The boreal forest is no stranger to fire. Each year, in the Northwest Territories alone, thousands of hectares of wilderness are consumed in flames – part of the natural process of forest regeneration.
But this year, as the region battles its worst fires since the 1990s and smoke drifts for thousands of kilometres to the U.S. border, a new set of questions is emerging: Is a warming climate amplifying the severity of northern wildfires? Will bad fire years like this become more common? Will the forest that regrows be different in character from the one burning away right now?
Fire on such a massive scale is a drama in three acts – and it's one that scientists are watching carefully for clues to a changing planet.
ACT 1 – THE SETUP
Weather is a key factor in forest fires. In Canada's northwest, the weather this summer has created optimal conditions for wildfires to spread.
The principal culprit is a ridge of warm, dry air that has been parked around the Mackenzie River valley and points east for weeks. The ridge acts as a roadblock to weather patterns that would otherwise carry moisture into the region.
Since mid-June, temperatures from Yellowknife to Tuktoyaktuk have been well above historic averages while precipitation has been sparse. As the forest dries out, there's less moisture around to slow fire down. If a fire breaks out it can go farther and faster than it would in a typical year, which makes all the difference. In Canada, just 3 per cent of fires are responsible for 97 per cent of the area burned.
"That's the tail that wags the dog – and why this event is having such an extreme effect," says Mike Flannigan, a professor at the University of Alberta who specializes in climate-fire interactions.
This summer's fire season is unusual but still within the normal range of variation for the Northwest Territories. What scientists are beginning to see, however, are signs that blocking patterns are becoming more pronounced in the North as the climate warms.
Years that seem out of the ordinary from a historical perspective may, in fact, represent the new normal. This year, the Northwest Territories could lose between one and two million hectares of boreal forest to wildfire. Last year, the northeast experienced similar conditions and Quebec lost 1.7 million hectares. This picture is reflected across the entire circumboreal region – the forested area that rings the Arctic. Preliminary results from a NASA-backed study reveal a seesaw pattern between eastern and western Siberia. When one is burning the other is not, indicating how the looping waves of the jet stream facilitate the persistence of ridges of dry air in some locations while moisture-laden troughs linger in others.
The data suggest a future of heightened fire extremes, says Prof. Flannigan, who is participating in the study, because "the ridges will be more long-lasting and perhaps more intense."
ACT 2 – THE INFERNO
Today, much of what is known about how Canadian forest fires unfold is based on an extensive series of experimental burns conducted in the Northwest Territories starting in the late 1990s.
"It's been a wealth of information for us," says Bill de Groot, a fire researcher with the Canadian Forest Service, based in Sault Ste. Marie, Ont.
Those carefully monitored experiments along with studies conducted in wind tunnels and other data can be used to develop fire behavioural models that show how a given forest fire will spread under a particular set of conditions. The models are intended to help predict where and when a dangerous fire is likely to arise.
A key element of the models is fuel mix. Currently models used in Canada distinguish among 16 different kinds of fuel based on forest type. For example, the boreal forest is densely packed with black spruce, which burns well and easily allows a ground fire to transition into an intense crown fire that spreads rapidly from treetop to treetop. This helps explain why this region burns more extensively than any other part of the country, even though locations farther south experience fire-friendly conditions more frequently.
Fires in the remote North are typically caused by lightning strikes. When a tree is struck, the charge travels down the trunk and ignites vegetation at or even below the surface.
A crucial factor in what happens next is the dryness of the litter along the forest floor. A top layer of twigs and needles can dry out quickly and help fuel a newly started fire. But the resulting blaze also depends on the state of the underlying layers of decaying vegetation and moss. These layers hold water like a sponge. When wet they can stop a fire before it gets started. When dry – as they have been across the Northwest Territories this summer – a single electrical storm can lead to many fires.
When deeper layers are dry, they can also harbour and sustain a smouldering fire below ground for days or weeks, even if the topmost layer has been temporarily dampened by passing showers. They can also lead to a more energetic fire.
"There's a whole lot of biomass in those deeper layers," says Mike Wotton, a federal research scientist based at the University of Toronto's Fire Management Systems Laboratory. "As they dry and start to contribute, that can really drive up flame intensity."
Ultimately, understanding this dynamic may prove key to gauging whether Canadian forest fires are part of a positive feedback loop in the global climate system. Many regions of the boreal forest sit atop peat that has been storing carbon for thousands of years. If a higher proportion of this material burns along with the forests it will add significantly to atmospheric carbon, which in turn will accelerate global warming and set the stage for more fire.
ACT 3 – THE AFTERMATH
All things being equal, a warming planet should create opportunities for more southerly plant species to migrate north. But while looking for clues as to whether this is happening, Jill Johnstone, a plant ecologist at the University of Saskatchewan, says she has found that two very different patterns can emerge once a patch of boreal forest has burned.
"We're seeing areas where the forest changes and spreads after a fire and we're seeing areas where the forest disappears," she says.
One example of the former type of shift can be found near Inuvik, NWT, where a stretch of boreal forest cleared by forest fire in 1968 has been replaced by stands of aspen and birch.
The amount of fuel consumed in the 1968 fire may have played a role in the change. When fire is less severe, the organic layer on the ground is only partly burned. It forms a loose black fluff that gets hot and dry in direct sunlight and tends to keep seeds separated from moisture lower down. Black spruce seedlings are adapted to this and tend to do better under such circumstances. But when fires become more severe, the organic layer is completely stripped away and aspen or birch seeds can gain a foothold.
This has an effect on wildlife. Woodland caribou, already under pressure from road building and other human activity, are among the losers when lichen-rich spruce forests give way to a mixed deciduous bush. In contrast, moose stand to gain.
Permafrost is another casualty of big fires. When permafrost melts away under a fire, the ground can shift, sometimes dramatically, creating crevices and eroded areas where shrubs and other plants more readily take over instead of the trees that were there previously.
The takeaway message is that context matters when trying to predict what will happen after fire sweeps through a northern forest. With comparatively little data to go on, researchers are still struggling to see the big picture.
That's why the current conflagration – once the smoke clears – represents a big opportunity for scientists to learn more, Dr. Johnstone says. Some of the most useful data will come from looking at burned areas within a year of the blaze, since it becomes harder to reconstruct the ecological impact a fire has made as time passes.
"I think it's really important for us to take advantage of studying these big disturbance events," she says. "Because, if we can say anything, we can say that we think they're going to be more common."