Influence of rainfall and grazing on the compositional change of the herbaceous layer of a sandveld savanna

Abstract

The influence of heavy grazing by cattle for three years, and of rainfall interception for one year, on the herbaceous composition of a sandveld savanna in Gazankulu was investigated. The composition and the abundance of the predominant species changed substantially, owing mainly to rainfall. Pogonarthria squarrosa, considered to increase under grazing, was initially the most abundant species, but declined most rapidly on the heavily‐grazed, rainfall interception treatment, owing to a high mortality. Digitaria eriantha increased markedly on the heavily‐grazed treatment, despite up to a 24% tuft mortality, because of extensive vegetative recruitment from stoloniferous growth. Aristida congesta subsp. barbicollis and Aristida stipitata, considered to increase under heavy grazing, increased on the lightly‐grazed treatment. There was an explosion of annual forbs, predominantly Tephrosia purpurea, Indigofera costata and Cassia absus, in the third year. The trend of rainfall during the study, initially wet and successively drier, apparently had a greater influence on compositional trends than the imposed treatments.