KNAU and Arizona News
9:55 am
Wed July 7, 2010

Thinning trees to help slow climate change

Flagstaff, AZ – It seems counter-intuitive. But burning and cutting trees -in the long run can actually reduce carbon dioxide emissions.

"Thinning is very effective in preventing large losses of co2 to the atmosphere caused by these intense fires."

Tom Kolb is a professor of forest ecology at Northern Arizona University.

"The most detrimental impacts that we could have on climate change, is to allow large, forested landscapes to lose their capacity to store and take up co2, and these very intense fires will cause those losses to occur."

For the past several years Kolb has measured the exchange of carbon dioxide at three patches of forest around Flagstaff: a site that's been thinned of small diameter trees, one that's barely been touched for a century, and a third where the Horseshoe fire burned back in 1996. Kolb has found that right after the forest is thinned, it does release a bit more co2 than it absorbs.

"But 2-3 years after the thinning, the residual trees are growing so fast, that the annual carbon exchange is indistinguishable from a non thinned forest."

The big carbon savings come when a thinned forest prevents a catastrophic wildfire. In recent years those fires have made up as much as five percent of human caused carbon emissions.

Prescribed burns are another tool foresters use to keep ponderosa forests healthy, and to keep huge fires from happening. Now those intentional fires have also been found to have climate benefits. NAU forest ecologist Matt Hurteau has found as much as a 60 percent reduction in co2 emissions when prescribed fire replaces wildfire.

"The idea is when your forest is more open, you've aggregated the carbon in fewer, larger trees, so it's more resistant to high severity wildfire. That's like putting your money in a low risk account, where your yield is lower, but chances are you won't lose that money."

Whereas if you pack as much carbon as you can in an overly dense forest, you might store a little bit more, but if a wildfire hits, there will be enormous emissions of co2.

And Tom Kolb has found that wildfires continue to emit that carbon long after they've been extinguished.

"What we found is that even 10 to 15 years after that Horseshoe fire, that site still is not taking up co2 on an annual basis."

That's because a ponderosa pine forest can take a long time to regenerate. The grasses and shrubs that move in suck up a lot less co2 than big trees. And Kolb says it can take 100 years for the dead trees and roots to decompose, all the while slowly releasing more carbon dioxide.

The research is important as climate planners increasingly promote forests as giant carbon sinks to keep it out of the atmosphere. It shows it's not so simple as just growing trees sometimes you have to cut, or even burn them.