Post-thaw vegetation changes and carbon accumulation in polygonal peatlands and treed plateau bogs in Hudson Bay Lowlands

The Hudson Bay Lowlands peatland complex in Canada is experiencing the impacts of climate change, which potentially threaten its carbon sink capacity and biodiversity. However, the direction and magnitude of recent changes remain uncertain, particularly regarding post-thaw accumulation of Sphagnum moss biomass and carbon (C). We documented vegetation changes (decadal to centennial) and C accumulation, following permafrost thaw, in Wapusk National Park (WNP), located at the ecotone between the continuous and discontinuous permafrost zones. During the 2023–2025 growing seasons, we collected 25 surface peat monoliths (30-50 cm) along transects at the edges of ten thermokarst ponds, both from polygonal peatlands and treed plateau bogs. Peat monoliths were analyzed for total organic carbon, C/N ratio, plant macrofossils, and diatom communities, and were dated with coupled 14C and lead-210 (210Pb) chronologies.Our results indicate a variable timing of permafrost thaw and a subsequent increase in Sphagnum moss biomass across WNP. At most sites in polygonal peatlands, different wet-habitat Sphagnum moss species (mainly section Cuspidata) took over shortly after the end of the Little Ice Age (ca. 1850 CE), while treed peat plateaus showed evidence of ongoing subsidence. Preliminary results from three sites showed that, on average, a total mass of 4.2 kg of C per m2 accumulated at polygonal sites since 1950 CE. During the same period, 4.7 kg of C per m2 accumulated post-thaw in a treed plateau bog. Furthermore, our results suggest contrasting processes behind thermokarst pond formation between polygonal peatlands and plateau bogs. In treed plateau bogs, thermokarst ponds form via peat subsidence, which is rapidly followed by increases in Sphagnum riparium and S. lindbergii. In polygonal peatlands, the increase of Sphagnum mosses after thaw appeared to be connected to either changes in active layer thickness or degradation of ice wedges.