In 2008, Canadian researchers made a scary prediction: In our warming world, boreal forests would stop absorbing excess carbon and start contributing to climate change as soon as 2020.
What would cause this switch? The mountain pine beetles that have been eating their way though tens of millions of acres of alpine forests, leaving swaths of decaying trees in their wake. "When trees are killed, they no longer are able to take carbon from the atmosphere. Then when dead trees start to decompose, that releases carbon dioxide into the atmosphere," study author Werner Kurz of the Canadian Forest Service told the Associated Press.
That could exacerbate the global warming that contributed to the outbreaks in the first place. Warmer temperatures have allowed beetles to survive farther north, at higher elevations and make it through the winter.
"This is the kind of feedback we're all very worried about in the carbon cycle — a warming planet leading to, in this case, an insect outbreak that increases carbon dioxide into the atmosphere, which can increase warming," Andy Jacobson, a carbon cycle scientist for the National Oceanic and Atmospheric Administration in Boulder, Colo., told The Associated Press.
But wait! Before you stop reading this blog and complain to your friends about how depressing environmental journalism is, hear this: A new study suggests dead trees don’t release as much carbon into the atmosphere as previously thought. Yay! Now we can get back to worrying about China’s coal consumption, methane released from natural gas wells and deforestation.
It turns out that beetle-killed trees actually stop releasing carbon dioxide when they die, and don’t decay right away. Nicole Trahan, a University of Colorado researcher who co-authored the study, says that a large portion of the CO2 trees release has been recently taken in through photosynthesis. When trees die, they stop photosynthesizing, so respiration (the act of turning oxygen and sugar into water, CO2 and energy) stops, too. Living trees also put carbon into the soil, supporting a community of soil microorganisms. These soil microbe populations crash after the trees die, and no longer release CO2 to atmosphere.
To summarize: when trees die, less carbon dioxide is removed from the atmosphere, but less is released from the soil as well. (Want to know more about the study? Check out this handy video made by the researchers).
Carbon emissions do pick up a few years after the trees die, as needles start breaking down, but researchers found that was not enough to offset the loss of the trees and soil microbes respiring. Also, dead trees take a long time to decompose in the cold, dry climate of the Colorado alpine forests where Trahan did her research. So carbon will likely be stored in dead trees for a while, unless a big fire sweeps through and burns all the trees up (although contrary to conventional wisdom, fire is no more prevalent in beetle-killed forests than anywhere else, meaning dead trees will likely decompose before they burn).
In a related, unpublished study, Trahan and her research team also found that beetles kill selectively, meaning some trees survive the attack. Those that live actually do better after their neighbors die, because there are more nutrients, sunlight and water available. So they grow faster, off-setting CO2 released by decaying dead trees.
The findings are significant because they show how important it is to study predictions like the ones made by the Canadian team on the ground. The study should also dispel fears, like those expressed by NOAA scientist Jacobson, that bark beetle outbreaks are both caused by climate change and also make it worse. Trahan is quick to point out that her findings should not be applied to warmer, moister forests, where dead trees decay faster. But for alpine forests, she says beetle kill “will have less climate change implications than we originally supposed.” Phew!
Emily Guerin is the assistant online editor at High Country News.
Photo courtesy Flickr user vickie j