Progress 02/01/05 to 01/31/10
Outputs
OUTPUTS: Our objectives were to (a) develop a spatial data base that will enable a landscape-scale accounting of ecosystem carbon (C) pools on sites with contrasting management histories; and (b) use a dynamic ecosystem model (CENTURY) to (i) reconstruct how ecosystem C pools have changed with woody plant proliferation, grazing, and brush management; and (ii) project how future pools might change. Our study site (Santa Rita Experimental Range, southerb AZ) is semi-desert grassland with a well-documented history of livestock grazing and shrub (Prosopis velutina) encroachment. Allometric equations enabled us to quantify aboveground grass and shrub C pools using non-destructive measurements. These equations were applied in field surveys to quantify aboveground grass C mass under shrub canopies and within inter-canopy zones. These data, in combination with soil organic C (SOC) data, were used to quantify how ecosystem C pools change with woody plant encroachment into grassland, parameterize CENTURY, and evaluate its performance. Studies assessing soil responses to woody plant proliferation seldom account for shrub size and sub-canopy spatial gradients. We found that bulk density (BD) increased with distance from the P. velutina bole and decreased with increasing shrub size, while both SOC increased with shrub size and decreased with distance from the bole. Significant (but predictable) spatial variation in BD suggests use of generic values would generate unreliable estimates of SOC mass, and subcanopy SOC pools could be overestimated by nearly 30% if intercanopy BD values were applied to subcanopy sites. Predictive models based on field-documented spatial patterns were used to generate integrated estimates of under-shrub SOC pools and these were compared to 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 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 We adapted the CENTURY model for dryland systems, parameterized it for a mid-elevation site at the Santa Rita Experimental Range and evaluated its performance. The model will be used to simulate ecosystem C and N responses to changes in perennial grasses and shrub abundance in the context of changes in livestock grazing, climate, brush management and fire regimes. The model can now be used to reconstruct past ecosystem C pools for a range of land use scenarios and project future changes that may occur with changes in climate and land use practices. An unexpected outcome of this project was an analysis of the implications of methods for quantifying bulk density on soil C accounting. A large variation in the methods used for calculating bulk density currently exists, and this methodological inconsistency has considerable implications for calculated SOC pools, particularly in rocky soils. PARTICIPANTS: Dr. Steve Archer, PI. Overall project management; Dr. Mitch McClaran, Co-PI. Selection of field sites, shrub and grass allometry, grass ANPP, and data interpretation; Dr. Heather Throop, Co-PI. C/N analyses and CENTURY modelling; Mr. Mark Heitlinger, Collaborator. Assist with field logistics at Santa Rita Experimental Range; Dr. R. Steidl, Collaborator. Statistical consultation; Mr. Chad McMurtry, Research Associate, supervising field campaigns, sample processing, maintaining databases, and statistical and graphical data summaries; Ms. Chloe Tewksbury, PhD student, plant and soil field sampling, sample processing, data analysis. Ms. Aleta Nafus, MS student, grass allometry; Ms. F. Mashiri, PhD student, field assistant; Undergraduate student workers: K. Boyce, E. Hoggatt, E. Hurd, D. King, J. Mudd, A. Vogel, M. Castillo, J. Root and R. Wu (University of Arizona) and M. Calkins, J. Lewis, L. Ebbs, J. Sprague, J. Fitzgerald, J. Perez, T. Clawson, D. Lyman (New Mexico State University) TARGET AUDIENCES: Rangeland managers, Land Management Professionals, Soil Scientists, Biogeochemists, Dryland Ecologists, Plant Ecologists, Remote Sensing Scientists, Ecosystem Scientists, Society for Range Management, Ecological Society of America, International Association of Landscape Ecologists, International Association of Vegetation Scientists, American Geophysical Union PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Results from the grass allometry study suggest multi-species models provide a robust, non-destructive allometric method for estimating grass ANPP in semi-desert rangeland. Field-time efficiency will be maximized when using the diameter-only multi-species model because fewer measures will be needed and species identification will be unnecessary. Grazing history was a significant explanatory variable for 2 of the 8 species examined. Furthermore, it explained only an additional 1% of the variance in biomass estimates and was significant only when height was not included in the model. Inclusion of height in allometric models may therefore be warranted in situations where grazing is spatially variable and when grazing history is unknown. Remote sensing approaches have made considerable progress in estimating aboveground C stores; and linking aboveground cover to SOC pools would provide important information on woody plant impacts on C sequestration. Results of our study illustrate, however, that simple estimates of SOC stores based on area-weighted woody plant cover alone will not provide accurate estimates of SOC stores. Knowledge of stand age-structure is critical for forecasting ecosystem C sequestration potential. Along these lines, our data suggest remote sensing approaches that derive shrub size-class distributions in drylands could be coupled with algorithms that relate spatial patterns of SOC to shrub canopy area to estimate ecosystem C (aboveground woody plant + SOC) pools over large areas. A library of SOC-canopy area algorithms would be needed to account for specific differences likely to occur for soils, land forms, woody species or functional groups, land management scenarios, and climate regimes. Recent state, federal and international initiatives aimed at C flux regulation suggest C emission reduction credits may soon be a reality. We suggest that landscape-scale SOC estimates in drylands based on woody plant presence or absence alone will be inaccurate unless additional information on shrub size and SOC distribution is included. Current C accounting tools (e.g., Comet-VR, a USDA-NRCS voluntary reporting C management tool; http://www.cometvr.colostate.edu/) provide crude estimates of changes in SOC in response to management based on information such as soil type. These coarse-scale tools are currently limited, however, by lack of inclusion of spatial and temporal data on the structure of vegetation cover and land management history. Development of algorithms that can represent or account for these factors will be needed to advance our ability to conduct large-scale C accounting programs.
