Trees improve water storage and reduce soil evaporation in agroforestry systems on bench terraces in SW Uganda

The success of agroforestry in semi-arid areas depends on efficient use of available water and effective strategies to limit tree/crop competition and maximise productivity. On hillsides, planting improved tree fallows on the degraded upper section of bench terraces is a recommended practice to improve soil fertility while cropping continues on the lower terrace to maintain food production. This study examined the influence of tree fallows on soil water content (theta (w) ) and evaporation (E (s) ). Alnus acuminata Kunth (alnus), Calliandra calothyrsus Meissner (calliandra), Sesbania sesban L. (sesbania), a mixture of all three species, or sole crops (beans (Phaseolus vulgaris L.) or maize (Zea mays L.)) were grown on the upper terrace. The same sole crops were grown on the lower terrace. Four management regimes (unpruned, root, shoot and root + shoot pruned) were applied to the tree rows adjacent to the cropping area. Neutron probe and microlysimeter approaches were used to determine theta (w) and E (s) when the trees were c. 3.5 years old. Sesbania and alnus increased theta (w) by 9-18 % in the cropping area on the lower terrace but calliandra reduced theta (w) by 3-15 %. After heavy rain, E (s) comprised 29-38 % of precipitation in the tree-based treatments and 53 % under sole crops. Absolute values declined as rainfall decreased, but E (s) as a proportion of rainfall increased to 39-45 % in the tree-based treatments and 62 % for sole crops. Root + shoot pruning of alnus and the tree mixture increased theta (w) in the cropping area but had no significant effect in the other tree-based treatments. The results suggest that sesbania and alnus can be planted on smallholdings without compromising water supply to adjacent crops, whereas calliandra decreased water availability despite reducing E (s) . These results provide a mechanistic understanding of reported effects on crop yield in the same site.