Rocky Mountain National Park
April 22, 2014

Bark Beetle Infestations Affect Water Quality, Stream Flows

April Flowers for - Your Universe Online

An infestation of bark beetles is killing trees in the mountains across the western US. The beetles all reproduce in the inner bark of the trees, though they kill the trees in different ways. For example, the mountain pine beetle attacks and kills live trees, while other species live in dead, weakened or dying trees. In fact, more than 3.4 million acres of pine trees in Colorado alone have fallen to the mountain pine beetle.

Researchers question what effect this massive tree die-off has on stream flow and water quality. According to a new study published in Nature Climate Change, the effect is pretty big.

"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 like those of the Colorado and Platte Rivers," hydrologist Reed Maxwell of the Colorado School of Mines, said in a recent statement.

Dying trees affect the water cycle in a variety of ways. For example, living trees take up water from the soil, move it throughout the plant, and evaporate it from the leaves, stems and flowers, in a process known as transpiration. Dying trees cease transpiration, leaving more in the local groundwater system and leading to increased water flow in nearby streams.

"Large-scale tree death due to pine beetles has many negative effects," said 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."

"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?"

Unfortunately, the answers aren't always good.

As trees move from a healthy green phase (using shallow groundwater) to a red- or gray-phase (caused by the beetle infestation), transpiration stops. This leads to higher water tables and greater water availability for groundwater to flow into streams.

After beetles infest and affect a watershed area, the amount of late-summer groundwater flowing into the streams is approximately 30 percent higher than normal, according to the new study.

"Water budget analysis confirms that transpiration loss resulting from beetle kill can account for the increase in groundwater contributions to streams," according to Maxwell and his colleagues Lindsay Bearup and John McCray of the Colorado School of Mines, and David Clow of the U.S. Geological Survey.

"Using 'fingerprints' of different water sources, defined by the sources' water chemistry, we found that a higher fraction of late-summer streamflow in affected watersheds comes from groundwater rather than surface flows," says Bearup.

"Increases in stream flow and groundwater levels are very hard to detect because of fluctuations from changes in climate and in topography. Our approach using water chemistry allows us to 'dissect' the water in streams and better understand its source."

With millions of dead trees, adds Maxwell, "we asked: What's the potential effect if the trees stop using water? Our findings not only identify this change, but quantify how much water trees use."

Bearup adds that a crucial implication of their study is the knowledge that the change can alter water quality.

The new findings support prior research by Colorado School of Mines researchers. "That research found an unexpected spike in carcinogenic disinfection by-products in late summer in water treatment plants," said Bearup, who noted that the water treatment plants were located in beetle-infested watersheds.

The study was funded by the National Science Foundation's (NSF) Water, Sustainability and Climate (WSC) Program.