Winter droughts in Colorado’s mountains stress trees, leave forests vulnerable to fire and insects, and provide less water for the state’s thirsty cities.
But low-snowpack years also slow the release of the greenhouse gas carbon dioxide from Western forests into the atmosphere, a new study shows.
“It’s a little bit of good news in a cloud of bad,” said Russell Monson, an ecologist at the University of Colorado at Boulder and lead author of a paper published in Nature today.
Snowpack levels have dropped significantly across the West in the last two decades, according to several studies.
“The same trend has been found in Europe, in the Swiss Alps,” Monson said. “It’s a Northern Hemisphere issue.”
In snow-rich winters, the snow insulates mountain soils, letting microorganisms thrive in the relative warmth below.
Those communities of microbes pump carbon dioxide out of the soil into the air, Monson and his colleagues found in the study, conducted at the Niwot Ridge Long Term Ecological Research Station west of Boulder.
In low-snow winters, soils were colder, microbes were less active and the researchers found about 25 percent less carbon dioxide in the air above the forests, according to the new paper.
The team took measurements for six years, with instruments mounted on 100-foot towers scattered among the trees.
Dave Schimel, an ecologist at the National Center for Atmospheric Research in Boulder, said the new paper may make scientists rethink the role of Western forests in global warming.
Many researchers assumed that in winter, high-altitude forests essentially shut down, neither contributing much of the heat-trapping carbon dioxide to the air nor pulling much out. In the summer, healthy forests tend to remove carbon dioxide from the atmosphere, using it to fuel their growth.
Now, it appears that winter is a significant player in carbon-dioxide dynamics, Schimel said, with forest ecosystems, depending on snowpack, sometimes releasing large amounts of the gas and at other times very little.
The effect is likely to be complicated, however, and will require more research to sort out, he and Monson said.
Low-snow winters mean less runoff, which stresses trees during the growing season. Those trees may grow less, pulling less carbon dioxide from the air.
So, winter’s reduced carbon-dioxide emissions may not always balance summer’s weaker carbon-dioxide removal.
Stressed forests also are more vulnerable to wildfires, which pump large amounts of the greenhouse gas into the atmosphere, the scientists said.
“The question is which of those effects will dominate,” said Schimel.
Staff writer Katy Human can be reached at 303-820-1910 or khuman@denverpost.com.
