara and Berberis trifoliolata, growing in patches with a live Prosopis. These findings indicate that the presence of Prosopis was not enhancing the growth or activity of mature understorey shrubs; facilitation may. therefore, be important only during early stages of cluster development. In addition, we found no indication that the loss of Prosopis has initiated a downward phase in a cyclic succession of patch initiation, growth and death. Rather, the understorey shrubs appear to be able to maintain growth and productivity in the absence of a Prosopis overstorey. and may, therefore, represent persistent components of woody patches on these savanna landscapes.
- Ocumpaugh, W. R., Archer, S., & Stuth, J. W. (1996). Switchgrass recruitment from broadcast seed vs. seed fed to cattle. Journal of Range Management, 49(4), 368-371.More infoAbstract: Fecal seeding by livestock may be an effective, low-cost means of rangeland restoration. We compared recruitment of switch-grass (Panicum virgatum L.) from seed fed to cattle and deposited in dung to that of broadcast-seeded plots receiving a comparable number of unfed seed. Although germinability of seed passed through livestock (52 to 62%) was reduced relative to that of broadcast seed (85 to 91%), recruitment of switchgrass from seed in cattle feces was equal to or superior to that of broadcast seed in terms of establishment (frequency of occurrence and density), plant growth and final plant size. The frequency of plots with emerging switchgrass plants ranged from 62 to 100% when seeds were delivered in feces, but only 2 to 40% when seeds were broadcast. After 1 year, the frequency of occurrence of switch-grass plants in fecal vs. broadcast-seeded plots was comparable for autumn trials. However, evaluations 1 year after the spring trials continue to result in higher frequency of plots with switch-grass plants from seed delivered in feces than of broadcast seedings (56 vs. 4% for May 1990, P0.10 for October 1991). Results suggest significant advantages of fecal seeding over conventional broadcast seeding in terms of seedling emergence, establishment and growth.
- Zitzer, S. F., Archer, S. R., & Boutton, T. W. (1996). Spatial variability in the potential for symbiotic N2 fixation by woody plants in a subtropical savanna ecosystem. Journal of Applied Ecology, 33(5), 1125-1136.More infoAbstract: 1. Root infection by symbiotic N2-fixing Frankia and Rhizobium strains was quantified in relation to light and soil properties for seedlings of 12 woody species from a subtropical savanna in southern Texas, USA. 2. None of four rhamnaceous species nodulated, despite the fact that bioassays with a known actinorhizal species yielded 13 nodules per seedling. Celtis pallida (Ulmaceae), Acacia greggii and Acacia berlandieri (Leguminosae) also failed to nodulate even though field populations of these species were characterized by high (2.7 4.2%) foliar nitrogen concentration. 3. Infective rhizobia occurred in all soils studied regardless of soil depth, distance from a host plant or type of plant cover. Plant growth in N-free media and acetylene reduction activity suggested that all nodules were capable of N2-fixation. 4. The extent of nodulation varied by species. However, nodulated seedlings were taller, produced more biomass and allocated less biomass to root systems than their non-nodulated counterparts. 5. Numbers of nodules on seedlings of Prasopis glandulosa, the dominant woody species in this subtropical savanna and throughout the south-western USA, were reduced by low light (15% full sunlight) regardless of soil N level; at medium and full sunlight nodule biomass expressed as a fraction of whole plant biomass decreased with increasing soil N. Nodulation of field-grown P. glandulosa appears to be ephemeral, apparently varying with changes in soil moisture. 6. Nodulation and N2 fixation among woody legumes in subtropical savannas can occur across a broad range of soil conditions and depths with significant impacts on local and regional N-cycles. 7. Field levels of foliar N in species that failed to nodulate in the laboratory were comparable to or greater than those in species capable of nodulation, suggesting that leaf N is not a reliable indicator of N2 fixation.
- Archer, S. (1995). Tree-grass dynamics in a Prosopis-thornscrub savanna parkland: Reconstructing the past and predicting the future. Ecoscience, 2(1), 83-99.
- Archer, S., Schimel, D. S., & Holland, E. A. (1995). Mechanisms of shrubland expansion: land use, climate or CO2?. Climatic Change, 29(1), 91-99.More infoAbstract: Encroachment of trees and shrubs into grasslands and the "thicketization' of savannas has occurred worldwide over the past century. Traditional explanations offered to account for the historic displacement of grasses by woody plants in many arid and semi-arid ecosystems have centered around changes in climatic, livestock grazing and fire regimes. We evaluate the CO2 enrichment hypotheses and argue that historic, positive correlations between woody plant expansion and atmospheric CO2 are not cause and effect. -from Authors
- Owens, M. K., Wallace, R. B., & Archer, S. (1995). Seed dormancy and persistence of Acacia berlandieri andLeucaena pulverulenta in a semi-arid environment. Journal of Arid Environments, 29(1), 15-23.More infoAbstract: Seed longevity of the leguminous shrubs Acacia berlandieri and Leucaenapulverulenta was evaluated under field conditions at the soil surface and at 3-5 cm depth. Abiotic influences on seed longevity and dormancy were investigated by comparing seeds maintained at 5°C with seeds experiencing fluctuating day/night temperatures (40°C/20°C vs. 50°C/30°C) under dry and moist conditions. Acacia seed demonstrated no dormancy and initial germinability was > 82% in the laboratory. Viability of Leucaena seed was >97%, but seed coat dormancy limited germination to
- Owens, M. K., Wallace, R. B., & Archer, S. R. (1995). Landscape and microsite influences on shrub recruitment in a disturbed semi-arid Quercus-Juniperus woodland. Oikos, 74(3), 493-502.
- Boutton, T. W., Archer, S. R., Nordt, L. C., Monger, H. C., & Cole, D. R. (1994). Climate, CO2 and plant abundance . Nature, 372(6507), 625-626.
- Flinn, R. C., Archer, S., Boutton, T. W., & Harlan, T. (1994). Identification of annual rings in an arid-land woody plant, Prosopis glandulosa. Ecology, 75(3), 850-853.
