Tree phenology and water availability in semi-arid agroforestry systems

Four common agroforestry trees, including both exotic and native species, were used to provide a range of leafing phenologies to test the hypothesis that temporal complementarity between trees and crops reduces competition for water in agroforestry systems during the cropping period and improves utilisation of annual rainfall. Species examined included Melia volkensii, which sheds its leaves twice a year, Senna spectabilis and Gliricidia sepium, which shed their leaves once during the long dry season, and the evergreen Croton megalocarpus. Phenological patterns were examined in relation to climatic conditions in the bimodal rainfall regions of Kenya to identify factors which dictate the intensity of competition between trees and crops. The main differences in leaf cover patterns were between indigenous and exotic tree species. The Central American species, S. spectabilis and G. sepium, shed their foliage during the dry season before the short rains, whereas the native species, M. volkensii and C. megalocarpus, exhibited reduced leaf cover during both dry seasons. C. megalocarpus was the only species to maintain leaf cover throughout the 2-year experimental period. M. volkensii and S. spectabilis exhibited similar leafing phenology, losing almost all leaf cover during the long dry season (July–October) and flushing before the onset of the ensuing rains. S. spectabilis lost few leaves during the short dry season, whereas M. volkensii shed a greater proportion of its foliage before flushing prior to the long rains (March–July). M. volkensii lost much of its leaf cover during the 1997/1998 short rains (October–February), when soil water content was unusually high. Although essentially evergreen, leaf cover in C. megalocarpus decreased during the dry season and increased rapidly during periods of high rainfall. G. sepium exhibited a period of low leaf cover during the long dry season and did not regain full leaf cover until mid-way through the short rains. The mechanisms responsible for these phenological changes and the implications of tree phenology for resource utilisation and competition with crops are discussed.