Ellen Whitman is a forest ecologist at Natural Resources Canada and the University of Alberta and is the co-author of a study published in the journal Nature Scientific Reports that examined changes to boreal forests when they are burned more often due to climate change.
Canada’s Boreal forest comprises about one-third of a band of green in the Northern Hemisphere that reaches across North America, Europe, and Asia, mostly north of the 50th parallel. Canada’s boreal forests cover almost 60 percent of the country’s land area.
In Canada, this band of green, consisting of a matrix of forest, wetlands, and waters is the country’s biggest defense against climate change. The forests are a gigantic carbon sponge, pulling carbon dioxide out of the atmosphere to fuel photosynthesis.
In June of 2019, the Chuckegg Creek Fire in Northern Alberta grew to over 280,000 hectares (691,895 acres) and was just one of about 10 wildfires burning out of control in a region of the old-growth boreal forest. This area of old-growth forests had not seen a fire in 80 or 90 years.
Frequency and interval of wildfires
In the study, Whitman and her colleagues examined paired forest sites that had similar climate and soil conditions and had last been burned by the same fire. Half of the sites had been burned less than 17 years ago, while the other sites had last experienced a wildfire more than 30 years ago.
Observed fire-frequencies are called “fire-free intervals,” and in the boreal forest can range from hundreds of years to approximately 30 years. Generally, a recent wildfire would mean the forest could resist reburning for upwards of 30 years due to a lack of fuel.
This is because the coniferous and broadleaf trees of the boreal forest have adaptions to wildfires, like vegetative regeneration, and seed banking in soils, making them self-generating and more resistant to wildfire damage.
However, with climate conditions changing, “extreme fire weather conditions can override these controls, allowing fires to spread in fuel-limited recently burned areas,” says the study. This is due to the frequency of the fires creating a loss of resiliency. This, in turn, can shift the balance of vegetation types on the landscape.
The differences were striking
The researchers found that the short-interval stands had far fewer trees and less undergrowth. Gone were the lush shrubs and grasses that cover a normal forest floor. They also documented large areas of exposed mineral soil, where all organic material had been burned off, including the natural seed bank.
“You have a landscape where you’re surrounded by short, stunted trees,” Whitman said. “You have a crust of lichen or some sparse grasses. It’s almost like walking through the edge of a prairie where you’re shifting from a grassland into a forest edge.
“At a lot of the long-interval sites, you’ve got quite dense conifers, closer together. You’ve got moss on the ground and flowers and shrubs. It’s more what looks like a young forest.” This is where climate change is breaking all the rules, Whitman added.
“We’re experiencing more hot, dry windy days — the main trigger for large fire years. As more years experience more extreme fire weather, (the blazes) are able to overwhelm that resistance that recently burned sites have.”
Extreme wildfires have changed the landscape in a permanent way. Usually, forests will bounce back. But with today’s fires, boreal forests are not likely to come back as a conventional forest. “Immediate post-fire condition is an extremely strong predictor of what the stand will look like further down the road,” said Whitman.
“With a longer fire season, larger fires, more of the landscape burning each year, the likelihood of encountering a recently burned area increases. We’re undergoing a shortening of the fire frequency in the boreal forest.”
