Multistrata agroforestry with beans, bananas and Grevillea robusta in the highlands of Burundi

Some agronomic, economic and ecological aspects of an agroforestry system combining Grevillea robusta trees, bananas (Musa spp.) and beans (Phaseolus vulgaris) were studied in the subhumid highlands of Burundi. Three densities of G. robusta, 208, 313 and 625 trees ha(-1) were interplanted in plots of bananas, beans and a banana-bean mixture. When G. robustia was interplanted with bananas, the tree had a positive effect on banana yield with maximum yield occurring at 300 trees ha(-1). In the presence of beans, banana yield was not influenced by the densities of the tree. Under low Fertility regimes, the yields of beans interplanted with G, robusta were equal to or greater than those of the no-tree plots. Maximum bean yields were observed at between 283 and 295 trees ha(-1), representing a 25-135% yield increase above the no-tree control. When soil fertility was raised by the application of fertilizers, bean yields declined with an increasing density of G. robusta. Three years after planting, the growth of G, robusta was not affected by tree density. Mean height and mean basal diameter were 7.1 m and 13.2 cm respectively. Below ground competition between beans on the one hand, and bananas or bananas and G. robusta on the other, was more important than competition for light. Compared with the crops alone treatments, cumulative net present values of tree plots improved from being negative in the first year to being positive in the third year. Net present value was highest in two of three instances when the density of G. robusta was 625 trees ha(-1). A multistrata arrangement of bananas and beans with G. robusta is viable for the low input agriculture system of the highlands of Burundi.