Publications
- Throop, H., and S. Archer. 2008. Shrub (Prosopis velutina) encroachment in a semi-desert grassland: spatial-temporal changes in soil organic carbon and nitrogen pools. Global Change Biology 14:2420-2431. Browning, DM, Archer, SR, Byrne, AT. 2009. Field validation of 1930s aerial photography: What are we missing Journal of Arid Environments 73:844-853.
- Nafus, A., M. P. McClaran, S. R. Archer, and H. L. Throop. 2009. Multi-species allometric models predict grass biomass in semi-Desert rangeland. Rangeland Ecology & Management 62:68-72
- Archer, S, KW Davies, TE Fulbright, K McDaniel, and BP Wilcox. 2009. Brush Management and Conservation: New Perspectives on an Old Problem in "Conservation Effects Assessment Project (CEAP) Rangeland Literature Synthesis: An Initial Report" Symposium. Society for Range Management Annual Meetings, Albuquerque, NM.
- Boutton,TW, JD Liao, TR. Filley,SR Archer. 2009. Belowground carbon storage and dynamics following woody plant encroachment in a subtropical savanna, pp. 181-205. In: Soil Carbon Sequestration and the Greenhouse Effect (R. Lal and R. Follett, eds.). Soil Science Society of America, Madison, WI.
- Asner, GP, SR Archer. 2010. Livestock and the global carbon cycle, pp. 69-82. In: Livestock in a Changing Landscape: Volume 1, Drivers, Consequences and Responses (H Steinfeld, HA Mooney, F Schneider, LE Neville, eds.). Island Press, Washington, D.C.
- Archer, SR. 2010. Rangeland conservation and shrub encroachment: new perspectives on an old problem, pp. 53-97. In: Wild Rangelands: Conserving Wildlife While Maintaining Livestock in Semi-Arid Ecosystems (Johan du Toit, R. Kock, J. Deutsch, eds.). Wiley-Blackwell, Oxford.
- Browning, DM, Archer, SR. 2011. Protection from livestock fails to deter shrub proliferation in a desert landscape with a history of heavy grazing. Ecological Applications 21:1629-1642.
- Throop, HL, SR Archer, HC Monger, S Waltman. 2012. When bulk density methods matter: implications for estimating soil organic carbon pools in coarse soils. Journal of Arid Environments 77:66-71.
- Archer, SR, Pierce, N, Lamanna, CA. 2010. Lateral roots and lignotubers: overlooked components of ecosystem carbon pools in drylands. Ecological Soc. America Annual Meetings, Pittsburgh, PA.
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Progress 02/01/08 to 01/31/09
Outputs
OUTPUTS: The objectives of this project are to (a) develop a spatial data base that will enable a landscape-scale accounting of ecosystem carbon (C) and nitrogen (N) pools on sites with contrasting management histories; and (b) use a dynamic ecosystem model (CENTURY) to (i) reconstruct how ecosystem C and N pools have changed with woody plant proliferation, grazing, and brush management; and (ii) project how future pools might change. Our study site (Santa Rita Experimental Range, southeastern AZ) is a semi-desert grassland with a well-documented history of livestock grazing and woody plant (Prosopis velutina) encroachment. Prior studies at the Santa Rita Experimental Range in southern Arizona found that values obtained using traditional area-weighting approaches 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. In contrast, we have recently explored spatial patterns of SOC around P. glandulosa coppice dunes in the Chihuahuan Desert. In this system we find a significant difference in SOC pools on and off dunes, but no significant within-dune spatial patterns or differences with dune size. We are currently in the process of sample and data analysis to assess (a) how these spatial and temporal patterns of SOC and TN around P. velutina in the Sonoran Desert are affected by grazing and brush management; and (b) if observed canopy-to-dripline gradients associated with individual, widely spaced P. velutina shrubs can be reliably applied to multi-plant patches of P. velutina; or if different algorithms will be required to deal with these cases. We have adapted the CENTURY model for dryland systems and have parameterized it for our mid-elevation site at the Santa Rita Experimental Range. We are currently using the model to simulate biogeochemical pool responses to changes in the abundance of (perennial grasses and mesquite shrubs. We have reconstructed past soil C pools by running model simulations for a range of historical grazing and shrub encroachment scenarios. We have then coupled these historical scenarios with a variety of current grazing and brush management scenarios to explore the implications of management on C pools. Future modeling will explore the impacts of climate change on biogeochemical responses to management. The initial stages of this work are summarized in a manuscript that is currently in preparation. An unexpected outcome of this project was an analysis of the implications of methods for quantifying bulk density on soil C accounting. A large variation in the methods used for calculating bulk density currently exists, and this methodological inconsistency has considerable implications for calculated SOC pools, particularly in rocky soils. We have completed an analysis of how different methods affect calculated SOC pools and have drafted a manuscript on this topic that will be submitted early in 2009. PARTICIPANTS: Dr. Steve Archer, PI. Overall project management; Dr. Mitch McClaran, Co-PI. Selection of field sites, grass ANPP, and data interpretation; Dr. Heather Throop, Co-PI. C/N analyses and CENTURY modelling; Dr. Mr. Mark Heitlinger, Collaborator. Assist with field logistics at Santa Rita Experimental