Trait Complementarity Enhances Native Plant Restoration in an Invaded Urban Landscape

Developing strategies to reintroduce native species in invaded landscapes is a major challenge for ecological restoration, particularly in urban areas. Although complete eradication of invasive exotics is a common restoration goal, an emerging approach in heavily invaded landscapes is to plant native species likely to persist, even in the presence of exotic competition. Functional traits may be used to inform restoration strategies by indicating whether native and exotic species are likely to occupy different niches (i.e., dissimilar traits indicate different resource strategies). We adopted a functional trait approach to test whether planting species with dissimilar, as opposed to similar, traits to exotic species enhanced native species cover and diversity for an urban restoration project that is heavily invaded by Hedera helix (English ivy) and Hedera canariensis (canary ivy). We conducted a trait screening of Hedera and 37 candidate native understory plants and used trait values to select three species that were functionally dissimilar and three that were functionally similar to Hedera. We then tested their survival and growth when planted in and out of competition with Hedera over two years. Species with dissimilar traits to Hedera had high survival and growth across time and competitive environments, whereas species with similar traits to Hedera had significantly reduced survival when in competition with Hedera. Our results suggest that, in heavily invaded landscapes, restoration projects that plant species with complementary resource strategies to exotics may be most successful and that functional traits are a useful tool to select these species.