Impacts of permafrost thaw and methane cycling in a Northern Peatland

Climate change is disproportionately impacting high latitude regions, resulting in thawing permafrost, changes in vegetation, hydrology and greenhouse gas emissions. As permafrost thaws in peatlands, ground subsidence occurs, resulting in a transition from elevated palsa to lower, wetter bogs and fens. Changes in vegetation and hydrology with thaw result in increased methane (CH4) fluxes and distinctive shifts in the carbon isotope signature of porewater and emitted CH4. Previous research has established the differences between these landcover classes; however, we know little about how CH4 cycling processes change during land class transitions. To address this gap, we compared recently formed bogs and fens with more established sites at Stordalen Mire, a sub-arctic peatland in Abisko, Sweden that is undergoing rapid thaw. In 2023 and 2024 we revisited and reclassified 69 plots established in 2015; 23% of the surveyed plots had changed land class due to permafrost thaw. Vegetation data shows a distinct pattern that precedes landcover change. In thawing palsas, shrub and herbaceous cover declines and bryophyte cover expands before other signs of thaw become evident. Similarly, in thawing bogs, bryophyte cover is reduced, and open water increases before landcover change occurs. Porewater samples suggest that changes in pH, dissolved CH4 and d13C-CH also precede landcover change resulting from thaw. In 2015, palsa and bog sites that would experience thaw by 2024 already had distinct porewater chemistry profiles compared to sites that did not transition during the study time frame. The magnitude of CH4 emissions responds rapidly to thaw, with emissions increasing alongside landcover change. There was no difference observed between newer and older bogs or fens in terms of emitted CH4 magnitude or d13C-CH signature.