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Title: Ecosystem carbon and nitrogen pools in managed rangelands: a spatial accounting of management influences.
Funding Agency: USDA-NRI Managed Ecosystems
PI: Steve Archer, School of Natural Resources, University of Arizona
Co-PIs: Mitch McClaran, School of Natural Resources, University of Arizona; Heather Throop, New Mexico State University

Project Summary:

Management of rangelands has traditionally focused on livestock/wildlife production and soil/water conservation. However, with the emergence of ‘global change’ issues in recent years, there is a pressing need to improve our understanding of how rangelands and rangeland management practices influence carbon sequestration. Our goal is to link field measurements and modeling approaches to reconstruct and project the influence of rangeland management practices on the size and spatial distribution of ecosystem C and N pools.

Land cover change is a key element of global change. Increases in woody plant abundance in recent history has caused major land cover change on rangelands. Although this phenomenon has been widely reported, the rates and patterns of change have not been well quantified. Hence, we know little about how this vegetation change has affected plant and soil organic C (SOC) and total N (TN) pools. A recent synthesis reveals these pools may increase, decrease or remain unchanged in response to woody plant encroachment. Current theory or understanding cannot account for these variable responses. We contend that these discrepancies are due, at least in part, to a failure to account for spatial/temporal patterns. We propose to develop a spatiotemporal data base that will enable us to test this hypothesis and at the same time compute area-weighting algorithms that can be used in conjunction with time-series aerial photos to generate a landscape-scale accounting of ecosystem C and N pools on sites with contrasting grazing and brush encroachment/management histories along a precipitation gradient.

Rangelands are often mosaics of landscapes at different stages of recovery from brush management (BM). Little is known about how BM activities impact ecosystem C and N pools. Therefore, one of our objectives is to quantify changes in ecosystem C and N pools and their spatial distributions in the context of BM and livestock grazing interactions across a precipitation gradient. Our proposed study site, the Santa Rita Experimental Range, has a long (back to 1903), well-documented history of contrasting grazing and BM treatment combinations. As such, it is uniquely suited to address this objective.

Dynamic, process-based simulation models are a potentially powerful tool for assessing, reconstructing and extrapolating the interactive effects of shrub encroachment, brush and grazing management and precipitation variability on ecosystem C and N pools. Such models exist, but have seldom been used in this context. We will use the CENTURY model to assess how ecosystem C and N pools changes in response to woody encroachment, brush management and livestock grazing; and to project changes in ecosystem C and N pools that might accompany future changes in grazing/brush management practices.

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