The 'debt' is in the detail: A synthesis of recent temporal forest carbon analyses on woody biomass for energy

The temporal imbalance between the release and sequestration of forest carbon has raised a fundamental concern about the climate mitigation potential of forest biomass for energy. The potential carbon debt caused by harvest and the resulting time spans needed to reach pre-harvest carbon levels (payback) or those of a reference case (parity) have become important parameters for climate and bioenergy policy developments. The present range of analyses however varies in assumptions, regional scopes, and conclusions. Comparing these modeling efforts, we reveal that they apply different principle modeling frameworks while results are largely affected by the same parameters. The size of the carbon debt is mostly determined by the type and amount of biomass harvested and whether land-use change emissions need to be accounted for. Payback times are mainly determined by plant growth rates, i.e. the forest biome, tree species, site productivity and management. Parity times are primarily influenced by the choice and construction of the reference scenario and fossil carbon displacement efficiencies. Using small residual biomass (harvesting/processing), deadwood from highly insect-infected sites, or new plantations on highly productive or marginal land offers (almost) immediate net carbon benefits. Their eventual climate mitigation potential however is determined by the effectiveness of the fossil fuel displacement. We deem it therefore unsuitable to define political guidance by feedstock alone. Current global wood pellet production is predominantly residue based. Production increases based on low-grade stemwood are expected in regions with a downturn in the local wood product sector, highlighting the importance of accounting for regional forest carbon trends. © 2013 Society of Chemical Industry and John Wiley & Sons, Ltd