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Native seed reintroduction experiment

Old growth longleaf pine savannas harbor a diverse array of plant functional groups, including mast-producing shrubs, legumes, wildflowers (also known as forbs), warm-season grasses (i.e. grasses that use the C4 photosynthetic pathway as an adaptation to hot and/or dry weather), and cool-season grasses. All of these functional groups are essential components of wildlife habitat and ecosystem functioning, but native C4 grasses (in particular wiregrass, Aristida stricta) are especially important, due to their role in creating a fine fuel layer that helps carry the low-intensity, fast-moving ground fires upon which the system depends. For this reason, restoration efforts in the region typically focus heavily on establishing wiregrass and other C4 grasses. However, research in other regions has shown that 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, 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 one 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.

Bar graph showing the relative proportions of C4 grasses and forbs, including leguminous forbs, and the relative proportions of planted and volunteer species within each functional group.