Introduction

Agricultural land use and management present major development challenges throughout sub-Saharan Africa. The area under cultivation has expanded notably, total yields are rising, and there is large-scale conversion from fallow-based cropping systems to continuous cultivation. None the less, per capita food production has declined by about 2% per year since 1960 (World Bank, 1996), and constraints on growth in agricultural sectors, which remain prominent in most African economies, are an important factor explaining a 1% per year decline in per capita incomes between 1983 and 1993 (Cleaver and Schreiber, 1994; World Bank, 1994). Environmental problems associated with agricultural production have also become a major concern. With the marked expansion and intensification of farming, total forested area in Africa declined by 50 million hectares during the 1980s (Dembner, 1991), reducing the availability of wood products for fuel and construction, degrading range resources, and exposing vulnerable soils to degradation. In many areas, particularly in the densely populated highlands and in drylands, soil degradation due to inadequate agricultural practices and nutrient depletion threatens long-term productive potential (Scherr and Yadav, 1995; Buresh et al., 1997). Agroforestry is defined as a dynamic, ecologically based, natural resource management system that, through the integration of trees on farms and in the agricultural landscape, diversifies and sustains production for increased social, economic and environmental benefits for land users at all levels (Leakey, 1996). Agroforestry has considerable potential to contribute towards solving some of these problems. Nitrogen-fixing trees, as substitutes or complements for chemical fertilizer, can increase smallholder incomes, conserve foreign exchange and improve regional food security. By providing a supply of fuelwood from the farm, agroforestry can help reduce pressure on forests and communal woodlands. Moreover, agroforestry trees can supply farm households with a wide range of other products, including food, medicine, livestock feed, and timber for home use and sale. Other services that trees provide, such as boundary markers, windbreaks, soil erosion barriers, beauty and shade, are difficult to quantify but are none the less of substantial importance to farm families and for natural resource protection. Indeed, indigenous agroforestry practices are already widespread in Africa, as is the planting and protection of many tree species introduced during the colonial period and later through forestry and agroforestry extension projects (Le Houerou, 1987; Kerkhof et al., 1990; Warner, 1993). However, most are suited to low-intensity agricultural systems, and function well below their potential productivity as a result of using poorly adapted species, provenances and management systems. Over the past 20 years, research and extension systems working in Africa have sought to improve the productivity of existing systems and develop new practices to meet new land-use challenges and opportunities (Scherr and Müller, 1990; Kang, 1993; Cooper et al., 1996).

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