Tree leafing phenology and crop productivity in semi-arid agroforestry systems in Kenya

The hypothesis that temporal separation of resource use between trees and crops minimises competition for wa ter in agroforestry systems during the cropping period and increases utilisation of annual rainfall was tested at Machakos in semi-arid Kenya. Four popular tree species were chosen to provide a range of leafing phenologies. These included Melia volkensii, which sheds its leaves twice a year, Senna spectabilis and Gliricidia sepium, which shed their leaves during the long dry season, and the evergreen Croton megalocarpus. All four species retained their foliage during the long rains, offering little scope for temporal separation of resource use. Maize (Zea mays) yields were reduced by 50–70% in the agroforestry treatments. Reductions in crop yield were strongly correlated with tree growth (r 2 =0.94) and available soil moisture (r 2 =0.88). G. sepium remained leafless for much of the short rains despite the presence of available soil water, and was least competitive with the bean crops (Phaseolus vulgaris) grown at this time. Reductions in crop yield in the agroforestry treatments were closely correlated with tree growth (r 2 =0.99) and available moisture (r 2 =0.79) during the 1996/97 short rains (158 mm), but not during the much wetter 1997/98 season (608 mm). Shading by trees or shade nets reduced crop yield, in contrast to previous studies in the semi-arid tropics. Low off-season rainfall during the study period (9% of annual rainfall compared to the long-term average of 20%) limited the potential for temporal separation of growing periods. Where the prospects for temporal or spatial separation in resource use are limited, shoot and/or root pruning may be necessary to manage competition between trees and crops.

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