| Summary: | <i>Sphagnum</i>-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 <i>Sphagnum</i> 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 CO<sub>2</sub> 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, <i>Sphagnum</i> peat compression was found to stabilize GPP, indicating that this mechanism of resilience to drought may transmit across the landscape depending on which <i>Sphagnum</i> landform types are dominant. This study draws direct connections between <i>Sphagnum</i> traits and peatland hydrology and carbon cycling.
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