Finding the complementarity: trees improve water availability to crops in southwest Uganda

On the steep and crowded terraced hillsides of southwest Uganda, farmers are challenged with trying to produce crops while at the same time needing tree products. Their terraces are often degraded, with the upper terrace typically having poor soil fertility and producing low crop yields. In this dry environment, it is important to use as much of the available rainfall as possible.

The results of four years research into the way trees may increase water productivity in these conditions have been published in the scientific journal, Agroforestry Systems, with the authors claiming such approaches may also be applicable in other dry areas with steep terraced slopes.

Co-author and Principal Scientist with the World Agroforestry Centre, Ric Coe says, “The key is to identify appropriate species and management practices that reduce water losses while ensuring crop and tree production.”

Researchers from the World Agroforestry Centre, the UK Centre for Ecology and Hydrology, Makerere University in Uganda and the University of Nottingham set out to determine how trees on the upper level affect evaporation and soil water content, and what role root and shoot pruning plays in improving the compatibility between trees and crops.

The scientists found that some tree species have beneficial effects on evaporation and soil water content in the adjacent cropping area and that the effects of pruning on crop yield were associated with reductions in evaporation and increases in soil water content.

Traditional cropping systems often cannot fully utilise available rainfall because of losses through evaporation from the soil surface, runoff and drainage. In areas such as southwest Uganda where crop yields are less than 35 per cent of potential production and there is a shortage of wood, agroforestry is seen as a viable option to sustaining crop productivity while also providing essential tree products and ecological services.

Agroforestry can improve productivity in several ways: increasing soil organic matter, infiltration and water storage; improving soil physical properties and biological activity; and enhancing nutrient supplies through nitrogen fixation and reduced leaching and soil erosion.

But, warns Coe, agroforestry does not always provide a solution as competition with crops is common. “There is a need to develop effective strategies which limit competition and maximise complementarity and resource use efficiency.”

The study was carried out at Kigezi High School, Kabale District in southwest Uganda. Much of the land in the surrounding area is terraced to control runoff and erosion, and farmers grow sorghum, maize, beans, peas, sweet potato and Irish potatoes.

The scientists grew sole stands of Alnus acuminata Kunth (alnus), Calliandra calothyrsus Meissner (calliandra), Sesbania sesban (L.) Merr. var. sesban (sesbania) as well as a mixture of all three, and a sole crop control treatment on the upper terrace. The tree species were chosen for their Nitrogen fixing capacity and ability to produce fuelwood and above-ground biomass under local climatic conditions. Cropping of maize or beans continued on the middle and lower terraces during the long and short cropping seasons.

In the tree mixture, a single row of each species was grown; sesbania, the least competitive of the species was located next to the cropping area, calliandra in the central row and alnus, believed to be the most competitive, planted furthest from the cropping area.

Four tree management sub-treatments were applied: unpruned, root pruned, shoot pruned and root + shoot pruned.

Climatic conditions were monitored, including solar radiation, air temperature, atmospheric saturation deficit, rainfall, wind speed, soil water evaporation and volumetric soil moisture content.

The scientists found that sesbania and alnus increased soil water content by 9–18 % in the cropping area on the lower terrace but calliandra reduced soil water content by 3–15 %. This suggests that sesbania and alnus can be incorporated into smallholdings without compromising water supplies to adjacent crops, but the extensive lateral rooting of calliandra deprived adjacent crops of water even though evaporation was reduced.

Root + shoot pruning of the tree mixture and alnus increased soil water content in the cropping area compared to unpruned trees, but had no significant effect in the other tree-based treatments.

Says Coe, “The results demonstrate that some agroforestry systems in semiarid and sub-humid tropics can improve water use efficiency and provide valuable tree products to smallholder farmers”.

The full study is available from:

Siriri D, Wilson J, Coe R, Tenywa M M, Bekunda M A, Ong C K, Black C R (2013) Trees improve water storage and reduce soil evaporation in agroforestry systems on bench terraces in SW Uganda. Agroforestry systems 87: 45-58