Think you can trust that burn area?

We get it. Wildfires are changing. But you’d think we could at least rely on the stuff burned by wildfires to stay burned.

And it does stay burned – but increasingly, the wildfires don’t seem to care.

Accepted wisdom is that for up to a few decades after a wildfire, a burn area doesn’t meaningfully burn again. That’s valuable knowledge if you’re figuring out how to protect a community.

Now, that wisdom is being tossed out of the window. Those burn areas are no longer reliable. Some are burning again, so soon that scientists’ eyes are popping out.

This is called short-interval reburn. Last year, the numbers were off the scale.

“There were about 450,000 hectares of those short-interval reburns just in the Northwest Territories last year, which is really quite exceptional. It’s mind-blowing,” said Marc-André Parisien, a research scientist at the Canadian Forest Service who specializes in this kind of wildfire science.

Put another way, the amount of NWT burn area that ordinarily wouldn’t burn but did burn again last year is more than 40 times the size of Yellowknife. And that’s being generous to Yellowknife.

In an average year, the territory sees about 15,000 hectares of short-interval reburn, according to Parisien’s colleague Ellen Whitman. Last year was around 30 times as bad.

One single patch burned 116,000 hectares of land that had already burned in the past 20 years, Parisien said.

“Had I heard that 12 months ago, I would have told my colleague, ‘No, you’re wrong. The data is wrong. You did something wrong, check your analysis because it’s wrong.’ But it did happen.”

OK, let’s say it burns again. So?

This is important because firefighters want to trust burn areas.

If you just saw the thing burn, and it looks really burned, the obvious conclusion is it’s not gonna burn again.

“It’s a big factor,” said NWT wildfire operations manager Richard Olsen – who makes a lot of the territory’s big wildfire decisions – in a briefing with reporters last month.

“Within one year of a fire burning under extreme conditions, where it’s burning into mineral soil, we don’t expect fire to be moving through those areas significantly,” Olsen said.

“There were large areas, unfortunately, last year that burned right into communities. That will cause a far lesser concern than unburned areas.”

The huge burn areas west and south of Hay River, for example, are much less of a worry for the immediate future than any parts of the South Slave that remain forested, with lots of fuel around for a fire, in the middle of this intense drought the region is experiencing.

Parisien, though, says in some extreme examples, even burn areas that went up in flames one summer ago are burning again as a new fire rips through.

The same areas might not burn with the same intensity, because huge amounts of fuel are gone, but it could be enough to sustain a fire driven by the wind – and that could mean the burn area isn’t the protection a community thought it was.

To make matters more confounding, sometimes an old burn area will resist a new fire remarkably well. Outside Tulita last year, for example, a decades-old burn area held fast with a fresh fire lapping at its edges. Outside Fort Smith, much newer burn areas weren’t nearly as resilient.

So far, Olsen says, his experience is that most one or two-year burn areas are “holding very good.”

“I think there is some fair confidence in some of those areas. We can use those in our strategies and tactics for existing fires or potential fire growth,” he said.

But he’s well aware that scientists’ understanding of burn areas is evolving and, with it, the traditional approach has to be discarded.

“Years ago, we used to have a general rule of 10, 20 or 30 years,” he said, describing the period for which a burn area was considered to be a natural fire break if another fire came along.

“Last year and even in 2014” – the territory’s other recent horror fire season – “we saw some burning through recent burns, areas where we would normally expect that burn area to stop things,” Olsen said.

“Fires were really pushing through those kinds of areas. It was so dry that everything was available to burn and very strong winds pushed things over barren areas from one spot to the other.”

Parisien says researchers are starting to understand what pushes a burn area over the edge from being fire-resistant to going up in smoke again.

Recent research suggests there’s a tipping point, he said, and just a tiny extra shove from the right mix of conditions can trigger short-interval reburn.

The weather plays a role, he said. So does the weather fires sometimes create themselves, giant storm clouds generated by some wildfires that northerners have recently seen. Meanwhile, drier conditions can mean a fire doesn’t have to flank around wetlands so much, so it packs more force – and more embers, helping it advance in the wind – when it hits a recent burn area.

In 2023, all of these things seem to have been turned up to the max. The result had never been seen before.

During the previous worst summer of 2014, Parisien said, “there were these short-interval reburns but it was a bite here and there, a nibble on the greater landscape.”

“What we saw last year is much more than a nibble,” he continued. “These are big chunks of the land. And we will see some ecological effects.”

Wait, what effects?

Burning, and burning, and burning again does things to the landscape. Ultimately, Parisien says it could turn parts of the boreal forest into grassland, at least for a time.

The more things burn, he says, the fewer coniferous trees are left behind, to the point where there aren’t even any seedlings. Instead, a more open and broad-leafed forest begins to develop.

“In some cases, you’re looking at a vegetation type conversion. You’re going from a forest to a grassland-shrubland for a bit. Is that going to be permanent? Probably not. But if it burns again? Well, maybe that’s going to endure for a long, long time – decades or centuries,” said Parisien.

“Typically, when everything’s going hunky-dory, you have a fire, there’s fairly vigorous regeneration … between three and five years after a fire, you kind-of know the stand you’re going to get. The trees have established, it’s going to be a forest again.

“What we’re seeing in these short-interval reburns is something quite different. It’s much more open. The trees are gone. Often what comes back in is not conifers, it’s aspen.”

Is that all bad? Probably not, Parisien said, noting that bison would probably vote for this outcome.

“The bison love it. Almost all of the species [moving into terrain like this] are native. We’re not talking about exotic species coming in, taking over and degrading the regional ecology,” he said.

“So it becomes a little bit philosophical, whether these striking ecological effects are good or bad.

“It depends on our objectives. It depends on how we define ecological integrity. I try not to be too apocalyptic, because I do believe that nature has this incredible capacity of taking care of itself.”