Snow dynamics and forest structure interact to increase wildfire burn severity in the boreal forest

Climate change in boreal regions is leading to warmer, drier conditions which amplify wildfire activity by altering fuel moisture, weather conditions, as well as the timing and duration of snow cover. Reduced snowpack and earlier snowmelt can lower fuel moisture, extend wildfire seasons, and increase burn severity. However, the effects of snow cover on burn severity under different environmental conditions remain uncertain. We examined how forest structure and snow cover dynamics affect burn severity using structural equation models and remotely sensed burn severity data from 689 wildfires in Ontario’s boreal forest from 2002 to 2019. Longer snow-free periods were associated with more extreme burn severity but, contrary to our expectations, lower median severity. Earlier snowmelt also decreased median severity. Forest structure indirectly affected burn severity through snow disappearance date and snow-free duration, but directly influenced only extreme cases. In Ontario’s western ecoregion, these factors had a stronger impact compared to the eastern ecoregion where with the length of the snow-free period had the most significant effect on burn severity. Our findings suggest that earlier snow disappearance and longer snow-free periods, driven by ongoing climate change, is increasing the likelihood of extreme burn severity.