Spatial and functional niche overlap between invasive Ligustrum lucidum and native woody species in an urban shrine forest in Japan

Preventing invasion of exotic species into urban forests is important for sustaining native species diversity. In Japan, Ligustrum lucidum, an invasive tree, has become widespread in urban areas. Here, we investigated the spread of L. lucidum in a semi-natural, urban shrine forest to infer which native species’ preferred habitats may be affected. L. lucidum was most abundant near the artificially created forest edge where canopy trees are pruned regularly. Spatial distribution of L. lucidum overlapped with those of two native species (Cinnamomum japonicum, Quercus glauca), but was segregated from the other three native species. We also compared leaf functional traits between L. lucidum and the native species to infer functional niche overlap. L. lucidum exhibited greater plasticity of leaf functional traits compared to native species and its leaf functional traits were similar with Ci. japonicum, Ilex rotunda and Q. glauca, but not with Camelia japonica and Ternstroemia gymnanthera. Our results suggest that, currently, the preferred habitat and realized functional niche of L. lucidum overlaps with those of Ci. japonicum and Q. glauca. High plasticity of leaf functional traits contributing to shade tolerance may allow L. lucidum to expand its spatial distribution in this forest, affecting other native species in the future. Our results emphasize the importance of removing invasive species during early stages of invasion while their distribution is still relatively confined and negative effects on native species are limited.