What research tells us about ‘zombie’ forest fires

KITCHENER, ONT. — A new study from Wilfrid Laurier University examined the impacts of overwintering fires, also known as zombie fires. Jennifer Baltzer, Canada research chair in forests and global change, published this first-of-its-kind data, which found that these fires were less destructive than initially thought.

“These are fires that start in one fire season, and they continue to burn into the fall and over winter, under the snow in the ground, and then can reignite the following spring,” said Baltzer regarding overwintering fires.

Though these fires don’t always reignite, they will continue smouldering for long periods. She told The Observer about a fire she was informed of that has been burning for multiple years. Her colleagues in the Northwest Territories have been monitoring this fire for three summers now.

“We’re investigating these overwintering fires. They’re not necessarily a new phenomenon in terms of fire behaviour, but they’re becoming increasingly frequent as we have drier, more flammable landscapes where fire can take hold in the soil,” said Baltzer.

Previous data on these fires were gathered through sensing data and a general expectation of what should happen.

Between 2022 and 2023, Baltzer and her colleagues were the first in the world to collect samples at overwintering fire sites. They visited 20 locations in the boreal forests of Alaska and the Northwest Territories that had originally burned in 2009 or 2014.

They thought most of these fires would occur in peatlands, which are carbon-rich soils and, therefore, have the highest risk of potential fires. However, that was not the case with most of these overwintering fires, which occurred in ‘upland’ locations ‘with productive forests on higher ground, characterized by good drainage and thinner layers of organic, carbon-rich soil.’

The researchers paired their overwintering with an adjacent single-season fire, aiming to characterize the differences in soil, combustion, tree species diversity, and regeneration outcomes.

“There were some differences in terms of combustion severity with overwintering fires being a little bit more severe than single season fires, but it wasn’t a huge impact,” said Baltzer.

In the same sense, they thought that in terms of regenerations, if you have a fire burning over multiple years, then it could be assumed that there would be some regeneration failure.

“At the time, we thought there was a potential for these kinds of fires to lead to regeneration failure, because, you know, that second fire could kill all the seeds that had arrived on site. So, we didn’t find that. We found very strong regeneration in all of the sites,” noted Baltzer.

Another notable finding was that Baltzer observed a change in the types of trees that were growing back following overwintering fires.

“This is a transition we’ve documented before across western North America, where short interval fires and severe burning can lead to a transition from spruce to aspen,” said Baltzer regarding the shift in species.

They followed a 2014 fire, which was the biggest forest fire on record at the time of the study, discovering that 40 per cent of sites were transitioning away from spruce to aspen or pine.

“Following over-wintering fires, we found that 100 per cent of the sites were transitioning from spruce to aspen. So, it seems as though this, this fire behaviour, really exacerbates that pattern of a shift from conifer to deciduous dominated,” Baltzer explained. “Which has impacts for landscape, vulnerability, for wildlife, habitat, for carbon storage, any of your favourite ecosystem services are altered by that, and we’ll still have forests in those locations, but the forest will look different.”

The shift in these tree species is something that can be either beneficial or detrimental to the ecosystem as a whole. Baltzer said that it depends on the lens through which you are looking at the ecosystem.

From a caribou habitat perspective, loss of spruce forests is a problem because deciduous trees don’t support the growth of caribou lichen, which they forage on in the winter, for example.

“From a fire perspective, deciduous aspen trees are less flammable than conifers and can slow or stop the progression of fire, serving as a natural firebreak on the landscape. So, the impacts vary, but what is important is that we haven’t seen a species shift like this in these forests in thousands of years. More than anything, it’s an indicator of climate change rapidly affecting this biome.”

The information collected thus far encourages more study, she said, noting that expanding the sample size of research would contribute to a better understanding of changing fire behavior.