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Carbon accounting of forest bioenergy and stand thinning

March 16th, 2012 by Rachel Perlman,

Article: Hudiburg, T.W. et al. (2011). Regional carbon dioxide implications of forest bioenergy production. Nature Climate Change: 1-5.

Background: Forests are valuable carbon sinks, since atmospheric carbon dioxide is taken up and stored as carbon in tree biomass (trunks, branches, foliage, and roots). However, tree thinning, which makes forests less crowded, is a strategy for preventing forest fires (which release carbon dioxide). Forests also can provide energy (as a biofuel) in the form of firewood, denser wood pellets, wood charcoal, wood-derived liquid fuel. Some strategies for reducing CO2 emissions include substituting forest biofuels for fossil fuels, which is assumed to have zero net emissions (i.e. carbon stored in growth of new tree biomass equals carbon emitted in burning).

Summary: Hudiburg et al. used LCA and inventory data from 80 forests on the US West Coast (California, Washington, and Oregon) to do carbon accounting analyses for current management practices, as well as for the following three forest management practices: (1) fire prevention by removing fuel ladders, (2) removing fuel ladders and enough marketable wood in fire-prone areas to be economically feasible (3) thinning all forestland to support energy production while contributing to fire prevention.

They found that in 90% of the diverse forest area studied, the carbon sink cannot be matched or exceeded by replacing fossil fuels with forest bioenergy. Thinning reduced net biome production in 90% of the region’s forest area. They show that the three scenarios lead to a 2-14% (46-405 Tg C) increase in carbon emissions over a 20-yr period. Forest bioenergy production could only reduce both fire risk and carbon emissions in the near future if forests in the region naturally became weaker sinks, trapping 30-60 g C/m2/yr less (due to insect infestations or higher rates of fires). At present, only three of the 19 ecoregions studied have high enough fire emissions that thinning for fire prevention and bioenergy would likely have a net emission savings.

CATF take-away message: Forests provide a wide variety of valuable services, and it is important to consider various management options in any given forested region by doing proper carbon accounting. Hudiburg et al. wrongly assume that biomass-based energy production is “carbon neutral” (an outdated assumption that neglects how the delay in re-sequestration affects climate, improperly credits bioenergy with sequstration that may have happened anyway, and fails to account for the possibility that regrowth (and thus re-sequestration) may not occur due to development decisions, etc.).  Nevertheless, the article still suggests that it is typically not in our best interests from a carbon emissions standpoint to use temperate forests for bioenergy, unless net biome production is weakened there; generally, more carbon will be stored by minimal harvesting.