The Consequences Of Ligustrum Sinense (Chinese Privet) Invasion Of Piedmont Floodplain Forests In South Carolina: Effects On Plant Species And Factors Influencing Distribution.

摘要

Invasive species and human land use are related global change drivers that can confound interpretations of native plant declines. Invasive plant species are reported to negatively affect native plant species, but recent research questioned the interpretation that invasive plants are the sole cause of native plant species decline. At the same time human land use has resulted in direct and indirect changes to habitat and disturbance regimes. Riparian habitats are closely associated with human dominated systems and changes to the natural flow regime have been observed with development. In the southeastern United States floodplain forests are being rapidly invaded by Ligustrum sinense (Chinese Privet) a non-native shrub and provide a model system to investigate mechanisms driving invasive species dynamics. This study examined (1) if L. sinense is the direct cause of native plant decline and (2) what factors influence its distribution across the landscape. This project used both observational and experimental designs to investigate the effect of L. sinense on plant species. I conducted a vegetation survey of twelve floodplain forests in the Piedmont ecoregion of South Carolina. Additionally, at one site an experiment was conducted for two growing seasons (63 weeks) to compare the growth and survival of native plants under mature L. sinense stands. Results of the vegetation survey show that sites with mature L. sinense had significantly lower herbaceous plant cover, total plant abundance, and native species richness. At the local level results from the transplant experiment show drastic effects of L. sinense on native plant seedlings with both decreased survival and growth. The parallel results of landscape and local scales reveal that L. sinense is a cause of native species decline. Invasion of L. sinense inhibits the herbaceous understory and prevents the regeneration of canopy trees by suppressing seedlings. This research supports the hypothesis that certain invasive plants can have severe and dramatic impacts on native species and their associated ecosystems. Ligustrum sinense presence is correlated to urban areas, but the reason for this correlation is unknown. If L. sinense is taking advantage of disturbance created by human development than abiotic conditions should differ by dominant land cover and L. sinense should have higher growth or survival in developed watersheds. If L. sinense is competitively dominant then it should have higher survival and growth than native species in all watersheds. To test these questions I conducted a transplant seedling experiment where I monitored the growth and survival of L. sinense against three native species for two growing seasons in nine watersheds with different land covers. By monitoring both abiotic conditions along with the biotic responses of the transplant seedlings I can infer what is the mechanism driving L. sinense invasion. The abiotic conditions where similar between watersheds and do not support a disturbance gradient. Likewise growth and survival of L. sinense was not related to watershed development. These results indicate that L. sinense is not driven by disturbance and that no floodplain forests are resistant to invasion. In comparison to the native species, L. sinense had the highest mean survival. Ligustrum sinense also had significantly less herbivory than native species. These results indicate that L. sinense is a strong invader that does not need disturbance events to invade habitats. Further surveys revealed that sites that had been invaded earliest had the highest amounts of L. sinense. This shows that past propagule pressure is an extremely important element for explaining current distributions. Management of both invaded areas and prevention of new propagule introductions are needed to avert further spread of L. sinense.