Seeding Density Alters the Assembly of a Restored Plant Community after the Removal of a Dam in Southern Wisconsin, USA

Recently exposed reservoir sediments, prone to colonization by invasive species, provide novel settings to test hypotheses related to soil conditions and propagule supply as potential drivers of plant assembly in disturbed ecosystems. We used a dam removal site in southwestern Wisconsin to examine the relationship between the physiochemical properties of dewatered sediments, seeding density, and plant community assembly. The plant communities from five seed densities (1000, 500, 250, 125, and 0 seed m−2) were annually assessed over four years. We hypothesized (1) that the native aboveground biomass and the proportion of native to invasive (non-seeded species) aboveground biomass would increase with the seeding density and (2) that the diversity of seeded native species would increase with a higher seeding density. We found evidence that sowing at least 500 seeds m−2 of prairie species increased their abundance, establishment, and plot diversity compared to non-seeded plants that persisted four years after seeding (p < 0.05). The seeding density treatments led to the assembly of two distinct communities: “native” and “invasive”. The “native” community, assembled in plots seeded with at least 500 seeds m−2, had a greater aboveground biomass and diversity (i.e., richness) of seeded plants compared to plots with lower seed densities, and its productivity was positively related to this richness. In the “weedy” community, the diversity of invasive species had no relationship to their aboveground biomass, likely because these species share similar traits (i.e., redundancy) and may have performed similar functions within the plant community. These findings suggest that the seeding density interacted with the disturbed soil resources to increase the diversity and productivity of seeded native species and may serve as a positive feedback mechanism for the establishment of native communities in dewatered sediments.