The Effects of Urbanization on Riparian Forest Plant Communities in the Research Triangle Area of North Carolina

Ongoing urban development is having profound impacts on ecosystems worldwide, by reducing the amount and connectivity of natural habitat and changing the local environmental conditions. Despite the ubiquity and the continued growth of urban areas, the effects of urban development on the diversity and composition of plant communities remain poorly understood. In this dissertation, I address this issue by investigating the effects of urban development on patches of remnant riparian forest in the Research Triangle area of North Carolina. To do this, I used a rural-to-urban gradient approach, comparing the plant species and environmental conditions of sites surrounded by different amounts of urban development. I first explored the effects of different "filters" created by urban development on plant biodiversity using structural equation modeling. In this, I tested the predictions of an established conceptual framework and developed a general model that could be used to compare the effects of urbanization on plant biodiversity across cities. I then examined changes in plant species' functional traits along the rural-to-urban gradient and compared the species and traits found at these sites to high-quality riparian plant communities that represent potential targets for restoration. I found that plant traits change somewhat predictably along an urbanization gradient, particularly with regard to species' seed dispersal modes, but that traits may not be as useful for predicting which species will establish in urban restoration efforts. Finally, I assessed the ability of environmental variables and spatial variables describing habitat connectivity to explain variation in plant species composition across sites. I found that species with limited dispersal ability showed the strongest signal of dispersal limitation across sites, and that urban development between sites appears to reduce connectivity for these species. This dissertation provides insight into the ways that urban development is altering riparian forests in the Research Triangle, which may have important impacts on biodiversity and ecosystem services. My results have implications for land management, restoration, and urban planning in this landscape.