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.

Introduction

Agroforestry is defined as a land-use system in which woody plants [often multipurpose tree species] are grown in association with agricultural crops, pastures or livestock (Breman and Kessler, 1997). Such systems have the potential to increase yields by effectively growing two crops at the same time on a single piece of land (Nair, 1991), and have been promoted as more sustainable agricultural production systems appropriate for developing countries where the use of external inputs is not feasible (Winterbottom and Hazelwood, 1987). Several reasons have been suggested for combining trees and crops on the same piece of land, as opposed to more traditional systems of crop fields and wood lots. These include, among others, reducing soil erosion, limited availability of land, beneficial shading of some crop species, the distance needed to travel to the nearest source of fuel wood, and a shift in ownership of woodland from communal to private tenure.

However, in many tropical areas the main factors determining the success or failure of agroforestry systems are usually either water availability (Singh et al., 1989), or physical constraints such as poor drainage, shallow soil depth, steep slopes and gravelly soils (Sanchez and Leakey, 1997). In certain systems where studies have shown a significant fraction of the annual rainfall is unused by current land management practices (Cooper et al., 1983; Ong et al., 1991; Wallace, 1991), being lost both through soil evaporation and percolation to beyond the tree or crop rooting zones, it is conceivable that trees could utilise part of the rainfall that would otherwise be lost (Huda and Ong, 1989; Nizinski et al., 1994). In the case of the African highlands, steep slopes and highly erodible soils, together with the removal of forest cover through increased population pressure has lead to severe problems of soil erosion. Agroforestry can provide a reliable tool for soil and water conservation at much lower cost than with traditional techniques such as banks, ditches or terraces (ICRAF, 1993).

The addition of trees to a conventional cropping system may increase the water-use of the system directly, through the utilisation of rainfall which cannot be used by the crop alone, or in the case of steeply sloping areas, a reduction in the amount of water lost through runoff (Young, 1989). Increased plant water-use can also occur indirectly through a modification of the crop micro-climate so that less water is lost through soil evaporation, and hence more is available for tree and crop transpiration. However, combining trees and crops on the same piece of land may lead to competition between the trees and crops for available soil water, particularly in areas where the soil depth is shallow.

Many researchers are currently investigating the processes by which agroforestry systems can be sustained, adopting modelling approaches such as those outlined by Lawson et al. (1995)to determine the factors limiting the success of such systems in the field. Successful agroforestry systems may succeed where the tree and crop components are complementary to each other in the way they exploit the available resources. Anderson and Sinclair (1993)considered agroforestry systems in these terms, suggesting that success depended largely on niche differentiation—either temporal, for example, where the trees accessed resources like rainfall occurring outside the conventional cropping seasons—or spatial, for example, where deep-rooted tree species could utilise resources outside the reach of crop roots. In the case of intensive systems, such as the Grevillea/maize system reported in this paper, it may be the case that such complementarity has to be imposed on the system, through site management practices such as canopy pruning, or root pruning by trenching or deep root ploughing. However, such below-ground practices are labour-intensive or require powerful machinery.

As yet, the hydrological reasons for the success or failure of incorporating trees and crops in agroforestry systems are still largely unknown (Wallace et al., 1995). Since quantitative investigations of water-use in agroforestry systems are rare (Ong et al., 1991), we decided to investigate the water-use of a combined Grevillea/maize hillslope agroforestry system. It was intended that the experiment would provide data on how rainfall distribution is modified by both tree and crop canopies in an agroforestry system, as well as demonstrating how these processes could be controlled by regular manipulation of the tree canopy, achieved through pruning. To achieve this, changes in soil moisture deficits were investigated within the agroforestry system, and both the tree and crop components grown separately, using neutron probe access tubes to cover the entire soil profile. It is intended that the results will be incorporated in process-based agroforestry models currently under development (Lawson et al., 1995). This paper gives details of the experimental design used to monitor the changes in soil moisture in an agroforestry system (where there is important small-scale spatial variability in soil water input and output), and where the effects of pruning are examined.