This issue was highlighted in a controversial 2010 report entitled “Biomass Sustainability and Carbon Policy” from the Manomet Center for Conservation Sciences. Its in-depth study of Massachusetts forests concluded that the greenhouse gas (GHG) emissions caused by burning trees creates an immediate “carbon debt” that lasts between 20 to 30 years before the carbon dioxide recycling by new living trees offsets that and creates “carbon dividends.”
Other researchers have expanded on that finding. In a 2012 study in Global Change Biology/Bioenergy, researchers concluded that ramping up forest bioenergy to even 20 percent of the global energy supply would increase greenhouse gas emissions and was unsustainable. One recent study, published in Energy Policy, asserts that many scientists have made a “fundamental error” in greenhouse gas accounting related to forest bioenergy – treating biogenic carbon as “neutral”:
“This line of thought makes a ‘baseline error’ because it fails to recognize that if bioenergy were not produced, (trees) not harvested would continue to absorb carbon and help to reduce carbon in the atmosphere,” the researchers note. “Because that carbon reduction … is counted in global projections of atmospheric carbon, counting bioenergy that uses this carbon as carbon-neutral is double counting.”
Other researchers and forestry experts say the Manomet study starts with flawed understandings about forest management: It focuses only on stands of trees that are harvested in any given year, ignoring stands that are not disturbed by harvesting and the carbon already sequestered by the trees. As a result, the model creates the false impression that forest carbon stocks are always depleted by harvesting, wrote William Straus, a director of Maine Energy Systems, in Biomass Magazine.
“Manomet got it backwards because it’s a timing issue,” says Strauss. “They begin carbon accounting the day the tree is harvested and carbon released.”
But trees have been taking CO2 out of the atmosphere and fixing it since they were seedlings, so they already have a carbon dividend or “credit” when they are harvested, Strauss says. Wood-to-energy from sustainably managed forests can provide net-zero carbon emission if the woody biomass stock is not depleted, says Strauss. In addition, forests are producing new growth every year. In a sustainably managed forest, only a percentage of that new growth is taken, so the stock of trees in never reduced. “You just harvest the ‘interest,’” he says.
One thing is certain: Ensuring that forest biomass actually lowers our carbon footprint – rather than increasing it – demands long-term research and planning.
In a 2012 analysis for Forests, for example, scientists investigated four scenarios for biomass harvest in the eastern U.S.: partial harvests of mixed hardwood forests, pine plantation management, short-rotation woody cropping systems, and forest residue removal.
Using validated models and past literature, they simulated the harvest of poplar grown on a plantation in Mondovi, Wisc., at either four or eight-year intervals over a 60-year period. If harvested every eight years, poplar was a net “sink” of carbon after about 25 years – that is, it served to lower carbon emissions that contribute to global warming.
If harvested every four years, however, the poplar plantation was a net source of carbon to the atmosphere over the entire 60-year period – that is, it raised carbon emissions that contribute to global warming.
The scientists found equally striking results for pine plantations. Using a landscape-level model developed for the southeastern U.S., they simulated wood production and net carbon stored in a pine plantation that used no fertilizer and was completely harvested at either 12 or 20-year intervals over the next 60 years. In the short-term, the pine plantation was a net sink of carbon. Unfortunately, if it were harvested regularly every 12 or 20 years, the plantation was transformed into a net source of carbon to the atmosphere.
And with other types of feedstock – including selective cutting in managed forests –the more intense the harvest, the greater the risk to the environment, the researchers found. Removing 10 to 17 percent of forest biomass during harvests allowed the forest to remain a carbon sink, researchers found, but harvesting 34 percent of the biomass led the forest to gradually become a source of carbon emissions. Still more troubling, removing 51 percent caused the forest to be a net source of carbon throughout most of the simulated 60-year period.
But there was a promising finding: the researchers also found that partial harvests of mixed-hardwood forests and wood residue removal resulted in greater carbon storage than plantation management and short-rotation cropping.
And in a model that reduced the amount of land managed for pine plantations to 17 percent of the abandoned land in the southeast, and by combining plantation harvests and short-rotation cropping with partial harvests of standing secondary hardwood forests, researchers found a sustainable solution. In this scenario, about 176 tons a year of wood biomass – representing 330,000 gigawatt-hours of electricity or about 16 billion gallons of liquid fuel – could be sustainably harvested and reduce carbon emissions in the temperate United States over a 60-year time frame.
The bottom line: Carbon accounting for forest bioenergy is complicated. A robust life-cycle analysis is critical to sustainability, and that analysis will be different for each forest.