CONICET | Buscador de Institutos y Recursos Humanos

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: We tested the intermediate disturbance hypothesis whereby intermediate grazing intensities should maximise species diversity. 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 (separated as that contained in the soil and litter fractions) were assessed in plots with 2 and 12-years of grazing exclusion and in nearby grazed areas. Multivariate analyses of plant functional groups and species composition were used to evaluate treatment effects.Results: Grazing increased bare ground and reduced plant cover. Grazing reduced the specific richness in the seed bank of grass seeds, while increased the richness and density of the seeds of shrubs. The soil fraction was more species-rich and had more seeds than the litter fraction. 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 composition and cover (reducing bare soil and thus erosion) and seed bank density. The observed trend in reduction of species and life form diversity may be countered by controlled grazing should maintaining the full spectrum of life forms be deemed desirable for conservation purposes.

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