Drivers of change in vegetation and soil in the Gamka Karoo: grazing versus rainfall

Abstract

The Gamka Karoo is a low-lying vegetation type in the semi-arid Nama Karoo in the south-western part of South Africa with extensive rangelands used for livestock ranching and wildlife. Although the Nama Karoo is generally resilient to grazing and most responsive to variable rainfall, chronic overstocking, particularly in the driest regions and around water points, could degrade vegetation and soils. The interactive effects of rainfall (MAP) and grazing (stocking rate, percentage overstocking, resting system) on plant species composition and soil chemical characteristics were examined on 36 farms across a regional rainfall gradient (120–300 mm y⁻¹) and around water points (n = 3) on farms. Multivariate direct gradient analyses revealed regional species compositional differences were related most strongly to MAP (r = 0.736), but not to grazing, with no MAP × grazing interaction. On a smaller scale, grazing intensity (1/distance from water point) had a weak negative impact on the vegetation (r = 0.36) and soil chemistry (r = 0.49) with nutrient enrichment around water points in paddocks. Our findings suggest that the Karoo is responsive to rainfall, but loosely coupled to grazing. Adaptive management with livestock numbers matched to rainfall and forage supply is recommended to sustain animal production, vegetation and soils.

Keywords:

• overgrazing
• piosphere
• semi-arid
• stocking rate