Extremely large fires shape fire severity patterns across the diverse forests of British Columbia, Canada

Warming and drying conditions are driving increases in wildfire size and annual area burned across the forests of British Columbia, Canada. The impact of increasing fire activity on these forests remains unclear as examination of concurrent changes to fire severity is lacking. Here, we assess how fire severity patterns change with the amplification of wildfire size across the bioregions of British Columbia using fire severity mapping from 1986 to 2021. First, we examine trends in extremely large fires (i.e., largest 5% of fires) and their influence on annual area burned; then we examine scaling relationships between wildfire size and fire severity to determine if extremely large fires are more severe than smaller fires. Extremely large fires explained much of the variation in annual area burned and accounted for a large proportion of cumulative area burned (33%–71%) across the study area. Consequently, shifts in the size of extremely large fires, which increased by an order of magnitude over our study period, have driven a substantial increase in annual area burned. Scaling relationships revealed that bigger wildfires consisted of larger and more homogenous patches of high-severity fire than smaller fires, resulting in a greater proportional contribution of high-severity fire to fire extent. Patterns in scaling relationships were qualitatively similar for all bioregions, indicating shifts in fire regimes are widespread across the province. Our results demonstrate that recent increases in the extremes of wildfire size across the forests of British Columbia have driven a sharp increase in area burned, which was associated with a disproportionate increase in the size and extent of patches of high-severity fire.