The Impact of Seed Mix Weight on Diversity and Species Composition in a Tallgrass Prairie Restoration Planting, Nachusa Grasslands, Illinois, USA

We evaluated the effects of varying bulk seed mix weight (kg/ha) applications on species diversity and species composition of a tallgrass prairie planting established at Nachusa Grasslands (north-central Illinois). Following the application of five seed sowing weights (0, 11.2, 33.6, 56.0, and 78.5 kg/ha) containing seeds from 128 species, we censused plot species richness, stem density, and percent cover of each species in the treatment and control plots (0 kg/ha). Overall there were significant differences in vegetation composition between seed weight treatments. Low seed density plots had low species evenness, while densities of 56.0 and 78.5 kg/ha showed significantly greater evenness. Based on germination and growth, the floristic quality index (FQI) was significantly lower in the control and lighter seed weight treatments. Detrended Correspondence Analysis (DCA) ordination suggests that 56.0 kg/ha yields compositionally similar results to 78.5 kg/ha. Considering all ecological metrics analyzed, there were few differences between the 56.0 and 78.5 kg/ha treatments. The applications of 33.6 kg/ha and below led to outcomes such as greater weed abundance and lower overall vegetation cover, potentially creating future management concerns. Our findings, two years post-planting, suggest that seed weights of at least 56.0 kg/ha should be used whenever possible to maximize diversity and increase presence of species with high conservation value.