Aspen trees provide a natural buffer against wildfires, even as fire season intensifies, study shows

Aspen trees could provide a natural buffer against wildfires, as new research shows that areas of forest where these trees dominate burn less intensely than those made of conifers.

The findings suggest that planting more of the right kinds of trees, such as trembling aspen (Populus tremuloides, also known as quaking aspen), could be an effective strategy for protecting communities as devastating wildfires become more common and intense in a warming world, researchers reported June 15 in the journal Forest Ecology and Management.

Individual aspen trees burn easily because of their thin bark, but large "stands" ‪—‬ contiguous communities of trees that are uniform in species, age and structure ‪—‬ of them are more resistant to fire than stands of conifers and can act like a firebreak.

The reasons for aspen stands' resilience are not certain. However, researchers think it likely stems from a combination of factors, including the high moisture content in the aspens' leaves and the undergrowth, the trees' lack of highly flammable resin, and the leaves' height, which makes it harder for fire to spread to the canopy and leap from tree to tree.

Flavie Pelletier, who conducted the research as part of her doctorate at McGill University in Montreal, wanted to see whether aspen stands have been able to maintain their fire-resistant properties as climate change makes fire seasons more intense and destructive.

She combined new maps of dominant tree species in forests across Canada with satellite maps of wildfires from three recent years, including 2023, which was the country's most destructive wildfire season ever recorded. She noted which tree species tended to be inside the fire zone and which were more common at the edges. She found that aspen trees were more than twice as common at the perimeters of fires than inside them, while conifers like spruce and pine were equally present in both locations.

Having more aspen trees in a particular area also decreased the amount of forest that burned each day during a fire, so it also appears to help slow the spread of the fire. Notably, the trees still resisted fire spread even in the spring, before their leaves had fully sprouted.

"This is proof that aspen has an impact on fire perimeter formation and can slow fire progression," she told Live Science.

Anthony Taylor, a forest management specialist at the University of New Brunswick, said the study, with its use of large datasets on real fires, does a good job of analyzing and quantifying a phenomenon that Indigenous peoples and foresters have understood for a long time.

"The idea that deciduous trees burn less has been known for millennia, but knowing something through experience is different than testing it," he told Live Science.

Changing practices

The research supports the idea that planting aspen or other deciduous trees around vulnerable communities can help protect them from fires. Taylor said this buffer strategy is being considered or implemented in many areas.

A single line of trees will not do the trick. The stands must be deep enough to stop the flames from blowing over the top — probably 300 to 1,000 feet (100 to 300 meters), Taylor said.

Aspens can also play a role in fire control at larger scales. Pelletier first became interested in this topic when she learned that forestry companies often spray the forests they manage with the herbicide glyphosate to kill deciduous trees like aspen so they can grow monocultures of more economically valuable conifers, leaving those forests at much higher risk of fires. She hopes her work will help convince the companies and regulators to change their practices.

"There needs to be less of a war on aspen in commercial forests," she said. In the long term, it would actually benefit the companies, because it would reduce the risk of losing large numbers of trees to fire, even if it slightly reduces the value of the forest in the short term.

Taylor agrees that in places like New Brunswick, a province with a large forestry sector, officials should be promoting more mixed and hardwood forests, and less of the softwood monocultures that the forestry industry relies on.

"That would go a long way to making our landscape more resilient to wildfire," he said. "And that's what would have been here anyway, before we started converting the forests to a more industrial forest."