The tiny spruce beetle, aka Dendroctonus rufipennis (Kirby), is causing big problems not only in the San Juans, but throughout the state. A new study shows that persistent drought may be to blame for
the bug's decimation of 1 million acres

Spruce beetles tied to drought

Epidemic gripping San Juans likely drag on, study finds

by Missy Votel

The massive spruce beetle infestation gripping the San Juans may not be ending anytime soon. According to a new University of Colorado Boulder study, the primary trigger of the outbreak is drought, which is tied to long-term changes in sea-surface temperatures. Unfortunately for local high-elevation spruce, that trend is expected to continue for decades, according to the study, which was published in the online journal Ecology.

In 2012, the state's spruce-beetle infestation grew by 183,000 acres, bringing the total acreage affected to nearly 1 million acres. Much of that increase came in the San Juan and Rio Grande national forests, which have been the hardest hit.

The study is interesting in that it found drought to be a better forecaster of spruce beetle outbreaks than temperature alone, according to lead author Sarah Hart, a CU doctoral student.

 “It was interesting that drought was a better predictor,” said Hart, a member of the Geography Department. “The study suggests that spruce beetle outbreaks occur when warm and dry conditions cause stress in the host trees.”

Drought appears to decrease trees' defenses against spruce beetles, which attack the inner layers of bark. The insects feed and breed in the soft inner bark, which impedes growth and eventually kills the tree.

Spruce beetles are a cousin of the mountain pine beetle, which decimated the state's lodgepole pine forests in the 1990s. While waning, that pine beetle epidemic was considered to be the worst on record. However, the spruce beetle infestation has the potential to be even more devastating, said Hart.

 “In 2012, U.S. Forest Service surveys indicated that more area was under attack by spruce beetles than mountain pine beetles in the Southern Rocky Mountains, which includes southern Wyoming, Colorado and northern New Mexico,” she said. “The drought conditions that promote spruce beetle outbreak are expected to continue.”

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

Spruce beetles range from Alaska to Arizona and live in forests of Engelmann spruce and subalpine fir trees in Colorado, typically above 10,000 feet. The CU study included sites in the White River, Routt, Arapaho, Roosevelt and Grand Mesa national forests as well as Rocky Mountain National Park.

The study assembled a long-term record of spruce beetle outbreaks using a combination of historical documents and tree ring data reaching all the way back to 1650. Several broad-scale beetle outbreaks were noted during the years 1843-60, 1882-89, 1931-57 and 2004-10.

The researchers used a variety of methods to tease out variations in the data and finger drought as the main driver of the outbreak. “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.”

For example, when Colorado was in a warm, wet period from 1976-98, both spruce beetle populations and tree defenses like “pitching” beetles out of trees with resin, were likely high. But during that period of warming, outbreak was minimal.

However, the strongest climate correlation to spruce beetle outbreaks was above-average temperatures for the Atlantic Multi-decadal Oscillation, or AMO, a phenomenon in the North Atlantic that is believed to shift between cool and warm about every 60 years. Warmer AMO conditions, such as the one that started in the 1990s, are linked to warmer and drier conditions over much of North America. Veblen said the current warm AMO could continue for another 35 - 40 years. A 2006 tree-ring study also found that this warm phase has correlated to increased wildfires as well.

In addition to AMO, the researchers looked at other ocean phenomena, including El Niño and the Pacific Decadal Oscillation, as well as past temperatures and precipitation.

The National Science Foundation and National Geographic Society funded the study.

One big concern about spruce beetle outbreaks is their effect on water supplies, said Veblen. “In the long term, the absence of the trees killed by beetles may lead to less persistence of snow and earlier runoff.”