A 10-year sheep grazing experiment was conducted in southern Patagonia to test the hypothesis that high stocking rates would promote transitions from open grasslands to dwarf shrublands. Treatments included three stocking rates and an exclosure. Total aerial vegetation cover grew in all treatments, but especially under low stocking rates and exclosure. Short grasses and herbaceous dicots increased significantly in these last treatments. Bunchgrasses increased under low stocking rates, but decreased in the exclosure. Cover of dwarf shrubs did not change in any treatment. Diversity of the community, measured in Shannon-Wiener and species richness index, increased in the exclosure, and species richness was reduced under high stocking rates. Transitions were explored by means of PCA. While all treatments moved along the axes of the multivariate space, we detected no thresholds and found no indications of having induced transitions to alternate states with the grazing treatments imposed. The open grasslands at our experimental site could be considered stable states induced by a century of domestic grazing, and they withstood 10 years of grazing at 0.60 sheep ha(-1) without unfavourable transitions. Even though, loss of palatable species and diversity could indicate that moderate, variable stocking rates (average 0.40 sheep ha(-1)) would be closer to 'sustainable' in the long-term. Vegetation seems to retain the ability to build up cover and diversity after a stocking rate reduction. (C) 1998 Academic Press.
