Phragmites australis: Response to wave exposure gradients, substrate characteristics, and its influence on plant species diversity in a Lake Erie coastal marsh

The expansion of Phragmites australis throughout the marshes of East Harbor State Park, Ohio, over the past 50 years has been associated with a perceived decline in floristic diversity. Few studies explore the limits to the aggressive nature of Phragmites, especially in freshwater marshes on Lake Erie. This study explores the interactions between Phragmites and floristic diversity following substrate disturbance, the effects of Phragmites on species density in three proximate freshwater marshes, and Phragmites' response to key environmental variables.; Mechanical substrate disturbance successfully reduced Phragmites abundance in the short term and promoted rare and endangered species richness, but concurrently promoted aggressive species richness. This single disturbance event and subsequent hydrologic stress was insufficient to maintain extant and seed bank species richness over an eight-year period.; Species diversity declined with increasing Phragmites abundance, most effectively at a high standing crop. Species density did not differ between quadrats with and without Phragmites, however, indicating that factors other than Phragmites abundance are more influential in controlling species diversity.; The relationship between species data and environmental gradients was investigated using canonical correspondence analysis (CCA). Two gradients combined to explain 62% of the variation in the data: an elevation gradient (35.7%) and a gradient representing Phragmites standing crop and soil fertility (26.2%). Elevation was instrumental in describing species composition, distinguishing wet meadow species from marsh species. The Phragmites standing crop-soil fertility gradient was more closely associated with species density. Species density was highest at low to moderate soil fertility for both wet meadow and marsh species. The association of Phragmites with elevated wave exposure and elevated soil fertility was attributed to the plant's ability to withstand physical disturbance and dissipate wave energy, thereby contributing to increasing soil fertility via accumulation of soil organic matter.; The species diversity-Phragmites abundance relationship is driven by a complex interaction of environmental and biotic factors. The mere presence of Phragmites does not beget low plant species diversity in the freshwater, non-tidal marshes that I studied. Future research should investigate specific combinations of these variables, particularly water depth and soil fertility, and the effects these combinations have on the species diversity-Phragmites abundance relationship.