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Carol A. Wessman

Regional Production and Carbon Stocks in Southwestern USA Rangelands: Land-Use Impacts on the Grassland-Woodland Balance

Historical changes in grazing and fire regimes have resulted in an increase in woody plant abundance in the world’s drylands. These dynamic changes in tree/grass ratios have important implications for carbon cycle dynamics (sequestration and liberation) and for making regional source/sink assessments in a national carbon accounting (e.g., Kyoto Protocol). Rates and dynamics of woody plant increases are poorly understood, but are known to vary with soils and topography. Bush management practices add further complexity to carbon cycle trajectories by offsetting woody plant increases in some landscapes and not in others. In confronting these challenges, 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 and C and N pools and primary production at spatially complex local and regional scales. Our work to date in northern Texas indicates a net increase in regional aboveground plant carbon pools of 32% between 1937 and 1999. In addition, there has been a homogenization of woody plant cover due to woody plant encroachment into open grasslands and brush management-induced reductions of woody biomass in areas once characterized by high woody cover. However, within the region, changes in aboveground woody plant carbon stocks ranged from a >200% increase (landscape experiencing brush encroachment and no brush management since 1950) to ca. 50% decrease (landscape experiencing a high intensity fire 4 y prior to 1999 image). In future work multi-spatial remote sensing will be used to (a) quantify the relative abundance of woody plants in Southwestern rangelands, and (b) constrain an ecosystem process model (TerraFlux), in order to (c) estimate regional productivity and carbon storage in vegetation and soil. Land use scenarios that define different policy environments (e.g. subsidies for woody plant control, or carbon credit incentives that promote woody plant proliferation) will be evaluated with respect to impacts on regional carbon budgets.