Interactions of Woody and Herbaceous Vegetation in a Southern African Savanna

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Abstract
A number of plots were set up in 2 natural savanna communities, with identical climates but different soils, to examine possible competition between the woody and herbaceous components of the vegetation. The community on the more sandy soil is a broad-leaf woody savanna and the other a more open microphyllous Acacia community. Vegetative growth and soil water were monitored over a 2-yr period in control plots and in plots cleared one of the vegetation components. In the broad-leaf community the effect of the herbaceous vegetation on the woody plants is negligible. In the Acacia community with 7 times more herbaceous biomass, mature woody-plant growth was reduced by competition from the grass-dominated herb layer particularly in the first (wetter) year. The vertical root distributions and soil-water data indicate that the grasses take up topsoil (0-30 cm) water sufficiently rapidly to reduce drainage into the subsoil (30-130 cm), an that they also take up subsoil water directly, thus lowering the amount of subsoil water available to woody plants. The different herbaceous to woody-plant biomass ratios in the 2 sites and the different intensity of competition during the 2 yr can be explained in terms of the effects of the soil properties and of the rainfall intensity on the ratio of water in the topsoil to that in the subsoil.