Studies that suggest extended timeframes for carbon debt repayment make assumptions in direct conflict with how wood pellets are produced. These errors include claims that forests are slow growing, have high carbon stocks, and are managed and harvested strictly for bioenergy. In reality, raw materials used in the manufacture of wood pellets are sourced from harvest residues and low value timber in forests that grow relatively quickly.
Bioenergy Does Not Drive Forest Management
Papers that posit wood biomass creates a carbon debt, such as the infamous Manomet Center study, incorrectly assume forests are harvested for bioenergy. However, markets for high-value products like sawtimber (used to produce lumber) are the main drivers behind forest management and harvests.
During these harvests, the portions of trees that cannot be used to make boards (limbs, tops, bark, etc.) as well as crooked trees or those riddled with knots make up the residues that would otherwise be left to decompose on the forest floor. This harvest residue is produced regardless of whether or not a market for otherwise unmerchantable material exists. The fact that forestland owners can now sell a portion of this material provides further economic incentive for them to keep their forests as forests rather than convert their acreage to uses such as development or marginal cropland.
The argument that increased demand for bioenergy could lead to harvests in otherwise unmanaged forests, leading to more greenhouse gas (GHG) emissions over time, is extremely unlikely given the economic considerations of forestland owners discussed above. Although it may seem counterintuitive to think that harvesting trees could ultimately reduce GHG emissions, the ideal scenario is to do just that by opening markets for higher value forest products.
When a tree is harvested, it no longer absorbs carbon dioxide from the atmosphere, but it also does not immediately release the carbon it has sequestered. Wood products act as carbon stores in the form of lumber, furniture, paper and a number of other products. In addition, wood is a preferable substitute to carbon-intensive products like concrete and steel. Harvest residues that can be converted to produce energy are simply icing on the cake, so to speak.
Forest Growth Rates & Mature Carbon Stocks
Multiple scenarios considered throughout the AEBIOM report demonstrate zero to very little time must pass before the use of wood biomass results in net GHG reductions. On the other hand, studies that assume wood pellets are sourced from whole trees in slow growth forests like Canada’s boreal forest suggest carbon debt repayment times of 100 years or more. Not only does such an analysis fail to recognize an entire forest is not harvested for biomass purposes, it also neglects to consider regional variations (climate, soil, etc.) that affect forest growth rates.
In the Southeastern United States, pine is harvested approximately every 25 years and hardwood every 50 years. Accordingly, results that assume 100-year-old whole trees are harvested for use as wood pellets have no basis in reality and therefore cannot be considered a logical argument in carbon accounting.
Unlike purely fictional scenarios, the growth-to-drain (or growth-to-removal) ratio provides a method to consider carbon debts in light of forest sustainability. Measured as the annual growth in forest inventory divided by the annual volume removed, a growth-to-drain ratio equal to or greater than 1.0 indicates wood biomass is sourced from a sustainably managed forest. Because such forests sequester more carbon through growth than is extracted through harvest, carbon stocks are maintained or increase over time, creating zero carbon debt.
As these examples illustrate, counterfactual arguments – the expected future scenarios if wood biomass is not used for bioenergy – can lead to very different assumptions of future carbon debts. Studies that assume a “continued growth counterfactual” simply do not apply to forests managed to produce timber and pulpwood; these management plan never anticipate trees will continue to mature until they die out naturally.
The report asserts accurate counterfactuals recognize the need forest owners have to receive ongoing economic benefits from their lands. Careful consideration of both regional and market trends are needed to develop appropriate counterfactual arguments. In addition to growing conditions and existing and future forest products markets, these considerations include forest management practices, the policy environment and the potential for natural disturbances.
Results based on assumptions that do not correspond with common best practices are representative neither of the forestry nor bioenergy industries. Let us not forget that industrial wood pellets do not fill an energy vacuum. Instead, they replace the highly carbon-intensive coal presently used in power plants. In and of itself, this substitution results in fewer greenhouse gas emissions.
Carbon accounting is a complex concept, and a system built on anything other than forest science and economics is doomed to be inaccurate. When objective facts rather than subjective, emotional opinions are considered, it becomes apparent the emerging wood bioenergy market can give rise to better-managed and healthier forests, higher land values, and increased carbon sequestration.
The full report, “Forest sustainability and carbon balance of EU importation of North American Forest Biomass for Bioenergy Production” is available via the European Biomass Association website.