New study shows thinning forests for biomass is not a climate win-win

Demand for biomass is growing rapidly, as power companies come under increased pressure to find alternatives to fossil fuels like coal. (See NRDC's Forests not Fuel site.) One of the main sources they are targeting is trees harvested as part of wholesale thinning operations. Together with the forest biomass industry, they argue that using thinnings to fuel their power plants is an environmental win-win: good for forest management because, they assert, thinning reduces the risk of forest fires and good for the climate since trees can re-grow and are therefore a “carbon neutral” fuel source.

The pressing need for sustainable sources of energy make the industries’ claims tempting. But the results of a new report from researchers at Oregon State University strongly suggest that even if thinning needs to be done for non-climate reasons, there’s a price to pay in terms of climate pollution, not the win-win claimed by industry.

The study looks at the lifecycle carbon emissions impacts of different levels of thinning on forest plots in eastern and western Oregon. It finds that far from providing a “carbon neutral” fuel source, forest thinning increases net carbon pollution in the atmosphere for more than 50 years, even accounting for tree re-growth and the carbon emissions avoided when thinnings are used as biomass to displace fossil fuels. Carbon losses on-site account for the bulk of the effect of thinning on carbon.

These results hold for multiple kinds of thinning operations across a wide spectrum of forest locations and types in the Pacific Northwest. And while carbon stocks in the forest can, in time, rebound, it may be many centuries or longer before carbon stocks in a thinned forest catch up to one left unlogged.

And while the study doesn’t consider the impacts of thinning on fire, this factor would likely show further net emissions of carbon. Restoration thinning requires reintroduction of frequent low intensity fires. The total carbon emitted from this series of fires over time is likely to greatly exceed the carbon emissions from one intense fire every 200 years or so.

OSU’s findings are part of a growing body of science on the lifecycle impacts of biomass that points to the need for our bioenergy policies to distinguish amongst different sources of biomass. Some, like switchgrass grown on non-forested land and other short-rotation energy crops, can reduce carbon pollution or achieve carbon neutrality within 1 to 3 years. But others, like whole trees sourced from practices like thinning, will increase carbon pollution for decades or longer, and cannot be considered sustainable. (See our video on the carbon accounting for whole trees.)  And this is just looking at thinning. In addition, these operations risk degrading critical wildlife habitat, soil and water quality.

Unfortunately, both federal and state policies are failing to differentiate between the good, the bad, and the ugly when it comes to biomass, maintaining a blanket assumption that all biomass—even the worst kinds, like whole trees—are carbon neutral. With a rapidly growing bioenergy industry adding substantial new demand for biomass to the existing market for forest products, the pressures on our forests have never been greater.