New research finds a particular tree species can act as a wildfire barrier

As Ontario continues to experience its worst fire season on record, new research from McGill University reaffirms that aspen trees can act as a fire barrier.

Flavie Pelletier, a PhD candidate and the study’s lead author, said aspen have always anecdotally been considered a species that is especially resilient to fire.

“But the paradox is that aspen can burn very quickly by itself because it has thin bark,” she said.

By analyzing satellite imagery of Canada’s boreal forest before and after wildfires, Pelletier and her colleagues found that large patches of aspen can slow down wildfires.

The before-and-after images showed that larger patches of aspen were more prevalent at the perimeters of burned out areas. 

Pelletier said there are a couple theories as to why patches of aspen can act as a fire barrier.

“Aspen have much more moisture, like higher moisture content in the trees and in the leaves compared to coniferous trees,” she said.

The leaves on aspen also tend to start higher up on the trunk than the needles on coniferous trees like spruce and pine. That means a ground fire is more likely to climb other species first.

‘War against aspen’

While aspen can help contain wildfires, Pelletier said “there’s kind of a war against aspen in commercial forests.”

Every summer, forestry companies in Ontario spray thousands of hectares of Crown land with herbicides designed to kill broadleaf vegetation which competes with coniferous trees that are harvested for their lumber.

The most commonly used herbicide for this purpose is glyphosate.

“If you significantly reduce aspen in commercial forests, whether it’s through brushing or glyphosate, then I really believe that you’re increasing fire risk in the long term,” Pelletier said.

Fred Pinto, an adjunct professor of forestry at the University of Toronto and former executive director at the Ontario Professional Foresters Association, said some studies show that broad leaf trees like aspen return to the landscape after an area is sprayed with glyphosate.

The return of those broad leaf plants can take between five and 10 years, he said.

Pinto said that only a small percentage of Ontario’s boreal forest is harvested each year, and that he was not aware of any research that shows whether or not forests sprayed with glyphosate were more vulnerable to wildfires.

Many of the fires currently burning in northern Ontario, he added, are in remote parts of northwestern Ontario that have never been harvested, nor sprayed with glyphosate.

As for whether wildfire smoke from forests sprayed with glyphosate is more toxic, Pinto said the effect would be negligible.

He said the presence of glyphosate in the forest would be so small, compared to the total biomass, that measuring any chemicals in the air would be nearly impossible.

He added that wildfires burn so hot that they would theoretically vaporize any glyphosate residue in the air.