How the controlled burn approach is used to contain wildfires

In May, US forest officials set out to burn a patch of the Sante Fe National Forest in New Mexico. They were looking to thin some of the dried underbrush that gives rise to intense wildfires, but with high winds and low humidity, their burn got out of control. Instead of singing the woodland, they helped ignite the largest fire in state history; a massive blaze that tore through an area larger than Los Angeles.

In an autopsy of the fire, US Forest Service chief Randy Moore wrote, “Drought, extreme weather, wind conditions and unpredictable weather changes are challenging our ability to use prescribed fire as a tool to combat destructive fires.” In short, climate change is making it harder to fight fire with fire.

Forests in the American West are fire-adapted, reliant on regular, small burns to thin their ranks. For centuries, Indigenous people aided this process by igniting fires when woodlands grew especially unkempt. But European colonisers took a different and ultimately short-sighted tack, guarding forests from all manner of fire.

The practice allowed woods to become overgrown, with densely packed trees now competing for a dwindling supply of water as rising temperatures fuel rampant drought. Pines are dying or drying out, turning forests to tinder and fuelling ever-worsening blazes.

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Having deprived woodlands of needed fire for decades, forest managers are now working to pay down the accumulated debt by administering prescribed burns. But climate change is complicating this work. Fire season is growing longer, while the period during which officials can safely light burns is shifting or narrowing.

The New Mexico fire grew from two prescribed burns: the fire set in May, which met with unusually flammable conditions, and a fire set in January, which “smouldered underground for months, persisting through multiple snowstorms and freezing temperatures, before resurfacing as a wildfire,” Moore wrote, adding that such a phenomenon would have been “nearly unheard of until recently.”

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A recent study of California found the spring burn window is shrinking by around one day per year across much of the state. A similar shift is under way in forests, shrub lands, and grasslands in Australia and the Mediterranean. While burn conditions vary widely from year to year, there has been an overall downward trend in the number of days that are suitable for prescribed burning in Southern Europe, according to Paulo Fernandes, a wildfire researcher at the University of Trás-os-Montes and Alto Douro in Portugal. He estimates that, since 1980, the window for prescribed burns has shrunk by eight days per decade in forests and 15 days per decade in shrub land.

Even when weather conditions are favourable, the build-up of dry, woody fuel means that burns can easily get out of hand, according to Anabelle Cardoso, an ecologist with a Nasa project surveying biodiversity in South Africa, where fires are a regular feature of the landscape.

“At the moment there seems to be a move toward doing more prescribed burns. That is the right decision,” she points out, “But in certain areas that have been mismanaged for a very long time, I think it’s just very difficult.”

Many of those mismanaged lands are in the US, Australia, and southern Europe, where 19th-century foresters were heavily influenced by their German counterparts, who believed fire to be a wholly destructive force. The German view made sense in the temperate Black Forest, where rainfall varies little month to month and wildfires are relatively rare, but it had little application in more arid climates, where forests see little to no rain in the summer and a certain amount of fire is inevitable.

In dry regions, plants and wildlife have learned to live with fire, if not depend on it. Fires kill pests, destroy invasive weeds, and enrich the soil with nutrients. California’s giant sequoias, the world’s largest trees, need fire to reproduce. Wildfires crack open sequoia pine cones, releasing their seeds. Fires also clear the forest floor of shrubs and pine needles so those seeds can take hold.

Decades of fires suppression let many woods become overgrown — in parts of Arizona, forests that historically had 60 to 80 trees per acre now have up to 2,000 trees per acre. The challenge was compounded in southern Europe by farmers abandoning their lands. “As they entered the modern economy, people fled the countryside for the cities, so you have huge urban growth, and no one’s tending the land any more, and it overgrows,” said Steve Pyne a retired professor at Arizona State University specialising in fire.

The abundance of fuel has fed catastrophic megafires, which, Pyne said, “are pretty much a pathology of the developed world.”

Prescribed burning can help keep wildfires in check. Experts say that prescribed burns limited the damage from intense wildfires in California and Oregon last summer. In Portugal, the national fire agency urged rural communities to clear scrub and administer controlled burns, and in 2021, it saw the fewest rural fires in a decade. Even in Ireland and the UK, where upland field burning has been met with opposition, research suggests prescribed burns can mitigate the impact of more frequent and severe wildfires.

As heat and drought grow more extreme, however, prescribed burning is becoming riskier. And a new paper looking at fires in the grasslands of South Africa, Gabon, and the United States finds that fire risk doesn’t increase linearly as conditions worsen. Rather, fires go from “not being able to spread to, all of a sudden, you have a state shift into fire being able to spread everywhere,” said Cardoso, who undertook the study while a postdoctoral researcher at Yale University.

The study could help land managers identify when fires are prone to grow and when they are likely fizzle out, so they can time prescribed burns for when weather will limit their advance, she said.

Historically, burns get out of hand less than 2 per cent of the time on U.S. federal lands, but that proportion could grow “unless we get smarter about our prescribed fire,” Pyne said. That means, among other things, being flexible about when to burn. With climate change, “We can’t go by the traditional calendar.”

In Australia and the American West, forest managers accustomed to burning in the spring or fall are now looking to administer burns in the cold months. California saw this winter began with burst of heavy snowfall in the Sierra Nevada mountains before settling into a long stretch of unusually meagre precipitation, which created an opportunity for prescribed burning, says Lenya Quinn-Davidson, director of the Northern California Prescribed Fire Council. “We saw people burning around the state in February and March, which normally we wouldn’t be able to do.”

Quinn-Davidson notes forests officials will need to be more nimble, ready to set prescribed burns whenever conditions arise. “With climate change, we need to change the way we’re thinking about prescribed fire and really look for opportunities year-round. That’s a real challenge. Part of the reason that we don’t do as much of this work as we need to is because we don’t have a workforce that can be flexible,” she adds.

One risk of prescribed burns is that they can cloud nearby communities with smoke.

This is a concern of Dominick DellaSala, chief scientist at Wild Heritage, a US environmental non-profit, who contends that, as forests face unprecedented heat and drought, prescribed burns will do little to prevent megafires. “We have to have an effective climate change strategy that gets us off fossil fuels,” he says. “You could do some prescribed burning, but don’t expect that to change things.”

After the prescribed burns in New Mexico grew out of a control, the US Forest Service halted all burning on federal lands for 90 days, a decision that Quinn-Davidson criticises as “absolutely a political move.” While prescribed burns sparked disaster in New Mexico, elsewhere, they have proved instrumental in protecting woodlands.

In July, a fire erupted near Mariposa Grove in Yosemite National Park, home to hundreds of ancient sequoias. While the fire burned through 5,000 acres of woodland, the sequoias escaped unscathed. The iconic trees were spared, fire officials said, by a half-century of regular cutting and burning, which thinned the woods, slowing the fire, and giving firefighters enough time to contain the blaze. “The result,” they underlined, “is that the Mariposa Grove survived, remains in good health and a healthier habitat has been created for local flora and fauna.”