- Boutton, T. W., Nordt, L. C., Archer, S. R., Midwood, A. J., & Casar, I. (1993). Stable carbon isotope ratios of soil organic matter and their potential use as indicators of palaeoclimate. International Atomic Energy Agency, Proceedings Series, 445-459.More infoAbstract: The δ13C values of soil organic matter record information regarding the relative importance of C3 versus C4 plants in past plant communities. Because the geographical distribution of C4 plants is correlated strongly with temperature, δ13C of soil organic matter has the potential to enhance understanding of climate history. The purpose of the paper was to: (1) demonstrate that δ13C of soil organic matter responds to and records changes in the relative abundances of C3 and C4 plants in situations where plant community histories are well known; then (2) apply this technique to soil organic matter in palaeosols representing the past 15 000 years in order to reconstruct vegetation and climate change during that period. The δ13C values of soil organic matter accurately documented vegetation dynamics at two sites where land use and vegetation history had been quantified previously: a tallgrass prairie and a subtropical woodland. Changes in δ13C values of organic matter in palaeosols from central Texas indicated shifts in the relative abundance of C3 and C4 plants which tracked changes in climate indicated from accounts published previously and based on other climate reconstruction techniques. Results indicate that δ13C of soil organic matter, by reflecting the relative contribution of C3 and C4 species to plant community productivity, can be used to reconstruct vegetation history and can therefore serve as a proxy indicator of past climate.
- Midwood, A. J., Boutton, T. W., Watts, S. E., & Archer, S. R. (1993). Natural abundance of 2H and 18O in rainfall, soil moisture and plants in a subtropical thorn woodland ecosystem: implications for plant water use. International Atomic Energy Agency, Proceedings Series, 419-431.More infoAbstract: In the Rio Grande Plains of southern Texas, subtropical thorn woodland has replaced relatively open grassland/savanna during the past 200 years. To investigate potential changes in the hydrologic cycle of this system, the authors determined the natural abundance of 2H and 18O in rainfall, soil water and plant water in three habitats representing a successional chronosequence in the transition from open grassland to closed canopy woodland. Precipitation was isotopically identical with groundwater and fell on the meteoric water line with a weighted mean δ2H of -22per mill and a weighted mean δ18O of -4.3per mill. δ2H and δ18O of precipitation did not vary seasonally, but showed a significant 'amount effect'. Correlations between δ2H and δ18O of soil water indicated higher evaporation rates in wooded areas than in grassland. It is suggested that this result does not reflect higher evaporation rates, but rapid drying of the soil via transpiration followed by kinetic fractionation associated with evaporation in dry soils. δ2H and δ18O of plant and soil water indicated that more recently established woody plants associated with earlier successional stages had shallower root systems and obtained water from the upper 150 cm of the soil profile, while those associated with later successional stages had deeper root systems and acquired water below that depth. By reducing transpirational leaf area and root biomass in the grass layer, long term grazing in this former grassland may have enabled deeper infiltration of soil water, creating an opportunity for development of a community dominated by more deeply rooted woody plant species capable of exploiting this deeper water resource.
- Archer, S., & Pyke, D. A. (1991). Plant-animal interactions affecting plant establishment and persistence on revegetated rangeland. Journal of Range Management, 44(6), 558-565.More infoAbstract: Timing and site preparation are important in limiting belowground herbivory. Animals can serve as dispersal agents of seeds. Livestock dosed with desirable seeds can disperse them in their dung across the landscape, thereby creating patches of desirable plants. If revegetation sites will be grazed by livestock, then managers should choose plant species that tolerate rather than avoid grazing and should apply adequate management to establish and maintain plant populations. Seeds inoculated with mutualistic species such as mycorrhizae, nitrogen-fixing bacteria, or actinomycetes may enhance establishment, productivity, and nutrient quality of rangeland species while increasing rates of succession. -Authors
- Pyke, D. A., & Archer, S. (1991). Plant-plant interactions affecting plant establishment and persistence on revegetated rangeland. Journal of Range Management, 44(6), 550-557.More infoAbstract: At the intraspecific level the authors propose the development and use of density-yield diagrams for rangeland species, based on the self-thinning principle, that aboveground biomass is related to plant density and to the dynamic process of density-dependent mortality. The approach would be used to determine optimum seeding rates, and to predict future biomass of revegetated rangeland. At the interspecific level, competitive relationships of species used to reseed rangelands need to be identified to enhance the probability that species will coexist and thereby facilitate greater species diversity on the site. A diversity for species and growth forms may provide a more stable cover and productivity than the monoculture on sites characterized by environmental variability while potentially enhancing nutrient status for the site. -from Authors
- Archer, S. (1990). Development and stability of grass/woody mosaics in a subtropical savanna parkland, Texas, USA. Journal of Biogeography, 17(4-5), 453-462.More infoAbstract: Integrates a series of studies addressing the following questions: Have woodlands replaced grasslands or savannas? If there was a physiognomic conversion: 1) what successional processes were involved; 2) what time scale would have been required; and 3) what were the causes? -from Author
- Brown, J. R., & Archer, S. (1990). Water relations of a perennial grass and seedling vs adult woody plants in a subtropical savanna, Texas. Oikos, 57(3), 366-374.More infoAbstract: Over 52±16% (Mean ±SE) of the seeds of the arborescent legume Prosopis gladulosa var. glandulosa germinated within 2 wk of dissemination in plots dominated by a perennial grass (Chloris cucullata) in July 1984 and 63±7% of those germinating survived through September 1985. Over 60% of the herbaceous root biomass occurred in the upper 30 cm of soil. In contrast, tap roots of Prosopis seedlings had penetrated beyond 40 cm within 4 months of germination and their mean proportion of total biomass belowground increased from 0.27±0.09 in May to 0.52±0.15 in August. Net photosynthesis (Pn) and conductance (g) of Chloris were closely coupled to fluctuations in moisture in the upper soil horizons (90 cm. Rapid development of roots of Prosopis seedlings during their first year of growth apparently enhanced survival by enabling them to access soil moisture beyond the zone effectively utilized by grasses. On sites with a history of grazing, competition for water from herbaceous vegetation may not play a significant role in limiting establishment of P. glandulosa seedlings or the growth of mature plants in most years. -from Authors
- Archer, S. (1989). Have southern Texas savannas been converted to woodlands in recent history?. American Naturalist, 134(4), 545-561.More infoAbstract: Discrete clusters of woody plants form in herbaceous clearings following the invasion of mesquite Prosopis glandulosa var. glandulosa, an arborescent legume. The growth rate of these clusters varies with precipitation and size. A simulation model was developed in which the establishment of other woody species beneath invading Prosopis occurred within 10-15 yr. As a cluster developed around the Prosopis nucleus, species richness increased rapidly for 35-45 yr and became asymptotic at 10 species per cluster. Estimated age of the oldest Prosopis plant found in clusters was 172-217 yr, but model-derived size-age relationships predicted that 90% of clusters and mesquite plants at the site are
