Physical impact of sheep grazing on arid Karoo subshrub/grass rangeland, South Africa

Abstract

Grazing levels and rotational schemes need to be tailored to each individual farm or pasture, and more studies are needed on the resilience of rangelands and on separating the effects of grazing and climate. The direct short-term impact of three rates of stocking (4, 8 and 16 Small Stock Units-SSU/ha) was quantified in terms of composition and cover of arid Nama Karoo vegetation (subshrub/grass). Mature Merino wethers grazed in one hectare plots during May in 1995 and 1996 (the plots were not subjected to grazing at any other time). The basal cover of the Karoo bushes (shrubs) showed a decrease at the highest stocking rate only, with the species Phymaspermum parvifolium the most sensitive to intensive grazing. An increase in stocking rate caused a significant decrease in both canopy cover and canopy-spread cover. The canopy cover of palatable Karoo bushes such as Felicia muricata, Salsola calluna and Walafrida geniculata decreased most. Light stocking (4 SSU/ha) was apparently the least detrimental to the vegetation composition and cover. Regardless of stocking rate, an 11-month resting period was possibly sufficient for all the vegetation parameters concerned to be fully restored after grazing took place. The rangeland rapidly reacted to rainfall as the ephemeral cover increased temporarily. The higher the stocking rate was, the greater the increase in ephemerals occurring. The ecological sustainability of the Nama Karoo ecosystem, utilised by high stocking densities, is questioned.

Keywords: Basal cover; canopy cover; canopy-spread cover; ephemerals; Karoo bushes; stocking rate