Common assumptions about how vegetation affects wetland methane (CH 4) flux include acting as conduits for CH 4 release, providing carbon substrates for growth and activity of methanogenic organisms, and supplying oxygen to support CH 4 oxidation. However, these effects may change through time, especially in seasonal wetlands that experience drying and rewetting, or change across space, dependent on proximity to vegetation. In a mesocosm study, we assessed the impacts of Typha on CH 4 flux using clear flux chamber measurements directly over Typha plants ("whole-plant"), adjacent to Typha plants (where roots were present but no stems; "plant-adjacent"), and plant-free soils ("control"). During the establishment phase of the study (first 30 days), the whole-plant treatment had~5 times higher CH 4 flux rates (51.78 ± 8.16 mg-C m −2 day −1) than plant-adjacent or control treatments, which was primarily due to plant-mediated transport, with little contribution from diffusive-only flux. However, porewater CH 4 concentrations were relatively low directly below whole-plant and in neighboring plant-adjacent treatments, while controls accumulated a highly concentrated reservoir of porewater CH 4. When the water table was drawn down to simulate seasonal drying, reserve porewater CH 4 from control soil was released as a pulse, equaling the earlier higher CH 4 emissions from whole-plants. Plant-adjacent treatments, which had neither plant-mediated CH 4 transport nor a concentrated reservoir of porewater CH 4 , had low CH 4 flux throughout the study. Our findings indicate that in seasonal wetlands, vegetation affects the timing and location of CH 4 emissions. These results have important mechanistic and methodological implications for understanding the role of vegetation on wetland CH 4 flux.

Invasive plants, such as the hybrid cattail Typha × glauca, can reduce biodiversity and alter the ability of wetlands to provide critical ecosystem services, including nutrient cycling and carbon storage. Several approaches have been used to reduce Typha dominance and restore invaded wetlands, but long-term studies assessing benefits of these restoration efforts are limited. A previous study demonstrated that aboveground harvesting of Typha × glauca stems and litter reduced Typha dominance 2 years post-treatment in a Great Lakes coastal wetland. In the current study, we extended monitoring of experimental aboveground Typha harvest to 4 years post-treatment and added assessments of treatment effects on soil nutrients, carbon emissions, and microbial community composition. Aboveground harvest treatment resulted in a dramatic reduction in Typha litter cover that persisted for 4 years, increased soil temperature, and increased abundance of the native plant genus Carex. However, aboveground harvest treatment did not significantly reduce Typha abundance, nor did it have significant effects on soil nutrient concentrations, carbon fluxes, or the taxonomic composition of soil microbial communities. We did observe differences in bacterial community composition between plots based on time since Typha invasion, which may indicate some legacy effects of Typha invasion. At the scale of this experiment (4 × 4 m plots), our results indicate that a single aboveground removal of Typha × glauca is not sufficient to restore a heavily invaded freshwater wetland ecosystem, and that periodic harvesting of Typha stems and litter may be required to maintain native plant abundance.

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