- Brown, J. R., & Archer, S. (1989). Woody plant invasion of grasslands: establishment of honey mesquite (Prosopis glandulosa var. glandulosa) on sites differing in herbaceous biomass and grazing history. Oecologia, 80(1), 19-26.More infoPMID: 23494340;Abstract: Emergence and survival of honey mesquite (Prosopis glandulosa var. glandulosa Torr.) seedlings was quantified on sites with contrasting grazing histories: long-term continuous grazing (LTG) and long-term protection (LTP) from grazing by cattle. On each site, different levels of heroaceous defoliation were imposed at monthly intervals (no defoliation=ND, moderate=MD and heavy=HD). The two weeks following seed dissemination appeared to be the most critical to Prosopis establishment on LTP-ND plots. Openings in the herbaceous layer created by moderate defoliation of grasses on the LTP site increased germination and/or survival 7-to 8-fold during this period. However, increasing the degree of defoliation from moderate to heavy did not stimulate additional emergence on either the LTP or LTG site. Emergence from scarified seed placed in cattle dung (17 to 30%) was lower than that of bare seed placements in various microhabitats (43-60%). However, deposition of scarified Prosopis seed in dung in conjunction with graminoid defoliation may be the most likely combination of events when livestock are present. Emergence from seeds transported into grasslands by other fauna likely would be low, unless seeds were deposited in areas where grasses had been defoliated. Prosopis survival was comparably high in dung and bare seed placements after one growing season. survival of seedlings present two weeks after seed dissemination ranged from 74 to 97% at the end of the second growing season. Seedling survival and shoot development (biomass, leaf area and height) were similar on LTP and LTG sites, regardless of the level of herbaceous defoliation or seed placement. In addition, the magnitude and patterns of net photosynthesis, stomatal conductance and xylem water potential were comparable among one-year-old seedtings on ND, MD and HD plots, even though differences in herbaceous species composition and above- and below-ground biomass between these treatments were substantial. Such data suggest competition for soil resources between grasses and Prosopis may be minimal early in the life cycle of Prosopis. High rates of Prosopis emergence and establishment on LTP-MD plots are counter to the widespread assumption that long-term and/or heavy grazing is requisite for Prosopis encroachment into grasslands. Results are discussed with regard to factors contributing to the recent, widespread invasion of this woody legume into grasslands of southwestern North America. © 1989 Springer-Verlag.
- Archer, S., Scifres, C., Bassham, C. R., & Maggio, R. (1988). Autogenic succession in a subtropical savanna: conversion of grassland to thorn woodland. Ecological Monographs, 58(2), 111-127.More infoAbstract: Dense thorn woodlands occupy what are thought to have been grasslands and savannas prior to settlement of the Rio Grande Plains of Texas. To assess the physiognomic stability of the two-phase landscapes, cluster size, density and cover were quantified for 1941, 1960, and 1983 from aerial photographs. Results indicate: 1) mesquite Prosopis glandulosa invaded grasslands and served as the nucleus of cluster organization on upland sites; 2) woody plant community development has been highly punctuated by variations in precipitation; 3) clusters >5 m2 in area are persistent features of the landscape; and 4) the present two-phase pattern is moving toward a monophasic woodland as new clusters are initiated and existing clusters expand and coalesce. As a result, 5) shrub clusters on uplands represent an intermediate stage in the conversion of grassland to woodland, and 6) closed-canopy woodlands on more mesic sites appear to represent portions of the landscape where this has already occurred. -from Authors
- Brown, J. R., & Archer, S. (1988). Woody plant seed dispersal and gap formation in a North American subtropical savanna woodland: the role of domestic herbivores. Vegetatio, 73(2), 73-80.More infoAbstract: The relationship between domestic cattle and vegetation change in a savanna woodland was evaluated with respect to dung deposition and the dispersal and establishment of mesquite (Prosopis glandulosa var. glandulosa, Mimosaceae), a cosmopolitan woody invader of grasslands in the southwestern USA. Dung deposited in autumn disintegrated rapidly, leaving patches of bare ground ranging from 50 to 900 cm2. Herbaceous cover on gaps created by dung deposition recovered to levels comparable to neighboring vegetation by the end of the following growing season. Vegetation colonizing gaps consisted primarily of grasses not found in the surrounding vegetation. Dung deposition increased species diversity and spatial heterogeneity of the herbaceous vegetation and contributed to the development of a fine-grain mosaic of small patches of varying successional age-states. The role of cattle in facilitating the ingress and establishment of mesquite has broader implications with regard to the conversion of grasslands to woodlands. On the site with cattle, mesquite seedlings were found in 75% of dung pats surveyed in September (mean =4.2 seedlings per pat; maximum =50). Although seedling survival in dung (79%) was only 16% greater than that of mesquite emerging from seeds experimentally sown away from dung, no seedlings were found on areas without cattle. Mean (± SE) density of mesquite seedlings ranged from 12±2 to 15±2 m-2 on the site with cattle. Seed densities away from parent plants averaged 10.7 m-2 and 0.0 m-2 on areas with and without cattle, respectively. Seed densities beneath adult plants were comparable between sites. The high density of seedlings on areas with cattle, in contrast to absence of seedlings on the area without cattle, suggests rates of invasion of grasslands by mesquite would have increased substantially in North America following the settlement and introduction of domestic ungulates. Prior to the introduction of livestock, poor seed dissemination and germination may have limited its Holocene spread. © 1987 Dr W. Junk Publishers.
- Archer, S., Garrett, M. G., & Detling, J. K. (1987). Rates of vegetation change associated with prairie dog (Cynomys ludovicianus) grazing in North American mixed-grass prairie. Vegetatio, 72(3), 159-166.More infoAbstract: A prairie dog (Cynomys ludovicianus) colony with a known history of habitation was studied to quantify the effects of herbivory on plant species composition, dominance, stature and diversity in a North American mixedgrass prairie. Gradient analysis was used to quantify the relationship between plant community structure, prairie dog density, burrow density and habitation history and to document community-level responses of plants subjected to heavy grazing pressure. The results quantify the type, rate and extent of change which plant populations and communities may undergo in response to the differential grazing of plants variously tolerant of defoliation. Detrended correspondence analysis indicated that 69% of the between-sample floristic variance on the site was attributable to prairie dog habitation. Perennial grasses were rapidly displaced from the site within 3 yr of colonization and were replaced by annual forbs. The net result was an increase in species richness and diversity on the prairie dog colony. Within the colony, however, the number of species was more a function of stand size than colonization history. Significant decreases in canopy stature after 2 yr of habitation resulted from replacement of mid-height grass species by shortgrass species and forbs. In addition, there was a shift from tall growth forms of off-colony species to dwarf growth forms of the same species on the colony. Decreases in litter and increases in bare soil cover were substantial during the first 2 yr of habitation but changed little thereafter. © 1987 Dr W. Junk Publishers.
