Associate Professor University of Toronto, Forestry, Ontario, Canada
Increases in wildfire severity are expected to reduce forest ecosystem resilience as fire-adapted species are not necessarily adapted to high severity burning events. Climate warming is expected to increase the frequency of such high severity fires with uncertain consequences for species resilience. Humid eastern boreal forests are predicted to act as climate change refugia due to their potential to buffer the effects of warming. These areas may also serve as a refugia for fire-adapted species by sustaining the historic range of natural variability of disturbance regimes. The boreal shield ecozone exhibits an east-west gradient in moisture availability, whereby the eastern section may act as refugia for fire-adapted species. To better understand whether the humid eastern boreal forest can act as a refugia for fire-adapted species from increases in high-severity burning events, we examined trends in burn severity across an east-west gradient in moisture availability. Using remotely sensed burn severity maps generated in Google Earth Engine and historical climate data from 1986-2020, we quantified the effects of moisture availability (Climate Moisture Index; CMI) on trends in burn severity for 1433 fires ( > 40 ha) that burned in the Ontario shield.
Burn severity is expected to increase over time from decreases in moisture availability, with moisture availability predicted to be lowest in western-central regions of the boreal forest where drastic decreases in precipitation coupled with increases in temperature are expected to occur. Drier conditions in the west should therefore support more high-severity fire compared to the east. Year-to-year effects in moisture availability are expected to lead to more occurrences of extreme high-severity fire within each fire compared to increasing the overall median of high-severity fire observed within an individual fire. Burn severity has broad and persisting impacts on ecological succession, biodiversity and forest management. This relationship makes characterizing shifts in burn severity over time crucial when trying to identify where and when forest systems can buffer the effects of climate change with respect to wildfire.