Rising levels of greenhouse gases, predominantly carbon dioxide and methane, are the main drivers of the current climate crisis. Intact peatlands are natural carbon sinks, but once drained, become a significant source of carbon dioxide. Fens, a common type of peatland in northern Europe, capture and store carbon in their roots and accumulate peat through slow decomposition rates. This degree project investigates the responses of peatland plants to varying environmental conditions, with a focus on plant growth and decomposition in fens under different water regimes and species compositions.Data was collected from a mesocosm experiment which monitored plant growth, root growth, and decomposition rates from 2019 to 2021. Using linear mixed effect models, the influence of water regime and number of species on above and below ground biomass, as well as decomposition rates was analysed. The results indicated that while above ground biomass increased with the number of species (1 to 2) by 34 % and root growth by 37 %, there was no significant effect from water regime. However, root growth shifted to deeper soil layers under a low water regime suggesting adaptive capabilities of fen ecosystems. Litter decomposition was found to be mainly depending on litter quality rather than the number of species or water regime. Green tea decomposition was highest among mesocosms with a high water regime , while rooibos tea decomposition conversely was highest with a low water regime.The degree project provides insights into the carbon fluxes within peatland ecosystems. These findings suggest that fen ecosystems are able to adapt to low water tables and species composition might be more important for carbon cycling in fens than previously thought.