Assembly history and resource levels modulate local, turnover and regional diversity in synthetic grassland communities
Modern landscapes contain native and exotic plant species, which in turn may strongly affect community structure and dynamics. It is not completely clear how, under the same regional species pool, assembly history and environmental differences may modulate the final state of these mixed native-exotic communities. Our objective was to evaluate the relative importance of resources availability and initial floristic composition on the local (alfa), turnover (beta) and regional (gamma) diversity in synthetic old-field grassland communities. We “constructed” 10 different plant communities resulting from two nitrogen levels and five different functional group compositions. Initial seed composition comprised native perennial grasses sown alone (5spp) or accompanied by forbs (10 spp), annual grasses (3spp) or exotic perennial grasses (5spp), plus an unsown control. Each initial composition by resource level treatment was assigned to 2m2 plots and replicated in six blocks. Total plant richness per treatmen was surveyed during 6 growing seasons.
After six years, all sown species have been established and produced seeds within experimental plots. At the local scale, plots sown with native grasses supported on average eight species, while adding exotic perennial grasses reduced richness to four species per plot. Here, nitrogen addition reduced local richness, in particular in those plots sown with exotic grasses. Turnover rate was significantly lower when exotic grasses were added to the plots. While plots with native perennial grasses incorporated on average ten species across the blocks independently of nitrogen level, plots with exotics changed on average ten species without N addition and only in three species when nitrogen was added. In terms of total richness, communities sown with native perennial grasses alone supported 13 mores species than those where exotic grasses were added into the initial species mix. In those plots where forbs species were also added supported the highest regional richness. Taken together, these results suggest that under the same regional species pool, initial functional group composition may modify the species richness at different scales. Moreover, early establishment of exotic perennial grasses may reduce not only the local number of species, but also community heterogeneity and regional species richness.