Pine beetles contributing to forest smog, study shows
Beetle-infested trees release gases that form harmful aerosols in the atmosphere
Anyone who has seen the damage caused by the mountain pine beetle infestation, which has devastated millions of hectares of forest in B.C., Alberta and parts of the U.S., doesn't need any more evidence of the destructive powers of this pest.
In B.C. alone, 18.1 million hectares, or about five Vancouver Islands' worth, of pine forest have been attacked by the beetle since the current epidemic began in 1996. The province expects the infestation to eliminate 80 per cent of its mature lodgepole pine trees by 2013.
Early on, scientists found that the infestation was contributing to an increase in greenhouse gas emissions as the dead trees release carbon dioxide when they decay, turning the infested forests from carbon sinks to carbon emitters.
But now, new research shows that when the beetles bore into the bark of lodgepole pine trees, they also increase emissions of other types of gases that contribute to harmful air pollution in forested areas.
Smog alerts common in national parks
Chemists and atmospheric scientists at the Desert Research Institute in Colorado and Southern Illinois University Carbondale have shown that infested trees release as much as 21 times more volatile organic compounds, or VOCs, than healthy ones. These VOCs are known to create harmful particulate matter that makes up part of the atmospheric substance known as haze or smog.
Haze is a mix of gases, particulate matter, water, small amounts of metal and other substances that are released into the air as a result of biological and man-made activities, such as the burning of fossil fuels.
Although more familiar in large urban centres, haze is an increasingly common presence in many forested areas and parks throughout North America, where it impedes visibility, reduces air quality and can contribute to health problems such as heart and lung diseases.
Just this week, The Associated Press reported that Sequoia National Park in California, which is home to majestic, 3,000-year-old Sequoia redwood trees, has ozone levels comparable to those in urban areas like Los Angeles. The park regularly issues smog alerts when poor visibility makes hiking unsafe and warns park staff of the health risks of haze.
"What other people have shown is that the haze, if it contains particulate matter, the particulate matter … increases cardiovascular symptoms, it leads to asthma, it leads to premature death," said Kara Huff Hartz, assistant professor in the department of chemistry and biochemistry at Southern Illinois University Carbondale.
"The other issue is it's really closely tied into climate change and how the earth deals with the sun's radiation and how it reflects the light back into space or absorbs it in the atmosphere."
Forest devastation hit home
Hartz's colleague, Gannet Hallar, asked Hartz to help analyse the gases released from the bore holes that pine beetles make when attacking a tree after she witnessed first-hand the devastation the beetles had wrought on a cross-country ski area in Colorado that she had been visiting since childhood.
"They, unfortunately, had to clear-cut the whole area because all the trees died, and it was a fire danger for all the cabins," said Hallar, who runs the Storm Peak Laboratory at the Desert Research Institute in Steamboat Springs, Colo. "So, that was just shocking to me, to be honest; that had a big impact."
Hallar, an atmospheric scientist and research professor at the institute, knew about a previous study done by Danish researcher Marianne Glasius that had found a several-fold increase in emissions of volatile organic compounds in spruce trees infested with weevils.
She wanted to see if the same held for pine beetle-infested trees in the part of Medicine Bow-Routt National Forest in northwestern Colorado where her mountain-top atmospheric research facility is located and which is one of the areas hardest hit by the infestation.
She and her colleagues took samples of the gases coming from the resin that oozes out of the beetle bore holes of damaged tress and compared them with air samples taken near healthy trees growing at the same elevation and slope — although Hallar said they had trouble finding any healthy trees at all in the devastated forest.
They found that the concentration of VOCs was significantly higher near the damaged trees and even found evidence of the same compounds when they took so-called grab samples of air throughout the forest.
"What's really interesting about the study is we found the compounds even away from the tree in the background air," said Hallar. "Even above the forest, above the treeline, we still found those compounds."
VOCs react to form harmful aerosols
One compound in particular that drew the researchers' interest was β-phellandrene. Its levels were on average 35 times higher near the trunk of infested trees than at the trunk of healthy trees
"That's the one that was most predominantly found in comparison to the healthy tree, so [it] almost could be used as a signature, essentially," Hallar said.
"What Kara and I are really interested in is whether or not these compounds will allow for the formation of more particles in the air … and this is a compound that's known to do that."
VOCs released by the infested trees react with oxidants like ozone that are present in the atmosphere naturally at low levels but are enhanced by human activities. They form a secondary aerosol, a mixture of solid or liquid particles suspended in air that makes up one source of atmospheric haze and, in high concentrations, can be hazardous to human health.
Pine trees release some of these volatile organic compounds naturally — to signal other trees, as a defence mechanism to ward off predators, or as part of the process of photosynthesis.
"If you've been in a pine forest, that piney smell, those are the VOCs that you're smelling," said Hartz.
Normally, though, the gases are released at much lower concentrations and through the needles, not bark.
"This might be one of the reasons why the beetles affect the VOC emissions so much, because normally the emissions don't come from the trunk itself, but the beetle actually opens up the trunk and basically makes a port for those emissions to come out of the tree," Hartz said.
Quantifying these emissions, as Hartz and Hallar have done, can help scientists better understand how environmental stresses change the chemistry of the atmosphere and how to improve the models they use to make observations about air quality, visibility and climate.
"What modelers know is we haven't identified all the sources of particulate matter [in the atmosphere], so where this could be helpful to modelers is that they may say, 'Oh, look there's a source of particulate matter that we hadn't considered before. Let's try to include that in our model and see if our observations match better'."
Air quality insight could impact policy
"In the U.S., you have regulations pertaining to forest service land that require a certain amount of visibility — that you can see essentially from Point A to Point B clearly and that your visual experience isn't impacted by air quality," Hallar said. "So, one aspect of this is that these compounds would create more haze and thus reduce the air quality."
Having more information about what contributes to bad air quality could help parks tackle the problem and meet their legal obligations.
Hallar and her colleagues hope to incorporate their data into a wider model that would apply to pine beetle-infested areas across North America, although Hallar said she expects to find the same effect on VOC emissions in other beetle-infested areas as she did in the Colorado forest.
"With confidence, I would say it's happening in B.C.," she said.
The researchers who worked on the recent study, who also included scientists from the University of New Hampshire and Appalachian State University in North Carolina, were also hoping to repeat the original experiment, but the beetle infestation wiped out the pine trees in Colorado so completely that that won't be possible.
"I had a student hike for several days and could not find a healthy tree," Hallar said.
Instead, she and her colleagues will repeat the experiment in a controlled environment, where they will be able to observe how the emissions are affected by changes in temperature or humidity.