Impact of sheep grazing intensity on vegetation at the Arid Karoo Stocking Rate Trial after 27 years, Carnarvon, South Africa

Sustained heavy grazing is expected to result in degradation and loss of biodiversity in drylands but long-term experiments which assess the impact of management practises on biodiversity are necessary. The effects of stocking rate (SR) on vegetation composition, abundance of different plant functional groups (PFGs), abundance of dominant species, and plant diversity were investigated after 27 years at a long-term SR trial. Vegetation composition was investigated using Canonical Correspondence Analysis and General Linear Models. Stocking rate and time had significant effects on species composition. Increases over time were apparent for total plant cover, palatable shrub cover, and perennial grass cover, annual grass cover decreased over time, whereas other PFGs and rangeland condition showed no trend over time. Greater stocking rates resulted in lower total plant cover, palatable and unpalatable shrub cover, perennial grasses, and annual herbaceous species. Higher annual rainfall resulted in higher total plant cover, while greater preceding three-month rainfall benefitted annual grasses and annual herbaceous species. Plant diversity seemed unaffected by SR. Vegetation structure did not seem to be influenced by SR after 27 years. Our study confirms the slow rate at which vegetation change occurs in drylands and highlights the importance of long-term monitoring trials.