Dynamics of woody vegetation in a semi-arid savanna, with a focus on bush encroachment

Abstract

Increases in the tree: grass ratio with accompanying changes in herbaceous composition, called bush or shrub encroachment, is a worldwide phenomenon in savanna regions. Although heavy grazing by livestock is often believed to be the cause, it is clear that the factors and processes involved in bush encroachment are poorly understood. The effects of soil type and grazing on the tree: grass ratio in encroached Acacia mellifera-dominated savannas in the Northern Cape, South Africa, were investigated by means of vegetation classification and analyses of sequential aerial photographs. Acacia mellifera never occurred on clay pans, and in low numbers only in other clay and sandy soils. This trend is ascribed to the soil texture, soil water regimes and/or heavy utilisation in these areas. Soil rockiness was important in determining the long-term presence and abundance of A. mellifera, perhaps due to better soil moisture conditions. Long-term increases in A. mellifera in non-rocky habitats were also found, although the causes of this remain unclear. Grazing strategy did not prove to be important in determining presence or abundance of A. mellifera in the short- or long-term.