A massive spruce beetle outbreak has been attacking and killing large swaths of coniferous forests across the West for years. Now a recent study from the University of Colorado Boulder indicates that, at least in Colorado's northern mountains, the main trigger of the outbreak is due to drought and long-term changes in sea-surface temperatures -- a trend that is expected to continue for decades.
“It was interesting that drought was a better predictor for spruce beetle outbreaks than temperature,” said Sarah Hart, CU-Boulder doctoral student and lead study author, in a statement. "The study suggests that spruce beetle outbreaks occur when warm and dry conditions cause stress in the host trees.”
The study's authors analyzed periods of broad-scale outbreak and reconstructed those using historical records and tree-rings dated back to 1843 and through various periods all the way to 2010, via ecological research journal Ecology:
Periods of outbreak were compared with seasonal temperature, precipitation, vapor pressure deficit (VPD), the Palmer Drought Severity Index, and indices of ocean-atmosphere oscillation that include the El Niño Southern Oscillation, Pacific Decadal Oscillation, and Atlantic Multidecadal Oscillation (AMO). Classification trees showed that outbreaks can be predicted most successfully from above average annual AMO values and above average summer VPD values, indicators of drought across Colorado. Given the persistence of the AMO, the shift to a positive AMO phase in the late 1990s is likely to promote continued spruce beetle disturbance.
The most devastating outbreak in the region's history is believed to have been by mountain pine beetles -- relatives of the spruce beetle -- in the Southern Rocky Mountains. But the study suggests that the lesser known spruce beetle infestation in northern Colorado has the potential to be equally or even more devastating.
And the area of high-elevation forests that are affected by spruce beetles is only growing in the West, researcher Hart said in a press statement about the study.
“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,” Hart said. “The drought conditions that promote spruce beetle outbreak are expected to continue.”
The mountain pine beetle epidemic, although still lingering on the Front Range and Western Slope of Colorado, does appear to be waning while the spruce beetle outbreak continues to expand, according to the U.S. Forest Service.
In 2012, 183,000 new acres of spruce beetle infested trees were detected which brought the total acreage affected to nearly one million acres, with the San Juan and Rio Grande National Forests experiencing the most increased activity -- the home of the West Fork Complex Fire which burned more than 100,000 acres in 2013.
During that same period, the mountain pine beetle infestation expanded by only 31,000 acres, down from 140,000 acres of expansion the year prior.
2012 and 2013 saw two of the worst wildfire seasons in Colorado history with dozens of large wildfires burning across the state, many of which were fueled by the beetle-killed trees.
The effect that spruce beetle infestations have on forests with regard to extreme fire vulnerability varies. One recent study published in the journal PLOS ONE suggested that beetle infested forests increased fire risk. However another recent study in the journal of Ecoscience suggested that a spruce beetle infestation should reduce a region's vulnerability to extreme fires like the ones Colorado has faced.
Alan Boyle, NBC News' science editor, explained the inconsistency. "If a beetle-killed tree stands long enough to lose its highly flammable pine needles, it becomes less prone to catch fire," Boyle wrote. "And when the dead tree falls to the forest floor, that increases the gap between surviving trees, making it more difficult for a blaze to jump from the crown of one tree to the next. However, that gap could let gusts of winds blow through more easily, whipping up a forest fire once it does get started."