By Hillary Rosner
October 1, 2020,
The photos and news from the past month’s devastating fires across parts of the West—charred towns, dramatic helicopter rescues, apocalyptic skies—have provoked fear, anger, and an understandable search for blame. It’s clear that fires are getting worse. But why? Scientists point to a number of reasons. Hotter and drier conditions brought on by climate change can prime vegetation to burn, and decades of fire suppression have allowed fuel to accumulate in forests. Millions of people now live closer to those dry forests than ever. And then there is the matter of the beetles.
There are 600 species of bark beetles in the United States, and they’ve evolved with their various host trees over millennia. Many bark beetles infest already dead or dying trees, but some, like the mountain pine beetle, attack living ones. The mountain pine beetle alone has killed roughly 100,000 square miles of trees across western North America over the past 20 years, from New Mexico all the way up to northern British Columbia. Climate change has instigated this dramatic spread, by eliminating the cold spells that kill off the beetles and by leaving the trees stressed by drought, unable to defend themselves.
The fear is that these enormous expanses of dead trees could, as a recent headline in the Los Angeles Times put it, “fuel unprecedented firestorms” across the West. In California’s Sierra Nevada this year, the fear seems to have been realized: The Creek Fire has so far burned more than 309,033 acres and destroyed 855 structures, and the U.S. Forest Service estimates that 80 to 90 percent of what’s burning is beetle-killed timber. During California’s crippling drought from 2012 to 2017, the Forest Service says, roughly 142 million trees died, mostly in low-elevation ponderosa-pine forests.
But while it may seem obvious that a hillside covered in dead trees is more likely to go up in a deadly blaze than a hillside covered in green, living trees, the connection between beetle-kill and wildfire seems to depend on the type of forest. “You can’t necessarily say that conclusions drawn from fires in California would apply to forests in the Rockies or the Pacific Northwest,” says Emily Francis, a postdoctoral researcher in forest ecology at the University of Texas at Austin. Her doctorate was on the impacts of extreme drought on California’s forests, and her field sites burned in both the CZU Lightning Complex and Creek fires this summer.
In some forests, beetle infestations don’t seem to make wildfires any worse—and might even limit their severity.
A matter of elevation
In a study published in 2018, a group of fire scientists led by Scott Stephens of the University of California at Berkeley more or less predicted what has happened this year in parts of the Sierra Nevada. In the southern and central Sierras, they warned, “the scale of present tree mortality is so large that greater potential for ‘mass fire’ exists in the coming decades, driven by the amount and continuity of dry, combustible, large woody material that could produce large, severe fires.”
“I am afraid it pretty much described what happened in the Creek Fire,” Stephens says now of that study.
Countdown to tree death—see how pine beetles do their damage.
For centuries the relationship was mutually beneficial: Pine beetles culled older, weaker trees, producing new beetles but also a healthier forest. Climate change, with its warmer, drier conditions, has upset that balance, leaving even healthy trees vulnerable to attack.
Satellite imagery generally shows the highest heat at the edge of a wildfire, where it moves into unburned areas. But during the first few days of the Creek Fire, the intensity deep inside the burning area was equal to that at the perimeter, Stephens says. “All that dead and downed material was burning and creating a signature equivalent to the flaming edge. We call that a mass fire.”
Lower-elevation ponderosa pine forests, like those in the Sierra Nevada, are especially vulnerable to this type of event because they evolved with frequent, low-intensity fires, burning every five to 25 years. Their natural state was open grass between widely spaced, large trees, so there was less to burn when fires came through. But a century of fire suppression by humans has filled the forests with fuel and more densely packed trees that would otherwise have been thinned out by the periodic fires. Competing for resources, the trees are stressed and more susceptible to beetle outbreaks—which makes the forest more prone to extreme blazes in hot, dry, windy weather.
In the lodgepole-pine forests typical of high elevations in the Rocky Mountains, on the other hand, the story is different. Those forests naturally burn only once every 75 to 300 years, in severe fire events. There, fire suppression hasn’t had much of an impact; whether the trees are alive or killed by beetles, plenty of fuel to burn has built up over time when a fire does come through.
The northern Rocky Mountains and the Pacific Northwest are yet other entities. In interior forests, east of the Cascades, research has shown that beetle outbreaks don’t always impact the severity of fires, and in some cases may actually reduce it. That’s because in those forests, typical fire behavior involves so-called crown fires, which burn through the forest canopy. But following a beetle outbreak, the canopy fuels become much less abundant, as needles and branches fall off the dead trees. It can take a long time after an outbreak for those canopy fuels to come back to levels that are important for active crown fires.
On the west side of the Cascades, meanwhile, from northern Oregon through Washington, the forests naturally have extraordinarily high amounts of biomass—giant trees, and lots of them, thanks in part to lots of rain. The dominant trees here, such as Douglas firs, are also fast-growing and long-lived. These forests have historically burned very infrequently, with fires sometimes occurring hundreds of years apart. But when they do burn, they burn big.
“It has nothing to do with bark beetle outbreaks or unnaturally high fuel loads,” says Brian Harvey, a forest and fire ecologist at the University of Washington. What propels those fires, he says, is a combination of warm, dry conditions that dry out the trees and other plants, some form of “ignition on the landscape—human caused or lightning,” and east winds that can drive the fires to travel at extreme speeds.
In the forests of the Rockies and Pacific Northwest, this weather seems to be a far bigger driver of fire severity than beetles. When it’s hot and dry enough, whether the trees are alive or dead doesn’t “make a huge difference relative to how these fires would normally burn,” says Sarah Hart, a forest ecologist at the University of Wisconsin.
She has analyzed the relationships of hundreds of large wildfires to mountain pine beetle outbreaks in Rocky Mountain lodgepole pine forests. She and colleagues at the University of Colorado found that the area burned each year “has not increased in direct response to bark beetle activity.”