Effects of prairie restoration on weed invasions

Restoration of late-successional plant communities may reduce weed invasion in non-cropland. In studies of natural succession and invasion, however, the presence of late-successional vegetation is often confounded with lower weed propagule pressure and greater time since disturbance. This paper reports a study in which propagule pressure and time since disturbance were controlled by adding weed species to experimental plots dominated by early and late-successional perennial grass species. Seeds of 12 weed species were added to randomized, 6-year-old plots of restored prairie and non-restored, old-field vegetation. Restoration reduced added weed biomass by 92% in year one and 72% in year two, and reduced biomass of four individual weed species, each by more than 82%. Likely mechanisms through which restoration may have reduced invasion include competition and establishment limitation. To examine these mechanisms, subplots were treated with N addition and burning, to reduce competition and remove litter, respectively. Several patterns suggest that competition may have mediated reductions in weed invasion: restoration increased biomass of resident vegetation, decreased light levels, and decreased weed size. Furthermore, adding N reduced effects of restoration on resident and added weeds. The importance of establishment limitation in reducing invasion is suggested by increased litter mass and decreased weed density with restoration. Burning removed litter, and reduced effects of restoration on added weeds. These results suggest that tallgrass prairie restoration can reduce weed invasion, and that this reduction may involve multiple mechanisms, including competition and establishment limitation.