On Earth Week--and in fact, every week now--trees in mountains across the western United States are dying, thanks to an infestation of bark beetles that reproduce in the trees' inner bark. Some species of the beetles, such as the mountain pine beetle, attack and kill live trees. Others live in dead, weakened or dying hosts.

In Colorado alone, the mountain pine beetle has caused the deaths of more than 3.4 million acres of pine trees.

What effect do all these dead trees have on stream flow and water quality? Plenty, according to new research findings reported this week.

Dead trees don't drink water

"The unprecedented tree deaths caused by these beetles provided a new approach to estimating the interaction of trees with the water cycle in mountain headwaters（源头） like those of the Colorado and Platte Rivers," says hydrologist（水文学者） Reed Maxwell of the Colorado School of Mines.

Maxwell and colleagues have published results of their study of beetle effects on stream flows in this week's issue of the journal Nature Climate Change.

As the trees die, they stop taking up water from the soil, known as transpiration（蒸发，散发）. Transpiration is the process of water movement through a plant and its evaporation from leaves, stems and flowers.

The "unused" water then becomes part of the local groundwater and leads to increased water flows in nearby streams.

The research is funded by the National Science Foundation's (NSF) Water, Sustainability and Climate (WSC) Program. WSC is part of NSF's Science, Engineering and Education for Sustainability initiative.

"Large-scale tree death due to pine beetles has many negative effects," says Tom Torgersen of NSF's Directorate for Geosciences and lead WSC program director.

"This loss of trees increases groundwater flow and water availability, seemingly a positive," Torgersen says.

"The total effect, however, of the extensive tree death and increased water flow has to be evaluated for how much of an increase, when does such an increase occur, and what's the water quality of the resulting flow?"

The answers aren't always good ones.