Passive recovery of an urban forest in the Pacific Northwest after removal of invasive plants

Abstract

Urban natural areas are often heavily invaded by non-native plants. To restore the biodiversity and ecosystem function of these areas, land managers commonly remove invasive species. Partnering with Portland Parks and Recreation, I studied a management-scale removal of Hedera spp., Clematis vitalba, Ilex aquifolium and Prunus laurocerasus from a 59 ha forested natural area in Portland, OR. Over four years, I compared the community composition of this area with contiguous control areas where invasive species remained intact. Hedera, the most abundant invader, was dramatically reduced in removal areas, along with Clematis and Ilex. Both the abundance and the diversity of native herbs and woody seedlings were substantially higher in removal areas, with Claytonia sibirica, Galium spp., Hydrophyllum tenuipes, Trillium ovatum, and Acer macrophyllum seedlings all showing significant increases. By 2015, the minimum native species richness for removal areas was 37, as determined by the Chao 2 estimator, versus 10 native species for control areas. In contrast, native shrubs and ferns showed no increase in response to invasive removal; in fact, control plots typically contained significantly higher stem densities of native shrubs and ferns than removal plots did. Few non-native species recolonized removal areas, except for seedlings of Clematis. Collectively, these results indicate that passive recovery of the herbaceous layer is possible at large scales following invasive species removal, but that active replanting of woody species may be needed. Partnerships between land managers and academic researchers are key to documenting the effects of management-scale removals.