- Archer, S. (1984). The distribution of phytosynthetic pathway types on a mixed-grass prairie hillside.. American Midland Naturalist, 111(1), 138-142.More infoAbstract: An Agropyron smithii-dominated community was found at the hilltop. A Poa-Hordeum-dominated community was located at the base of the slope. Midslope areas supported a Bouteloua-Agropyron-dominated community. Each topographic location had different soil moistures and textures. Grasses with the C4 photosynthetic pathway had highest importance values on well- drained sites where soil moisture was lowest and soils were coarsest. C3 graminoids predominated on the relatively moist upper and lower portions of the hillside.-from Author
- Archer, S., & Detling, J. K. (1984). The effects of defoliation and competition on regrowth of tillers of two North American mixed-grass prairie graminoids.. Oikos, 43(3), 351-357.More infoAbstract: Tillers (ramets) of Andropogon gerardi (big bluestem) and Carex filifolia (threadleaf sedge) were subjected to various defoliation regimes under full and reduced competition. Tillers defoliated under conditions of full competition produced significantly less leaf biomass than tillers defoliated under reduced competition. Apparent mortality of tillers defoliated bi-weekly was 95% under full competition and 20% under reduced competition. Andropogon tillers subjected to multiple defoliation under reduced competition produced c3 more leaves per tiller than nondefoliated tillers or tillers similarly defoliated under full competition. All defoliated tillers had substantiated reductions in biomass of storage organs relative to nondefoliated tillers. Tillers defoliated under full competition had stembase biomass reductions comparable to those of tillers defoliated under reduced competition but produced significantly less leaf biomass. Results suggest that the positive aspects of resource sharing among interconnected tillers were outweighed by the negative aspects of intra- and interspecific competition. At the community level, herbivores may mediate competitive interactions among primary producers through the differential defoliation of plants.-from Authors
- Fuhlendorf, S. D., Wilcox, B., Archer, S. R., Birt, A., Sorice, M., Kreuter, U., & Scholtz, R. (2018, February). Socio-ecological transformation of the Great Plains through Juniperus invasion. Annual Meetings, Soc Range Management, Native Invasives Symposium. Sparks, NV: Soc Range Management Annual Meetings.
- Naito, A. T., Archer, S. R., Heilman, P., & Predick, K. I. (2018, December). Brush management and grassland conservation: an ecosystem services perspective. American Geophysical Union Fall Meeting. Washington, DC.
- Naito, A., & Archer, S. R. (2018, April). Brush management and ecosystem services: a quantification of trade-offs. Altar Valley Conservation Alliance symposium. Buenos Aires National Wildlife Refuge.
- Archer, S. R. (2017, July). Shrub-shrub transitions in the Jornada Basin: where have we been, where are we going?. Jornada Basin LTER Annual Symposium. Las Cruces, NM: NSF Long Term Ecological Research Program.
- Archer, S. R., Schultz, R., & Fuhlendorf, S. (2017, March). Social and ecological challenges in understanding fire dynamics and woodland expansion in the Great Plains, USA. 5th Annual Savanna Science Networking Meeting. Skukuza, South Africa: South African National Parks.
- Archer, S. R., Wilcox, B., Birt, A., Fuhlendorf, S., Kreuter, U., Sorice, M., van Leeuwen, W., & Zou, C. (2017, April). Slowing the expansion of woodlands and increasing the resilience of grasslands in the Southern Great Plains. American Association of Geographers Annual Meetings. Boston, MA.
- Archer, S. R., Wilcox, B., Fuhlendorf, S., Bouttoj, T., West, J., & Barboza, P. (2017, July). Savanna Long-term Research Initiative in the Southern Great Plains. 52nd Annual Grassland Soc Southern Africa Congress “Advancing Rangeland Ecology and Pasture Management in Africa. Hoedspruit, South Africa.
- McCulley, R., Nelson, J., Predick, K. I., Levi, E., Barnes, P., Throop, H., & Archer, S. R. (2017, August). UV radiation stimulates but soil-litter mixing reduces fungal role in dryland litter decomposition. Organized oral session “Fungi and Global Change – Linking Diversity, Community Composition, and Ecosystem Function in a Changing World”, Ecological Soc America Annual Meetings. Portland, OR.
- McIntyre, C. L., Archer, S. R., & Belnap, J. (2017, August). Talk: Biocrusts and grass germination – what do awns have to do with it?. Ecological Soc. America Annual Meetings. Portland, OR.
- McIntyre, C. L., Archer, S. R., & Belnap, J. (2017, September). Invited Talk: Grass germination and establishment on biocrusts - the role of awns and seed placement. 14th Biennial Conference of Science & Management on the Colorado Plateau & Southwest Region. Flagstaff, AZ.
- Naito, A. T., Archer, S. R., Barron-Gafford, G. A., Heilman, P., & et al., . (2017, June). Brush management: an ecosystem services perspective. Science in the Sonoita Plain Symposium. Appleton-Whittell Research Ranch, Elgin, AZ.
- Naito, A., Archer, S. R., Barron-Gafford, G. A., Heilman, P., Katharine, P. I., & Throop, H. (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.
- 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.
- Naito, A., Archer, S. R., Barron-Gafford, G. A., Heilman, P., Predick, K. I., & Throop, H. (2017, January). Evaluating ecosystem services and trade-offs in the context of brush management. Society for Range Management Annual Meeting. St. George, UT.
- Archer, S. R. (2016, March). Invited Talk: Shrubs in the Anthropocene: A Box of Chocolates. EJ Dyksterhuis Distinguished Lecture. Texas A&M University: Texas A&M University.
- Archer, S. R. (2016, May). Invited Talk: Grassland-Woodland Transitions: It's Complicated. Graduate/Faculty Seminar. Northern Arizona University, Flagstaff: Forestry Graduate Student Association.
- Barnes, P. W., Throop, H. L., & Archer, S. R. (2016, August). Invited Talk: Sunlight and soil-litter mixing: Drivers of dryland litter decomposition now and in the future. Symposium 11716. Litter Decomposition in the Anthropocene: Do we understand the regulators of decomposition well enough to predict future consequences?. Orlando, FL: Ecological Society of America Annual Meeting.
