The combined effects of afforestation and grazing on Uruguayan grassland vegetation at multiple spatiotemporal scales

Abstract

The Campos grasslands of southern South America have been affected by long-term livestock grazing and increasingly widespread afforestation, but the effects of multiple disturbances have not been well studied, especially at multiple spatiotemporal scales. To determine vegetation response to these disturbances, we constructed grazing exclosures bisecting the forest-grassland ecotone at five sites and measured species richness in nested plots for four treatments (grazed forest, ungrazed forest, grazed grassland, and ungrazed grassland) over 18 months. Species-area curves revealed greater small-scale species richness in grasslands than forests but greater rates of species accumulation in forests and thus little difference in richness between habitats at larger scales. Temporally, small-scale richness differed among sample dates, but rates of species accumulation did not. The interactive effect of habitat and management on total species richness varied with times since the exclosures were built. Grazing and afforestation interactively affect the vegetation of the Campos, as plants respond to changes in both disturbances; over the duration of this study, the removal of grazing decreased species richness in grasslands but increased richness in forests. Our results illustrate that the response of plant species richness to grazing and afforestation varies by the nature of the disturbance mechanism, and that multiple disturbances have complex interactive effects on vegetative responses.