A recent decrease in Rocky Mountain snowpack has slowed the release of heat-trapping carbon dioxide gases from forest soils into the atmosphere during the dead of winter, according to a new University of Colorado at Boulder study.
Professor Russell Monson of CU-Boulder's ecology and evolutionary biology department said the lack of snow has decreased the winter insulation of the soils, cooling them and slowing the metabolism of microbes that release large amounts of CO2. But the discovery of what Monson called a "serendipitous effect" of reduced snowfall does little to bolster the overall environmental health picture of the West's mountain forests in recent years, he said.
"I view this as a small amount of good news in a large cloud of bad news," said Monson, chief author of a paper appearing in the Feb. 9 issue of Nature. "While winter CO2 emissions from forest soils have slowed, the lack of winter moisture is stressing the trees during the spring and summer and inhibiting the much larger amount of CO2 they absorb during their growing season," he said.
Co-authors on the study included David Lipson of San Diego State University, Sean Burns, Mark Williams and Steven Schmidt of CU-Boulder and Andrew Turnipseed and Anthony Delany of the National Center for Atmospheric Research.
The study was undertaken at the Niwot Ridge Long Term Ecological Research site west of Boulder. The site is home to one of several dozen so-called AmeriFlux installations on the continent that measure CO2 activity. The Niwot Ridge AmeriFlux site features five towers studded with climate instruments that are funded by the U.S. Department of Energy, the National Science Foundation and the National Center for Atmospheric Research.
The researchers used the 100-foot-high towers -- which were erected at 10,000 feet in a forest of lodgepole pine, sub-alpine fir and Englemann spruce adjacent to CU-Boulder's Mountain Research Station -- to measure CO2, water and energy exchanges between the biosphere and atmosphere, said Monson. They used the instruments to zero in on the subtle, swirling winds drifting over the rugged terrain and took millions of individual CO2 data readings from 1998 to 2004.
"The deeper the snowpack, the more CO2 we observed leaving the forest," he said. "This forced us to look at the wintertime period more closely than before."
The researchers discovered a unique collection of microbes under the snow soils with life spans of only hours to days thriving at temperatures hovering around zero, Monson said. They used DNA fingerprinting techniques to show the winter microbe community was very different genetically from the summer microbe community.
Recent studies have shown that some mountain ranges in Europe and the Western United States, including parts of the Rocky Mountains and Sierra Nevada Mountains, have experienced 50 percent to 75 percent decreases in snowpack in recent decades, he said. The declining snowpack trend has been correlated with rising temperatures.
During several recent winters, decreased snowpack has caused the trees to go for extended periods during spring without the snowmelt moisture that normally tides them over until the summer monsoon begins in late July and August, said Monson. The environmental stress from lack of moisture not only inhibits the ability of trees to take up CO2 in the summer, but also makes them more susceptible to threats like insect infestations, diseases and forest fires, he said.
Separate research undertaken by researchers from the National Oceanic and Atmospheric Administration, CU-Boulder and NCAR indicate spring is now arriving up to a month earlier on Niwot Ridge due to warmer temperatures, Monson said. This puts additional stress on the trees, with the end result being that they absorb less CO2 over the course of a year, he said.
The measurement of CO2 transport in temperate mountain forests like Niwot Ridge is critical to understanding the bigger environmental picture, said Monson. "Most carbon sequestration occurs in hilly or mountainous terrain, and forests store more carbon dioxide than other ecosystems," he said. "The only way to understand what is truly happening with carbon dioxide in the Northern Hemisphere is with monitoring systems like the one at Niwot Ridge."
Levels of CO2 in Earth's atmosphere, blamed in part for rising temperatures in recent years, hovered around 250 parts per million for thousands of years. Since the Industrial Revolution, however, they have been steadily climbing and currently are approaching 380 parts per million. Atmospheric CO2 levels have been monitored on Niwot Ridge since 1968, providing the third longest record in the world.
The Nature study also involved four CU-Boulder graduate students and four postdoctoral researchers, as well as students from Colorado State University and NCAR. The research was funded by NSF, DOE and NCAR.
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