Wetland Successional State Affects Fire Severity in a Boreal Shield Landscape

Boreal peatland ecosystems are generally resilient to low severity wildfire. However, climate change may increase wildfire intensity and frequency, potentially shifting wetlands to less wildfire resistant states. Peatlands formed by infilling generally undergo a succession from open water to grounded peat, with spatially complex intermediate states which may impact wildfire resistance. We explored the relationship between wetland successional states and fire severity following a > 11,000 ha wildfire in Ontario’s Boreal Shield landscape. We digitized 144 wetlands of varying successional states from aerial imagery and assessed fire severity using the Relative differenced Normalized Burn Ratio calculated from Sentinel-2A satellite imagery. Completely peat-filled wetlands were small in area (0.7 ± 2 ha) but were most frequent on the landscape (n = 99/144) compared to a smaller number (n = 8) of large (19 ± 2 ha) spatially complex wetlands that comprised > 30% of the total wetland area. In wetlands covered completely, or nearly completely, by peat, fire severity was significantly higher compared to wetlands with interspersed patches of shallow open water. Moreover, > 90% of fire resistant wetlands with open water were associated with beaver dams. Wetlands with more complex surface cover experienced lower mean fire severity but greater variability in fire severity suggesting that variable fuel configuration (i.e., spatially heterogeneous surface cover) in complex wetlands limit wildfire propagation across the wetland surface. Our findings are important for landscape conservation and wildfire management, as spatially complex wetlands host a diverse array of habitats for at-risk species and may offer protection from severe wildfires as fire refugia.