Pine beetles' impact leaves the ground, affects the air
TheScience
CIRES researchers study how a pine beetle outbreak may affect gas emissions by trees, litter, and soil microbes.
some of the trees girdled a decade ago (they’re now dead); and others girdled just last year (stressed, but still alive).
It seemed like an ideal setup for studying how VOC emissions change as trees die, Monson said. “Joost had pioneered the development of an instrument that pretty much captures the whole range of VOCs, and he had the experience with similar data. We had the site, and Noah (Fierer) had a graduate student who was interested.” The three Fellows applied for and received an Innovative Research Grant from CIRES, and set up a proton transfer reaction mass spectrometer to collect emissions data from mountain forests during the summer of 2009.
CIRES graduate student Chris Gray is beginning to pore through the data. In theory, he said, one might expect VOC emissions from soils to spike up as trees drop needles and branches and microbes in the soil begin to decompose some of that litter. There’s some evidence, from other studies, that plants under certain kinds of stress emit more VOC. Within a few years, as the “easy-to-decompose” material is broken down, VOC emissions may drop back down again, Gray hypothesized. “But this is all kind of new research, so we’re not sure,” he said.
Monson said it’s still not certain that the current beetle outbreak is a result of climate change, “but there is strong reason to believe that trees that are more stressed are more vulnerable to attack by beetles.” He and others suspect that increasing temperature and decreasing moisture availability in the mountains are stressing the trees—and girdling clearly has stressed them, too, Monson said.
During the summer of 2009, he and a graduate student found beetles “zeroed in on the trees that had just been girdled... those trees acted like magnets.” 
A famous U.S. politician once quipped, “Trees cause more pollution than automobiles do.” The statement was widely derided, but it contained a kernel of truth: trees and the soil surrounding their roots do produce carbon compounds called volatile organic compounds (VOCs). VOCs can interact with anthropogenic emissions—nitrogen compounds from car tailpipes and power plant stacks, for example—to form lung-damaging ozone smog.
CIRES researchers are trying to understand how VOC emissions from Colorado’s high-country forests will change as mountain pine beetles invade, killing lodgepole pines and dropping needles and branches to the ground.
“When you have a significant part of the state overrun with beetles, as Colorado is, it’s going to affect the atmosphere,” said CIRES Fellow Joost de Gouw. “The question is, how?” De Gouw is working with CIRES Fellows Russ Monson and Noah Fierer to take advantage of a decade-long history of forest experiments at the University of Colorado’s Mountain Research Station near Nederland, Colorado. Mountain pine beetles are just beginning to arrive at the site, but Monson’s group has been working there for more than 10 years, trying to understand how climate change alters the carbon dioxide (CO2) cycle in forests.
Part of the ecological research has involved girdling groups of trees—removing wide strips of bark to slowly kill trees by breaking the link between their needles and roots. The idea was to distinguish between CO2 emissions by tree roots—which would be affected by girdling—and emissions by soil microbes, which would not.
Serendipitously, the research created a timeline of beetle-like disturbances, with
