SEATTLE — Don’t blame bark beetles for catastrophic wildfires such as the blaze that blackened more than 23,000 acres of Washington’s Kittitas County last summer, some scientists say.
In a peer-reviewed paper published this week in Natural Areas Journal, scientists say they found through a literature review that bark beetles do not substantially increase the risk of crown fire in lodgepole pine and spruce forests, as commonly assumed.
Instead, they concluded, the fires are primarily caused by dry conditions exacerbated by climate change. And as long as severe droughts continue, so will wildfires, regardless of beetle populations, the scientists determined.
The paper’s findings are similar to those reached by University of Wisconsin researchers in 2010. That research team used NASA satellite data to identify large swaths of beetle-killed forests near Yellowstone National Park. The team compared maps of recent fires with the maps of beetle-killed forests.
They were surprised to learn large fires did not appear to occur more often or with greater severity in forest tracts with beetle damage. In fact, in some cases, beetle-killed forest swaths seemed less likely to burn because the fire stalled out in the dead trees that had lost their needles and branches.
The common link between beetles and fire wasn’t what they had assumed — beetle-killed trees stoking fire — but something else: climate change. Warmer, drier weather was fueling both drought and beetle populations.
After combing through hundreds of scientific studies, another team of scientists has reached a similar conclusion, that beetle-killed trees don’t necessarily stoke crown fires. They, too, concluded that depleted stands of beetle-killed trees might pose less, not more, risk of crown fire because of gaps caused in the forest canopy as branches drop. And the scientists found that drought, not beetle damage, was the real fire risk.
“It’s natural enough to look at a landscape that is covered with trees recently killed by bark beetles and, therefore, worry about fire risk,” said Dominik Kulakowski, assistant professor of geography and biology at Clark University in Massachusetts and a co-author on the Natural Areas Journal paper. “But if you look into the long-term ecology of these forests, there is a high fire risk under drought conditions, even when the trees are green and the landscape looks beautiful. Conversely, if we are not in drought conditions, the actual risk of wildfire is going to be fairly low, regardless of the effect of bark beetles.”
Further, beetle damage can have the effect of thinning forests, reducing the ability of crown fires to spread.
“If you look at a forest that was affected by beetle outbreak five or 10 years ago, you are talking about something like trying to set fire to a row of telephone poles; there is less to carry the fire,” Kulakowski said.
The authors of the Natural Areas Journal paper, including another scientist from Colorado State University, also found that as long as severe droughts persist, so will the risk of wildfire — beetles or not — as forests dry out. Climate change poses the real threat to forest health because of both drought and warmer winters.
Beetle populations can flourish without a killing winter cold. Surveys from the Washington state Department of Natural Resources have found more acres of bug-infested forests in the state than in the past 40 years, with about a third of the state’s forestland east of the Cascades at risk for die off and tree damage from bugs and disease.