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Chronic overgrazing in the central Andes alters vegetation and may cause erosion and loss of productivity, but quantitative studies are lacking. We measured the relative influence of environmental factors and sheep grazing on local plant species composition, diversity, and soil organic matter in a remote site in northwestern Argentina. Using redundancy analysis, we found that environmental variables explained 22% of variation in species composition between sites, while grazing-related variables explained 24% of variation. The complete model, incorporating all significant variables, explained 33% of variation. Aspect, season of grazing (wet vs. dry) combined with total vegetative cover, and soil type formed the basis for the first 3 ordination axes. Unpalatable or toxic species and very low-growing species were significantly more abundant on heavily grazed sites compared to relatively protected sites. Stocking rate in wet season pastures was negatively correlated with total cover, forage volume, soil organic matter, and species richness. Season of grazing had a more dramatic effect on total cover, forage volume, species diversity and soil organic matter, which were all significantly lower in wet season pastures compared to dry season pastures. Season of grazing and aspect interacted strongly: wet season pastures on north aspects appear more susceptible to degradation and changes in species composition than south-facing sites. Our results suggest that protecting pastures during the summer rainy season may be an important complement to traditional management efforts to reduce stocking rates.
Adler et al. (Wed,) studied this question.