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Rangelands in Africa (i.e., grasslands, savannas, and woodlands, which contain both grasses and woody plants) cover approximately 2.1x109 ha. Africa's livestock population of about 184 million cattle, 3.72 million small ruminants (sheep and goats), and 17 million camels extracts about 80% of its nutrition from these vast rangelands (IPCC 1996, WG II, Table 2-1). In addition, Africa's rangelands support a vibrant tourist industry that, in many countries, is the leading contributor to gross national product (GNP). Because Africa's population has been growing at about 3% annually, the rangelands recently have become an arena for intense human and animal conflict, leading to serious reduction in spatial distribution and diversity of species. This reduction is likely to be exacerbated by projected changes in climate.
From a land-use perspective, there are differences between west Africa and east Africa in the way rangelands are used. In arid and semi-arid areas of west Africa (rainfall 5-600 mm), millet (or another crop) is planted over a unimodal (one peak in rainfall per year) rainy season (3-4 months); then fields remain fallow over the 8- to 9-month dry season. Livestock eat crop residues. Land use is dominated by cultivation, with livestock playing a subsidiary role in the village economy (Ellis and Galvin, 1994). In east Africa, by contrast, areas with higher rainfall (up to 600 mm) are inhabited by pastoral people rather than farmers. The Ngisonyoka of northern Kenya, for example, are pastoral nomads, completely dependent on their livestock for food, livelihood, and life (Galvin, 1992). In dry parts of eastern Africa, cultivation is uncommon and occurs mainly where irrigation is possible or where water can otherwise be sequestered and stored for cropping (Ellis and Galvin, 1994). Rainfall is bimodal (two peaks in rainfall per year) in most east African rangelands, resulting in two plant-growing seasons. This pattern has important implications for natural vegetation and rain-fed agriculture (de Ridder et al., 1982). According to Ellis et al. (1987), Turkana pastoralists in northern Kenya say that the best years for livestock production are not necessarily those with the greatest rainfall. Rather, years in which moderate rainfall extends over several months, resulting in a long period of foliage production and livestock milk production, are good years. Thus, the distribution and timing of rainfall will be at least as important as total annual amounts projected under climate change.
Rangelands are noted for high climatic variability and high frequency of drought events. They have a long history of human use. The combination of climatic variability and human land use make rangeland ecosystems more susceptible to rapid degeneration of ecosystem properties (Parton et al., 1996). Because these systems develop under highly variable rainfall regimes, they are conducive to rapid changes in ecosystem structure given modifications in fire and grazing patterns (Archer et al., 1994; Ojima et al., 1994) and altered climate regimes (OIES, 1991).
At the very broad scale, simulations with the biome models (MAPSS and BIOME3) projected increases in the extent of rangelands, mainly as a result of a reduction in the area of arid and semi-arid desert resulting from the reduction of drought stress with projected higher rainfall. However, ecosystem process models are more appropriate in analysis of this specific ecosystem type. Ojima et al. (1996) used the CENTURY (Parton et al., 1992) ecosystem process model of plant-soil interactions to analyze the impact of climate and atmospheric CO2 changes on grasslands of the world, including 7 of 31 sites in Africa. Ojima et al. (1996) looked at the effects of increasing CO2 and climate, using climate change scenarios based on the Canadian Climate Centre (CCC) and GFDL GCMs. They found that changes in total plant productivity were positively correlated to changes in precipitation and nitrogen mineralization (with nitrogen mineralization the most important factor). The response to nitrogen mineralization was consistent with the general observation that grasslands respond positively to addition of nitrogen fertilizer (Rains et al., 1975; Lauenroth and Dodd, 1978). Plant responses to CO2 were modified in complex ways by moisture and nutrient availabilities; their results generally indicated that CO2 enrichment had a greater effect with higher moisture stress. However, nutrient limitations reduced CO2 responses. Ojima et al. (1996) concluded that increased atmospheric CO2 will offset the negative effects of periodic droughts, making grasslands more resilient to natural (and human-induced changes in) climate variability. The strength of this beneficial effect, however, is controlled by the availability of nitrogen and other nutrients, which tend to be limited in many African landscapes.
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