Responses in the Physical Structure and Density of Bush Clumps on Piosphere Gradients in an Arid Thicket Mosaic, South Africa

Abstract The hypothesis that transformation of rangelands by domestic mesoherbivores follows state-and-transition models allows for the prediction that the vegetation could cross an ecological threshold. We examine this hypothesis by attempting to identify structural thresholds in Gamka Thicket, a structurally heterogeneous variation of Albany Thicket in South Africa. We relate the physical structure and density of thicket bush clumps to distance from artificial watering points in four piosphere treatments differing in rangeland management history. Furthermore, we compare the pattern, extent, and variation in transformation at the end regions of our piosphere treatments with each other and with a transformed and untransformed reference site. Changes to the density and leaf mass below 1.5 m of bush clumps indicate that their structure is profoundly altered by mesoherbivores. Nevertheless, the evidence is not sufficient to indicate that irreversible changes in the physical structure of bush clumps have taken place and therefore that structural thresholds have been crossed. A useful addition to our sampling design would be to include exclosure plots along the piosphere gradients.