Range; Dr. R. Steidl, Collaborator. Statistical consultation; Katie Predick, Research Associate, supervising field campaigns, sample processing, maintaining databases, and statistical and graphical data summaries. Ms. Aleta Nafus, MS student, grass allometry; Ms. F. Mashiri, PhD student, field assistant; Undergraduate student workers: B.Aragon, S. Bluemer, M. Ferman A. Habgood, M. Hanna, C. Picraux, S. Raghailligh, A. Sheperd, F. Sunanto and R. Wu (University of Arizona) and M. Calkins, J. Lewis, L. Ebbs, J. Sprague, J. Fitzgerald, J. Perez, T. Clawson, D. Lyman (New Mexico State University) TARGET AUDIENCES: Rangeland managers, Land Management Professionals, Soil Scientists, Biogeochemists, Dryland Ecologists, Plant Ecologists, Remote Sensing Scientists, Ecosystem Scientists, Society for Range Management, Ecological Society of America, International Association of Landscape Ecologists, International Association of Vegetation Scientists, American Geophysical Union PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Numerous factors affect uncertainties in terrestrial C budgets. Some reflect inconsistencies in how carbon mass data are collected and expressed. In the case of soil organic carbon (SOC), substantial variation in pool sizes can result from methodological inconsistencies in estimating bulk density. Our results suggest these inconsistencies are largely the result of differences in how coarse fragments are handled in bulk density calculations. For example, the commonly used core method may under-sample coarse fragments if they are too large to fit in the corer. The core method may also yield inaccurate results in soils where compaction occurs as a result of inserting the corer in fine-textured soils. While there are limitations to the core method, the benefits of ease and large number of samples that can be collected suggest it will remain an important method. We advocate that papers (i) explicitly articulate of how coarse fragment mass and volume are treated in bulk density calculations; and (ii) report the proportional volume of coarse fragments in their samples. These refinements will facilitate future comparisons among studies. Remote sensing approaches have made considerable progress in estimating aboveground C stores; and linking aboveground cover to SOC pools would provide important information on woody plant impacts on C sequestration. Results of our study illustrate, however, that simple estimates of SOC based on area-weighted woody plant cover alone will not provide accurate estimates of SOC pools. Knowledge of stand age-structure is critical for forecasting ecosystem C sequestration potential. Along these lines, our data suggest remote sensing approaches that derive shrub size-class distributions in drylands could be coupled with algorithms that relate spatial patterns of SOC to shrub canopy area to estimate ecosystem C (aboveground woody plant + SOC) pools over large areas. A library of SOC-canopy area algorithms would be needed to account for specific differences likely to occur for soils, land forms, woody species or functional groups, land management scenarios, and climate regimes. Recent state, federal and international initiatives aimed at C flux regulation suggest C emission reduction credits may soon be a reality. We suggest that landscape-scale SOC estimates in drylands based on woody plant presence or absence alone will be inaccurate unless additional information on shrub size and SOC distribution is included. Current C accounting tools (e.g., Comet-VR, a USDA-NRCS voluntary reporting C management tool; http://www.cometvr.colostate.edu/) provide crude estimates of changes in SOC in response to management based on information such as soil type. These coarse-scale tools are currently limited, however, by lack of inclusion of spatial and temporal data on the structure of vegetation cover and land management history. Development of algorithms that can represent or account for these factors will be needed to advance our ability to conduct large-scale C accounting programs.
Publications
- Throop, H.L. 2008. Outreach to a inner-city high school: Five 50 minute lectures on vegetation change, the carbon cycle, and climate to Oceanography and Astronomy classes at Cleveland High School, Portland, OR.
- Ebbs, L. and H.L. Throop. 2008. Spatial Patterns of Soil Organic Carbon and Total Nitrogen in Mesquite Coppice Dunes. American Geophysical Union Annual Fall Meeting. San Francisco, CA. (abstract, poster)
- Throop, H.L., K. Lajtha, P. Sollins H.C. Monger. 2008. Assessing Organic Carbon Stabilization in Chihuahuan Desert Soils Using Sequential Density Fractionation. American Geophysical Union Annual Fall Meeting. San Francisco, CA. (abstract, poster)
- Archer S, Throop H, McClaran M. 2008. Ecosystem carbon and nitrogen pools in managed rangelands: a spatial accounting of management influences. In: Project Directors Workshop, USDA Managed Ecosystems Workshop, Madison, WI (abstract)
- Archer, S. 2008. Grasslands in Transition: Emerging Issues and Challenges in the Western USA. In: Symposium on Assessing the Multi-Functionality of Grasslands-Future Research Priorities to Address Global Change. SA-CSSA-SSSA annual meeting in Houston, Texas (Abstract)
- Boutton,TW, JD Liao, TR. Filley,SR Archer. 2008. Belowground carbon storage and dynamics following woody plant encroachment in a subtropical savanna, pp. 181-205. In: Soil Carbon Sequestration and the Greenhouse Effect (R. Lal and R. Follett, eds.). Soil Science Society of America, Madison, WI.
