Study: Beetle invasions dampen, not intensify, wildfire risk
LOS ANGELES - Dire warnings have accompanied the armies of bark beetles that have 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, co-author 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 in the western United States 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 the authority to designate "insect and disease emergency" areas on national forest land, giving priority to thinning projects.
But Turner and co-authors 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 fire, which is fed by fallen limbs and branches.
The researchers used satellite imagery to map lodgepole stands attacked by mountain pine beetles, hiked into the areas to confirm the beetle damage and measured 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."