Study: Forest thinning increases carbon releases

Assumptions that forest thinning treatments help sequester carbon are not grounded in science — instead, recent research shows that thinning actually releases more carbon to the atmosphere than any amount saved by successful fire prevention.

The study acknowledges that there are many valid reasons to thin forests, but increased carbon sequestration is not one of them, according to scientists from Oregon State University.

“Some researchers have suggested that various levels of tree removal are consistent with efforts to sequester carbon in forest biomass, and reduce atmospheric carbon dioxide levels,” said John Campbell, an OSU research associate in the Department of Forest Ecosystems and Society. “That may make common sense, but it’s based on unrealistic assumptions and not supported by the science.”

Even in fire-prone forests, it’s necessary to treat about 10 locations to influence fire behavior in one. There are high carbon losses associated with fuel treatment and only modest savings in reducing the severity of fire, they found.

“There is no doubt you can change fire behavior by managing fuels and there may be other reasons to do it,” said Mark Harmon, holder of the Richardson Chair in Forest Science at OSU. “But the carbon does not just disappear, even if it’s used for wood products or other purposes. We have to be honest about the carbon cost and consider it along with the other reasons for this type of forest management.”

Even if wood removed by thinning is used for biofuels it will not eliminate the concern. Previous studies at OSU have indicated that, in most of western Oregon, use of wood for biofuels will result in a net loss of carbon sequestration for at least 100 years, and probably much longer.

In the new analysis, researchers analyzed the effect of fuel treatments on wildfire and carbon stocks in several scenarios, including a single forest patch or disturbance, an entire forest landscape and multiple disturbances.

One key finding was that even a low-severity fire released 70 percent as much carbon as did a high-severity fire that killed most trees. The majority of carbon emissions result from combustion of surface fuels, which occur in any type of fire.

The researchers also said that the basic principles in these evaluations would apply to a wide range of forest types and conditions, and are not specific to just a few locations.

“People want to believe that every situation is different, but in fact the basic relationships are consistent,”Campbellsaid. “We may want to do fuel reduction across much of the West, these are real concerns. But if so we’ll have to accept that it will likely increase carbon emissions.”