ABSTRACT
In shifting-cultivation systems, farmers often identify and retain useful tree species when they open and clear their fallows. However, in many parts of the Asian uplands a human and environmental tragedy is unfolding. Unfortunately, the farm-gate cost of fertilizers relative to crop prices is high in Africa, particularly for food crops. The trees are an integral part of the agricultural system, providing food, fuel, fodder, medicines, wood for construction and cash commodities, as well as contributing to soil fertility, water conservation and other environmental services. The alley-cropping model was modified to reduce the labour burden while still taking advantage of the biofertilizer and soil-regenerating benefits of fast-growing fertilizer trees. The evolution of a diverse portfolio of practices by which trees are integrated into croplands has given rise to the vision of an 'evergreen agriculture', in which much of the world's annual crop production may one day be managed with trees.
