Short-Term responses to sheep grazing in a Patagonian steppe

Abstract

Grazing modifies ecosystem function through direct effects on plants, but also through indirect effects mediated by floristic changes induced by grazing. Although both types of effects occur in the long term, only the direct effects are evident in the short term. We evaluated the short-term direct effects of sheep (Ovis aries) grazing on a Patagonian steppe during one growing season. We measured plant aerial cover in permanent transects located at increasing distances from a watering point in three paddocks with different stocking rates through the growing season. We also measured frequency of defoliation for vegetative and reproductive phases of different plant species located along these transects. Sheep grazing directly (a) reduced aerial cover and/or increased frequency of defoliation of certain preferred grasses and perennial forbs, (b) did not increase the aerial cover of any life form, but only the proportion of bare soil, (c) did not change the litter aerial cover, and (d) defoliated the flowers of even the least preferred shrub. Result a) was coincident with previous plant aerial cover long-term studies; but results (b) and (c) were contrary to long-term studies, probably because they resulted from indirect rather than direct grazing effects. Result (d) was not detected by long-term studies, probably because flower defoliation through grazing is undetectable when measuring shrub plant aerial cover. Our study showed that grazing has short-term direct effects mainly on the most preferred species. This could be useful for rangeland management and conservation of Patagonian steppes because short-term effects may be more easily reversible than long-term ones, and may provide early warning of rangeland condition deterioration.