Climate change is a pressure on key goods and services in a system that is responsive to climatic fluctuations. It is possible that climate change would lead to increased frequency of extremes of climatic events (such as drought and floods) driven through change in the frequency of ENSO events. Apart from climate change, other direct and indirect pressures from human activities can be important in the delivery of services from rangelands. These pressures include land-use change, which often leads to fragmentation (Allen-Diaz, 1996; Gitay and Noble, 1998; WRI, 2000); changes in the densities of livestock (WRI, 2000); competition for land and water; and altered fire regimes (Russell-Smith et al., 1997). These pressures would have subsequent impacts that cannot be disaggregated from impacts of climate change.
It has been suggested that for rangelands the possible effect of climate change may be trivial compared with the past and present impacts of human activities, including livestock grazing (Le Houérou, 1996). This may not be entirely true for all rangelands, but it does suggest that it would be difficult to separate the impacts of climate change from the impacts of many of the other pressures that are acting and will continue to act on the system. Subsequent sections assess the direct impacts of climate change; impacts from other aspects of global change, especially the pressures listed above, also are assessed (see Box 5-9). Major impacts of climate change on key goods and services are mediated through changes in NPP; changes in plant community composition, structure, and forage quality (e.g., through changes in C3 and C4 plants); and changes in plant herbivory and phenology.
Box 5-9. Impacts of Some Pressures on Rangelands
Major factors in land-use change are conversion of rangelands to croplands (Allen-Diaz, 1996; WRI, 2000) and increased human settlements, especially urbanization (Gitay and Noble, 1998), which lead to fragmentation (WRI, 2000). There have been large-scale changes in land use: For example, in the South Platte Basin in the United States, 40% of the land cover has been converted from rangelands to croplands. This can alter carbon stores, sometimes leading to soil carbon loss of as much as 50% (Allen-Diaz, 1996), but it also can lead to increased plant productivity through irrigated grain production (Baron et al., 1998). Baron et al. (1998) conclude that subsequent impacts on the biogeochemical cycles of the basin and on land-atmosphere interactions can affect many rangelands. In many rangelands, native species that occur at low density are used for fuelwood. In some cases, fuelwood collection can lead to decreases in woody vegetation cover and possible land degradation.
Some rangelands have high densities (>100 km-2) of livestock, with livestock being moved to take advantage of the periodic growth, especially after rain and/or fires (WRI, 2000). Modeling studies show that increased grazing pressure (i.e., overgrazing) would cause grass and herbaceous productivity to fall below a certain threshold, resulting in increased and rapid rates of land degradation especially under drier and/or hotter climate conditions (Abel, 1997).
Competition for Land and Water
There is increased demand for water for direct human consumption and for irrigation (WRI, 2000). Food production obviously is positively affected by increased water use (see Section 5.3 and Chapter 4), but this water use is an added pressure on many rangeland ecosystems. Nomadic pastoralism, which was common in many rangelands until recently, allowed pastoralists to cope with the variable climate of the rangelands they inhabited. Land-use changes (to permanent agriculture, urban areas, conservation, and game reserves that have included loss of sources of permanent water) have led to overall loss of land available. Together with increased human population, this has led to competition for land and changes from pastoral communities to more market-orientated and cash-based economies (Allen-Diaz, 1996).
Altered Fire Regimes
The SAR did an extensive review of fires and rangelands. The projected increase in variability in climate led to the conclusion that the frequency and severity of fires will increase in rangelands (Allen-Diaz, 1996), provided that drought and grazing do not lead to a reduction in vegetation biomass. There is historical evidence that fire frequency has changed (increased and decreased, depending on vegetation biomass) in recent decades (see, e.g., Russell-Smith et al., 1997), leading to changes in vegetation composition. Fires also are started by humans; for example, in Africa 25-80% of rangelands are burned every year, often to induce new plant growth (WRI, 2000). This has implications for short-term productivity but possibly long-term land degradation.
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