Fuel loads and peat smouldering carbon loss increase following peatland drainage

Northern peatlands store ~500 Pg C and are important ecosystems for global climate regulation. Wildfire is the largest natural disturbance to peatlands within the Boreal Plains of western Canada. Historically, low-severity fires in this region release less carbon than accumulates over a fire return interval (~120 years), allowing peatlands to maintain their carbon sink function. While peat combustion (measured as the depth of burn; DOB) is typically low, ranging from 5-10 cm (representing carbon emissions of ~1 kg C m^-2), during prolonged drought, or in drained peatlands, peat burn severity can reach depths >1 m (~100 kg C m^-2), threatening the carbon sink function of boreal peatlands. We aimed to assess how peatland drainage altered the spatiotemporal variability in forest cover, aboveground biomass, and tree productivity and how these changes related to the spatial variability in peat burn severity from a fire 24 years post-drainage. Using remote sensing techniques, forest cover and biomass were estimated through time and with distance from the nearest ditch. Field surveys and a LiDAR-based analysis were conducted to measure the spatial variability in peat burn severity. Peatland drainage increased forest cover and aboveground biomass. Drained peatland margins had the greatest peat burn severity with a mean depth of burn of 26.9 ± 12.6 cm (34.0 ± 10.1 kg C m^-2) and some locations experienced DOB >90 cm (>87 kg C m^-2), where peat burn severity increased with proximity to drainage ditches and greater aboveground biomass. Peatland drainage increases both aboveground and peat fuel loads through the triggering of positive peatland drying feedbacks which increase peatland vulnerability to deep smouldering, with peatland margins experiencing the greatest peat burn severity. Drained peatlands represent a severe fire risk that can be challenging for communities and fire management agencies. Peatland restoration should be integrated into fuel management strategies to reduce the fire risk that drained peatlands pose.