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-dominated to vascular-plant-dominated peatlands may occur , with a potential alteration in their C-sink function. To investigate how the main GHG fluxes (CO 2 and CH 4) are affected by a plant community change (shift from dominance of Sphagnum mosses to vascular plants, i.e., Molinia caerulea), a mesocosm experiment was set up. Gross primary production (GPP), ecosystem respiration (ER) and CH 4 emission models were used to estimate the annual C balance and global warming potential under both vegetation covers. While the ER and CH 4 emission models estimated an output of, respectively, 376 ± 108 and 7 ± 4 g C m −2 yr −1 in Sphagnum mesocosms, this reached 1018 ± 362 and 33 ± 8 g C m −2 yr −1 in mesocosms with Sphagnum rubellum and Molinia caerulea. Annual modeled GPP was estimated at −414±122 and −1273±482 g C m −2 yr −1 in Sphagnum and Sphagnum + Molinia plots, respectively, leading to an annual CO 2 and CH 4 budget of −30 g C m −2 yr −1 in Sphagnum plots and of −223 g C m −2 yr −1 in Sphagnum + Molinia ones (i.e., a C sink). Even if CH 4 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 sequestra-tion at a gaseous level. However, roots and litter of Molinia caerulea could provide substrates for C emissions that were not taken into account in the short measurement period studied here.