Agroforestry and the Mitigation of Land Degradation in the Humid and Sub-humid Tropics of Africa

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Abstract
In the last 35 years, the population of sub-Saharan Africa has increased nearly threefold and is expected to reach 681 million by the year 2000, with nearly 50% of the population living in urban centres. Such population pressures, exacerbated by a range of social and political factors, have already resulted in widespread land degradation in areas of high population densities and the expansion of agriculture on to marginal and sloping land. Declining soil fertility and soil erosion are increasingly threatening the sustainability of small scale farming systems throughout Africa, and affordable external nutrient inputs are seldom available to farmers. In addition, shortages of wood for construction and fuel and high-quality dry-season fodder for livestock are widespread and serious constraints to farm productivity.Agroforestry, the deliberate integration of woody perennials into crop and livestock systems, has the potential to mitigate many of these constraints through both the service and production functions played by trees. In recent decades much agroforestry research has been undertaken in sub-Saharan Africa. In this review we focus specifically on research which addresses the potential of agroforestry systems to enhance soil fertility, prevent soil erosion, provide high-quality dry-season fodder or generate much needed income through the production of high-value goods.Much emphasis has been placed on a wide range of agroforestry systems for the maintenance of soil fertility and the prevention of soil erosion losses, and encouraging results, both in technical performance and farmer enthusiasm, have occurred. However, it is clear that agroforestry solutions to land degradation are always likely to be location-specific in their relevance, performance and farmer acceptability. It is essential that farmers are included as research partners to determine what is appropriate for their conditions.Good progress has also been made on identifying fast-growing leguminous trees and shrubs for high-quality livestock fodder supplements. Where livestock enterprises, such as peri-urban milk production, are market-oriented the adoption and impact of such systems have been high. Given population and urbanization projections, it is likely that fodder trees and shrubs will have a major role to play in meeting future feed demands for both milk and meat production. Research on the potential of high-value indigenous and exotic trees to generate income has been less extensive in Africa, although the huge potential of this approach has been clearly demonstrated by farmers in south-east Asia. We suggest that there is a need for increased research emphasis on the domestication of high value indigenous trees, and their integration into more sustainable, diverse and intensive land use systems.We conclude that, although good progress has been made in agroforestry research in Africa and farmer adoption is occurring, future population projections pose a clear challenge. Agroforestry systems which provide solutions for today's land degradation problems will need to evolve in both diversity and intensity if they are to remain relevant and effective for tomorrow's Africa.