Seed sowing shifts native–exotic richness relationships in favor of natives during restoration.

A central goal of ecological restoration is to promote diverse ecosystems dominated by native species, but restorations are often plagued by exotic species. A better understanding of factors underlying positive correlations between native and exotic species richness, a pattern that is nearly ubiquitous at large scales in plant communities, may help managers modify these correlations to favor native plant species during restoration. Across 29 tallgrass prairie sites restored through seed sowing onto former agricultural lands, we examined whether the relationship between native and exotic richness is (1) altered by management, such as seed additions and prescribed fire; (2) controlled instead by environmental conditions and successional processes; or (3) altered by management in certain environments and not in others. As is commonly found, native and exotic richness were positively correlated at large scales (i.e., across sites) in this study. Management actions explained much of the remaining variation in native richness, while environmental conditions explained very little. Sites sown with more species at higher seeding rates, especially forb species, had higher native richness than predicted by the native–exotic richness relationship. In contrast, native richness was lower in older restorations than predicted by the native–exotic richness relationship, because native richness, and not exotic richness, declined with restoration age. We show that management actions such as seed sowing can modify the native–exotic richness relationship to favor native species during restoration. The development of management actions that mitigate native species richness declines over time will further benefit native species restoration. [ABSTRACT FROM AUTHOR]