Effects of grazing exclusion on vegetation and seed bank composition in a mesic mountain grassland in Argentina

Background: Grazing is a major factor that affects grassland composition and species richness. While the long-term effects of grazing are well documented, information about the short-term effects of grazing is necessary for adaptive conservation management. Aims: Our objective was to ascertain how this held over varying time periods after herbivore removal vs. exclosures of different ages for grassland and soil seed bank composition in the Argentine Pampas. Methods: The composition of the vegetation and the seed bank assessed in plots with 2 and 12 years of grazing exclusion and in nearby grazed areas. Multivariate analyses were used to evaluate treatment effects. Results: Grazing increased bare ground and reduced plant cover. Grazing reduced grass specific richness in the seed bank, while increased the richness and density of shrubs. The composition of the vegetation and the seed bank was not related. There was a trend whereby grazed areas were characterised by shrub species and non-palatable or annual grasses; the 2-year-old exclosures by forb species and the 12-year-old exclosures were dominated by perennial grasses. Conclusions: The removal of herbivores has changed vegetation and seed bank composition. The observed trend in reduction of species and life form diversity may be countered by controlled grazing for conservation purposes.