Is Biomass Carbon Neutral?

Evelyne Thiffault
February 25, 2010
Written by Evelyne Thiffault
For my last New Year’s resolution, I decided to visit schools to give presentations about trees and forestry. At my first outing, in a class of six-year-olds, I easily convinced the kids of the benefits of forests—clean air, stable soils, habitat for furry and feathery little things, etc. I then moved on to the products humans can get from trees, including biomass for bioenergy. That’s when Mrs. J., the teacher, interrupted to say, “Dear, we don’t need to cut trees anymore; we have all those wonderful substitution products to replace wood.” I looked at her, aghast, and then launched into a fumbling diatribe about greenhouse gas emissions and carbon footprints. I quickly realized that had I lost the toddlers’ interest after my third mention of “life-cycle analysis”.

Once I escaped from Mrs. J.’s class, I wondered if there was an easy answer to her comment. Is it sufficient to say that bioenergy from forest biomass is carbon neutral and hence good for the environment? Is there such a thing as carbon neutrality?

An interesting concept to consider is that of “carbon debt”, introduced by Fargione and collaborators in Science in 2008: The carbon (C) debt is the amount of C released from the soil and plant C stocks following the conversion of an area for bioenergy production. Over time, bioenergy from this area can repay the C debt if its production and combustion have net C emissions that are less than the life-cycle C emissions of the fossil fuels it displaces. For example, converting a peatland rainforest into a palm oil plantation releases huge amounts of C stored in the plants and soil of the native ecosystem, incurring a C debt that would take 423 years to repay. Thus, to have environmental benefits, a bioenergy product’s C debt should be small enough to be repaid on a timescale that avoids severe climate change effects.

What would be the C debt of bioenergy from Canadian forest biomass? Let’s take as a starting point a landscape of unmanaged boreal forest and convert it into a normalized managed forest, with regular harvests producing a sustained yield of forest products. This would take the landscape away from the natural distribution of stand age classes, lowering the proportion of older stands and increasing that of younger ones. This would decrease the amount of standing C present on the site. However, this C lost from the area is not annihilated; it is removed in products for further use. And the loss of on-site C happens just once. If managed properly, the new landscape should keep a relatively stable stock of standing C through time, albeit at a lower level than in the pristine landscape, while creating a flow of forest products.

A great unknown in this equation is the C loss from soil stores. This is critical to consider; for example, it represents close to 85% of the C debt in the case of peatland rainforest conversion to palm plantation. However, boreal forest conversion to managed forest is arguably a much milder form of ecosystem conversion. The managed forest still retains some characteristics of its native state such as soil C accumulation and storage. Also, in Canada, as in many Nordic countries, the greatest source of forest biomass for bioenergy will come from waste produced by harvesting for conventional wood products such as timber and pulp. Thus, the actual C debt allocated to bioenergy is less than the total amount of C lost from the conversion of the natural ecosystem; the other forest products should foot their part of the C bill.

So, is bioenergy from forest biomass carbon neutral? Probably not. Carbon lean would be a more appropriate expression. Woody bioenergy does not necessitate a dramatic conversion of a native ecosystem. If managed with sound forest practices, the new converted system can still offer some of the services provided by natural ecosystems—the clean air, stable soils, and habitat for furry and feathery things. And the bioenergy is produced from waste resulting from harvesting other products. Thus, when produced as part of a sustainable forest management strategy, carbon-lean energy from the forest can provide environmental benefits—this was acknowledged by the Intergovernmental Panel on Climate Change. Aha, now back to you, Mrs. J.

Dr. Evelyne Thiffault is a research scientist in forest biomass at Natural Resources Canada and provides Canadian Biomass with her thoughts on sustainable biomass harvesting.

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