Do more pine beetles mean more groundwater? Perhaps—but that's not necessarily a good thing, say the authors of a new study.

The bark beetle outbreak that has plagued North America's Rocky Mountains is having significant ripple effects on the region's hydrology, researchers from Colorado's School of Mines in Golden, Colo., report in a study published this week in the journal Nature Climate Change.

Compared with a watershed where a beetle infestation's impacts were less intense and occurred less recently, a watershed with more beetle-killed trees absorbs about 30 percent more groundwater, the researchers found.

"A 30 percent increase in groundwater is substantial, particularly in the late summer when other contributions are low," said lead author Lindsay Bearup, a doctoral candidate at the School of Mines' Hydrological Science and Engineering Program.

However, Bearup cautioned that it's too early to celebrate this side effect of an otherwise ecologically devastating trend.

"From a quantity standpoint, it's obviously not a bad thing," Bearup said, but she added that this is perhaps a simplistic way to look at her study's outcome.

"As we're changing where the water comes from, it can change the quality as well," she said.

Dead trees don't take up water
Above-average temperatures caused the "perfect storm" of conditions leading to the loss of millions of acres of Colorado's forests, explained paper co-author Reed Maxwell, who directs the School of Mines' Integrated GroundWater Modeling Center.

Winters aren't getting cold enough to kill off the beetles' larval population. Longer summers have lengthened the insect's reproductive seasons. And recent research has also found that drought could be a factor in lowering trees' defenses to the beetles (ClimateWire, Oct. 11, 2013).

Aerial surveys by the Forest Service found that the mountain pine beetle alone has affected about 3.4 million acres in Colorado since the outbreak started in 1996.

The 2013 survey found that areas affected by mountain pine beetles spread by only 8,000 acres, a sharp drop from 31,000 acres in 2012 and 140,000 acres in 2011. But this isn't because conditions are getting better — it's because the beetles have eaten themselves out of house and home, having destroyed the majority of mature lodgepole pine trees. Spruce beetles seem to have moved in to eat the remainders, exploding in population to affect 216,000 acres of Colorado's forests in 2013.

To learn how such a dramatic ecological change might affect groundwater supplies, Bearup examined two watersheds in Colorado's Rocky Mountain National Park with similar geology, soils and mix of tree species.

Although both watersheds had been hit with a pine beetle infestation, in one watershed the impacts were less recent and less dramatic.

"The beetle's impacted most of the forested area in Colorado that's lodgepole pine," Bearup said. "Finding something unimpacted is really unrealistic, so lower impact is really the best you can do."

Bearup's hypothesis was relatively simple: With fewer trees around to drink up groundwater with their roots, streams in a watershed that had seen a bigger, more recent beetle outbreak would end up with more groundwater in them.

But confirming this hypothesis was a little more difficult. Bearup traveled around Rocky Mountain National Park taking samples of rainwater, snowpack, groundwater and streams at various spots throughout the two watersheds. These different water sources have different combinations of hydrogen and oxygen isotopes, so these samples provided Bearup with "fingerprints" of where the water in the streams had come from.

In late summer 2012, when trees are most dependent on groundwater, Bearup took samples of stream water in the two different watersheds. She found that in the watershed that experienced a later and more severe mountain pine beetle outbreak, groundwater made up a significantly greater proportion of the stream water's source.

Is water quality impaired?
Bearup said that her study was one of the first to use the isotope and chemical "fingerprinting" method instead of simply examining changes in peak stream flows to learn how a bark beetle infestation affects a watershed.

Like Bearup, co-author Maxwell also said it is too early to jump to any positive conclusions: "It's probably true that there is more groundwater in these mountain headwaters," he said. "How much is that really a boon to water resources? That's very, very hard to say."

In fact, the study was motivated by a paper published in Nature Climate Change in October of 2012 that found that Colorado water treatment facilities in beetle-impacted areas dealt with higher organic carbon concentrations in their source water — along with significantly more disinfection byproducts in the drinking water, which can be harmful to human health.

More research is needed to support the hypothesis that the increase in groundwater flowing into streams is leading to more disinfection byproducts in Colorado's water supply, Bearup cautioned.

But Maxwell said this research is yet another lesson in the unintended and sometimes surprising consequences of climate change.

"In some ways, when you have this land cover change at an unprecedented scale and all the trees of a certain type are killed, it's a very interesting kind of experiment," Maxwell said. "... You wouldn't necessarily think that there's going to be different carbon in the system because all these trees are dead so the natural bio-ecological, geochemical systems are disrupted."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500