Pattern and process in wetland vegetation across estuarine salinity gradients

Wetland plant communities vary across estuarine salinity gradients with low salinity tidal marshes having different dominants and higher species diversity than salt marshes. In this thesis, I conducted a number of studies to examine how the processes driving these various plant community patterns shift across estuarine salinity gradients. First, I present experimental evidence that plant species distributions across this landscape-scale salinity gradient are not only driven physical stress, but additionally by competitive interactions with other plants. Plants persist where they do based on trade-offs in stress tolerance and competitive ability. Next, I examined how the impacts of ecosystem engineers, organisms that alter their physical setting, vary across the estuarine gradient. I found that in salt marshes, hummock forming plants facilitate the vegetative community by ameliorating physical stressors. In contrast, engineering plants in tidal freshwater marshes drive plant distribution patterns through indirect negative effects of wrack buildup and secondarily offer competitor- and predator-free space on tops of tussocks. This experimental evidence was integrated into general environmental stress models to create a conceptual model predicting which types of engineering pathways will be most important in habitats that vary in background environmental stress. I then investigated how nutrient limitation of primary productivity shifts across the estuarine salinity gradient and how eutrophication impacts plant communities. Through nutrient addition experiments, I discovered that limiting nutrients shift from nitrogen in salt and brackish marshes to nitrogen and phosphorus co-limitation in low salinity marshes. Eutrophication changes species composition in low salinity marshes, likely driving biodiversity loss over time. Finally, species interactions between plants were examined across gradients in both physical environment (salinity) and herbivory by small marsh mammals. Plant interactions were found to change direction depending on the environmental context, with positive interactions prevailing in harsh physical and high consumer environments, and negative interactions in intermediate environments. Together, these studies lay a foundation of knowledge of how vegetation patterns change across estuarine salinity gradients and how both biotic and abiotic factors drive these patterns, information essential for appropriately managing and protecting the full complement of habitat types in these critical coastal ecosystems.