Northern peatlands act as globally important carbon (C) sinks (Nichols & Peteet, 2019). Changes in climate and large-scale drainage for agriculture and forestry increase the susceptibility of these C sinks to oxidation, which has the potential to shift some peatlands from being net C sinks to sources (Dinsmore et al., 2010;Trettin et al., 2006), although the warming and drying-to-C-loss scenario is far from certain across all boreal peatlands (Charman et al., 2013;Gallego-Sala et al., 2018). Beyond simply acting as net C sinks, peatlands play

Sphagnum-dominated peatlands store more carbon than all of Earth’s forests, playing a large role in the balance of carbon dioxide. However, these carbon sinks face an uncertain future as the changing climate is likely to cause water stress, potentially reducing Sphagnum productivity and transitioning peatlands to carbon sources. A mesocosm experiment was performed on thirty-two peat cores collected from two peatland landforms: elevated mounds (hummocks) and lower, flat areas of the peatland (hollows). Both rainfall treatments and water tables were manipulated, and CO2 fluxes were measured. Other studies have observed peat subsiding and tracking the water table downward when experiencing water stress, thought to be a self-preservation technique termed ‘Mire-breathing’. However, we found that hummocks tended to compress inwards, rather than subsiding towards the lowered water table as significantly as hollows. Lower peat height was linearly associated with reduced gross primary production (GPP) in response to lowered water tables, indicating that peat subsidence did not significantly enhance the resistance of GPP to drought. Conversely, Sphagnum peat compression was found to stabilize GPP, indicating that this mechanism of resilience to drought may transmit across the landscape depending on which Sphagnum landform types are dominant. This study draws direct connections between Sphagnum traits and peatland hydrology and carbon cycling.