Study Links Massive Spruce Beetle Outbreak In Colorado To Drought

According to researchers drought in the high northern Colorado Mountains is a key factor responsible for massive outbreak of spruce beetles in the region.

The latest finding by a team of researchers at the University of Colorado Boulder indicates that drought in the northern Colorado mountains is responsible for triggering  massive spruce beetle outbreak, which is further linked to the long term alterations in sea surface temperature from the Northern Atlantic Ocean.

Drought conditions lower the number of host trees' defenses against spruce beetles. The beetles attack the phloem, which kills the tree and eventually the entire forest.

According to the study author Sarah Hart, a CU-Boulder doctoral student in geography, the finding is important as it highlights drought as a better predictor for spruce beetle outbreak in the northern Colorado than only temperature.

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

This finding challenges assumptions that fire suppression in the West helps spruce beetle spread.

For the study, researchers investigated sites at the White River, Routt Arapaho, Roosevelt and Grand Mesa national forests as well as Rocky Mountain National Park.

"The extent to which we could distinguish between the warming signals and the drought signals was surprising," CU-Boulder geography Professor Thomas Veblen explained. "These are two things that easily can get mixed together in most tree ring analyses."

According to the study, during the warm and wet period from 1976 to 1998, both the rate of spruce beetle reproduction and tree defenses were high. But during this warm, "ideal" period, beetle- outbreak was minimal.

The team identified another effective predicator of drought conditions i.e. summer vapour pressure deficit- a measurement of atmospheric dryness.

However, it was the above-average annual variation in the Atlantic Multi-decadal Oscillation, or AMO that was strongly co-related with the spruce beetle outbreaks. AMO is a long-term phenomenon that is associated with sea-surface temperature changes in North Atlantic. Every 60 years, a positive AMO leads to warmer and drier conditions over much of North America, according to a news release

Researchers also found that number of high-elevation forests affected by spruce beetles is increasing in the West. Spruce beetle outbreak remains a menace as they impact the headwater streams that are crucial for water resources.

Spruce beetles are known to invade the large areas of coniferous forests across the West. Ranging from Alaska to Arizona the spruce beetles live in forests of Engelmann spruce and subalpine fir trees in Colorado.

The study was published in the journal Ecology.