Repeated large-scale mechanical treatment of invasive Typha under increasing water levels promotes floating mat formation and wetland methane emissions

Invasive species management typically aims to promote diversity and wildlife habitat, but little is known about how management techniques affect wetland carbon (C) dynamics. Since wetland C uptake is largely influenced by water levels and highly productive plants, the interplay of hydrologic extremes and invasive species is fundamental to understanding and managing these ecosystems. During a period of rapid water level rise in the Laurentian Great Lakes, we tested how mechanical treatment of invasive plant Typha × glauca shifts plant-mediated wetland C metrics. From 2015 to 2017, we implemented large-scale treatment plots (0.36-ha) of harvest (i.e., cut above water surface, removed biomass twice a season), crush (i.e., ran over biomass once mid-season with a tracked vehicle), and Typha-dominated controls. Treated Typha regrew with approximately half as much biomass as unmanipulated controls each year, and Typha production in control stands increased from 500 to 1500 g-dry mass m⁻² yr⁻¹ with rising water levels (~10 to 75 cm) across five years. Harvested stands had total in-situ methane (CH₄) flux rates twice as high as in controls, and this increase was likely via transport through cut stems because crushing did not change total CH₄ flux. In 2018, one year after final treatment implementation, crushed stands had greater surface water diffusive CH₄ flux rates than controls (measured using dissolved gas in water), likely due to anaerobic decomposition of flattened biomass. Legacy effects of treatments were evident in 2019; floating Typha mats were present only in harvested and crushed stands, with higher frequency in deeper water and a positive correlation with surface water diffusive CH₄ flux. Our study demonstrates that two mechanical treatments have differential effects on Typha structure and consequent wetland CH₄ emissions, suggesting that C-based responses and multi-year monitoring in variable water conditions are necessary to accurately assess how management impacts ecological function.