Priority effects and competitive exclusion by C4 grasses on longleaf pine savanna restoration sites

Abstract

On grassland and savanna restoration sites, planted native C4 grasses can competitively exclude other co‐introduced plant functional groups, with negative implications for ecosystem functioning and restoration success. Previous studies have suggested that C4 grass competitive exclusion does not occur in longleaf pine savannas of the North American Coastal Plain; however these studies were primarily conducted on old growth xeric sites. We designed an experiment to test competitive interactions between C4 grasses and forbs introduced by seed to mesic savanna restoration sites. Research installations were established on three former bahiagrass (Paspalum notatum) pastures in Florida, within which six different seed treatments were applied: (1) C4 grasses and forbs together in years 1 and 2, (2) C4 grasses alone in year 1 and forbs alone in year 2, (3) forbs alone in year 1 and C4 grasses alone in year 2, (4) C4 grasses alone in both years, (5) forbs alone in both years, and (6) an unplanted control. All plots were sampled 1 year after the second seeding event. We found that C4 grasses competitively excluded forbs, reducing cover of forbs arriving on the site at the same time, as well as those arriving in the second year. The highest forb species richness and cover was found in the three treatments that did not contain C4 grasses in year 1, and different C4 grass species exhibited different community associations. These findings suggest that restoration seed mixes for longleaf pine savannas should account for the competitive interactions of C4 grasses.