Dispersal, not Understory Light Competition, Limits Restoration of Iowa Woodland Understory Herbs

Abstract

Plant community restoration may be limited by the inability of target species to disperse to a restoration site (i.e. dispersal limitation) or by the failure of species to establish once seeds do arrive (i.e. establishment limitation). Seed additions attempt to overcome dispersal limitations; however, seed additions may fail due to establishment limitations resulting from competition with pre‐existing vegetation. We tested whether dispersal and/or competition with the pre‐existing understory limited restoration of 22 native deciduous woodland herbs in Iowa, United States. We employed an experiment that varied seed mix composition to contain either high densities of forbs or graminoids, coupled with clipping of pre‐existing understory vegetation to reduce competition for light during establishment. We found strong support for dispersal limitations, as seed additions increased understory richness by 4–8 species/m2. This was due to the addition of species from the seed mix, not modification to the naturally occurring plant community, which suggests the existence of an open (unsaturated) plant community. Most species in our seed mix established at high rates and there was no effect of seed mix (high vs. low density of graminoid vs. forb seeds) on the richness or persistence of species added by seed. Light competition with the pre‐existing understory vegetation did not limit establishment. We recommend simple seed sowing to restore the majority of species in our study; 19/22 species readily established. Our results support the use of less expensive seed mixes containing high density of gramoids, relative to forbs.