Vast armies of tiny, tree-killing insects called bark beetles have eaten their way through millions of hectares of pine forest in the western United States since the mid-1990s, leaving the mountains of Colorado, Wyoming, Montana, Idaho, and other states riddled with dead trees. But contrary to popular belief, say the authors of a new study, the beetles aren’t to blame for the record-breaking extent of the wildfires that have torched the region’s forests in recent years.
The study, published online on 23 March in the Proceedings of the National Academy of Sciences, is the first to take a large-scale look at how beetles are affecting the acreage burned across the western United States. Its findings are consistent with earlier research looking at smaller regions and are likely to fuel an ongoing debate over the wisdom of relatively expensive federal efforts to cull insect-damaged trees from western forests, in part to reduce fire risk.
“We examined [an] assumption that’s been made in a lot of policy and management discussions,” says co-author Tom Veblen, a forest and fire ecologist at the University of Colorado, Boulder. “That, as a result of tree-kill by bark beetles, there should be an increase in the aggregate area burned in the western U.S.”
To see if that assumption is correct, the researchers compared U.S. Forest Service maps of forests affected by the mountain pine beetle—the beetle species responsible for the most tree death—with maps of acreage burned during the peak wildfire seasons since 2002. “We were looking to see if there was more area burned in forests that had been infested by mountain pine beetle rather than the live forests,” says lead author Sarah Hart, a University of Colorado postdoctoral researcher.
The result, Hart says: “We did not find any difference.” Specifically, the researchers found that beetles had recently attacked just 5% of the more than 100,000 square kilometers of forest that burned during the 2006, 2007, and 2012 wildfire seasons.
Intuitively, it might seem that mountainsides pincushioned with dead, dry trees should burn more readily than those still awash in green. News headlines have frequently linked beetle-kill with fire risk, as have lawmakers. The 2014 Farm Bill passed by Congress, for instance, authorized spending up to $200 million annually to combat insect and disease threats to trees on some 18 million hectares of public land. Such efforts—including removing, or “thinning,” insect-damaged trees from forests—would “help reduce the risk of wildfires and the threats they pose to our communities and natural resources,” Senator Michael Bennet (D–CO) predicted at the time.
Some researchers, however, worry that poorly planned thinning efforts could be at best a waste of money and at worst do more ecological damage to forests than they prevent. Veblen, for instance, says that the evidence suggests that thinning beetle-kill at a “landscape-scale” isn’t likely to prevent big fires. In part, that’s because hot, dry weather plays a much bigger role than beetles in determining when and where forests burn, he and his colleagues have concluded. Under those conditions—which have been common in recent years—both live and dead stands of trees burn spectacularly well. (Veblen acknowledges that smaller scale thinning efforts are necessary to reduce fire risk around homes and critical infrastructure, such as reservoirs, but notes that such strategies are often useful whether or not trees have been killed by beetles.)
Although the study’s main finding is sound, says Matt Jolly, a research ecologist with the U.S. Forest Service’s Fire Sciences Laboratory in Missoula, Montana, the research does not address another issue that’s frequently overlooked: how insect damage might influence the behavior of wildfires, making them more dangerous and difficult to fight. “There are really two ways of looking at how mountain pine beetles affect actual fires,” Jolly says. One is to go in after the fact and study how beetles affected forest ecology or the area burned, as Hart and Veblen did. “The other side of the story,” Jolly says, “is to consider how those fires burned when they were burning.”
Lab studies and observational reports from firefighters show that the presence of beetle-kill can change fire behavior, he says, making the flames erratic, and therefore more difficult to control. “The flame lengths and rates of spread in the beetle-kill is significantly higher,” Jolly says, and fires may “spot” more, shooting out sparks that light new fires, and move in unpredictable directions—a major hazard for firefighters on the ground. Those differences can influence decision-making during firefighting operations, and they also matter for communities at the edge of forests ravaged by beetles. In such places, clearing insect-damaged trees may be a wise investment. Even if thinning doesn’t prevent wildfire altogether, it could increase firefighters’ ability to stop a fire advancing on homes, and reduce the safety risks they face on the job.
Hart agrees that such concerns are real. “We don’t want to say [beetles are] unimportant for fire,” she says. “But we do want to say that what burns isn’t strongly affected [by beetles].” And that has policy implications, she adds. “If you’re trying to manage for fire, it’s probably a better strategy to manage for the underlying driver of fire across the West. There we’re talking about warm and dry conditions.”