- Archer, S. 2008. Rangeland conservation and shrub encroachment: new perspectives on an old problem. In: Rangelands or Wildlands Livestock and Wildlife in Semi-Arid Ecosystems (Johan du Toit, R. Kock, J. Deutsch, eds.). Blackwell Publishing Ltd. (In Press).
- Archer, S. Boutton TW, McClaran, MP, Throop, HL Wu, XB. 2008. Grasses, shrubs and the functional consequences of ecosystem state changes. In: Current Ecological Issues n Grassland Science Symposium (RL McCulley, organizer), Society for Range Management Annual Meetings, Louisville, KY (Abstract)
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Progress 02/01/07 to 01/31/08
Outputs
OUTPUTS: The objectives of this project are to (a) develop a spatial data base that will enable a landscape-scale accounting of ecosystem carbon (C) and nitrogen (N) pools on sites with contrasting management histories; and (b) use a dynamic ecosystem model (CENTURY) to (i) reconstruct how ecosystem C and N pools have changed with woody plant proliferation, grazing, and brush management; and (ii) project how future pools might change. Our study site (Santa Rita Experimental Range, southeastern AZ) is a semi-desert grassland with a well-documented history of livestock grazing and woody plant (Prosopis velutina) encroachment. Allometric equations developed previously enable us to quantify aboveground shrub C pools using non-destructive measurements. In this study, we developed algorithms for non-destructively estimating grass biomass. Field measurements (2005, 2006) were used to develop regression models to predict aboveground biomass for 8 common grass species. Predictive ability of
multi-species general models (R2 = 0.53-0.88) compared favorably with species-specific models (R2 = 0.57-0.95). Basal diameter had the strongest relationship with biomass; and height the weakest. Grazing history was either insignificant (6 species) or of little consequence (increased R2 by 1%). Results suggest that a single biomass-size relationship is robust across different grass growth forms. These allometric equations will next be applied to recently completed field surveys to quantify aboveground grass C and N mass under shrub canopies and within inter-canopy zones. These data, in combination with existing data on shrub biomass will enable us to quantify ecosystem (grass plus shrub) aboveground C and N pools, parameterize CENTURY, and evaluate its performance. Prior studies assessing soil responses to woody plant proliferation have not explicitly accounted for shrub size and sub-canopy spatial gradients and the effects these may have on estimates of soil organic carbon (SOC) and
total nitrogen (TN) pools. We found that bulk density (BD) increased with distance from the P. velutina bole and decreased with increasing shrub size, while both SOC and TN increased with shrub size and decreased with distance from the bole. Significant (but predictable) spatial variation in BD suggests 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 BD 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 to 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 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.
PARTICIPANTS: Dr. Steve Archer, PI. Overall project management; Dr. Mitch McClaran, Co-PI. Selection of field sites, grass ANPP, and data interpretation; Dr. Heather Throop, Co-PI. C/N analyses and CENTURY modelling; Dr. Mr. Mark Heitlinger, Collaborator. Assist with field logistics at Santa Rita Experimental Range; Dr. R. Steidl, Collaborator. Statistical consultation; Mr. Chad McMurtry, Research Associate, supervising field campaigns, sample processing, maintaining databases, and statistical and graphical data summaries; Ms. Chloe Tewksbury, PhD student, plant and soil field sampling, sample processing, data analysis. Ms. Aleta Nafus, MS student, grass allometry; Ms. F. Mashiri, PhD student, field assistant; Undergraduate student workers: K. Boyce, E. Hoggatt, E. Hurd, D. King, J. Mudd, A. Vogel, M. Castillo, J. Root and R. Wu (University of Arizona) and M. Calkins, J. Lewis, L. Ebbs, J. Sprague (New Mexico State University)
TARGET AUDIENCES: Rangeland managers, Land Management Professionals, Soil Scientists, Biogeochemists, Dryland Ecologists, Plant Ecologists, Remote Sensing Scientists, Ecosystem Scientists, Society for Range Management, Ecological Society of America, International Association of Landscape Ecologists, International Association of Vegetation Scientists, American Geophysical Union
PROJECT MODIFICATIONS: When the proposal was funded, Co-PI Throop was a NOAA post-doctoral fellow at the University of Arizona. When Dr. Throop's NOAA funding terminated in September 2005, her postdoctoral salary was supported in full by the Managed Ecosystems Award. In August 2006 Dr. Throop began a tenure-track Assistant Professor position at New Mexico State University, Las Cruces (4 hour drive from Tucson). Dr. Throop has remained actively involved in the research, spending much of her non-teaching time on this project. Co-PIs McClaran and Archer (University of Arizona) are in regular contact with Dr. Throop via phone and email; and she visits the UA campus and the field site on a regular basis. Her current activities include: plant/soil sample processing and C/N analyses, full responsibility for CENTURY modeling, data organization and analyses and dissemination of research results (presentations at American Geophysical Union, Ecological Soc. America, and to the general public
(Las Cruces Natural History Museum). Another personnel change has necessitated making some adjustments. Chloe Tewksbury, a PhD student supported on this grant, left the project in May 2007 to pursue an opportunity with an environmental consulting firm. We deemed it unrealistic to find another graduate student at this juncture in the project, in large part because the remaining funds would not cover the stipend of a graduate research assistant (GRA) for the full graduate program of a new student. Dr. Throop, now at NMSU (see above) has taken over much of the sample processing and all of the sample analyses that were being done at UA by this graduate student; and we plan on using the funds remaining in the GRA stipend to hire a part-time technician and undergraduate students to assist with the remaining work. Getting NMSU officially involved in this project will provide research opportunities for students at this federally-designated Hispanic-serving and EPSCoR institution.
