Dire warnings have accompanied the armies of bark beetles that bored their way across the Mountain West in the past decade: Millions of acres had turned into a tinderbox of scraggly, dead trees ready to explode in flames.
But scientists are pouring water on that conventional wisdom.
A new study in the lodgepole pine forests of the greater Yellowstone region concludes that rather than increasing the wildfire risk, beetle attacks reduce it by thinning tree crowns.
"It's really counterintuitive," said University of Wisconsin ecology professor Monica Turner, coauthor of a paper that has been accepted for publication in Ecological Monographs. "The beetles are good foresters, thinning the forests for us in a way."
Beetle infestations have spread across more than 100 million acres of the Western U.S. and British Columbia since the 1990s, staining the rich green of conifer forests with the grays and rusts of dead and dying pine, spruce and fir trees.
Experts say drought and a century of fire suppression have left forests more vulnerable to the insects' cyclical outbreaks, while rising temperatures mean there are fewer fall and spring cold snaps to help keep the bugs in check.
The beetle destruction has prompted calls for stepped-up efforts to remove the dead trees and reduce the wildfire hazard. A Senate bill introduced last year would give the U.S. Agriculture Department authority to designate "insect and disease emergency" areas on national forestland, giving priority to thinning projects.
But Turner and coauthors Martin Simard, William Romme and Jacob Griffin concluded that overall, mountain pine beetle damage "generally results in a dampening rather than an amplification of fire behavior and intensity."
When the dead pines drop their needles, they are shedding fuel that can drive fast-moving fires in the tree crowns. "It's the decline in the canopy fuels that is responsible for the reduction in the fire hazard," Turner explained. "The perception people have about the increase in fire risk really did not have much of a basis."
The study also found that the increase in dead needles on the forest floor had little effect on the intensity and spread of surface fires, which are fed by fallen limbs and branches.
The researchers used satellite imagery to map lodgepole stands attacked by mountain pine beetles, a type of bark beetle, then hiked into the areas to confirm the beetle damage and measure fuel loads. Then they ran computer models to predict fire behavior.
"The overall message that bark beetles do not increase the risk of wildfire is something that has been coming out of the scientific literature for a number of years," said Dominik Kulakowski, an assistant geography professor at Clark University in Massachusetts who was not involved in the Yellowstone study. "What has been missing is the mechanism explaining that."
He has testified before Congress that climate, not the beetle epidemic, is what's driving the growth in Western wildfires. "The risk of wildfire is real and present, but that risk is associated with drought conditions," he said. "If we chase after the bark beetle outbreaks … we're going to waste very precious resources."
Michael Jenkins, an associate ecology professor at Utah State University, said his own research findings largely agree with the Yellowstone work, but there are two exceptions.
His studies in lodgepole forests in Utah and Idaho found that there is a relatively short-lived increase in the potential for surface fires when dying needles that have not yet lost all of their resin pile up on the ground. He also said the potential for a crown fire briefly increases in the early stages of a beetle epidemic, when the forest is a mixture of dying, dead and living trees.
But by the time the beetles have finished their work, leaving behind a sea of ghostly gray pine corpses, Jenkins said, the wildfire "hazard is gone." At that stage, he added, there is no point in cutting down the dead trees. "We're in a post-outbreak phase. That's the phase we're in now in the Colorado Rockies," a hot spot of lodgepole infestation.
The research on beetles and wildfire has focused on lodgepole and spruce-fir stands, so scientists can't say whether the results apply to other forest types that have also experienced beetle damage.
While the intermountain West has been hit the hardest, bark beetles carved a swath of destruction through Southern California's San Bernardino National Forest in 2003 and 2004. Citing fire risk, forest managers there have spent millions removing dead trees on about 29,000 acres near communities, roads and recreation areas.
"Our situation is considerably different" from Yellowstone's, deputy forest supervisor Kurt Winchester said.
The San Bernardino infestations were in stands of mixed conifers that included species not attacked by the bark beetles — creating a mosaic of dead and live trees. "These standing dead trees will act like torches," carrying fire into the live vegetation, he said.