The bark beetle feedback loop


Trees, you might say, are nature's ultimate do-gooders. A compound in the bark of Pacific yew trees fights cancer. Dead trees become nurse logs, nurturing forests' next generation of fungi and vegetation. In the ocean, rotting leaves boost the growth of plankton, fortifying the foundation of the sea's food chain. Living leaves scrub the air of the little nasties we humans like to overload it with: throat-irritating particulates and nitrogen oxides; world warming carbon dioxide. They are so skilled at sucking up CO2 that forests generally act as carbon sinks -- absorbing more CO2 than they put out, thereby tempering the effect of the climate changing gases we emit. 

In Western North America, though, bark beetles -- which proliferated during a series of warm winters -- have been steadily nibbling away at forests' ability to deliver this service. A few years ago, scientists predicted that by 2020, the beetles' tree killing spree in Canadian forests would have grown so prolific that the forests would become net carbon emitters. (When trees die, they not only stop absorbing CO2, they release what they've sequestered.) In other words, instead of mitigating climate change, they'd begin contributing to it. 

"This is the kind of feedback we're all very worried about in the carbon cycle," Andy Jacobson, a National Oceanic and Atmospheric Administration carbon cycle expert told the Associated Press in 2008. "A warming planet leading to, in this case, an insect outbreak that increases carbon dioxide in the atmosphere which can increase warming."

Now, new research out of British Columbia has delved deeper into the climate impact of all those dead trees, finding that summertime temperatures in beetle-killed B.C. forests have risen by about 1 degree Celsius, or about 1.8 degrees Fahrenheit. That's because live trees help to cool their local environment: "Trees sweat to help cool themselves in the same way that humans do," Holly Maness, an author of the study, told the Montreal Gazette. That "sweat" evaporates -- a process technically termed evapotranspiration -- moderating summer heat. "When you kill a tree, it's going to stop sweating. That means that solar radiation that was previously spent evaporating water from those trees is now going into heating the surface." 

The cascading effects of this uptick in temperature on local weather patterns and on things like the timing of spring runoff are, as yet, uncertain, according to the Gazette. "We've shown that the surface temperature increases that we've seen are sufficient to drive changes in cloud cover and precipitation, but it needs to be directly measured," Maness said.

Lest you finish this blog feeling totally depressed about the trajectory of our planet, consider what the study reinforces: the simple values of trees that we often take for granted. It says that there's something to programs in cities like L.A. to increase their urban canopy in anticipation of a warmer future, which to the the cynics among us might at first blush seem like mere feel-good stunts. Recognizing all that we're losing to the beetles also allows us to see what we have to gain by protecting the forests we still have and increasing tree cover in the places most of us live. Perhaps that age old tenet of DIY environmentalism -- to save the planet, plant a tree -- wasn't so dewy eyed after all. 

Cally Carswell is HCN's assistant editor. 

Photo: Beetle kill in Montana, courtesy flickr user LostDogPhotos, licensed under Creative Commons. 

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