Elevated atmospheric CO2 may create greater methane (CH4) emissions from subarctic wetlands. To date such ecosystem feedbacks remain poorly understood, particularly in relation to how different wetland plant species will control such feedbacks. In this study we exposed plant-peat mesocosms planted with four Cyperaceae species to 400 and 800 ppm atmospheric CO2 concentrations and measured plant and peat properties as well as CH4 fluxes. Above ground biomass for plants grown at 800 ppm CO2 increased for E. angustifolium, Eriophorum vaginatum and Carex brunnescens, but the total biomass of C. acuta decreased relative to the ambient CO2 treatment. The plant species and elevated CO2 treatment affected both peat redox potential and pore water chemistry. There was no overall effect of the elevated CO2 on CH4 emissions, however, CH4 emissions were related to above ground biomass and redox potential, both of which were significantly altered by elevated CO2. Our study shows that species composition poses an important control on how wetland communities will respond to elevated CO2 and that plant mediated changes of peat biogeochemical processes, in response to elevated CO2 levels, may affect CH4 emissions from sub-arctic wetlands, but any such responses will differ among species.