- Benally, R. K., Naito, A., & Archer, S. R. (2016, April). Land cover change in a Southwestern Desert. Arizona Space Grant Consortium Symposium. University of Arizona: NASA.
- Hale, S. L., Koprowski, J. L., & Archer, S. R. (2016, January). Invited Talk: Prairie Dogs: Rangeland Pests or Unexpected Allies in Grassland Management? In: Symposium: Rangeland Habitat, Wildlife Coexistence and Relationships. Soc. Range Management Annual Meetings. Corpus Christi, TX.More infoApproved for presentation on Feb 3, 2016
- Hale, S. L., Koprowski, J. L., & Archer, S. R. (2016, July). Re-assembling the pieces: The role of a recently reestablished keystone herbivore in restoring a degraded semidesert grassland ecosystem. North American Prairie Conference. Normal, IL.
- Hale, S., Koprowski, J. L., & Archer, S. R. (2016, February). Resuming a keystone role: the return of black-tailed prairie dogs to Arizona. Joint Annual Meeting, Arizona and New Mexico Chapters of The Wildlife Society and the American Fisheries Society. Flagstaff, AZ.
- Maghran, L. A., Falk, D. A., Malusa, J. R., & Archer, S. R. (2016, November). Resilience and recovery from two large fires in the Santa Catalina Mountains. AFE Southwest Conference. Tucson, AZ: Association for Fire Ecology (AFE).
- Pierce, N., Archer, S. R., & B, B. T. (2016, August). Grassland-to-shrubland state transitions in arid lands: Competition matters. Ecological Soc. America Annual Meetings. Ft Lauderdale, FL.
- Archer, S. R. (2015, December). Invited Talk: Stewardship of Wild Drylands: Land Cover Perspectives. International Symposium on “Stewardship for Future Drylands”. San Luis Potosi, Mexico: Mexican Scientific Ecological Society and National Network on Socioecosystems and Sustainability.
- Archer, S. R. (2015, December). Stewardship of Wild Drylands: Land Cover Perspectives. International Symposium on “Stewardship for Future Drylands”. San Luis Potosi, Mexico: Mexican Scientific Ecological Society and National Network for Socioecosystems and Sustainability.
- Archer, S. R. (2015, February). Invited Talk:Land cover state-change and ecosystem function: perspectives from grasslands, savannas and woodlands. UA Herbarium Seminar Series.
- Archer, S. R. (2015, June). Invited Talk: Shrub encroachment and brush management: research priorities for competing land-use objectives. Science in the Sonoita Plains Symposium. Appleton-Whittell Research Ranch, Elgin, Arizona: Cienega Watershed Partnership.
- Archer, S. R. (2015, March). Ecological intervention in National Parks: when, where and under what circumstances?. Science for Parks, Parks for Science: The Next Century. University of California-Berkeley: National Park Service and National Geographic Society.
- Archer, S. R. (2015, November). Grassland-Shrubland Transitions: The Rest of the Story. Graduate/Faculty Colloquium. University of Arizona: Soil Water & Environmental Sciences.
- Archer, S. R. (2015, October). Do biocrusts differentially influence native and non-native grass establishment?. 13th Biennial Conference of Science & Management on the Colorado Plateau & Southwest Region. Northern Arizona University High Country Conference Center.More infoConservation Management and Planning, Nonnaitive Species Management Session, October 5-8, 2015,
- Archer, S. R. (2015, October). Invited Talk: Imagining a New Long-Term Ecological Research Site for Arid Lands. 12th Annual RISE Symposium (Research Insights in SemiaridEcosystems). University of Arizona: UA/USDA-ARS.
- Archer, S. R. (2015, October). Invited Talk: Shrub encroachment, brush management and competing land use objectives: tough choices. 2015 Southwest Vegetation Management Association Annual Meeting. Sierra Vista, AZ.
- Archer, S. R., Peters, D. P., Yao, J., Burriss, D., & Monger, C. (2015, October). Shrub-shrub transitions. NSF Site Review: Jornada LTER. New Mexico State University, Las Cruces, NM.
- Archer, S. R., Pierce, N., & Bestelmeyer, B. T. (2015, October). Interactions between and within plant functional types during grassland-to-shrubland state transition. NSF Site Review: Jornada LTER. New Mexico State University, Las Cruces, NM: NSF.
- Hale, S. L., Koprowski, J. L., & Archer, S. R. (2015, June). The potential role of a native grazer and keystone species in managing woody encroachment. American Society of Mammalogists 95th Annual Meeting. Jacksonville, FL.
- Jones, S., & Archer, S. R. (2015, June). Quantifying rates and patterns of mesquite cover in Las Cienegas National Conservation Area using repeat aerial imagery and land use records from 1936 to 2014. Science in the Sonoita Plains Symposium. Appleton-Whittell Research Ranch, Elgin, Arizona: Cienega Watershed Partnership.
- McIntyre, C., & Archer, S. R. (2015, October). Invited talk: Influence of biocrusts on buffelgrass and native grass germination and establishment. Southern Arizona Buffelgrass Coordination Center.
- Pierce, N., Bestelmeyer, B. B., & Archer, S. R. (2015, August). Location, location, location: the influence of plant neighborhoods configuration on grass-shrub interactions. Ecological Society of America Annual Meetings. Baltimore, MD.
- Archer, S. R. (2014, April). Invited Talk: Grasslands. Yale Club. Arizona Inn, Tucson, AZ: Yale Club.
- Archer, S. R. (2014, August). Invited Talk: Interactions between soil carbon cycling and vegetation in a changing world. Organized Oral Session, Ecological Soc America Annual Medetings.More infoschedule conflict prevented my participation
- Archer, S. R. (2014, July). Invited Talk: Woody plants in grasslands: an ecosystem services perspective. Desert Ecology Short Course. Las Cruces, NM: New Mexico State University.
- Archer, S. R. (2014, October). Invited Talk: Land cover state-change and ecosystem function: perspectives from savannas, woodlands and forests. XXIV IURFO (International Union of Forest Research Organizations) World Congress. Cedar City, UT: International Union of Forest Research Organizations.
- Archer, S. R. (2014, October). Invited Talk: Planned Shrub Removal Experiments on Instrumented Watersheds: Opportunities for Collaboration. 11th Annual Research Insights in Semi-Arid Ecosystems (RISE) Symposium. University of Arizona: SNRE & ARS Southwest Watershed Research Center.
