Shifting boreal forest fire regimes in northern Alberta, Canada

Abstract:

Recent research indicates that fire activity in some boreal forests is increasing; however, there is substantial regional variability in these trends and it is unknown if increasing fire activity has translated to increasing fire severity (combustion and vegetation mortality). We sought to determine whether wildfire activity and severity have increased in the boreal forest of Alberta, Canada in the past 50 years. Using singular spectrum analysis we characterized non-parametric trends in the area burned and number of fires (1970 - 2019), and remotely sensed burn severity (1985 - 2018) in boreal Alberta. The area burned and number of large fires in the study area have significantly increased since the 1970s, and these increases in fire activity are significantly related to ongoing warming and drying in the region. As fire activity has increased, so have the frequency and extent of wildfires burning at unexpectedly short intervals (reburns; ≤ 20 years between fires), which often initiates rapid and pronounced shifts in vegetation composition towards increased dominance of broadleaf trees, to the detriment of conifers. Burn severity has not increased in the past 30 years for most vegetation types in the study area; however, we identified significant increases in severity in broadleaf forests, mixed forests, and forested wetlands. These changes in fire severity and activity may have significant implications for the carbon balance of boreal forests. As the climate of this region continues to warm and fire activity accelerates, we expect broadleaf forests to become increasingly dominant in the boreal region. The anticipated prevalence of this forest type and ongoing severe burning in broadleaf and mixed stands merits further investigation into the environmental conditions under which broadleaf forests’ resilience to fire and climate change may be overwhelmed.