Woodland Expansion in U.S. Grasslands: Assessing Land-Cover Change and Biogeochemical Impacts.

While the environmental impacts of tropical deforestation have received considerable attention, reductions in biomass are in stark contrast to significant increases in woody plant abundance in many grasslands worldwide. Though not well quantified on a global scale, this vegetation change has been widely reported in tropical, temperate and high-latitude rangelands worldwide (Archer 1994; Archer et al., 2001). Land-cover change of this type and magnitude is likely to affect key ecosystem processes in grasslands, and may significantly alter carbon cycling and feedbacks to climate change. Moreover, the proliferation of woody vegetation at the expense of grasses threatens to render substantial portions of these areas incapable of supporting pastoral, subsistence, or commercial livestock grazing, thus adversely affecting ≈20% of the world's population inhabiting these lands (Turner et al., 1990; Campbell and Stafford Smith, 2000). While interannual climate variability, atmospheric CO2 enrichment, and nitrogen deposition are also likely contributing factors (Archer et al., 1995; Kochy and Wilson, 2001), land use practices associated with livestock grazing and reductions in fire frequency have been implicated as proximate causes for this widespread land cover change (Archer 1995; Caspersen et al., 2000; Van Auken 2000). [ABSTRACT FROM AUTHOR]