Impacts
Results from the grass allometry study suggest multi-species models provide a robust, non-destructive allometric method for estimating grass ANPP in semi-desert rangeland. Field-time efficiency will be maximized when using the diameter-only multi-species model because fewer measures will be needed and species identification will be unnecessary. Grazing history was a significant explanatory variable for 2 of the 8 species examined. Furthermore, it explained only an additional 1% of the variance in biomass estimates and was significant only when height was not included in the model. Inclusion of height in allometric models may therefore be warranted in situations where grazing is spatially variable and when grazing history is unknown. These allometric equations will next be applied to recently completed field surveys to quantify aboveground grass C and N mass under shrub canopies (along bole-to-dripline gradients) and within inter-canopy zones. These data, in combination
with existing data on shrub biomass will enable us to quantify ecosystem (grass plus shrub) aboveground C and N pools, parameterize CENTURY and evaluate its performance with respect to predicting aboveground primary production. Remote sensing approaches have made considerable progress in estimating aboveground C stores; and linking aboveground cover to SOC pools would provide important information on woody plant impacts on C sequestration. Results of our study illustrate, however, that simple estimates of SOC stores based on area-weighted woody plant cover alone will not provide accurate estimates of SOC stores. Knowledge of stand age-structure is critical for forecasting ecosystem C sequestration potential. Along these lines, our data suggest remote sensing approaches that derive shrub size-class distributions in drylands could be coupled with algorithms that relate spatial patterns of SOC to shrub canopy area to estimate ecosystem C (aboveground woody plant + SOC) pools over large
areas. A library of SOC-canopy area algorithms would be needed to account for specific differences likely to occur for soils, land forms, woody species or functional groups, land management scenarios, and climate regimes. Recent state, federal and international initiatives aimed at C flux regulation suggest C emission reduction credits may soon be a reality. We suggest that landscape-scale SOC estimates in drylands based on woody plant presence or absence alone will be inaccurate unless additional information on shrub size and SOC distribution is included. Current C accounting tools (e.g., Comet-VR, a USDA-NRCS voluntary reporting C management tool; http://www.cometvr.colostate.edu/) provide crude estimates of changes in SOC in response to management based on information such as soil type. These coarse-scale tools are currently limited, however, by lack of inclusion of spatial and temporal data on the structure of vegetation cover and land management history. Development of algorithms
that can represent or account for these factors will be needed to advance our ability to conduct large-scale C accounting programs.
Publications
- Throop, H., and S. Archer. 2008. Shrub (Prosopis velutina) encroachment in a semi-desert grassland: spatial-temporal changes in soil organic carbon and nitrogen pools. Global Change Biology (In Press).
- Nafus, A., M. P. McClaran, S. R. Archer, and H. L. Throop. 2008. Multi-species allometric models predict grass biomass in semi-Desert rangeland. Rangeland Ecology & Management (submitted)
- Asner, G., and S. Archer. 2008. Livestock and the Global Carbon Cycle. In H. Mooney, H. Steinfeld, F. Schneider, and L. E. Neville, eds. Livestock in a Changing Landscape: Drivers, Consequences and Responses. Island Press, Washington, D.C. (In Press)
- Nafus, A. 2007. Using allometry to predict biomass for eight semi-desert grass species. MS Thesis. School of Natural Resources. University of Arizona.
- Nafus, A., M.P. McClaran, and C. McMurtry. 2007. A general model for estimating biomass of desert grassland grasses.Abstract, Society for Range Management Annual Meetings in Reno, NV.
- Throop, H.L., S.R. Archer, C.R. McMurtry, and M.P. McClaran. 2006. Relationships between aboveground woody biomass and soil organic carbon in a semi-desert grassland. Abstract, Ecological Society of America Annual Meeting, Memphis, TN.
- Throop, H.L., S.R. Archer, C.R. McMurtry, and M.P. McClaran. 2006. Predicting soil organic carbon from woody biomass. Abstract, LTER All Scientists Meeting, Estes Park, CO.