- Archer, S. R., & Levi, E. (2014, March). Invited Talk: Turf wars: causes and consequences of shrub invasion on the Santa Rita Experimental Range and grasslands around the globe. Discovery Saturday, Santa Rita Experimental Range.
- Pierce, N., & Archer, S. R. (2014, July). Invited Talk: Location, location, location: the influence of plant neighborhoods configuration on grass-shrub interactions. Jornada LTER Desert Ecology Short Course. Las Cruces, NM: New Mexico State University.
- Archer, S. R. (2013, Spring). Desert Decomposition. LTER Science Council. Las Cruces, NM: NSF LTER.
- Archer, S. R. (2013, Spring). Grassland Ecology. Tucson Museum of Art and Biosphere 2 Desert Initiative Project. Biosphere 2: Biosphere 2.
- Archer, S. R. (2013, Winter). Grasslands: The Shape of Things to Come. Arizona Native Plant Society. Tucson, AZ: Tucson Chapter, Arizona Native Plant Society.
- Breshears, D. D., Archer, S. R., Bojóquerez Ochoa, M., Field, J. P., Huxman, T. E., Law, D. J., Logie, C., Reynoso, E. D., Villegas, J. C., & Whicker, J. J. (2013, December). Consequences in change in vegetation cover for the Critical Zone: Example implications for hydropedology. American Geophysical Union. H32D. Hydropedology: Synergistic Integration of Soil Science and Hydrology in the Critical Zone. San Franciso.
- Ochoa, M. B., Logie, C., Reynoso, E. D., Law, D. J., Field, J. P., Breshears, D. D., & Archer, S. R. (2013, August). Determining a new dryland decomposition input term: small simulated fluvial redistribution of sediment to litter surfaces. Research Experience for Undergraduates Workshop. Biosphere 2: B2 REU Program.
- Niu, F., Nadoum, S., Fishella, M., Pierce, N., Okin, G., & Archer, S. R. (2019, Auguste). Sandblasting: An overlooked driver of shrub encroachment?. Ecological Soc. America Annual Meetings. Louisville, KY.
- Rutherford, A., Archer, S. R., Weber-Grullon, L., & Sala, O. E. (2019, February). Shrub recruitment in a Sonoran Desert grassland: variation in intra-seasonal precipitation trumps level of grazing. Soc Range Management Annual Meetings. Minneapolis, MN.
- McIntyre, C. L., Archer, S. R., & Belnap, J. (2018, August). Native grass emergence on biocrusts. Ecological Society of America Annual Meeting. New Orleans.
- Pierce, N., Archer, S. R., & Bestelmeyer, B. (2018, February). Grassland-shrubland state transitions in arid rangelands: Competition matters. Annual Meetings, Soc Range Management. Sparks, NV.
- Scholtz, R., Fuhlendorf, S. D., Archer, S. R., Buitenwerf, J. A., Polo, J. A., & Tanner, E. P. (2018, February). Woody plant dynamics in fragmented landscapes of the Great Plains, USA. Annual Meetings, Society for Range Managment. Reno, NV: Society for Range Managment.
- Archer, S. R., Belnap, J., & McIntyere, C. (2017, October). Biocrusts and the emergence and establishment of Southwestern grasses. Project Director’s Meeting. Washington, DC: USDA NIFA Weedy and Invasive Species Program.
- Archer, S. R., Biederman, J. A., Barron-Gafford, G. A., Guertin, D. P., Heilman, P., & Naito, A. T. (2017, October). Brush management and ecosytem services: quantification of trade-offs on western rangelands. Project Director's Meeting. Tampa, FL: USDA NIFA Agroecosystems Program.
- McIntyre, C. L., Archer, S. R., & Belnap, J. (2017, January). Influence of biological soil crusts on grass germination and establishment of native and non-native grasses. Soc Range Management Annual Meeting. St. George, UT.
- Peters, D. P., Havstat, K., Bestelmeyer, B., Archer, S. R., & et al., . (2017, July). Jornada Basin LTER: Landscape linkages and state changes across spatial and temporal scales. Jornada Basin LTER Annual Symposium. Las Cruces, NM: NSF Long Term Ecological Research Program.
- Weber-Grullon, L., Sala, O. E., Rutherford, A., & Archer, S. R. (2017, August). Woody-plant encroachment in the Chihuahuan Desert: Mechanisms of invasion and opportunities for containment. Ecological Soc. America Annual Meetings. Portland, OR.
- McIntyre, C. L., Archer, S. R., & Belnap, J. (2016, April). Establishment of native and non-native grasses on biocrusts in two North American deserts. Achievement Rewards for College Scientists Foundation. Phoenix Country Club, Phoenix, AZ: Achievement Rewards for College Scientists Foundation.
- Papuga, S. A., Rotunno, M., Schaller, A., Pope, A., Archer, S. R., & Smith, S. E. (2016, Spring). Understanding source water dynamics for a deep-rooted desert shrub using stable water isotopes: an experimental two-layer approach. UA Earth Week Student Symposium.
- Peters, D. P., Archer, S. R., Sala, O. E., Vivoni, E., Havstad, K. M., Monger, C., & Yao, J. (2016, August). Complex landscapes in the American Southwest: are desertified systems “novel”?. Ecological Soc. America Annual Meetings. Ft Lauderdale, FL.
- Rotunno, M. A., Papuga, S. A., Archer, S. R., Schaller, A., Pope, A. J., & Smith, S. E. (2016, April). Understanding source water dynamics for a deep-rooted desert shrub using stable water isotopes: an experimental two-layer approach. Earth Week. University of Arizona.
- Archer, S. R., Papuga, S. A., & Rotunno, M. (2015, March). Water use by semiarid shrubs: an experimental two-layer perspective. El Dia Del Agua Program, Earth Week.
- Archer, S. R., Pierce, N., & Bestelmeyer, B. T. (2015, October). Shrub dynamics in pre- and post-encroachment phases of grassland-to-shrubland transitions.. NSF Site Review: Jornada LTER. New Mexico State University, Las Cruces, NM: NSF.
- Archer, S. R., Pierce, N., & Bestelmeyer, B. T. (2015, September). The spatially explicit influence of neighborhood-scale shrub-grass interactions during grassland-to-shrubland state transition. LTER All Scientists Meeting. Estes Park, CO: National Science Foundation.
- Goodrich, D. C., Heilman, P., Scott, R. L., Nearing, M. A., Moran, S., Nichols, M., Vivoni, E. R., Archer, S. R., Biederman, J., & Naito, A. T. (2015, Dec). The Walnut Gulch – Santa Rita Wildland Watershed-Scale LTAR Sites. American Geophysical Union Fall Meetings. San Francisco, CA: American Geophysical Union (AGU).
