A new University of Colorado Boulder study indicates drought high in the northern Colorado mountains is the primary trigger of a massive spruce beetle outbreak that is tied to long-term changes in sea-surface temperatures from the Northern Atlantic Ocean, a trend that is expected to continue for decades. The new study is important because it shows that drought is a better predictor of spruce beetle outbreaks in northern Colorado than temperature alone, said lead study author Sarah Hart, a CU-Boulder doctoral student in geography. Drought conditions appear to decrease host tree defenses against spruce beetles, which attack the inner layers of bark, feeding and breeding in the phloem, a soft inner bark tissue, which impedes tree growth and eventually kills vast swaths of forest.

Spruce beetles, like their close relatives, mountain pine beetles, are attacking large areas of coniferous forests（针叶林） across the West. While the mountain pine beetle outbreak in the Southern Rocky Mountains is the best known and appears to be the worst in the historical record, the lesser known spruce beetle infestation has the potential to be equally or even more devastating in Colorado, said Hart, lead author on the new study.

"It was interesting that drought was a better predictor for spruce beetle outbreaks than temperature," said Hart of the geography department. "The study suggests that spruce beetle outbreaks occur when warm and dry conditions cause stress in the host trees."

A paper on the subject was published online in the journal Ecology. Co-authors include CU-Boulder geography Professor Thomas Veblen; former CU-Boulder graduate student Karen Eisenhart, now at Edinboro University of Pennsylvania; and former CU-Boulder students Daniel Jarvis and Dominik Kulakowski, now at Clark University in Worcester, Mass. The National Science Foundation and the National Geographic Society funded the study.

The new study also puts to rest false claims that fire suppression in the West is the trigger for spruce beetle outbreaks, said Veblen.

Spruce beetles range from Alaska to Arizona and live in forests of Engelmann spruce and subalpine（亚高山带的） fir trees in Colorado. The CU-Boulder study area included sites in the White River, Routt, Arapaho, Roosevelt and Grand Mesa national forests as well as in Rocky Mountain National Park.

The CU-Boulder team assembled a long-term record of spruce beetle outbreaks from the northern Front Range to the Grand Mesa in western Colorado using a combination of historical documents and tree ring data from 1650 to 2011. Broad-scale outbreaks were charted by the team from 1843-1860, 1882-1889, 1931-1957 and 2004 to 2010.

The researchers used a variety of statistical methods to tease out causes for variations in the dataset at 18 sites in Colorado. "The extent to which we could distinguish between the warming signals and the drought signals was surprising," said Veblen. "These are two things that easily can get mixed together in most tree ring analyses."