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 exist in the literature to confirm this relationship. Fewer studies explore the autecology or the limits to the aggressive nature of Phragmites, especially in the context of freshwater marshes on Lake Erie. This study attempts to explore 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 and biotic variables. The results from this study suggest that mechanical substrate disturbance successfully created an environment conducive to the proliferation of a variety of wetland plant species but did not differentially promote rare and endangered species over aggressive species. While the abundance of Phragmites was reduced in the short term, this single disturbance event and subsequent hydrologie stress was insufficient or too infrequent to maintain extant species richness. It was also Inadequate to maintain seed bank richness and did not iimit the richness and abundance of aggressive species over an eight-year period. Vegetation surveys conducted in 2000 in three proximate East Harbor marshes indicate that the abundance of Phragmites negatively affected species diversity, most effectively at a high standing crop (1400-2000 g W ). Species density was unrelated to the presence or absence of Phragmites within each marsh, suggesting that other fectors such as nutrient availability, stress and disturbance, and topographic heterogeneity may be directly responsible for species diversity and Phragmites abundance. The relationship between species data and environmental variables was investigated by using canonical correspondence analysis (CCA). Sediment chemistry, wave exposure, distance to shoreline, elevation, light interference, species density, and Phragmites standing crop were considered as possible predictor variables. Two gradients combined to explain 62% of the variation in the species-environment relationships: a gradient representing elevation (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. This study suggests that the species diversity-PhmgmAes 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 diversityPhragmites abundance relationship.