- Throop, H.L., S.R. Archer, C.R. McMurtry, and M.P. McClaran. 2006. Relationships between aboveground woody biomass and soil organic carbon in a semi-desert grassland. Abstract, Research Insights in Semi-Arid Ecosystems 3rd Annual Symposium. Tucson, AZ.
- McMurtry, C., S. Archer, M. McClaran, D. Browning, and H. Throop. Poster: Estimating Prosopis velutina aboveground biomass: field and remote sensing approaches. Research Insights in Semi-Arid Ecosystems 3rd Annual Symposium. Tucson, AZ. October 2006.
- Throop, H., S. Archer, and M. McClaran. 2007. Long-term influence of woody plant removal on soil organic carbon pools in a semi-desert grassland. Abstract, Ecological Society of America Annual Meetings, San Jose, CA.
- Throop, H., S. Archer, M. McClaran, D. Ojima, R. Keough, and W. Parton. 2006. Soil organic carbon responses to grazing and woody plant encroachment in a semi-desert grassland. Abstract, American Geophysical Union Meetings, San Francisco, CA.
- Throop, H., S. Archer, M. McClaran, D. Ojima, W. Parton. 2007. Modeled soil organic carbon responses to grazing and woody encroachment in a semi-desert grassland. Abstract, Society for Range Management Annual Meeting. Reno, NV.
- Tewksbury, C., and S. Archer. 2007. Coarse woody debris in a semi-desert grassland, Ariel Appleton Research Fellowship Report.
- Throop, H. 2007. Shifting vegetation patterns and global climate change in the desert Southwest. Presentation, Las Cruces Museum of Natural History.
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Progress 02/01/06 to 01/31/07
Outputs
OUTPUTS: The objectives of this project are to (a) develop a spatial data base that will enable a landscape-scale accounting of ecosystem carbon (C) and nitrogen (N) pools on sites with contrasting management histories; and (b) use a dynamic ecosystem model (CENTURY) to (i) reconstruct how ecosystem C and N pools have changed with woody plant proliferation, grazing, and brush management; and (ii) project how future pools might change. Our study site (Santa Rita Experimental Range, southeastern AZ) is a semi-desert grassland with a well-documented history of livestock grazing and woody plant (Prosopis velutina) encroachment. Allometric equations developed previously enable us to quantify aboveground shrub C pools using non-destructive measurements. In this study, we developed algorithms for non-destructively estimating grass biomass. Field measurements (2005, 2006) were used to develop regression models to predict aboveground biomass for 8 common grass species. Predictive ability of
multi-species general models (R2 = 0.53-0.88) compared favorably with species-specific models (R2 = 0.57-0.95). Basal diameter had the strongest relationship with biomass; and height the weakest. Grazing history was either insignificant (6 species) or of little consequence (increased R2 by 1%). Results suggest that a single biomass-size relationship is robust across different grass growth forms. These allometric equations will next be applied to recently completed field surveys to quantify aboveground grass C and N mass under shrub canopies and within inter-canopy zones. These data, in combination with existing data on shrub biomass will enable us to quantify ecosystem (grass plus shrub) aboveground C and N pools, parameterize CENTURY, and evaluate its performance. Prior studies assessing soil responses to woody plant proliferation have not explicitly accounted for shrub size and sub-canopy spatial gradients and the effects these may have on estimates of soil organic carbon (SOC) and
total nitrogen (TN) pools. We found that bulk density (BD) increased with distance from the P. velutina bole and decreased with increasing shrub size, while both SOC and TN increased with shrub size and decreased with distance from the bole. Significant (but predictable) spatial variation in BD suggests 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 BD 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 to 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 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.
