Populus tremuloides as a natural fire barrier in Canada’s boreal forest under a changing climate

Aspen ( Populus tremuloides ) stands have historically been considered a barrier to wildfire progression across Canada. However, as the climate changes and negatively impacts fire weather conditions, the established relationship between aspen, weather, and wildfires may also be changing. We explored this relationship using annual maps of dominant tree species extent and wildfire occurrence for three recent active fire years (2021–2023) within four Canadian forested ecozones (275 Mha), where most interactions between aspen stands and wildfires take place. We compared the proportion of aspen at burned perimeters with the proportion of aspen within the burned perimeters and found that aspen was more than twice as common at fire perimeters (ratio of 2.42). Increasing aspen cover also decreased daily burned area, from a median of 717 ha/day to 646 ha/day when aspen cover increased from less than 10% to more than 25%. Our analysis indicated that the increase in daily burned area following a rise in the fire weather index was reduced when greater aspen cover was present. Additionally, comparison of burn severity in spruce- and pine-dominated stands showed that aspen burned at a significantly lower severity than spruce in the two ecozones where aspen presence is greater. Our results indicate that despite a warming climate and an increase in the number of days conducive to severe fires, aspen continues to function as a barrier to the progression of wildfire and mitigates increases in daily burn area under extreme weather conditions. • Aspen act as a fire barrier: it is twice as common at fire perimeters than inside. • Increasing aspen cover reduces daily burned area. • Greater aspen cover moderates increased burned area caused by extreme fire weather. • Aspen burn severity was lower than spruce and pine where aspen presence was greater. • The difference in fire activity between leaf and leafless aspen is mixed.