Aspen trees don’t fuel wildfires, they can stop them, two studies find

When a wildfire tears through a conifer forest, the usual assumption is that more vegetation means a bigger, faster-moving fire. Two recent studies — one covering the southwestern United States, another examining Canada’s entire boreal forest — suggest that assumption does not hold for every tree species.

Aspen, a fast-growing deciduous tree found across much of North America, appears to genuinely slow fires down and sometimes stop them altogether, according to large-scale satellite and fire behaviour data rather than scattered anecdotal reports. A study published in the journal Ecological Applications, led by researchers at Western Colorado University, Colorado State University and the US Forest Service, examined 314 wildfires that burned across the southwestern US between 2001 and 2020.

Where aspen cover on the landscape was below 10%, fires grew by an average of 1,112 hectares a day with a maximum spread rate of 2.1 kilometres a day. Where aspen cover exceeded 25%, those figures dropped sharply to 368 hectares a day and 1.3 kilometres a day. Aspen was also found to be more abundant along the very edges of burned areas than within their interior, indicating fires do not just slow down near aspen stands — they sometimes stop or change direction entirely.

A separate Canadian study, led by Flavie Pelletier at McGill University with the Canadian Forest Service, examined dominant tree species maps against wildfire data across four major forested ecozones and found the same pattern at a national scale, with the proportion of aspen at fire perimeters running more than twice as high as within burned interiors.