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How Bark-Beetle Infestations Could Intensify Spring Runoff

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newwest-blogger | Jun 21, 2011 05:00 AM

By Matthew H. Davis

As the spring runoff leaves behind a trail of destruction in parts of the the northern Rockies, a new University of Colorado study points to how beetle-infested trees—which have affected more than 4 million acres in Colorado and southern Wyoming alone—could lead to deeper snowpack and speed up snowmelt in the future.

Beetle trees snowpackFaster snowmelt season has the potential to intensify runoff, which in turn could help fill reservoirs, said geological science doctoral student Evan Pugh, who led the study.

Pugh and his team studied trees near Grand Lake, Colorado, on the western side of Rocky Mountain National Park, over a three-year period. They selected eight pairs, including healthy trees and nearby groups of “red-phase” trees on roughly one-acre parcels. The red phase begins about nine months after infestation, when the needles begin turning a rusty, brick color and are likely to retain needles for another 18 months.

During the course of the study, some trees entered the “gray phase,” when they slowly lose all needles, twigs and smaller branches, leaving a tree skeleton, of sorts. Six of the eight healthy tree stands studied were made up primarily of lodgepole pines, while two were made up of mixed conifer trees.

Under gray-phase trees, snow accumulation was about 15 percent deeper than under healthy and red-phase trees, researchers found. This happens in part because the canopies of healthy and red-phase trees tend to accumulate more snow during the winter, and much of that snow sublimates into the atmosphere without reaching the ground.

The team also observed faster snowmelt under gray-phase trees.

Also, snow under red-phase trees melted at a faster rate than snow under healthy trees, usually one week earlier. The study attributed the faster melt rates under infected trees to a lower surface albedo, or the surface’s ability to reflect sunlight. The lower albedo was caused by more debris on the snow surface, which increases as the trees shed needles.

Eric Small, a co-author of the study, was surprised by the results, and said he had expected the differences in runoff between living and dead stands to be more dramatic.

“The changes were measurable, but not enormous,” Small said.

In fact, he pointed to a number of factors that could help offset heavier snowmelt.

As more trees begin to die, Small said, more water will soak into the soil, which could slow spring runoff. Also, as red-phase trees lose their needles, new plants will sprout beneath them, using more of the snowmelt water.

In recent years, according to Pugh, water managers in Salt Lake City have observed an unexplained increase in water in river basin, which could indicate increased runoff from beetle-infested forests.

This was the first study to look at the potential effects pine-beetle kill could have on snowmelt in Colorado. The study focused only on hydrology, Small said, which leaves more questions about the role infected trees could have on Rocky Mountain ecology.

The forests in parts of Colorado’s high country have been hit so hard by beetles that the research team ran into challenges while preparing to begin their work.

“One of the hardest parts of this study was to find stands of healthy trees in this area,” Pugh said.

To read the report, click here.

Essays in the Range blog are not written by High Country News. The authors are solely responsible for the content.

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Image courtesy Flickr user Greg Younger

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