Overwintering ‘zombie fires’ aren’t doing what we thought they’d do

A major new study of northern wildfires that smoulder through the winter comes with some findings that surprised the scientists involved.

Wilfrid Laurier University’s Jennifer Baltzer and colleagues spent 2022 and 2023 visiting sites of former overwintering fires – so-called “zombie fires” – in the Northwest Territories and Alaska.

Their paper was published in the journal Nature Ecology and Evolution this week.

The research is important as it’s the first time the scenes of overwintering fires have been studied in person, with scientists arriving via helicopter to understand the landscape in which those fires burned.

“Everything else had been based on remote sensing” – study via satellite data – “or just hypotheses as to how we think these fires probably work,” said Baltzer.

The NWT’s wildfire agency says the research contains lessons for its firefighters, particularly with the surge in overwintering fires that followed the severe fire summers of 2023 and 2024.

Here are some of the researchers’ key findings – and why some of the outcomes were a surprise.

Zombie fires happen in places we didn’t expect

The scientists expected the sites of old overwintering fires to be mostly peatland. That wasn’t what they found.

“The idea in the scientific literature was that these fires happen typically in peatlands, where deep peat soils can support smouldering combustion,” said Baltzer.

“We went into this thinking we were going to be arriving in all of these locations that were peatlands.

“The big surprising finding was that, in fact, these overwintering fires were occurring in lots of different landscapes.”

Many of the overwintering fires had actually happened in upland forest. In those locations, the winter burning wasn’t happening deep in peat soils. It was being sustained by tree roots and trunks smouldering for months beneath the snowpack.

“It seems like it can happen anywhere on the landscape. That was surprising,” said Baltzer.

Why a zombie fire’s location matters

There are two big reasons we care about where overwintering fires happen.

The first is emissions. A zombie fire in peatland would be expected to unlock a lot of carbon, which peatland stores in large amounts. The theory went that burning deep in the peat through the winter could add a heap of emissions to everything a bad fire summer had already spewed out.

But the research suggests fewer zombie fires are hanging out in peatlands than scientists thought.

“In some ways, this is a good news story,” Baltzer was quoted as saying in a Wilfrid Laurier University news release, because that means less of a carbon impact than anticipated.

But the presence of overwintering fires in forested areas creates a significant safety implication.

“If we have them in upland areas, it means forests become quite dangerous to be in, because it combusts the roots and you have these trees that fall over and make it quite dangerous to move around the land,” said Baltzer.

The message for anyone on the land is to be more careful than ever while moving through a forest in the months after a fire passed through.

“You have a lot of the smouldering happening in the roots of trees, so it really destabilizes them,” Baltzer continued.

“When we have these kinds of fires, it changes how long it takes before those trees come down.”

Baltzer uses an image of a smouldering zombie fire from 2024 to illustrate this.

“The trees are all down one year after fire,” she said. “This was something that happened really quickly after fire.”

Forests are growing back differently

Another finding of the study is that overwintering fires don’t necessarily have a huge impact on a forest’s ability to regenerate.

Scientists had thought winter-long smouldering might mean affected forests come back with sparser cover or simply don’t regenerate at all, changing the landscape over time.

Baltzer said her team found little to no evidence of that, but did notice a change in the type of tree at each site.

“Every single site we went into that had been conifer dominated pre-fire? After these overwintering fires, every site was converted to aspen,” she said.

This is a phenomenon scientists have noticed before. Along NWT Highway 3 between Fort Providence and Behchokǫ̀, for example, old spruce forests hit by the 2014 wildfires came back predominantly as aspen.

“Following 2014, we saw about 40 percent of sites doing that” along the highway, Baltzer said. At the overwintering sites researchers studied, “that was all of the sites.”

“It exacerbates this pattern that we’ve seen previously in the Northwest Territories and elsewhere in western Canada of conifer forest shifting to deciduous,” she said.

“When we have overwintering fires, that trajectory seems to be a little bit more locked-in.”

Intense fires don’t mean zombie fires

A sister paper due for publication will examine the relationship between fire intensity and overwintering fire.

“These overwintering fires weren’t happening where the really crazy, intense combustion was happening,” said Baltzer.

“These fires were really happening at the edges of burn perimeters for the most part, places where the fire had slowed down. The fires in those places weren’t intense, crowning fires.

“In most of the sites we visited, the branches and fine fuels on the trees were intact. It was really just that the fire had managed to get into that system, in the roots and the tree boles and in the soil, and slowly burn there, slowly smoulder through the winter.”

Lessons for firefighters

At NWT Fire, Mike Westwick said the news that forests were regenerating even after deep and sustained burning was “encouraging.”

He said the research also underlined “the importance of recognizing the hazards that overwinter burning can create and exacerbate” for the territory’s fire crews.

For example, Westwick said, the research demonstrated how overwintering fires can create pits covered by a thin veneer of leaves, needles and twigs. Zombie fires in forests, meanwhile, create a “huge hazard” from falling trees.

Many parts of the NWT have been in drought for several years, which is a contributing factor to the severity of the territory’s fire seasons and the number of overwintering fires.

“Those drought conditions are going to lead to us needing to do enhanced surveillance, as we have over the past couple of years, on overwinter fires to ensure we’re taking the right response steps,” Westwick said.

The next research

Baltzer said the sheer number of overwintering fires near highways from 2023 onward will help make them an easier phenomenon to study.

The sites used for this research required helicopter access for the most part. Sites near roads will be much simpler to reach.

“There’s going to be a lot of opportunity to explore these types of fires more, better understand impacts and evaluate the distribution of these kinds of fires on the landscape, to confirm some of what we found in this initial incursion,” she said.

However, her immediate research priority is some of the most unusual fire behaviour seen in the devastating 2023 season: short interval reburning.

This means areas hit by wildfires in the recent past that burned again in a fresh fire, much sooner than scientists might have imagined.

From 2024: Think you can trust that burn area?

Generally speaking, a wildfire’s burn area is expected to offer some protection to a community from future fires for a period of years. In the past, that period was thought to be up to 20 or 25 years.

Now, those burn areas appear to offer far less reliable protection as fire severity changes.

In 2023, said Baltzer, “we saw really short interval reburning where we had forests that were only 10 years old burning. Places that would have served as natural fire breaks for communities were burning.”

“Some of our next steps have to do with tackling some of those fire behaviours,” she said. “What are the conditions that lead to those young forests not protecting communities?”