Date of Award
Doctor of Philosophy
Global climate change is predicted to affect timing and severity of disturbance events (e.g., fire, drought, hurricanes, wind storms, and inundation), but the extent of these disturbance events and their impacts on natural ecosystems may vary regionally. Rising sea level, increased frequency and intensity of tropical storms, and altered inundation regimes are likely to create changing environmental conditions in low-lying coastal ecosystems. These large scale disturbances may increase resource availability and regeneration spaces, reduce competition, and possibly increase community vulnerability to invasion. Shifting disturbance regimes and invasion together are predicted to drive long-term shifts in coastal plant community structure and ecosystem processes. However, impacts of altered environmental conditions on native and invasive plant species and the species responses to changed environmental conditions are poorly understood. The aims of this study were: (i) to assess the probability of occurrence of juveniles of the invasive exotic Triadica sebifera and co-occurring native species, Baccharis halimifolia, Ilex vomitoria, and Morella cerifera in the field in relation to surrounding environmental factors, (ii) to assess the effects of elevated salinity across a typical coastal transition on germination of T. sebifera, B. halimifolia and M. cerifera, using controlled growth chamber and greenhouse experiments, (iii) to assess the effects of climate change and shifting inundation and tropical storms regimes on T. sebifera, B. halimifolia and M. cerifera, and (iv) to evaluate the role of vesicular arbuscular mycorrhizae (VAM) on spread of invasive T. sebifera in coastal transition ecosystems at the Grand Bay National Estuarine Research Reserve (GBNERR), Coastal Mississippi, southeastern USA. Results from assessing the probability of occurrence of juveniles of invasive and co-occurring native species showed that soil water conductivity (i.e., salinity) was the major factor related to the occurrence of invasive T. sebifera and native B. halimifolia, I. vomitoria and M. cerifera along the coastal transitions. Probability of occurrence of the invasive T. sebifera was significantly related to landscape factors and occurrence was highest in close proximity to roads, trails, power lines, and recreational sites, and water bodies. These results imply that future increases in salinity will negatively impact I. vomitoria, M. cerifera, and T. sebifera, leading to range contraction of these species away from the coast. However, natural and anthropogenic disturbances that often increase resource pulses and reduce competition, likely increase the dominance of T. sebifera in already invaded areas. Positive effects of landscape structures on T. sebifera occupancy highlight the role of landscape variables in promoting new invasions in coastal forests of the southeastern USA. Controlled growth chamber and greenhouse germination experiments demonstrated that germination of all species (i.e., T. sebifera, B. halimifolia, and M. cerifera) decreased with elevated salinity and that the reduction was most pronounced in soils from the most seaward zones along the coastal transition. Although native B. halimifolia was least sensitive to elevated salinity, invasive T. sebifera displayed plasticity of germination trait across different salinity levels in most inland soils. These results suggest that the phenotypic plasticity may facilitate spread of Triadica sebifera under some degree of salinity stress in more inland section of the coastal transition. A manipulative greenhouse experiment demonstrated that simulated canopy damage from intense hurricane winds and associated storm surge produced differential effects on survival and growth of native (B. halimifolia and M. cerifera) and invasive (T. sebifera) species at simulated different forest conditions common in the GBNERR. Invasive T. sebifera was by far the most shade tolerant of the three species and seedling survival under highly shaded conditions may provide it with a competitive edge over native species during community reassembly following tropical storms. T. sebifera may better utilize post-hurricane conditions (e.g., resource-rich empty spaces) and potentially increase its dominance in coastal forested ecosystems. The last experimental study revealed that invasive T. sebifera had higher VAM colonization compared to co-occurring native species both in controlled greenhouse and field experiments, and that the higher colonization leads to significant increases in aboveground biomass, supporting the hypothesis that VAM fungi strongly benefit the invasive species. These results suggest that the VAM colonization may be necessary for the initial establishment of T. sebifera along the coastal transitions. Furthermore, my research also suggested that T. sebifera was not allelopathic and did not interfere with growth of native species. Overall, the findings of this research provide insight into the impacts of climate change related shifts on performance of invasive and co-occurring native species across coastal transitions of the southeastern USA. Variation in invasive and co-occurring native species' performances under changed environmental conditions (e.g., elevated salinity and increased light availability) and improved mutualistic association between invasive T. sebifera and VAM fungi may drive increased invasion with frequent community reassembly of low-lying coastal ecosystems undergoing rapid climate change.
This dissertation is Open Access and may be downloaded by anyone.