- Hale, S. L., Koprowski, J. L., & Archer, S. R. (2015, April). Can a small, native grazer manage woody encroachment after reestablishment?. Earth Week. University of Arizona.
- Jones, S., Archer, S. R., & Predick, K. (2015, October). Shrub proliferation, brush management and ecosystem services. 12th Annual Research Insights in Semiarid Ecosystems (RISE) Symposium. University of Arizona: UA/USDA-ARS.
- McIntyre, C. L., & Archer, S. R. (2015, October). Do biocrusts differentially influence non-native and native grass establishment?. 12th Annual Research Insights in Semiarid Ecosystems (RISE) Symposium. University of Arizona: UA/USDA-ARS.
- Pierce, N., & Archer, S. R. (2015, October). Shrub dynamics in pre- and post-encroachment phases of grassland-to-shrubland transition. 12th Annual Research Insights in Semiarid Ecosystems (RISE) Symposium. University of Arizona: UA/USDA-ARS.
- Pierce, N., Archer, S. R., & Bestelmeyer, B. T. (2015, October). Do local-scale grass-shrub interactions influence grassland-to-shrubland state transition?. NSF Site Review: Jornada LTER. New Mexico State University, Las Cruces, NM: NSF.
- Rotunno, M. A., Papuga, S. A., & Archer, S. R. (2015, April). An experimental two-layer approach to understanding semiarid shrub water use. Earth Week. University of Arizona.
- Rotunno, M. A., Papuga, S. A., & Archer, S. R. (2015, April). An experimental two-layer approach to understanding semiarid shrub water use. UA Student Earth Week Symposium. University of Arizona.
- Rotunno, M., Papuga, S. A., & Archer, S. R. (2015, March). Water use by semiarid shrubs: an experimental two-layer perspective. El Dia Del Agua Program, Earth Week. University of Arizona.
- Gallery, R. E., O'Shea, C., Kwiecien, A., Predick, K. I., & Archer, S. R. (2014, December). Long-term effects of shrub encroachment and grazing on soil microbial composition and function. American Geophysical Union Annual Meetings. San Francisco, CA.
- Gallery, R. E., O'Shea, C., Kwiecien, A., Predick, K., & Archer, S. R. (2014, December). Long-term effects of shrub encroachment and grazing on soil microbial composition and function.. American Geophysical Union (AGU) Annual Meetings.. San Francisco, CA.: American Geophysical Union.
- Logie, C., Ochoa, E., Reynoso, E., Breshears, D. D., Law, D. J., Field, J., & Archer, S. R. (2014, November). Determining a new dryland decomposition input term: sediment redistribution to litter surfaces via wind and rain. UA Grad Blitz. University of Arizona.
- Logie, C., Ochoa, M., Reynoso, E., Law, D. J., Field, J., Breshears, D. D., & Archer, S. R. (2014, October). Determining a new dryland decomposition input term: Sediment redistribution to litter surfaces during fluvial event simulation. 11th Annual Research Insights in Semi-Arid Ecosystems (RISE) Symposium. University of Arizona: SNRE & ARS Southwest Watershed Research Center.
- O'Shea, C., Predick, K. I., Gallery, R. E., & Archer, S. R. (2014, April). Sonoran Desert soil microbial community response to vegetation change and cattle grazing. Earth Week. University of Arizona.
- O'Shea, C., Predick, K. I., Gallery, R. E., & Archer, S. R. (2014, August). Sonoran Desert soil microbial community response to vegetation change and cattle grazing. Annual Science Teacher Preparation Conference. University of Arizona: UA College of Education.
- O'Shea, C., Predick, K., Gallery, R. E., & Archer, S. R. (2014, April). Sonoran Desert soil microbial community response to vegetation change and cattle grazing.. Earth Week. University of Arizona..
- O'Shea, C., Predick, K., Gallery, R. E., & Archer, S. R. (2014, August). Sonoran Desert soil microbial community response to vegetation change and cattle grazing.. Annual Science Teacher Preparation Conference. University of Arizona: UA College of Education. University of Arizona.
- Kwicien, A., Predick, K. I., Archer, S. R., Rasmussen, C. -., & Gallery, R. E. (2013, October). Mesquite and cactus abundance on a grazed and protected Sonoran Desert grassland site. Research Insights in Semi-Arid Ecosystems (RISE) Symposium. University of Arizona: SNRE and USDA ARS.
- Levi, E. M., Archer, S. R., & Throop, H. L. (2013, August). Woody detritus decomposition in a shrub-invaded grassland: Interactions among soil deposition, termites and radiant energy. Ecological Soc America Annual Meetings. Minneapolis, MN: Ecological Soc America.
- Levi, E. M., Archer, S. R., Throop, H. L., Predick, K. I., Barnes, P. W., & Tobler, M. A. (2013, October). Soil deposition and UV radiation influence litter decomposition in a shrub-invaded dryland ecosystem. Annual Research Insights in Semi-Arid Ecosystems (RISE) Symposium. University of Arizona: SNRE and USDA ARS.
- Logie, C. J., Bojórquez Ochoa, M., Reynoso, E. D., Law, D. J., Field, J. P., Breshears, D. D., & Archer, S. R. (2013, November). Determining a new dryland decomposition input term: Sediment redistribution to litter surfaces via wind and rain. University of Arizona Grad Blitz. Tucson: Institute of the Environment.
- Logie, C. J., Bojórquez Ochoa, M., Reynoso, E. D., Law, D. J., Fiield, J. P., Breshears, D. D., & Archer, S. R. (2013, October). Determining a new dryland decomposition input term: Sediment redistribution to litter surfaces during fluvial event simulation. Research Insights in Semi-Arid Ecosystems (RISE) Symposium. Tucson: University of Arizona and Agricultural Research Service.
- Logie, C., Ochoa, M. B., Reynoso, E. D., Breshears, D. D., Law, D. J., Field, J. P., & Archer, S. R. (2013, November). Determining a new dryland decomposition input term: sediment redistribution to litter surfaces via wind and rain. UA Grad Blitz. University of Arizona: Office of Graduate Studies.
- O'Shea, C., Gallery, R. E., Predick, K. I., & Archer, S. R. (2013, October). Sonoran Desert soil microbial community responses to vegetation change and grazing. Annual Research Insights in Semi-Arid Ecosystems (RISE) Symposium. University of Arizona: SNRE and USDA ARS.
