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Why Bark Beetles are Chewing Through U.S. Forests

The conifer forests of the North American west have been under a massive assault over the past decade by bark beetles: one species alone, the mountain pine beetle, has killed more than 70,000 square miles’ worth of trees, equivalent to the area of Washington State, and two recent studies have shed some light on how climate change is helping fuel the assault, and what’s likely to happen in a world that continues to warm.

The first, published in the journal Ecology, shows how intense drought can bring on a population explosion in the voracious insects — and how this creates a vicious cycle of tree-killing even when drought subsides. The second, published in Proceedings of the National Academy of Sciences (PNAS) reveals that warming lets beetles move to higher elevations, where they’re encountering trees that are unusually susceptible to infestations.

Two recent studies shed some light on how climate change is helping fuel the assault of bark beetles, and what’s likely to happen in a world that continues to warm.
Credit: Colorado State Forest Service

Given the enormous destruction wrought by bark beetles in recent years, it’s natural to think of them as unstoppable eating machines. “Because we notice the big outbreaks,” said Ken Raffa, a University of Wisconsin entomologist who co-authored the PNAS study, in an interview, “it’s easy to picture forests as one big salad bar for the insects. But most of the time, they only go to the most stressed trees.”

That’s because trees have evolved defenses against the beetles in the form of natural chemicals that repel or even kill the attackers. In times of severe drought, however, the number of trees under stress in a forest will skyrocket. When that happens, as it did during a 2001-2002 Colorado drought documented in the Ecology study, the dramatic increase in relatively defenseless trees allows beetle populations to skyrocket.

“Once the beetles reach epidemic levels,” Teresa Chapman of the University of Colorado, the study’s lead author, said in an interview “the epidemic has a life of its own. Now, even if favorable conditions return, there are just too many beetles and [the trees] can’t defend themselves.”  

Favorable conditions will probably return less easily in years to come, however, as with a warming world, scientists expect droughts to be more intense, come more often and last longer.

Meanwhile the beetles, whose numbers would normally be held in check by cold winters that kill their larvae, are surviving in greater numbers from one year to the next as winters in the U.S. continue to get warmer. Not only that: as spring comes earlier and temperatures stay warm for longer, the beetles can fly further than they once did, allowing them to extend their range.

This range extension includes movement to higher altitudes, where beetles are encountering tree species they would normally not run into. That includes the white bark pine, the subject of the PNAS study. “What we find is that unlike the [lower-elevation] lodgepole pine, the white bark pine hasn’t evolved any defenses against the beetles,” Raffa said.

The combination of increasing stress on trees and decreasing limits on the beetles’ range and winter survival, say scientists, makes it almost inevitable that the insects will continue to experience a population boom — at the expense of the great conifer forests of America’s mountain west. 

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