Balancing bioenergy potential and carbon sink resources of forests

Forests can help mitigate the effects of global climate change by removing and storing CO₂ from the atmosphere. In addition, forest materials or biomass can be used as a source of renewable energy that can replace the use of fossil fuels. Decisions about how to use forest resources should therefore consider the balance between the benefits of using forests to store carbon with the potential of forest materials to reduce fossil fuel use.

This study integrated a life cycle assessment of GHG emissions with modelling of forest carbon to determine the overall GHG balances from harvesting biomass and carbon sequestration in forests over a 100 year period. The method was illustrated by examining the use of standing trees and harvest residues (tree tops and branches) to produce wood pellets for electricity generation and ethanol for transportation from forests in Ontario, Canada. To avoid market conflict, the study only considered biomass which is not currently used for any other type of product, such as pulp. GHGs considered were CO₂, methane and nitrous oxide, which were converted into CO₂ equivalents.

Other studies using life cycle assessment of forest biomass as an energy source typically assume the process is carbon neutral: CO₂ is released immediately to the air from using the harvested biomass and this is balanced by carbon uptake from forest regrowth. However, since forest regrowth takes time, carbon stock replacement will be delayed and the impact of harvest on forest carbon stocks should be considered.
The study found that, in scenarios investigated, when the impact on forest carbon stocks is taken into account, the use of forest biomass to produce bioenergy causes an initial increase in GHG emissions. Even though bioenergy reduces emissions from avoided use of fossil fuels, the reduction is not enough to compensate for the release of carbon from forest stocks, which are not immediately replaced.

However, the increase in GHG emissions is temporary: in time, the rate of loss of forest carbon is reduced as regrowth occurs and emissions saved from the use of biomass energy are increased. Over a sufficiently long period of time, this leads to an overall reduction in GHG emissions when forest biomass is used to produce bioenergy (provided that other conditions remain constant, and ignoring alternative uses of biomass).

Under the scenarios studied, using harvest residues as pellets for partial substitution (20 per cent replacement) of coal in electricity generation is estimated to increase emissions of GHGs (compared with the sole use of coal) for 16 years before overall GHG emissions begin to be reduced. In comparison, when harvest residues are used to produce ethanol, the impact of removing forest carbon stocks result in a 74 year delay in overall GHG savings, i.e., emissions will be higher (compared to using oil) for 74 years, after which savings will begin.

When standing trees are used to produce wood pellets for electricity generation, GHG emissions are estimated to increase for 38 years before overall emission reductions are achieved. Under assumptions in the study, using standing trees for ethanol production does not reduce GHG emissions even over a 100 year period, compared with emissions from petrol.

Under the conditions in this study, using forest biomass to replace coal in electricity generation is more effective in reducing GHG emissions (i.e., it will increase emissions for a shorter period) than the production of ethanol. However, other factors, including economic viability, contribution to energy security and other environmental impacts (e.g. on biodiversity), need to be considered when making decisions about forest bioenergy production. As every forest area is different, variations must be considered in each analysis of forest carbon dynamics.

Source: McKechnie, J., Colombo, S., Chen, J. et al. (2011). Forest Bioenergy or Forest Carbon? Assessing Trade-Offs in Greenhouse Gas Mitigation with Wood-Based Fuels. Environmental Science and Technology. 45: 789-795.

Contact: hmaclean@ecf.utoronto.ca