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Regional NPP and Carbon Stocks in Southwestern USA Rangelands: Land-use Impacts on the Grassland-Woodland Balance
Historical and on-going woody plant proliferation in the world’s extensive dryland ecosystems may have significant implications for the global carbon cycle. Factors contributing to these ecosystem transitions are subject to debate and include human-induced alterations of fire frequency, grazing intensity, atmospheric CO2 concentration and nitrogen deposition. Assessments of woody plant encroachment to the global carbon (C) budget are largely indirect and rudimentary. Documentation of changes in ecosystem C pools is of increasingly practical importance as federal land agencies and industry grapple with C emission standards and the development of C credit/offset programs. Our work with colleagues in Texas, Arizona, and California on carbon sequestration in Southwestern rangelands demonstrates that dryland regions are changing mosaics of woody plant classes whose trends through time are logistically difficult to track with traditional ground-based techniques. While shrub encroachment under "natural" conditions progresses on a decadal scale, management practices introduce a temporal complexity to the landscape as different areas or management units experience different land uses at different times. Our team has developed novel and viable approaches for coupling field data, isotope biogeochemistry, remote sensing, and modeling to quantify the impact of woody plant encroachment on aboveground biomass, carbon and nitrogen pools and primary production at spatially complex local and regional scales. Our data indicate that decadal accumulations of aboveground carbon by woody plants are highly significant relative to "background" grassland carbon stocks but can quickly be lost via natural disturbances and land management practices.
Our studies in grasslands in the Southwest have brought three important factors to the forefront. First, the encroachment phenomenon is of sufficient magnitude and extent that synoptic monitoring via remote sensing of the spatial distribution and temporal dynamics of woody plant abundance is imperative. Implementation of remotely sensed data with process models enables us to establish a fundamental connection between the spatial structure and the functional processes of a landscape. The ecosystem impacts of grassland to woodland transitions cannot be captured by ground measurements alone. Second, studies of the biogeochemical consequences of these transitions must recognize the importance of understanding local and landscape mechanisms in order to achieve accurate and prognostic regional assessments. This requires well-designed field studies, documentation and monitoring of land use practices, and the implementation of ecosystem simulation models to test our knowledge and build scenarios of change trajectories. We have demonstrated the importance of integrating fieldwork into the analysis and interpretation of remote sensing data and model development to achieve sufficient understanding of these complex landscapes. A third important factor regarding this problem is the fact that, traditionally, there have been strong policy, subsidy and economic incentives for brush clearing on rangelands. However, with the prospect of carbon credit/offset programs, 'brush' may become an income-generating commodity because of its potential to sequester more carbon above- and belowground relative to the grasslands it replaced. Our work demonstrates that linked remote sensing-modeling approaches will be a critical underpinning for the types of landscape and regional monitoring and assessment that will be required by policy makers seeking to make informed decisions.
We plan to continue work in the area of woody plant encroachment, focusing particularly on landscape- and regional-scale phenomena. Our lab will concentrate on understanding the dynamic and mutual feedbacks between ecosystem spatial structure and underlying processes, and will integrate field studies with landscape (structural) and ecosystem process models. In association with our colleagues, we will be extending this work to a regional-scale assessment of Southwestern carbon sequestration to project possible ecological outcomes associated with the grassland/woodland transition.