Long-term influence of season of grazing and rainfall on vegetation in the eastern Karoo, South Africa

In the Nama-Karoo, South Africa, rainfall and grazing by livestock are two important drivers of botanical composition. Summer rains are advantageous particularly to the grass layer, whereas the characteristic dwarf shrubs survive better than grasses during drought, and benefit from rainfall during cool months. Heavy grazing by livestock during summer reduces the abundance of grasses, whereas winter grazing increases it. This study examined the influences of season of grazing (summer vs winter) and rainfall on botanical composition at two long-term grazing trials at Grootfontein in the eastern Karoo from 1949 to 1967. Severe summer grazing strongly reduced grass abundance and slightly increased the abundance of both palatable and unpalatable dwarf shrubs. High previous-season rainfall strongly promoted short-lived grasses. Dwarf shrubs and grasses declined in response to a decline in annual rainfall over the study period, except for the creeping grass Tragus koelerioides that increased in abundance, possibly in response to decreased competition and an increase in available gaps in the vegetation layer. Results indicate that grazing systems that exclude repeated summer-only grazing maintain ecological health, and that rainfall is the main driver of composition unless extreme grazing systems are employed.