Numerous schemes have been used to describe Africa's vegetation and ecosystems.
White's (1983) classification system is used here, aggregated (after Justice
et al., 1994) to show the rainforest in central Africa (unmarked) and two major
categories of woodland savannas divided by moisture and nutrient level: broad-leafed,
nutrient-poor, moist savannas; and fine-leafed, nutrient-rich, arid savannas
(Figure 2-5). This aggregation summarizes current understanding
of the role of soil nutrients and moisture on vegetation distribution in Africa,
especially in savannas (Scholes and Walker, 1993). The broad-leafed savannas
include the extensive miombo woodlands of central and southern Africa. Nutritionally
poor soils support only low-quality grass for grazing, so the numbers of large
herbivores is low in miombo and other broad-leafed savannas (Frost, 1996). The
fine-leafed savannas include acacia-dominated thorn woodlands, which have higher-quality
grass and so support large numbers of large herbivores; these areas constitute
the main rangeland region.
|Figure 2-5: The distribution of broad-leafed and fine-leafed savannas in Africa. Dark-shaded areas are broad-leafed, nutrient-poor, moist savannas, and striped areas are fine-leafed, nutrient-rich, arid savannas. This map has been derived from White (1983) by reclassifying woodland and wooded grassland map units into one of the two savanna classes according to the dominant tree species.|
Africa is composed essentially of woodlands and grasslands, with rainforests occupying only about 7% of the land area. Africa's rainforests represent slightly less than one-fifth of the total remaining rainforest in the world; Asia and Latin America contain the rest (Sayer et al., 1992). Only about a third of Africa's historical forest extent remains, with west Africa's forests being lost faster than those of any other region. Annual deforestation rates average 0.7% per annum (FAO, 1997).
WRI (1996) indicates that only 8% (0.5 million km2) of Africa's regional forest remains as "frontier forest." (Frontier forest is essentially natural/primary forest of sufficient size to support ecologically viable populations of indigenous species.) More than 90% of west Africa's original forest has been lost, and only a small part of what remains qualifies as frontier forest. Of the remaining forest, 77% is under moderate or high threat from logging and commercial hunting to meet growing urban demand for bushmeat. Demands on forests also have escalated in some regions (e.g., as a result of civil unrest that has pushed hundreds of thousands of people into previously intact forest).
Many studies of African ecosystems emphasize particular vegetation types-savanna grasslands, miombo woodlands, mopane woodlands, rangelands, or rainforest-or particular regions: the Sahel, sub-Saharan Africa, or the Southern African Development Community (SADC). In general, the structure of Africa's vegetation is determined by climate at the large scale, then soil type (texture) and nutrient levels at the local scale (Scholes and Walker, 1993). Fire and herbivory are important disturbance factors. Increased moisture in drier areas will likely result in a complex set of feedbacks between nutrients, fire occurrence, decomposition, and competing vegetation.
Increased variability in rainfall and changes in temperature will likely disrupt key ecosystem processes such as phenology and will influence insect pests and diseases in mostly unknown ways. Direct effects on pests will involve disruption of insect life cycles or creation of more suitable conditions for new pests (or for old pests to expand their territory). Ticks, tsetse flies, and locusts are notable examples of serious insect pests in Africa. Although a lot of work has been done to study these insects, a lot remains to be done, especially in relation to how climate change may impact them.
Of the many climatic factors that are important for plant growth, among the most significant in relation to climate change are temperature, water availability (determined by precipitation and soil characteristics, as well as other meteorological variables), and carbon dioxide (CO2) concentrations. Consideration of the effects of climate change requires examination of the direct effects of changes in CO2 concentrations and climate variables on the growth of plants, as well as the ways in which these direct effects are modified by soil feedbacks and biological interactions among different organisms.
The effects of temperature changes will vary in different subregions and ecosystems. An increase in temperatures will reduce the incidence of frost damage in areas where this damage occurs and widen the potential geographical range of species that are limited by minimum temperatures. The extent of effects of higher temperatures on African vegetation (e.g., effects on respiration rates, membrane damage) is largely uncertain. Temperature is known to interact with CO2 concentration, so expected increases in respiration resulting from a temperature increase alone may be offset or even reduced by higher CO2 concentrations (Wullschleger and Norby, 1992).
In most of Africa, water availability is projected to have the greatest impact on plant processes. Individual species are adapted to particular water regimes and may perform poorly and possibly die out in conditions to which they are poorly adapted (e.g., Hinckley et al., 1981). The effects of climate change will vary-depending, for example, on how particular plant types use water (water-use efficiency, WUE) or the amount of water available in the soil. Plants are grouped into C3, C4, and CAM plant types depending on how the process of photosynthesis takes place (see IPCC 1996, WG II, Section A.2.2). C3 plants (which include most trees and crop species such as wheat, rice, barley, cassava, and potato) have relatively poor WUE, unlike C4 plants (most of the tropical grasses and agricultural species such as maize, sugarcane, and sorghum). Higher CO2 concentration will likely improve water-use efficiency and growth in C3 plants in water-limited environments. C4 and CAM plants (including desert plants such as cacti) are unlikely to be affected directly by changes in CO2 concentration.
The amount of water available to plants over the course of the year affects plant growth and location across soil and climate types. Available soil water (in combination, of course, with other factors) generally controls the growth cycle (beginning and end) and other events such as when to leaf, shed leaves, set buds, and so forth. Water availability and temperature indices and parameters (maxima and minima, heat sums, cold sums) have been used to relate the distribution of vegetation formations to climatic factors (for more details on these plant biogeography models, see Section 18.104.22.168). However, large uncertainties in GCM precipitation projections constrain our ability to project ecosystem responses to changes in climate. Thus, improving climate modeling at the regional scale is a priority in most of Africa, where ecosystem processes are limited by moisture.
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