Rainfall and Soils, Not Grazing Intensity, Determine the Composition and Productivity of Annual Plants in a Biodiverse Arid Winter Rainfall Region

Abstract

Concentration of grazer activity around watering points and stock posts has led to well-documented vegetation impacts in arid and semi-arid rangelands. Effect of grazing and abiotic factors on perennial plant diversity have been reported in the bio-diverse winter rainfall vegetation of the Succulent Karoo in South Africa, but the impact on annuals had not been investigated. The aim of this study was to assess the effect of rainfall, soil nutrients, land forms and grazing on forage depletion, soil chemistry and the composition, diversity, richness and biomass production of annual plants in the Richtersveld National park, which is a contractual national park used by seminomadic pastoralists to herd goats and sheep. A grazing gradient away from stock posts at 100m, 500m and 1000m on sandy plains and rocky foothills at five study sites with different mean annual rainfall and vegetation types were used. Distance from stock posts corresponded to a gradient of forage depletion and resulted in changes in soil chemistry with distance. Biomass production, richness and diversity of annuals were correlated with rainfall but not significantly affected by landform or distance from stock posts. Rainfall and soil variables had a greater influence on species composition than grazing in this arid ecosystem. The decrease in perennial cover and richness near the stock posts was not accompanied by increase in biomass production or richness of annuals, which is expected to have adverse effects on overall plant diversity and forage availability.