Patterns of Privet: Urbanizing Watersheds, Invasive Ligustrum sinense, and Performance of Native Plant Species in Piedmont Floodplain Forests

Floodplain forests in rapidly changing landscapes with increased urbanization may reshape habitat conditions to the detriment of native biota and favor invasive species. We assessed whether introduced Ligustrum sinense distribution and abundance are causally linked to urbanization and whether mechanisms that promote L. sinense cause the demise of native plant species. We surveyed vegetation in 12 independent floodplain forests along an urban to rural gradient in South Carolina, USA. We then used a seedling transplant experiment in nine watersheds to assess how increased urban development affects survival and growth of L. sinense and three native species over two growing seasons. Urban development ranged from 1 to 45% and L. sinense cover was positively associated with development. However, in our transplant experiment, growth and survival of L. sinense did not differ significantly among watersheds. Native species were able to survive at all sites, but performance varied greatly among sites and species but not as a function of urban development. Our results suggest that although L. sinense invasion and urbanization are related (likely due to proximity of propagules from urban sources), the demise of native species cannot be explained by increased urbanization or changes in edaphic conditions and points to L. sinense as an agent of change in our floodplain forests. These results indicate that all floodplain forests are at risk of invasion if propagules arrive and that land managers need to be vigilant against new species introductions regardless of the proximity to urban areas.