Beetle battle triggers earlier snowmelt

U. COLORADO-BOULDER (US) — Mountain pine beetles that have already killed more than 4 million acres of trees, could do further damage by forcing earlier snowmelt and increasing water yields.

Researchers monitored eight pairs of tree stands, each pair consisting of one live stand and one dead stand roughly an acre each in size and located adjacent to each other, sharing the same topography, elevation and slope.

The team monitored the two distinct phases of pine beetle tree death during the three-year study—the red phase in which dead trees still retained red needles, and the gray phase in which all of the tree needles and some small branches had been shed, says Evan Pugh, doctoral student of geological sciences at the University of Colorado-Boulder.

There was roughly 15 percent more snow accumulation under the gray phase stands than under living stands or red phase stands, probably largely due to a lack of  snow interception by needled tree branches that can cause snowflakes to sublimate into gas and return to the atmosphere.

Gray phase trees also allow more solar radiation through their canopies than live trees and red phase trees, increasing the potential for earlier melt.

Snowmelt rates were highest under red phase trees, with snow disappearing up to a week earlier than snow in adjacent, healthy stands even though both received the same amount of snowfall at their bases.

The earlier snowmelt in red phase tree stands is due in large part to the amount of needles and branches that drop or is blown from the trees onto on the snow surface, decreasing its solar reflecting power, and causing it to absorb more of the sun’s radiation and heat up slightly.

“This is the first study to look at the potential effects that different stages of mountain pine beetle tree death may have on snowmelt,” Pugh says. “What we are seeing is earlier snowmelt and more snow accumulation in dead forests.”

Details of the research appear in the journal Echohydrology.

The study took place at the headwaters of the Colorado River in north-central Colorado. Six of the eight healthy tree stands in the study were made up primarily of lodgepole pines, while two were made up of mixed conifer trees.  “One of the hardest parts of this study was to find stands of healthy trees in this area,” Pugh says.

The red phase that occurs following tree death usually lasts about 18 months, and the onset of the gray phase occurs about three or four years after tree death.

“One of the big surprises to me was that changes in snowpack depth and snowmelt timing as a result of the pine beetle outbreak were not larger,” says Eric Small, professor of geological sciences. But the continuing effects could become more significant in the coming decades.

In addition to avalanche poles used to periodically measure the snow depth at the 16 study stands, researchers also inserted tiny thermometers at various snow depths to help them predict when the snow would likely melt.  They dug snow pits in each of the tree stands and weighed known volumes of snow to calculate density and water content.

Tevis Blom takes a hemispheric photo of a tree canopy for a study of the mountain pine beetle infestation. (Credit: Evan Pugh)

Pyranometers were used to measure the snow surface albedo (solar reflecting power) and the transmission of sunlight through forest canopies and fisheye camera images taken from the snow surface helped calculate the size and structure of the various tree stand canopies.

A massive fire in the study area in the late 1800s resulted in most of the succeeding lodgepole pines to be about the same size and age, making them easier targets for pine beetles.

While mountain pine beetle infestations are natural events, climate change probably has played a role in the most recent outbreak. Drought conditions in the West in recent years have caused living pines to absorb less water, decreasing their ability to produce enough sap to pitch out beetles that are attacking them, Pugh says.

Water managers in Salt Lake City have reported extra water in river basins that hydrologic models had not predicted, an indication beetle-killed trees are having an impact on meltwater.

“Our study is the first to analyze the multiple stages of tree death from mountain pine beetles and their different impacts on snow accumulation and snowmelt,” says Pugh. “³There is no on/off switch here—only gradual changes.

“The effects of the beetle-killed tree stands in terms of snow accumulation are not going to affect ski resort seasons by any means,” he says.

“What we can say is there likely will be additional water resources for water managers. Additional snowpack coupled with dead trees that are no longer sucking up water will likely lead to more runoff.”

More news from University of Colorado: