Tree pruning as a means of controlling water use in an agroforestry system in Kenya

Abstract

This paper forms part of an investigation of the water use by a hillslope agroforestry system incorporating Grevillea robusta, a fast-growing tree species, and maize. Soil moisture deficits were monitored using a network of neutron probe access tubes monitored weekly over 5 years, in plots containing either trees or crops grown separately, as well as grown together as an agroforestry system. The agroforestry system had the highest water use, the trees grown alone used slightly less water and a conventional maize crop used significantly less water than either of the treatments incorporating trees, leading to substantial losses of water from the system through deep drainage. Significant differences in surface soil water input to and uptake from the soil profile were observed over the relatively small distances between trees. After rainfall, the soil water content close to the trees (0.3 m) increased more than it did at a 2.5 m distance. This was attributed to stemflow, and it was assumed that increased soil water uptake at the same position was due to higher root activity close to the trees. The importance of stemflow to the further development and refinement of existing process-based agroforestry models is discussed. It was demonstrated that moderate pruning of the tree canopy, although reducing competition for light between the trees and crops (data not shown), did little to limit the water demand of the tree component, resulting in little or no recharge to the soil profile. When the tree canopy was more heavily pruned, following the example of local farm management practices, the water requirement of the tree component was reduced and the soil profile water storage was able to recharge following rainfall. The rationale for including substantial tree pruning as a part of agroforestry site management practices is discussed.