Deforestation is defined by the FAO as 'the conversion of forest to another land use or the long-term reduction of the tree canopy cover below the minimum 10 percent threshold.' Forest degradation is defined as 'changes within the forest that negatively affect the structure or function of the stand or site, and thereby lower its capacity to supply products and/or services' (FAO 2001a). In 1999, the FAO reported that 10.5 per cent of Africa's forests had been lost between 1980 and 1995, the highest rate in the developing world and in sharp contrast to the net afforestation seen in developed countries. Forest loss between 1990 and 2000 was over 50 million hectares, representing an average deforestation rate of nearly 0.8 per cent per year over this period (FAO 2001a). As a consequence, availability of forest resources per capita declined from 1.22ha/person in 1980 to 0.74ha/person in 1995 (African Development Bank 2001). A regional comparison of changes in forest area is shown in Figure 2d.4. Deforestation can have a number of negative impacts on local agricultural production (see Figure 2d.5).
The pressures causing this decline are complex and operate at several different levels. They may be anthropogenic (caused by humans) or natural, and direct or indirect. Direct causes include commercial timber production, clearing of land for agriculture and urban expansion, and harvesting of wood for fuel and charcoal. These activities also open up forests by the construction of access roads to logging sites, fragmenting the forests and facilitating further clearance, resource extraction, and grazing by locals and commercial organizations. Forest fragmentation can lead to losses in biodiversity by cutting migratory routes for certain animal species, allowing invasion by alien species or changes in microclimate (UNDP and others). Indirect causes of deforestation include population growth, policies, agreements, legislation, lack of stakeholder participation and market factors that encourage the use of forest products leading to loss, fragmentation or degradation (Rodgers, Salehe & Olson 2000). Other causes of forest loss include conflicts, civil wars and lack of good governance (Verolme & Moussa 1999).
Commercial logging has caused the greatest rates of deforestation in Africa in the past 30 years or more, with governments hard-pressed to earn foreign currencies and stimulate their economies. The global demand for roundwood is set to increase by 1.7 per cent per year over the next decade and implementation of more sustainable harvesting methods is therefore a priority to prevent further deforestation (FAO 1999). Local processing and export of value-added products could raise export revenues and employment in timberproducing countries.
After commercial logging, consumption of wood as fuel is a major contributor to reduced forest cover and quality. Africa is the world's largest consumer of biomass energy (as a percentage of total energy consumption), which is largely wood and charcoal (see Figure 2d.6). Biomass consumption accounts for 5 per cent of total energy consumption in Northern Africa, 15 per cent in South Africa, and 86 per cent in sub-Saharan Africa excluding South Africa (EIA 1999). The demand for wood and charcoal in Africa is set to increase by over 45 per cent over the next 30 years, due to increases in population and demand for energy (FAO 2001b). Overharvesting of wood for fuel and charcoal production brings changes to the species composition of a forest or savanna. Local people are impacted by having to spend longer and search further to meet their daily fuel requirements. Development of alternative sources of energy is therefore a priority for the African region, and should be facilitated-for example, under the funding mechanisms of the Kyoto Protocol.
Slash and burn agriculture is another human activity contributing to declining forest cover and quality in Africa. This practice depletes the rich fertile forest soils of nutrients, and farming communities using this method have therefore traditionally shifted locations. However, the large and growing rural populations in Africa are making such shifts impossible and this breakdown in traditional management techniques is threatening the remaining forests with increasing rates of clearance and insufficient recovery time. Commercial agriculture, especially plantation agriculture, is also playing a significant part in this cycle of forest loss and soil depletion.
Natural causes of forest and woodland loss, fragmentation or degradation include landslides, volcanism, fire, wind, pests and changes in water tables that change the salinity of soils (IPCC 2000, Medley & Hughes 1996). Other natural causes are river meanders cutting off areas of riverine forest from the river, after which they degrade, and large herbivores such as elephants and rhino which can change the understorey vegetation and the composition of the forest or woodland community (Medley & Hughes 1996, Western 1997). Mangrove forests can undergo change as a result of pests, sedimentation, newly formed sand dunes, high marine pollution levels, and rises in sea levels (Wass 1995).
These natural events can also be precipitated, aggravated or in some cases mitigated by human activity. For example, landslides can be caused by loss of ground cover through clearing for agriculture or cutting of timber for commercial purposes, roads or mining activities. Fires can start in other areas through human activity, and then spread to forests. On the other hand, burning can also improve regeneration of some tree species (Medley & Hughes 1996). Changes in water tables or in salinity can be caused by clearing of trees, irrigation or interference with the hydrology of an area, including rivers flows. Large herbivores (such as elephant and rhino) may degrade their habitat if the size of continuous habitat is reduced. On the other hand, if these animals are excluded, seed dispersal and propagation of certain species may be interrupted (Ganzhorn, Fietz, Rakotovao, Schwab & Zinner 1999). Pests can become a problem if alien species are introduced by human activities, or if the predator-prey balance is disrupted by agrochemicals or other agents. Interestingly, some traditional activities, such as livestock grazing have been shown to control a serious pest (the bruchid beetle) on Acacia tortilis trees (Medley & Hughes 1996).
Fragmentation of forests exposes species adapted to the sheltered, moist, dark
forest interior to greater intensities of sunlight, greater wind speeds, or
increased levels of predation, reducing the number of species available to local
communities, both at present and in the future. The pace and duration of the
decline in species richness in forest fragments has been difficult to establish,
but fragments of tropical rainforest are high priority conservation areas as
they could form the basis for the regeneration of larger areas of forest (Turner
& Corlett 1996).