DENVER — Mountain pine beetles have left vast tracts of dead, dry trees in the West, raising fears that they’re more vulnerable to wildfire outbreaks, but a new study found no evidence that bug-infested forests are more likely to burn than healthy ones.
In a paper released Monday, University of Colorado researchers said weather and terrain are bigger factors in determining whether a forest will burn than beetle invasions.
The findings could provide some comfort to people who live near beetle-infested forests, if those trees are statistically no more likely to burn than healthy forests.
But the study acknowledged that other researchers have found that beetles pose different fire risks. Previous studies by the U.S. Forest Service found that once sparked, beetle-killed trees ignite faster and burn more quickly than healthy trees, posing a danger to firefighters.
The new findings were published in the Proceedings of the National Academy of Sciences. Researchers studied the three worst fire seasons in the past 12 years, when the weather was unusually hot and dry.
The study didn’t look at years with more moderate weather, but Sarah Hart, the lead author of the report, said she believes the results would be the same if it had.
The study said beetle outbreaks and wildfires increased at the same time but that drought was behind the worsening fires.
“The annual area burned in the western United States has not increased in direct response to bark beetle outbreaks,” the study says.
Matt Jolly, a Forest Service research ecologist at the Missoula Fire Sciences Laboratory in Montana, said the findings are valid but stressed that previous research has shown beetles affect fires in other, more threatening ways.
Trees infested with beetles have only about 10 percent of the moisture of healthy trees, Jolly said. Because they are drier, beetle-infested trees are more vulnerable to igniting from embers or burning branches carried by wind and heat waves, he said. “How that fire is burning is the most important thing,” he said.
The new study looked at fires in Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington and Wyoming. The Forest Service says the mountain pine beetle is also present in Nebraska and South Dakota, but those states weren’t part of the study.
Nearly 36,000 square miles of trees have been attacked by mountain pine beetles in the West since 1996.
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