The influence of tree thinning on the soil water in a semi-arid savanna of southern Africa

Abstract

The investigation was conducted on an area covered by a dense stand of Hardwickia mopane (Leguminosae: Caesalpinioideae). Seven plots (65×180 m) were subjected to different intensities of tree thinning, ranging from a totally cleared plot (0%) to plots thinned to the equivalent of 10, 20, 35, 50, and 75% of the leaf biomass of a control plot (100%) with a tree density of 2711 plants ha−1. Thinning was completed during 1989 and the soil water study was conducted during the 1990/91 and 1991/92 seasons with the aid of a neutron water meter. The study area, before thinning, was characterized by the virtual absence of herbaceous plants with severe soil degradation in the form of surface erosion and crust formations. Increased infiltration of rainwater was measured in the thinned plots and this was associated with the establishment of herbaceous plants, mainly grasses. The soil water was predominantly held at a very shallow depth (<450 mm) and only in the totally cleared (0%) plot did the soil water content of the >450–900 mm soil zone show marked increases associated with specific rainfall events. Mean evapotranspiration water losses varied seasonally, but were generally much higher in the 0% plot (grass only plot), which confirmed that grasses use soil water more rapidly than the H. mopane trees. Evidence is presented that the roots of the H. mopane trees are able to utilize soil water at a matric potential lower than that of grasses (ψ<−1500 kPa). This enables the H. mopane trees to compete successfully with herbaceous plants and to prevent their establishment at high tree densities.