Climate change, fires transform some of Canada's boreal forests into savannahs

In 2015, scientist Ellen Whitman set out on a visit to Wood Buffalo National Park, a vast wilderness spanning northeastern Alberta and the Northwest Territories.

At that time, the land had been subject to two major wildfires a decade apart — the most recent in 2014. 

"The first fire burned a very large, mature pine stand and it was regrowing back as pine with a little bit of aspen mixed in," recalled Whitman, a forest fire research specialist at Natural Resources Canada.

"Then that second fire killed all those seedlings and suddenly it's basically a grassland with a few scattered aspen trees."

Her team's findings, laid out in a recently published paper, are part of a growing body of evidence showing how the changing climate and increased severity of wildfires are altering the makeup of North American forests.

Her research compared forest areas that had similar climate and soil conditions, but half had been subjected to fire twice in a short time span, while the other half had a longer period of regrowth.

In the areas where fire had recurred more quickly, aspens dominated in the place of conifers, and growth beneath the trees was far less established. Areas of exposed mineral soil, where all organic material had been burned off, were also more common.

"When you've had a severe disturbance or a repeated burning on top of burning or a really severe drought in the year after a fire, we might kind of start to see these patches changing to be more southern-like in their ecosystem structure," Whitman said.

"Almost more like a savannah in some cases."

Scientists say such a transformation is likely to be seen elsewhere in Canada's boreal forest in the years to come.

Experts are quick to point out that fires are a crucial and natural aspect of a forest's life cycle; they have allowed Canada's boreal forest to flourish over millennia.

But there is also evidence that fires are becoming larger and more intense, changing what grows back after the flames go out.

"People talk about good fire and bad fire. The good fire is the fire that we've had historically that has helped to renew this landscape," said Jennifer Baltzer, an associate professor in the department of biology at Wilfrid Laurier University in Ontario.

"The bad fires [are] what we're seeing in the face of a combination of climate warming and really effective fire suppression in the past."