Medium-term changes in grass composition and diversity of Highland Sourveld grassland in the southern Drakensberg in response to fire and grazing management

Abstract

This study examined the compositional stability of Highland Sourveld in response to fire and grazing by wildlife (Coleford Nature Reserve) and by cattle on three properties over 20-25 years. A limited amount of compositional change took place except on a property stocked 1.5 times as heavily as the others, but no species were lost. In general, Decreaser species decreased and Increaser 2 species increased, although individual species of a group did not show a consistent pattern of change in abundance. Consistent heavy grazing favoured mtshiki species (Sporobolus africanus, Eragrostis plana), stoloniferous species (Paspalum notatum) and Alloteropsis semialata. The extent of compositional change was least at an intermediate (0.4 versus 0.1-0.75AU ha-1) stocking density. Clovelly soils were prone to twice as much change as Hutton or Mispah soils. On lands abandoned for >50 years, the dominant E. curvula declined by two thirds and small amounts of characteristic Highland Sourveld species established. Infrequent burning (every six years) resulted in twice as much compositional change as annual burning. Ordination techniques revealed three main groups of species, in terms of their amount and direction of change, identifiable with the Increaser-Decreaser classification. Important contradictions were, however, evident, such as a similar response for the Decreaser Themeda triandra and the Increaser 2 Diheteropogon filifolius, indicating review of this classification is warranted for the Highland Sourveld. Changes in composition reflected changes in grass diversity (evenness, species richness, Shannon-Weaver diversity). Heavy grazing increased evenness, hence Shannon-Weaver diversity, through reducing the mono-dominance of T. triandra. Grazing-induced changes in grassland composition may therefore reflect the conservation status of grasslands.