Can the Persistent Seed Bank Contribute to the Passive Restoration of Urban Forest Fragments After Invasive Species Removal?

Urban forest fragments are vulnerable to invasion by non-native species, and invaded forests are increasingly targeted for invasive species removals. Our goal was to determine the extent to which persistent seed banks can contribute to the recruitment of native forest species into urban forest fragments from which invasive plant species have been removed. In a greenhouse, we germinated seeds from soil samples from three forest fragments in Portland, Oregon, US. All sites had been invaded by Hedera hibernica (Irish ivy) and H. helix (English ivy), Clematis vitalba (virgin's bower), Ilex aquifolium (English holly), and Prunus laurocerasus (English laurel). At one site, these species had been removed three years prior to our study. Emergents represented 53 taxa, classified as: native forest species, native non-forest species, and non-native species. We observed few native forest species (5–12/site); 29–83% of samples contained these species, at median densities of 0–2 seeds/sample/site. Non-native species were more diverse (12–17/site), more frequent (75–89% of samples), and denser (median = 2–5 seeds/sample/site). Clematis vitalba seeds were especially abundant. Invasive removal had little effect on the persistent seed bank; however, the least-invaded site had the highest richness, frequency, and density of native forest species. The low richness and abundance of native forest species is not necessarily a concern, because many forest species do not form persistent seed banks. The annual seed rain can regenerate native species whose density has been diminished by invasive plant species and their removal. However, managers wishing to restore absent species should plan to follow removal efforts with active revegetation.