• Rocky Mountain Research Station
• 240 West Prospect
• Fort Collins, CO 80526
• (970) 498-1100

Consequences of Bark Beetle Outbreaks on Mountain Watersheds

At the Station's Fraser Experimental Forest in central Colorado, researchers rely on long-term climate, streamflow, sediment, vegetation and other data records to detect physical, biological and chemical changes resulting from extensive forest canopy mortality. In addition, new studies that link tree, hillslope and other processes will generate information about snow accumulation, streamflow and water quality, forest water use and carbon storage, soil productivity, riparian and wetland species composition, and large wood, sediment and stream channel dynamics in logged and untreated areas infested by bark beetles. For instance, sensors have been added to gaging and monitoring stations to assess changes in sediment transport in conjunction with streamflow and nutrient dynamics due to beetle outbreak. Studies were established along four streams to measure the impacts of the beetle kill on stream channel characteristics and large wood loading; co-located riparian plots will help to determine the vulnerability of riparian corridors to beetle impacts. Preliminary details are available in Increased Instream Wood Loading Attributed to Mountain Pine Beetle Outbreak in Subalpine Watersheds, American Geophysical Union, Fall Meeting 2006.

Scientists at Fraser are also looking at tree mortality following bark beetle attack. When mountain pine beetles attack a tree, they disturb two fundamental tree growth processes. First, the beetles and their larvae feed on the food-conducting tissue in the outer bark, resulting in a certain degree of girdling. Second, the beetles introduce blue-stain fungi that grow in the water conducting tissue of the tree, reducing the ability of the tree to conduct water from the soil to the canopy. Both processes have been suggested as the cause of tree death following bark beetle attack. Station scientists have initiated an experiment to examine the relative roles of these processes on tree mortality. In 2006, they began comparing water use and tree growth in infested, mechanically girdled (no beetle) and control trees (no beetle or girdle). Initial results indicate tree water use in MPB infested trees declines rapidly within three weeks following attack and is essentially zero at the beginning of the next growing season. Although tree water use also declined following the mechanical girdling treatments, girdled trees were still actively growing and using water as of August 2007 with similar growth rates of new foliage relative to controls. Results from this experiment indicate that the introduction of blue-stain fungi is the primary mortality agent following attack, killing trees long before girdling can. This work showcases the mechanism behind a widespread forest disturbance throughout North America. Results of this study are being sent to a peer reviewed journal for publication.

In addition, Station scientists are actively collaborating with the Arapaho-Roosevelt and Routt National Forests to evaluate harvesting practices and to assess how effective management practices are at sustaining forest production and protecting water quality and other watershed resources during extensive bark beetle outbreaks. Current studies on nutrient and sediment retention within riparian zones will help managers predict the impacts of fuel reduction projects on aquatic resources in areas infested by bark beetles. Such information will lead to land stewardship decisions that promote forest health objectives and are defensible to the public. Initial findings were presented to the Station's management partners at the US Forest Service Rocky Mountain Region's Hydrology, Fish and Soils meeting in July 2007.