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In Africa, forests—as defined and reported by FAO (1999a)—cover 5 million km2, one-sixth of the continent's land area. The moist tropical forests of the Congo constitute the second most extensive rainforest in the world and a globally important reserve of carbon. Trees and shrubs constitute an important component of the more than 12 million km2 of agricultural lands, pastures, shrublands, and savannas outside of closed-canopy forest areas.
Trees and shrubs provide ecosystem services of carbon sequestration, storing and transpiring water required for precipitation, maintaining soil fertility, and forming habitats for a diverse array of plant and animal species. Moreover, forest and woodland species also provide firewood, structural timber, traditional medicines, staple foods, and drought emergency foods. Because a large fraction of the population lives in rural areas, they depend on trees and shrubs for many of their subsistence needs. Indeed, firewood and charcoal provide approximately 70% of the energy used in Africa. Moreover, the export of timber, nuts, fruit, gum, and other forest products generates 6% of the economic product of African countries (FAO, 1999a). Thus, climate change renders vulnerable the large part of the African population that is dependent on forest species for subsistence needs and the nontrivial fraction of the economy that is based on forest products.
Because climate change alters the spatial and temporal patterns of temperature and precipitation, the two most fundamental factors determining the distribution and productivity of vegetation—geographical shifts in the ranges of individual species and changes in productivity—constitute the most likely impacts of CO2-induced climate change on forest species. Research in Senegal (Gonzalez, 1997, 2001) has documented retraction of mesic species to areas of higher rainfall and lower temperature as a result of desertification in the last half of the 20th century. These changes have caused a 25-30 km southwest shift of Sahel, Sudan, and Guinean vegetation zones in half a century, proceeding at an average rate of 500-600 m yr-1—foreshadowing the magnitude of projected vegetation shifts driven by CO2-induced climate change (Davis and Zabinski, 1992). In northwest Senegal, the human population density is 45 people km-2, whereas forest species can support only 13 people km-2 under altered conditions (Gonzalez, 1997).
Dry woodlands and savannas in semi-arid and subhumid areas will be increasingly
subjected to drying in the next century, as well as increasing land-use intensity—including
conversion to agriculture (Desanker et al., 1997). Moreover, CO2-induced
climate change is very likely to alter the frequency, intensity, seasonality,
and extent of vegetation fires that are critical to the maintenance of areas
such as the Serengeti grasslands of east Africa, the miombo woodlands of southern
Africa, and the fynbos of the Cape. Across the continent, farmers traditionally
use fire to clear agricultural fields in forest areas and areas outside closed-canopy
forest; pastoralists and hunter-gatherers use fire to improve the quality of
plant resources available during the dry season. Satellite remote sensing reveals
that more than half of the continent experiences a fire regime with a frequency
greater than once per decade (Kendall et al., 1997; Levine et al.,
1999).
Although the broad geographical pattern of fire-prone vegetation clearly is climatically related (van Wilgen and Scholes, 1997), the aspect of the fire regime that is most sensitive to the type and degree of climate change suggested for Africa is likely to be fire intensity, rather than its frequency or extent. Fire intensity is related largely to the available dry-season fuel load, which in turn is strongly and positively related to rainfall in the preceding wet season and nonlinearly related to woody plant cover. In the miombo woodlands, it is predicted that increased fire will expand savanna areas at the expense of wooded areas (Desanker et al., 1997). Because emissions of CH4, tropospheric O3 precursors, and aerosols from vegetation fires in Africa constitute a significant contribution to the global budgets of these species (Crutzen and Andreae, 1990; Hao et al., 1990; Scholes et al., 1996), changes in the African fire regime could have consequences for global and regional climate.
Modeling of the distribution of forest species on the basis of the Holdridge (1967) life zone classification has projected changes from mesic vegetation to xeric vegetation in Tanzania and The Gambia (Jallow and Danso, 1997) but a shift from arid vegetation to moist vegetation in Mozambique (Bila, 1999). It is not suggested that vegetation formations and their associated fauna (biomes) will migrate as a unit. It is more likely that species will respond to changing climate and disturbance regimes individualistically, with substantial time lags and periods of reorganization. The broad pattern of productive potential of vegetation zones is likely to move with greater spatial integrity because there is a degree of functional redundancy in ecosystems.
The most promising adaptation strategies to declining tree resources include natural regeneration of local species, energy-efficient cookstoves, sustainable forest management, and community-based natural resource management. The most effective adaptation to the decline of trees and shrubs in semi-arid areas is natural regeneration of local species. In addition, the ban ak suuk cookstove in Senegal and the jiko ceramic stove in Kenya have both produced energy-efficient gains in semi-arid areas (Dutt and Ravindranath, 1993). These practices generally depend on the ability of local people to exercise power to inventory and manage local resources in systems of community-based natural resource management. Decentralization of decisionmaking and revenue allocation authority has promoted efficient forest management in small areas of Niger, Madagascar, and Zimbabwe (FAO, 1999a). All of these practices constitute "no regrets" strategies that society would want to undertake under any climate scenario for their intrinsic environmental and economic benefits.
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