The mountain pine beetle has caused the death of millions of acres of trees in western states ranging from New Mexico to British Columbia. In Colorado alone, the beetle is responsible for 1.4 million ha (3.4 million ac) of dead pine trees, as reported by the National Science Foundation. The tree deaths not only increase the risk of mudslides and wildfires, but are also changing the hydrology of western watersheds, according to an article released in Nature Climate Change in late April.

The beetle population was once controlled by colder temperatures. But in a changing climate, the beetles have increased in number and expanded their range. Further, drought has weakened the trees’ defenses.

The swaths of dead trees no longer evapotranspire or intercept rain or snow. Changing tree canopies are resulting in shallower snowpack and earlier snowmelt. These changes can alter the timing and magnitude of spring runoff, according to researchers at the Colorado School of Mines, who are leading the study.

The snowpack effects are coupled with a decline in water taken up by trees, which has increased local groundwater and flows in nearby headwater streams. In late summer, healthy trees use shallow groundwater for transpiration. However, in watersheds affected by the mountain pine beetle, research shows that late-summer groundwater flows are 30% higher than in watersheds with healthier tree populations. Changes in groundwater flows have led to changes in water quality. In late summer, researchers have observed disinfection by-product spikes in drinking water from beetle-infested watersheds related to increased dissolved organic carbon levels. Read more.