Drought and the Spruce Bark Beetle: New CU-Boulder Study Makes the Connection

Editor’s note: The following article was submitted by CU-Boulder Geography Ph.D. student Sarah Hart.

Periods of drought and warm temperatures, are responsible for periods of spruce beetle outbreak in Colorado, says a new University of Colorado Boulder-led study.

A new study used tree-rings and documentary records of spruce beetle outbreak across much of the Rockies from the northern Front Range to Grand Mesa in southwestern Colorado over the past 300+ years to examine the climate variables associated with past outbreaks. Notably these multi-century tree-ring records of outbreaks documented that periods of outbreak occurred during periods of drought. The study suggests that spruce beetle outbreaks occur when warm and dry conditions cause drought stress in the host trees.

Previous studies had clearly documented the favorable influence of warming temperatures on bark beetle populations but the current study also highlights the association of past outbreaks with moisture stress affecting the host trees. The study also showed that outbreaks co-occur with periods of extended drought such as those linked to  a slowly changing oscillation in the Atlantic Ocean, known as the Atlantic Multidecadal Oscillation.

A paper on the subject was recently published in the journal Ecology. The authors are CU Geography Ph.D. student Sarah Hart, Geography Professor Thomas Veblen and three of Veblen’s former students – Karen Eisenhart, Dominik Kulakowski and Dan Jarvis. The National Science Foundation and National Geographic Society funded the study.

The best known bark beetle outbreak is the mountain pine beetle affecting primarily lodgepole pine and ponderosa pine forests from British Columbia through the U.S. Rockies since 1990. Like the mountain pine beetle, the spruce beetle is a native bark beetle that can result in significant mortality of North American coniferous forest. Spruce beetles range Alaska to Arizona, and in Colorado are the most damaging bark beetle in subalpine forests of Engelmann spruce and subalpine fir.

Recently, there has been growing area of high elevation forest affected by spruce beetles. So much so that 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 – Wyoming, Colorado and New Mexico. The drought conditions that promote spruce beetle outbreak are expected to continue.

It has been suggested that spruce beetle and mountain pine beetle outbreaks are the result of forest changes associated with fire suppression policy in the latter half of the 20th century or even are the result of reduced logging activities in recent decades in the Rocky Mountains.  The current study lends support to the argument that the key factor driving bark beetle outbreaks is climate variability, and specifically warming and drying trends, rather than past land-use practices.

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