Bark Beetles Kill Western Trees, Affect Mountain Streams

Forested mountain areas throughout the western United States are suffering from the effects of bark beetles. To put the damage in perspective, the mountain pine beetle has killed off over 3 million acres of pine trees just in the state of Colorado.

Several different species of bark beetle have been infesting and ultimately killing trees, leaving the area open to fire damage and other natural consequences. A study that was recently published in the journal Nature Climate Change gauged the effect of the dead and dying trees on the surrounding mountain streams, with respect to both water flow and water quality.

A research group from the Colorado School of Mines examined the effect of the dying trees on the streams that form the headwaters of nearby rivers. They found significant effects on the water table and the corresponding flow of groundwater into nearby streams.

As trees die, they cease to participate in the water cycle. Normally, water is drawn up from the ground through the roots and distributed throughout the tree. The water eventually evaporates through the leaves/needles and re-enters the atmosphere through transpiration. With fewer trees to remove groundwater, the water table rises and a higher percentage of the groundwater enter the streams.

The team quantified the effect of the beetles by analyzing the chemistry of the water sources and comparing infested with non-infested areas. This method effectively fingerprinted the different sources and allowed the team to perform an effective mass balance on the streams and their groundwater contributions.

The study concluded that the tree deaths caused by the bark beetles contribute an extra 30% to the amount of late-summer groundwater entering the streams. Not only does this quantify the amount of excess water in the streams, but it also serves as a rough guide for how much water new trees will consume, which could assist in the eventual reforestation process.

One might think that greater water content in the streams would be a good thing in the parched West, but there are downsides. A previous study from the Colorado School of Mines detected an unusual increase in contaminants in the late summer water flows - carcinogenic contaminants from disinfectants. They were found in water treatment plants servicing the watersheds of areas infested with bark beetles.

This puts the Forest Service and the National Parks in a difficult position - how do they combat the bark beetle without aggravating other issues such as water quality, and the potential for runoff and flood damages?

For example, while the recent massive flooding in north-central Colorado near Rocky Mountain National Park was caused by virtually unprecedented rains, the damage that the bark beetle has done certainly did not help the situation.
While the war against the bark beetle rages on, this study provides a template on how to quantify the effects - and hopefully before long it will be able to confirm that the war is won and that the water quality and groundwater levels are returning to normal. Otherwise, we may be in danger of permanent damage of some of our treasured National Parks.