Low passive restoration potential following invasive woody species removal in oak woodlands

The restoration of oak woodlands often requires removal and management of woody invasive plants. This can trigger germination of the soil seed bank, which can alter restoration trajectories. In degraded deciduous woodlands generally, it is unclear whether the soil seed bank will contribute native plant material in sufficient quantities to help achieve restoration goals and allow practitioners to rely on passive restoration without supplemental seeding. To support restoration decision-making around passive or active restoration in three Rhamnus cathartica-invaded forest preserves in the Chicago region, we asked: (1) Does the soil seed bank differ from standing aboveground vegetation at reference and unrestored sites? and (2) Can the species richness, Shannon diversity, floristic quality (measured by abundance-weighted mean coefficients of conservatism), or density of germinable seeds of native species in the soil seed bank be predicted by a site's restoration status (reference or unrestored)? We found that species composition differed significantly between aboveground vegetation and the soil seed bank at reference and unrestored sites, with a significant interaction between restoration status and location. Despite high variation among the three forest preserves, restoration status also predicted native species richness, diversity, floristic quality, and seed density in the soil seed bank, with unrestored sites significantly lower than reference sites in all measures. Results suggest that reintroduction of native seeds will be necessary to fully restore desired native plant communities in oak woodlands following the removal of invasive woody plants.