PARTICIPANTS: Dr. Steve Archer, PI. Overall project management; Dr. Mitch McClaran, Co-PI. Selection of field sites, grass ANPP, and data interpretation; Dr. Heather Throop, Co-PI. C/N analyses and CENTURY modelling; Dr. Mr. Mark Heitlinger, Collaborator. Assist with field logistics at Santa Rita Experimental Range; Dr. R. Steidl, Collaborator. Statistical consultation; Mr. Chad McMurtry, Research Associate, supervising field campaigns, sample processing, maintaining databases, and statistical and graphical data summaries; Ms. Chloe Tewksbury, PhD student, plant and soil field sampling, sample processing, data analysis. Ms. Aleta Nafus, MS student, grass allometry; Ms. F. Mashiri, PhD student, field assistant; Undergraduate student workers: K. Boyce, E. Hoggatt, E. Hurd, D. King, J. Mudd, A. Vogel, M. Castillo, J. Root and R. Wu (University of Arizona) and M. Calkins, J. Lewis, L. Ebbs, J. Sprague (New Mexico State University)
TARGET AUDIENCES: Rangeland managers, Land Management Professionals, Soil Scientists, Biogeochemists, Dryland Ecologists, Plant Ecologists, Remote Sensing Scientists, Ecosystem Scientists, Society for Range Management, Ecological Society of America, International Association of Landscape Ecologists, International Association of Vegetation Scientists, American Geophysical Union
PROJECT MODIFICATIONS: When the proposal was funded, Co-PI Throop was a NOAA post-doctoral fellow at the University of Arizona. When Dr. Throop's NOAA funding terminated in September 2005, her postdoctoral salary was supported in full by the Managed Ecosystems Award. In August 2006 Dr. Throop began a tenure-track Assistant Professor position at New Mexico State University, Las Cruces (4 hour drive from Tucson). Dr. Throop has remained actively involved in the research, spending much of her non-teaching time on this project. Co-PIs McClaran and Archer (University of Arizona) are in regular contact with Dr. Throop via phone and email; and she visits the UA campus and the field site on a regular basis. Her current activities include: plant/soil sample processing and C/N analyses, full responsibility for CENTURY modeling, data organization and analyses and dissemination of research results (presentations at American Geophysical Union, Ecological Soc. America, and to the general public
(Las Cruces Natural History Museum). Another personnel change has necessitated making some adjustments. Chloe Tewksbury, a PhD student supported on this grant, left the project in May 2007 to pursue an opportunity with an environmental consulting firm. We deemed it unrealistic to find another graduate student at this juncture in the project, in large part because the remaining funds would not cover the stipend of a graduate research assistant (GRA) for the full graduate program of a new student. Dr. Throop, now at NMSU (see above) has taken over much of the sample processing and all of the sample analyses that were being done at UA by this graduate student; and we plan on using the funds remaining in the GRA stipend to hire a part-time technician and undergraduate students to assist with the remaining work. Getting NMSU officially involved in this project will provide research opportunities for students at this federally-designated Hispanic-serving and EPSCoR institution.
Impacts
Results from the grass allometry study suggest multi-species models provide a robust, non-destructive allometric method for estimating grass ANPP in semi-desert rangeland. Field-time efficiency will be maximized when using the diameter-only multi-species model because fewer measures will be needed and species identification will be unnecessary. Grazing history was a significant explanatory variable for 2 of the 8 species examined. Furthermore, it explained only an additional 1% of the variance in biomass estimates and was significant only when height was not included in the model. Inclusion of height in allometric models may therefore be warranted in situations where grazing is spatially variable and when grazing history is unknown. These allometric equations will next be applied to recently completed field surveys to quantify aboveground grass C and N mass under shrub canopies (along bole-to-dripline gradients) and within inter-canopy zones. These data, in
combination with existing data on shrub biomass will enable us to quantify ecosystem (grass plus shrub) aboveground C and N pools, parameterize CENTURY and evaluate its performance with respect to predicting aboveground primary production. Remote sensing approaches have made considerable progress in estimating aboveground C stores; and linking aboveground cover to SOC pools would provide important information on woody plant impacts on C sequestration. Results of our study illustrate, however, that simple estimates of SOC stores based on area-weighted woody plant cover alone will not provide accurate estimates of SOC stores. Knowledge of stand age-structure is critical for forecasting ecosystem C sequestration potential. Along these lines, our data suggest remote sensing approaches that derive shrub size-class distributions in drylands could be coupled with algorithms that relate spatial patterns of SOC to shrub canopy area to estimate ecosystem C (aboveground woody plant + SOC) pools
over large areas. A library of SOC-canopy area algorithms would be needed to account for specific differences likely to occur for soils, land forms, woody species or functional groups, land management scenarios, and climate regimes. Recent state, federal and international initiatives aimed at C flux regulation suggest C emission reduction credits may soon be a reality. We suggest that landscape-scale SOC estimates in drylands based on woody plant presence or absence alone will be inaccurate unless additional information on shrub size and SOC distribution is included. Current C accounting tools (e.g., Comet-VR, a USDA-NRCS voluntary reporting C management tool; http://www.cometvr.colostate.edu/) provide crude estimates of changes in SOC in response to management based on information such as soil type. These coarse-scale tools are currently limited, however, by lack of inclusion of spatial and temporal data on the structure of vegetation cover and land management history. Development of
algorithms that can represent or account for these factors will be needed to advance our ability to conduct large-scale C accounting programs.
Publications
- Throop, H.L., S.R. Archer, C.R. McMurtry, and M.P. McClaran. 2006. Relationships between aboveground woody biomass and soil organic carbon in a semi-desert grassland. Abstract, Research Insights in Semi-Arid Ecosystems 3rd Annual Symposium. Tucson, AZ.
- McMurtry, C., S. Archer, M. McClaran, D. Browning, and H. Throop. Poster: Estimating Prosopis velutina aboveground biomass: field and remote sensing approaches. Research Insights in Semi-Arid Ecosystems 3rd Annual Symposium. Tucson, AZ. October 2006.
- Throop, H., S. Archer, and M. McClaran. 2007. Long-term influence of woody plant removal on soil organic carbon pools in a semi-desert grassland. Abstract, Ecological Society of America Annual Meetings, San Jose, CA.
- Throop, H., S. Archer, M. McClaran, D. Ojima, R. Keough, and W. Parton. 2006. Soil organic carbon responses to grazing and woody plant encroachment in a semi-desert grassland. Abstract, American Geophysical Union Meetings, San Francisco, CA.
