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Does beetle-kill increase runoff?

Forest scene in Frisco, Colorado.

Researchers are trying to determine whether runoff increases in beetle-killed forests.

University of Alberta researchers take a close look at pine beetle impacts to water quality, quantity

By Summit Voice

SUMMIT COUNTY — Canadian researchers at the University of Alberta are delving into the question of how the pine beetle epidemic in western forests could affect runoff and water quality.

The forestry professors — Uldis Silins and Ellen Macdonald — are  also looking toward a long-term goal of forest recovery, resilience and resistance.

Halfway through the three-year study, they’ve determined that the massive landscape-level die-off will have a significant affect on runoff.

Healthy pine forests use up virtually all of the precipitation during the growing season, which runs from May to September. Older pine forests that are most vulnerable to mountain pine beetle attack use even more water, relying on the extra water from spring snowmelt to provide for growth during the summer, the university said in a June 3 press release.

To track the effects, the scientists simulated areas of beetle kill in selected stands of lodgepole to study how much additional water will run off and where it goes after the trees lose their needles and are no longer able to capture rain or snowfall. They are also studying how the vegetation and tree seedlings in beetle-killed forests are re-growing after attack.

The research is providing some key information on water cycling and forest regrowth to help predict how mountain pine beetle attack will affect water supplies in much of central and southern Alberta, and how long those effects are likely to last.

“Once the needles fall off, all of that precipitation is going to hit the ground and there will be a lot more runoff,” Silins predicted. The loss of trees also means they aren’t consuming the water, which adds to the problem. That excess water, along with premature snowmelt after beetle attack, will soak into the ground and increased flow from rivers and streams will be a likely side effect of the beetle’s destruction of pine forests.

Higher runoff will likely pick up sediments and organic compounds like carbon which could lead to water quality issues.

The earlier runoff may also decrease water supplies later in the growing season, and could flush too many nutrients into rivers — increasing algae growth and reducing the oxygen supply for fish — after these plants decompose during the winter, Silins said.

Last year, researchers at the U.S. Forest Service Fraser Experimental Forest in Grand County said they haven’t yet been able to definitively pinpoint a connection between dead lodgepoles and increased runoff. And they said rapid regrowth of understory vegetation could soak up any water not being used by dead trees.

“There’s no statistically significant study that shows (an increase in runoff),” said Kelly Elder, administrator of the Fraser facility. “It’s a really complicated system. We’re seeing a lot of climate variability. That’s another signal on top of the beetle kill,” Elder said.

Anecdotally, at least, long-time local ranchers see it differently. Grady Culbreath said one sign that the beetle kill may be affecting runoff are flows in late summer and fall in small streams that are usually dry by that time of the year.

“The ranchers often know what’s going on before anybody else,” said Elder. “When the trees die, they stop intercepting snow. A healthy canopy catches snow and there’s more loss back to the atmosphere through sublimation (snow turning directly to water vapor without going through a water stage). So there’s more snow on the ground. And when the trees are dead, less of it gets used,” Elder said.

On the other hand, new grasses, shrubs and trees are springing up in the beetle-killed zones.

“We have a very healthy understory,” Elder said, explaining that young lodgepoles are growing at phenomenal rates now that they are getting plenty of sunlight and moisture. And like Culbreath, he also pointed to the growth of aspens in dead lodgepole areas, explaining that the new trees are using at least some of the water previously absorbed by the lodgepole forest.

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