Plant communities play a key role in regulating greenhouse gas (GHG) emissions in peatland ecosystems and therefore in their ability to act as carbon (C) sinks. However, in response to global change, a shift from Sphagnum to vascular plant-dominated peatlands may occur, with a potential alteration in their C-sink function. To investigate how the main GHG fluxes (CO2 and CH4) are affected by a plant community change (shift from dominance of Sphagnum mosses to 15 vascular plants, i.e. Molinia caerulea), a mesocosm experiment was set up. Gross primary production (GPP), ecosystem respiration (ER) and CH4 emission models were used to estimate the annual C balance and global warming potential under both vegetation covers. While the ER and CH4 emission models estimated an output of, respectively, 376 and 7 gC m-2 y-1 in Sphagnum mesocosms, this reached 1018 and 33 gC m-2 y-1 in mesocosms with Sphagnum rubellum and Molinia caerulea. Annual modelled GPP was estimated at-414 and-1273 gC m-2 y-1 in Sphagnum and Sphagnum + Molinia plots, respectively, 20 leading to an annual CO2 and CH4 budget of – 30 gC m-2 y-1 in Sphagnum plots and of-223 gC m-2 y-1 in Sphagnum + Molinia ones (i.e., a C-sink). Even if, CH4 emissions accounted for a small part of the gaseous C efflux (ca. 3%), their global warming potential value makes both plant communities have a climate warming effect. The shift of vegetation from Sphagnum mosses to Molinia caerulea seems beneficial for C sequestration at a gaseous level. However, roots and litters of Molinia caerulea could provide substrates for C emissions that were not taken into account in the short measurement period 25 studied here.