Riparian habitats are areas of significant importance due to their ecosystem services and close association with human dominated systems. In the southeastern United States floodplain forests are being rapidly invaded by Chinese privet (Ligustrum sinense Lour.), which negatively impacts native plant species diversity. Previous research has shown that L. sinense distribution and abundance is correlated with urbanization, but the reason for this correlation is unknown. Two core components of invasion theory, biotic resistance and disturbance theory, could be possible explanations for why L. sinense is more prevalent in urban watersheds. If urbanization is a driver for L. sinense invasions, than either the biotic or abiotic conditions of urbanized watersheds would favor survival or growth of L. sinense. To test this prediction we created nine transplant gardens along an urbanization gradient in the Piedmont ecoregion of South Carolina. Twenty seedlings of L. sinense along with twenty native seedlings each of a tree, grass, and forb species were planted in each extant floodplain forest plant community during the spring of 2008. Plants were monitored for survival, growth, and herbivory over the following twelve months. Each site was also monitored for soil nutrients, soil infiltration rates, and ground water table depths.
Preliminary results from the study indicate that the biotic and abiotic conditions of urban watersheds are not a direct driver of L. sinense invasion. Survival rates of L. sinense were not higher in urbanized watersheds (81.7% ± 6.7, mean ± S.E.) than more forested watersheds (86.7% ± 4.4). Similarly mean growth of L. sinense did not differ between urban and forested watersheds (1.38 cm ± 0.33 vs. 1.59 ± 1.14, respectively). The native plant species likewise had no spatial pattern of survival or growth, indicating they fare equally well in all conditions. The abiotic factors of soil nutrients, soil infiltration rates, and ground water table depth did not differ significantly among sites. We interpret these results to have strong ramifications for management of wetlands. First, it shows that all sites regardless of amount or distance from urbanization are in danger of being invaded by L. sinense and that floodplain forests have no inherent biotic resistance. Second, our results imply that a mechanism not directly related to biotic or abiotic alteration by urbanization is promoting L. sinense invasion. With this information land managers need to plan for protecting critical habitat no matter its spatial relation to urban areas.