- Throop, H., S. Archer, M. McClaran, D. Ojima, W. Parton. 2007. Modeled soil organic carbon responses to grazing and woody encroachment in a semi-desert grassland. Abstract, Society for Range Management Annual Meeting. Reno, NV.
- Throop, H., and S. Archer. 2008. Shrub (Prosopis velutina) encroachment in a semi-desert grassland: spatial-temporal changes in soil organic carbon and nitrogen pools. Global Change Biology (In Press).
- Nafus, A., M. P. McClaran, S. R. Archer, and H. L. Throop. 2008. Multi-species allometric models predict grass biomass in semi-Desert rangeland. Rangeland Ecology & Management (submitted)
- Asner, G., and S. Archer. 2008. Livestock and the Global Carbon Cycle. In H. Mooney, H. Steinfeld, F. Schneider, and L. E. Neville, eds. Livestock in a Changing Landscape: Drivers, Consequences and Responses. Island Press, Washington, D.C. (In Press)
- Nafus, A. 2007. Using allometry to predict biomass for eight semi-desert grass species. MS Thesis. School of Natural Resources. University of Arizona.
- Nafus, A., M.P. McClaran, and C. McMurtry. 2007. A general model for estimating biomass of desert grassland grasses.Abstract, Society for Range Management Annual Meetings in Reno, NV.
- Throop, H.L., S.R. Archer, C.R. McMurtry, and M.P. McClaran. 2006. Relationships between aboveground woody biomass and soil organic carbon in a semi-desert grassland. Abstract, Ecological Society of America Annual Meeting, Memphis, TN.
- Throop, H.L., S.R. Archer, C.R. McMurtry, and M.P. McClaran. 2006. Predicting soil organic carbon from woody biomass. Abstract, LTER All Scientists Meeting, Estes Park, CO.
- Tewksbury, C., and S. Archer. 2007. Coarse woody debris in a semi-desert grassland, Ariel Appleton Research Fellowship Report.
- Throop, H. 2007. Shifting vegetation patterns and global climate change in the desert Southwest. Presentation, Las Cruces Museum of Natural History.
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Progress 02/01/05 to 01/31/06
Outputs
The overall objectives of this project are to (a) develop a spatial data base that will enable a landscape-scale accounting of ecosystem carbon (C) and nitrogen (N) pools on sites with contrasting grazing and brush encroachment/management histories; and (b) use a dynamic ecosystem model (CENTURY) to (i) reconstruct how ecosystem C and N pools have changed with woody plant proliferation, grazing, and brush management; and (ii) project how future pools might change under altered grazing and brush management scenarios. Two primary field campaigns were conducted in the spring and summer of 2005, the first year of this study. Data from these field campaigns will be used to construct the spatial database of ecosystem C and N pools. Both field campaigns were designed to quantify spatial patterns of ecosystem C and N pools under different historical grazing and brush management regimes. An extensive collection of soil cores were obtained for determining spatial distribution
of soil organic carbon, total nitrogen, and root C pools in relation to mesquite trees of varyng basal areas. Laboratory processing and analyses of these samples has been initiated and will continue into 2006. Initial results suggest soil C and N pools are greatest towards the center of tree canopies and decrease with increasing soil depth. Grazing decreases overall soil C and N pools. We will complete soil sampling with an additional smaller field sampling campaign this winter; and an additional set of cores for a second root C and N determination will be made during summer 2006. The spatial distribution of herbaceous biomass, cover and density relative to mesquite size and distance from boles was quantified during August through October for live trees and tress killed in 1962-63 on sites with contrasting grazing histories. Herbaceous samples for biomass, C, and N determination were also obtained; and we are about half way through laboratory processing of these samples. We will
repeat the herbaceous biomass sampling in summer 2006. Adaptation and parameterization of the CENTURY model for our field site in nearly complete; and we are on track to begin using CENTURY in 2006 to reconstruct historical ecosystem pools and predict how pools may respond to alternative management scenarios. Two graduate students were recruited. Chloe Tewksbury, a doctoral student joined the project in September, and will work on the soil carbon aspect of the project. Aleta Nafus, a masters student joined the project in July, and is working on the herbaceous biomass portion of the project. We also hired several undergraduate research assistants to help with the field campaigns and assist with soil and herbaceous sample processing and analyses.
Impacts
Spatial heterogeneity and temporal variability in rangelands impose significant constraints on our ability to inventory, monitor, predict and manage vegetation and soils at scales of time (decades) and space (hundreds of hectares) relevant to management. The results of this study will help alleviate such constraints by providing a widely applicable conceptual framework from which to more effectively evaluate relationships between pattern (in our case, vegetation growthform distributions and land use) and processes (carbon and nitrogen pools) on landscapes. Results from the proposed study will at once be instructive for rangeland managers who will potentially be faced with making management decisions that will have to factor in emerging issues related to carbon sequestration and greenhouse gas emissions, for global change scientists grappling with resolving uncertainties in the carbon cycle, and to policy makers and economists seeking progressive and creative solutions
to global change.
Publications
- No publications reported this period
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