The growth form of dominant grasses regulates the invasibility of Uruguayan grasslands

In this paper we report the results of a small‐scale experiment on patches of natural grasslands in Uruguay. We analyzed whether growth form of C 4 grasses affects invasion success. The experimental plots represented three patch‐types arising from short‐term grazing effects and were dominated by prostrate C 4 grasses, erect C 4 grasses and codominated by prostrate C 4 grasses and forbs. We seeded the plots with four exotic species: one perennial C 4 grass, one annual C 3 grass, one perennial forb and one annual legume. Contrary to many studies conducted at small spatial scales, our results suggest that in our system, the processes that control invasibility are not directly linked with resident species richness or biomass, but rather are driven by growth form. Emergence of invaders did not differ among patch‐types, but seedlings survived less in plots dominated by erect grasses. We suggest that the erect growth form of C 4 grass species is more successful in preventing invasion by mediating seedling competition for light under the canopy for a longer period of time. Grazing management, through its ability to regulate canopy architecture, is a potentially important tool in controlling invasion.