Ecosystem-level effects of invasive species: A Phragmites case study in two freshwater tidal marsh ecosystems on the Connecticut River

Invasive species are currently one of the primary threats to biodiversity and habitat at global, regional and ecosystem scales. Research to gain a better understanding of invasion ecology has become a priority for conservation biology. One of the most understudied aspects of this field is the impact of invasive species on ecosystem processes. This dissertation used the invasive reed grass Phragmites australis as a case study to investigate whether Phragmites was altering nutrient cycles in two freshwater tidal marsh ecosystems.; Phragmites is an aggressive reed grass that invades marsh systems in the eastern United States. This species has expanded beyond its historical range during that last century in Atlantic coast marsh systems and has generally led to declines in marsh habitat. While the community effects of Phragmites invasion are relatively well documented, the ecosystem-level impacts of this species have only recently begun to be investigated.; This study compared various nutrient pools and fluxes of Phragmites -dominated sites with those sites dominated by Typha sp., a competitive macrophyte that Phragmites often displaces. The research included measurement of nutrient and trace metal concentrations (i.e., C, N, P, Al, K, Mn, Mg, Ca, Fe) in plant tissue, sediments, and interstitial porewater. In addition, aboveground biomass, decomposition rates of plant tissue and light were measured.; Total sediment pools of nutrients were significantly higher in most cases in Phragmites-dominated soils relative to Typha-dominated soils. However, there were few significant differences between Phragmites and Typha porewater nutrient and trace metals concentrations, suggesting that porewater was not a good indicator of Phragmites vigor. However, for particular nutrients, there was significant variability in concentrations over the growing season. On a mass basis (g/m2), total nutrient and trace metal content was higher in Phragmites tissues relative to Typha.; Aboveground biomass was significantly higher in Phragmites communities than Typha communities, and accounted for most of the total higher nutrient concentrations found when the two communities were compared. Decomposition rates were most rapid for Phragmites leaves and slowest for Phragmites stems. Decomposition rates of Typha leaves were intermediate between the Phragmites tissue types. Relative light levels were significantly lower in Phragmites stands relative to Typha.; The results of this research suggest that biomass accumulation and associated nutrient sequestration is one of the main mechanisms Phragmites employs to control the marsh ecosystems it invades. This competitive strategy may be common among invasive plants and warrants further investigation.