A massive spruce beetle outbreak that’s been decimating coniferous forests in the northern Colorado mountains has been caused mainly by drought, according to a new study.
The study, published this week in the journal Ecology by University of Colorado scientists, found that drought is a better predictor of beetle outbreaks in the West than temperature. The drought is tied to long-term changes in sea surface temperature from the Northern Atlantic Ocean — a pattern known as the Atlantic Multi-decadal Oscillation (AMO), which warms sea surface temperatures about every 60 years. The most current AMO began in the late 1990s, so the University of Colorado’s finding is troubling: since sea surface temperatures are expected to continue to rise for the next several years, drought could continue to persist in the West, a scenario which means the spruce beetles will continue to thrive.
The researchers found that dry conditions weakened trees’ defenses against spruce beetles. The beetles kill trees by burrowing into a tree’s soft inner bark and feeding and laying eggs there, impeding the tree’s growth and eventually killing it.
“These warm and dry conditions are both really good for the beetle,” said Sarah Hart, the study’s lead author. “It’s increasing beetle population and allowing them to survive winters. I think it’s reasonable to expect that a high proportion of Colorado’s forest — spruce fir forest — will be affected by the beetle in the next 10 years.”
The University of Colorado study links the outbreak to a natural occillation, but previous studies have also linked spruce beetle outbreaks to anthropogenic climate change. One study from Norway found warmer temperatures can lengthen the life cycle of the spruce beetle, meaning as temperatures rise, the beetle will have two attack periods on spruce trees — one in spring and one in late summer, instead of just one in the spring. And since climate change can exacerbate a naturally-occurring drought, trees may be made even more susceptible to beetle outbreaks.
“When global warming rears its head, these drying effects accumulate over time, making the drought more severe and more extreme,” Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research, told LiveScience.
And spruce beetles aren’t the only insect pest that are devastating forests. In Western North America, from New Mexico up through British Columbia, mountain pine beetles have killed large swaths of ponderosa, whitebark, lodgepole, Scotch, and limber pines, driven by warmer weather and milder winters that have caused populations of the beetle to explode and their ranges to expand. The mountain pine beetle outbreak may be the largest forest insect blight ever seen in North America, and it’s been tied to climate change. One study found that, like the spruce beetle, mountain pine beetle mating doubles under warmer conditions, meaning as temperatures increase, there could be up to 60 times as many beetles attacking trees each year. Dead trees killed off by beetles have the potential to increase fire risk in a forest: a federal study found that in 2006, a beetle outbreak created a “perfect storm” in Washington, where affected lodgepole pines burned “with exceptionally high intensity.”