Jeffrey D. Allred, Deseret News
A new first of its kind study says warmer than normal winters are not the only reason the West is experiencing devastating bark beetle outbreaks that have killed millions of acres of pine trees. Other factors might be forest management.

SALT LAKE CITY — Pine beetles that have killed enough trees to cover the state of Montana over the past 15 years are overrunning forests due to warmer winters, but temperatures aren't the sole culprit in the outbreaks.

A new, first of its kind climate study done by the U.S. Forest Service and Dartmouth College concluded that warming winters have fostered significant mountain pine beetle outbreaks in the coldest areas of the western United States, but regions with longstanding warm winters have had problems as well.

“Our analyses clearly show that winters have warmed across mountain pine beetle habitats in the western United States," said Barbara Bentz, co-author of the study from the U.S. Forest Service Rocky Mountain Research Station.

"Although warming winters have contributed to increased mountain pine beetle survival and outbreaks in the coldest areas, such as the northern Rocky Mountains, population outbreaks were not explained in areas with historically warm winters."

The findings, published in Landscape Ecology and announced Monday, were the result of the first climate study to evaluate warming winters across the entirety of the beetle's range in the Western United States, looking at 23 separate ecoregions.

Results showed that the coldest winter nights have warmed by 7 degrees since 1960 across Western forests.

In the 11 coldest regions, the study found that winter temperatures lethal to the beetle have occurred less frequently since the 1980s and beetle-killed trees have increased significantly in those areas. At the same time, the warmest regions have had problematic outbreaks, even though those winters have never been cold enough to have significant beetle population kills in the past.

The pervasiveness of the beetle infestations in Western forests suggests to researchers that factors beyond warming winters are at play with outbreaks, such as warmer summer temperatures speeding up beetle development and the effects of long-term forest management.

"We know that forest management is a big part of this. It is not just temperature," said Aaron Weed, study co-author and now an ecologist with the National Park Service. "The history of forestry practices in the West — fire suppression and lack of timber management — have largely created vast tracts with relatively uniform forest composition."

Forests with stands of trees similar in age are vulnerable to pine beetle infestations. Outbreaks are becoming more severe because cold weather isn't putting a brake on the bugs' survival.

"Outbreaks are typically stopped by cold weather events," Weed said. "The frequency of those events is basically diminishing."

Weed said the research involved gathering data from 700 weather stations from Washington down to Colorado and examining the coldest of winter temperatures over the last 40 to 50 years.

The evidence is clear, he added, that the trend of warmer winters has far-flung implications for pest management that go beyond pine beetles.

"This warming is going on everywhere," he said. "It is now permitting things to survive in areas where they have never been able to survive before."

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