Plant traits are often the central focus of ecological investigations into ecosystem structure and function because of the need to simplify complex plant communities to a few traits of importance. Invasions by invasive species can have major impacts on ecosystem function by altering the presence and/or dominance of plant traits that influence ecosystem energy flow and nutrient cycling. The broad goal of this dissertation was to investigate the ecosystem consequences of invasive plant species in temperate wetlands, which are important ecosystems for the cycling of carbon (C) and nitrogen (N), focusing on Phragmites australis (Cav) Trin. Ex Steud, Phalaris arundinacea L. and Typha glauca Godr. X. I hypothesized that within inland Michigan wetlands, the degree of invasion would be correlated with increased C and N stocks due to the high production of low quality litter from these invasive plants. I found evidence that both soil and ecosystem C stocks increased due to the presence of these invasive species. Additionally, I found significant differences for C and N mineralization among species linked to the quality of their litter (C:N ratios), with P. australis soil having the lowest C and N mineralization and P. arundinacea soil the highest.Phragmites australis is a tall, high biomass invasive species that is a relatively recent invader into the wetlands of the Great Lakes states. To investigate the effects of living biomass and litter on C and N cycling, I manipulated P. australis litter and biomass within plots at three wetland sites and then monitored abiotic conditions and performed a number of biogeochemical assays. Removing P. australis litter and biomass had the hypothesized effects of increasing light levels at the soil surface and increasing soil temperature, though these effects did not influence litter bag decomposition, in situ N mineralization, or potential denitrification rates. Biomass removal did affect porewater ion concentrations by decreasing Na+, Cl- and Ca2+ concentrations and increasing NO3- concentration. Though not initially hypothesized, all C and N cycling rates showed strong site effects caused by different hydrologic conditions among sites. To separate litter quality, litter diversity, and soil origin from other controls of decomposition, I performed two laboratory incubations using litter and soil collected from monospecific stands of the three focal invasive species plus Carex lacustris, a native sedge. I found support for my prediction that the four species differed in litter quality and that litter C:N ratio was negatively related to C and N mineralization rates. I also found strong soil origin effects related to soil nutrient availability, which have not been found before within a similar experimental framework. The second incubation showed that while litter diversity significantly affected litter decomposition rates, the effects were more dependent of the identity of the species than just the number of species. Taken together, these results suggest that invasive species can influence C and N cycling in inland Michigan wetlands, and that some of the biogeochemical effects, like increased C storage, could be a positive outcome of invasion. These effects can also be linked to key plant traits, such as litter quality and biomass production, and supports the conjecture that invasive species alter ecosystem function by changing the composition or dominance of plant traits within the community.
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