- O'Shea, C., Predick, K. I., Gallery, R. E., & Archer, S. R. (2013, July). Sonoran Desert soil microbial community response to vegetation change and cattle grazing. Natural Science for Teachers Program Annual Meeting. University of Arizona: UA Dept Education.
- Ochoa, M. B., Logie, C., Reynoso, E. D., Breshears, D. D., Law, D. J., Field, J. P., & Archer, S. R. (2013, August/summer). Determining a new dryland decomposition input term: small simulated fluvial redistribution of sediment to litter surfaces. Research Experience for Undergraduates Workshop. Biosphere 2: Biosphere 2 REU Program.
- Reynoso, D. D., Logie, C., Ochoa, M. B., Law, D. J., Field, J. P., Breshears, D. D., & Archer, S. R. (2013, August). Determining a new dryland decomposition input term: natural events of aeolian and fluvial redistribution of sediment to litter surfaces. Research Experience for Undergraduates Workshop. Biosphere 2: B2 REU Program.
- Wilcox, B., Archer, S. R., Berg, M., Boutton, T. W., Fuhlendorf, S., Heilman, J., McGinnies, K., Mohanty, B. K., Moore, G., Owens, M. K., Schwinning, S., & Walker, J. (2014. Texas State Water Supply Enhancement Plan(p. 5).More infothis report resulted in two members of the writing team (Wilcox, Moore) being invited to testify before the Texas Legislature’s Agriculture Committee. The report was well –received by that committee, and the Chair referred to it as “described it as “concise and powerful”.
- Archer, S. R., Mcclaran, M. P., & Gallery, R. E. (2019, February). Woody Plant Proliferation in Rangelands: Local Aboveground-Belowground Feedbacks with Landscape Consequences. Final Report, Hatch Project 233588 ARTZ-1360540-H12-199.
- Sala, O. E., & Archer, S. R. (2018, February). Exotic Grass and Woody Plant Encroachment in Southwestern Rangelands: Mechanisms of Invasion and Opportunities for Containment. USDA NIFA Annual Report, Award 2016-67013-24932, 6 pp..
- Archer, S. R., & McIntyre, C. (2017, March). Biocrusts, grass establishment and restoration on working rangelands. Final Report, Western Sustainable Agriculture Research & Education (SARE) Project GW15-006. 9 pp.
- Archer, S. R., & McIntyre, C. (2016, June). Annual Report: Biocrusts, grass establishment, and restoration of working rangelands. Western Sustainable Agricultural Research & Education (SARE) Program Project #GW15-006 (5 pp).
- Peters, D., Belstemeyer, B., Havstad, K., Archer, S. R., & et al., . (2016, September). Jornada Basin LTER VI Annual Report: Landscape linkages in arid and semiarid ecosystems. Annual Report (40 pp.). NSF LTER Program.
- Archer, S. R. (2015, April). Plan of Development for Proposed Research on BLM lands (Moab, UT).More infoApplication for permission to conduct the study "Can Biological Soil Crusts Inhibit Germination Or Establishment Of Invasive Grasses In Southwestern Rangelands?"
- Archer, S. R. (2015, January). TA Evaluation, RNR 200: Supervisor's Report.
- Archer, S. R., & McIntyre, C. (2015, Januart). Report: Biocrusts, grass establishment, and restoration of working rangelands: Annual Report. Western Sustainable Agricultural Research & Education Program.
- Archer, S. R., Scott, R., Guertin, D. P., Vivoni, E., Barron-Gafford, G. A., Heilman, P., & Throop, H. (2015, December). Report: Brush management and ecosystem services: a quantification of trade-offs on Western rangelands. USDA NIFA Project Initiation Report.
- McIntyre, C., & Archer, S. R. (2015, August). Plan of Development for Proposed Research on US Forest Service lands (Kaibab NF, Kaibab Ranger District)).More infoProposal for permission to conduct a project entitled "Biocrusts, grass establishment, and restoration of working rangelands"
- McIntyre, C., Archer, S. R., & Belnap, J. (2014, November). Request to conduct research on biocrust-grass interactions. DOI Bureau of Land Management.More infoBLM Moab Field Office. 7 pages.
- Peters, D., Havstad, K., Archer, S. R., Bestelmeyer, B., Okin, G., & Sala, O. (2015, October). Jornada Basin LTER VI Site Review Report (61 pp).
- Archer, S. R. (2014, December). Can biological soil crusts inhibit germination or establishment of invasive grasses in Southwestern Rangelands. USDA NIFA Project Initiation Report.More infoProject Initiation Report, Grant 11630349, DUNS No. 806345617 Controlling Weedy and Invasive Plants Program, USDA/NIFA. 3 pages
- Archer, S. R. (2014, Fall). Woody Plant Proliferation in Rangelands: Rates, Patterns and Consequences. USDA-NIFA Final Report.More infoUSDA-NIFA Final Report, ARZT-1360210-H12-181 (2008-2013). 5 pages.
- Cavallero, N., Ojima, D., Archer, S. R., Tanaka, J., & Bailey, D. (2014, Fall). Grassland Research Priorities. Report to the Indicator System Working Group, National Climate Assessment Program, Washington, DC.More info15 pages
- McIntyre, C., & Archer, S. R. (2014, December). Characterizing the biophysical drivers of biological soil crust distribution in the Sonoran Desert. Final Report, T&E Inc..More info10 pages
- Peters, D., Monger, C., Archer, S. R., Bestelmeyer, B., Havstad, K., & Okin, G. (2014, October). Annual Report: NSF Jornada Basin LTER VI.
- Archer, S. R. (2013, July). Grass-shrub and shrub-shrub interactions in semi-desert grassland. Final Report. National Science Foundation Subcontract Q01298, Prime Award No. DEB-0618210, 10 pp.
- Archer, S. R., & Breshears, D. D. (2013, December). Collaborative research: decomposition in drylands: soil erosion and UV interactions. Final Report, NSF Ecosystems Program, DEB 0816162. 17 pp..
- Peters, D., Bestelmeyer, B., Havstad, K., Herrick, J., Monger, C., Archer, S., Okin, G., Throop, H., & Schooley, R. (2013, November). Jornada Basin LTER V: Landscape linkages in arid and semiarid ecosystems. Final Report, NSF-LTER DEB-0618210. 60 pp.
- Peters, D., Monger, C., Havstad, K., Bestelmeer, B., & Archer, S. R. (2013, October). Jornada Basin LTER VI: Landscape linkages in arid and semiarid ecosystems. Annual Report, NSF LTER DEB-12-35828, 28 pp..