THE POTENTIAL OF AGROFORESTRY METHODS FOR EROSION CONTROL IN RWANDA

Based on four years of plot measurements in Butare (Rwanda), the potential of agroforestry practices for soil conservation was analysed. The agroforestry system investigated is based on Grevillea robusta planted at a density of 200 trees per hectare. Supplementary erosion control measures consisted of hedgerows planted on microterraces (two species of leguminous shrubs: Calliandra calothyrsus and Leucaena leucocephala as well as the fodder grass Setaria splendida). On the agroforestry plots, crop management was in accordance with the principles of appropriate ("ecologic") agriculture, including practices such as mixed cropping, the use of green manure, composting, and integration of livestock. Comparative measurements were carried out on bare fallow and on "traditional farming" plots with monocropped cassava. With a mean annual precipitation of 1279 mm, a rainfall erosivity factor of R = 385, and a slope of 28 percent, high annual soil losses were recorded on the bare fallow plot (557 tons per hectare per year) as well as on the cassava plot (303 t . ha-1 . y-1). The high soil losses recorded on the Grevillea plot (111 t . ha-1 . y-1) demonstrate that the problem of accelerated erosion on steep slopes cannot be solved by simply planting trees and by introducing management practices that provide a better ground cover for only part of the year. In contrast, on agroforestry plots with supplementary erosion control hedges, erosion rates lower than 12,5 t . ha-1 . y-1 were measured. Agroforestry systems that include the integration of leguminous perennials as contour hedges offer a promising method for soil conservation even on slopes threatened by severe erosion.