Implications for Ecosystem and Natural Resources Management

Woody plant encroachment has been and continues to be a major problem in grasslands and savannas worldwide (e.g., Grossman and Gandar, 1989). Because of its direct effects on livestock production,

FIGURE 11 Feedbacks between climate and atmospheric processes, ecosystem structure and function, and human activities. Dashed lines depict traditional research and public awareness; solid lines denote areas requiring increased research emphasis. Understanding global change will ultimately hinge on understanding how socioeconomics, policy, and government subsidies influence human activities and land use (see Figs. 12 and 13).

Marine Based Ice Sheet

FIGURE 12 LANDSAT image (1978) of the Canada (Saskatchewan/Alberta)-U.S. (Montana) border in the vicinity of the Milk River (from Knight, 1991). Subsequent to the drought and Dust Bowl of the 1930s, farmlands in Canada were repossessed by provincial or federal governments, withdrawn from cultivation, and underwent secondary succession. Intensive agriculture was maintained in the United States via elaborate farm subsidy programs. Striking contrasts in regional land cover were thus a direct result of changes in government policy. See also color insert.

FIGURE 12 LANDSAT image (1978) of the Canada (Saskatchewan/Alberta)-U.S. (Montana) border in the vicinity of the Milk River (from Knight, 1991). Subsequent to the drought and Dust Bowl of the 1930s, farmlands in Canada were repossessed by provincial or federal governments, withdrawn from cultivation, and underwent secondary succession. Intensive agriculture was maintained in the United States via elaborate farm subsidy programs. Striking contrasts in regional land cover were thus a direct result of changes in government policy. See also color insert.

encroachment of woody vegetation into grasslands has been one of the most important problems facing the ranching industry in the western United States and graziers and pastoralists in arid/semiarid regions throughout the world. This structural change in vegetation also has profound effects on the functional properties of ecosystems. Since woody plant encroachment into grasslands is occurring over large areas worldwide, ecosystem-level changes in nutrient pool sizes and fluxes will likely have important ramifications for regional and global biogeochemistry and climate. Thus, the replacement of grassland/savanna ecosystems by woodlands should be viewed not only as a local problem with economic impacts on livestock husbandry, but also in the context of longer-term, regional impacts on biogeochemistry and climate that will influence future land use options in arid and semiarid ecosystems worldwide.

Since the dawn of time, humans have been cognizant of the direct (e.g., catastrophic floods, wind storms, hail) and indirect (e.g., drought effects on food availability) effects of climate on their well-being. Ecosystem science in the 1960s and 70s focused on climatic and abiotic controls over ecosystem structure and function. During this same period, it became increasingly clear that human activities were directly responsible for significant changes in atmospheric chemistry which could feed back to affect ecosystem processes (e.g., acid rain) and human health (e.g., smog). While ecologists and natural resource managers have long been concerned with impacts of humans on ecosystems, we have only recently begun to assess how alterations of ecosystem structure and function might induce changes in climate and atmospheric chemistry, as shown in Figure 11 (Graetz, 1991; Bryant el at, 1990; Pielke el at, 1993, 1998). The case study presented here explicitly documents how human activities (specifically alteration of grazing and fire regimes) have modified the structure and function of a subtropical savanna grassland system in ways that may have significant impacts on climate and atmospheric processes. Anticipating future changes will largely depend on anticipating how human populations and land use will change. Land-use practices will be governed largely by socioeconomic conditions mediated by government policy and subsidies (Figs. 12 (see also color insert) and 13). Thus, the human dimension of global change is paramount and ecologists are challenged to interface ecosystem science with social science (Turner et al, 1990; Walker, 1993a, b; Walker and Steffen, 1993; Vitousek, 1994).

Changes in tree-shrub-grass ratios in drylands have policy implications for federal agencies grappling with designing and implementing carbon sequestration programs. The success of such endeavors will hinge on the ability to quantitatively monitor and inventory "carbon credits" associated with various land management practices. In arid and semiarid ecosystems, this means tracking changes in woody versus herbaceous cover and understanding how shifts between these growth forms influences aboveground and belowground C and N pools and fluxes. I Iowever, landscape-scale and regional quantification of grass-woody plant mass is challenging, because woody encroachment occurs relatively slowly (decadal time scales) in a nonlinear fashion, across large and often remote areas, and in a heterogeneous manner determined by topoedaphic constraints, climate, land-use, and disturbance regimes. In addition, reductions in woody biomass also occur in

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FIGURE 13 Differences in biophysical properties associated with contrasting land management policies in adjacent portions of the United States and Mexico (adapted from Bryant et al, 1990). Relaxation of grazing and range improvement programs occurred in the United States subsequent to the implementation of the Taylor Grazing Act in 1934. Changes in landscape cover resulting from a change in federal policy has had biophysical consequences.

drylands where trees and shrubs are cleared using fire, herbicides, or mechanical means (e.g., roller chopping, chaining). As a result, landscapes within a region may be a mosaic of variable-strength C and N sources and sinks. At present, we lack comprehensive information on the rate of change, areal extent, and pattern of woody plant abundance in the world's drylands. Hence, it is difficult to objectively assess the role of savannas in regional/global C and N cycling. Recent advances in remote sensing show promise for quantifying changes in grass and woody plant biomass in drylands (Asner et al, 1998b). These remote sensing tools, when used in conjunction with simulation modeling (Asner et al, 1998a), will potentially enable functional monitoring of land-use impacts on regional biogeochemistry in savanna regions.

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