Vegetation change in response to grazing exclusion in montane grasslands, Argentina

Background and aims - Natural montane grasslands in the Southern Pampas of Argentina are expected to show a high potential for recovery after heavy grazing due to their evolutionary history in the presence of large herbivores and their high productivity. The objective of this work is to compare plant diversity, bare soil percentage, biomass and botanical composition between grazed and non-grazed areas at different times following grazing exclusion. Methods - Vegetation was assessed on exclosures established in 2006 and on nearby areas open to grazing by feral horses in December 2006, 2007, 2008 and 2009. In 2009 we added 15 year old exclosures to the analysis. Key results - Species richness declined 24 months after grazing exclusion, mainly due to a decrease in the abundance of forb species. Bare soil percentage was significantly reduced 12 months after exclosures were built. Above-ground biomass showed a 3-fold increase 12 months after grazing exclusion. After three years, species richness and biomass were similar to those corresponding to older exclosures (15-years old). The composition of plant communities also changed following horse exclusion, with three and 15-years old exclosures dominated by perennial grasses typical of late seral stages. Conclusions - Our results support the hypothesis that natural montane grasslands in the Southern Pampas of Argentina may recover fast from grazing by large herbivores without application of specific restoration techniques.