Influence of Environmental Factors and Sheep Grazing on an Andean Grassland

Chronic overgrazing in the central Andes alters vegetationand may cause erosion and loss of productivity, but quantitativestudies are lacking. We measured the relative influence ofenvironmental factors and sheep grazing on local plant speciescomposition, diversity, and soil organic matter in a remote sitein northwestern Argentina. Using redundancy analysis, wefound that environmental variables explained 22% of variationin species composition between sites, while grazing-relatedvariables explained 24% of variation. The complete model,incorporating all significant variables, explained 33% of variation.Aspect, season of grazing (wet vs. dry) combined withtotal vegetative cover, and soil type formed the basis for thefirst 3 ordination axes. Unpalatable or toxic species and verylow-growing species were significantly more abundant onheavily grazed sites compared to relatively protected sites.Stocking rate in wet season pastures was negatively correlatedwith total cover, forage volume, soil organic matter, andspecies richness. Season of grazing had a more dramatic effecton total cover, forage volume, species diversity and soil organicmatter, which were all significantly lower in wet season pasturescompared to dry season pastures. Season of grazing andaspect interacted strongly: wet season pastures on northaspects appear more susceptible to degradation and changesin species composition than south-facing sites. Our results suggestthat protecting pastures during the summer rainy seasonmay be an important complement to traditional managementefforts to reduce stocking rates