AGROFORESTRY AND THE MITIGATION OF LAND DEGRADATION IN THE HUMID AND SUBHUMID TROPICS OF AFRICA

In the last 35 years, the population of sub-Saharan Africa has increas ed nearly threefold and is expected to reach 681 million by the year 2 000, with nearly 50% of the population living in urban centres. Such p opulation pressures, exacerbated by a range of social and political fa ctors, have already resulted in widespread land degradation in areas o f high population densities and the expansion of agriculture on to mar ginal and sloping land. Declining soil fertility and soil erosion are increasingly threatening the sustainability of small scale farming sys tems throughout Africa, and affordable external nutrient inputs are se ldom available to farmers. In addition, shortages of wood for construc tion and fuel and high-quality dry-season fodder for livestock are wid espread and serious constraints to farm productivity. Agroforestry, th e deliberate integration of woody perennials into crop and livestock s ystems, has the potential to mitigate many of these constraints throug h 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 addres ses the potential of agroforestry systems to enhance soil fertility, p revent soil erosion, provide high-quality dry-season fodder or generat e much needed income through the production of high-value goods. Much emphasis has been placed on a wide range of agroforestry systems for t he maintenance of soil fertility and the prevention of soil erosion lo sses, and encouraging results, both in technical performance and farme r enthusiasm, have occurred. However, it is clear that agroforestry so lutions to land degradation are always likely to be location-specific in their relevance, performance and farmer acceptability. It is essent ial chat farmers are included as research partners to determine what i s appropriate for their conditions. Good progress has also been made o n identifying fast-growing leguminous trees and shrubs for high-qualit y livestock fodder supplements. Where livestock enterprises, such as p eri-urban milk production, are market-oriented the adoption and impact of such systems have been high. Given population and urbanization pro jections, 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 pro duction. 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 F armers in south-east Asia. We suggest that there is a need for increas ed research emphasis on the domestication of high value indigenous tre es, and their integration into more sustainable, diverse and intensive land use systems. We conclude that, although good progress has been m ade in agroforestry research in Africa and farmer adoption is occurrin g, 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 rem ain relevant and effective for tomorrow's Africa.