Grassland plant species diversity decreases invasion by increasing resource use

Species richness of plant communities has been demonstrated to provide resistance to invasion by unsown species, though the relationship with resource availability varies between studies. The present work involved five grassland species grown in monocultures and in four-species mixtures sown in accordance with a simplex design. The species used represented different functional groups (i.e. grasses, legumes and non-N-2-fixing species), each of which differed internally in terms of competitiveness. I hypothesized that sown diversity would negatively affect invader performance by decreasing the availability of light and soil nitrogen (N) for invading species, and that functional composition of the sown diversity would affect the functional composition of the invading flora. The experimental plots were harvested for two years, and were fertilized with 100 kg N ha(-1) each year. The number of unsown species (classified into four functional groups) invading each plot and their proportion of the biomass harvested were recorded. The penetration of incoming light through the canopy, the apparent N uptake by the sown species from the soil, and the mineral N content in the soil were measured. I found that diverse communities captured more resources both above- and belowground, and the number of invading species and their biomass production were smaller in mixed than in monoculture plots. However, the sampling effect of one grass was also strong. These results suggest that increased resource use in diverse communities can reduce invasion.