Forest Carbon Assumptions

Recently CORRIM published their Trillion Trees Report which aims to flesh out the concept, and advocates for using timber as carbon storage. Unfortunately, right at the beginning of the report the bias of CORRIM is displayed with three assertions that are used to justify the remainder of the report:

  • the rate of a forest’s growth declines with age as more suppressed trees die, reducing the net rate of CO2 removal as the forest matures (Gholz, 1982; Lippke et al., 2011a; McArdle et al., 1961).
  • Carbon (C) in wood from dead forest trees and other organic matter combines with O2 by decomposing or burning to re-form CO2 that is released back to the atmosphere (Oneil & Lippke, 2010).
  • Consequently, in aging forests, CO2 sequestration rates slow down and may even release more CO2 than they are removing, especially if fires occur. In effect, growing forests provide a one-time reduction in CO2, not a sustained reduction.

Now what is really interesting is that when we look at the literature there is a divergence between climate scientists who posit that forests continue to sequester carbon with age, and forestry researchers who claim sequestration rates peak and decline within 100 years. Obviously this has huge implications for forestry management, and EPDs for timber based products. This is a serious discrepancy that has important implications for policy development. Deforestation accounts for 15% of global emissions and it is imperative that we get the balance right.

This difference appears to me to have arisen due to differences in methodology, climate scientists use Eddy-Covariance to measure carbon flux over large areas, whereas forestry research measures the total board-feet of saleable timber within individual stands and tries to extrapolate from there. Thus forestry research deliberately excludes biomass from the understorey and below ground. This paper attempts to reconcile this discrepancy.

Leaving aside that logging practices increase risks of fire and infestation, the figures used in the CORRIM report appear erroneous, the report references Lippke et. al. 2014 which claims that forest sequestration reaches a maximum of 200 TCO2-e/ha, however in surveys conducted in the 1970s Waring & Franklin, 1979 measured old growth forests in the same study region had biomass densities between 600 to 3,500 tons/ha - so roughly 300 to 1,750 TCO2-e/ha.

Forestry research also tries to impose limits on forest carbon sequestration on the basis of limited nitrogen availability, completely ignoring the nutrient exchange of nitrogen and carbohydrates between mycorrhizal networks and trees. Established and old growth forests can be thought of as giant carbon pumps that are continuously pushing carbon into the soil. It should also be recognized that these ecosystems take centuries to establish and cannot be simply replanted with genetically limited stock in a matter of a few decades.

Bellassen and Luysseart, 2014 provide a great summary and pragmatic analysis of the situation. In the meantime, the following should be made very clear:

  • Biogenic sequestration must include consideration of the feedstock of timber, and conversion of old growth forests would result in vastly increased embodied carbon of timber products. This also needs to consider the proportion of timber that makes its way into long term use compared to short term products / fuel, I have seen a figure of only 20% of logged timber is used for long term products. Other affects of logging operations (road clearing, degradation, etc.) should also be included.
  • FSC actually allow old growth timber to be certified as sustainable when there is no other economic opportunities for the local people, this is a perverse extension of the colonial mindset, effectively the west can extract resources from peoples that it has severely disadvantaged.
  • Reliable supply chain information is absolutely imperative and specifiers should require that timber products are only ever sourced from sustainable plantation forests (or better yet salvaged timber).
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Thanks Will. You make some good points. The Trillion Trees proposal has been around for a while. The Sierra Club had some discussions about it over a year ago and declined to support it. It does not distinguish between proforestation, aforestation, and reforestation. Proforestation is the process of encouraging forests to become more mature. Aforestation is putting forests where they were not previously. Reforestation is replanting forests where trees have been removed. Proforestation is one of our top terrestrial carbon sinks. It was good to see this acknowledged recently at the COP26. There is concern that the Trillion Trees proposal is open to abuse from the timber industry. They do want credit for plantation reforestation as well as credit for not converting land use. Their arguments about old growth carbon versus young tree carbon undermines regeneration and proforestation. If you think about the volume of a tree ring, it is much larger in older trees. However, the cubic meters of consumer product is far from the full story. Ecosystems and their services are responsible for large scale carbon sequesterization. I liked your points about soils as well. There is a lot of acknowledgement about blind spots in LCA and this is one of them. I see the timber industry as undermining the legitimacy of WBLCA and other EPDs. Their mandate is to get out the cut. The 30x30 Proposal has the potential to sequester vast amounts of carbon banks and to promote proforestation. Water and quality of life are much bigger economic engines than industrial extraction, especially on public lands. The CORRIM Report is exactly the type of industrial abuse that makes the Trillion Trees proposal difficult to support. I am also uncertain as to how the Million Trees effort in Ethiopia actually went down. However, much of Ethiopia used to be forested.

My apologies for not making a scientific contribution with this comment: but we really need to make clear the issues with CORRIM’s report (and others who have made similar claims) and do so prominently!! As an architect I see wood promoted everywhere. Now is the time to get better forestry practices in place if we really do plan to use more wood.

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Hi Sara, I think CORRIM need to state their assumptions and the limitations of the research very clearly. In my view their conclusions are only applicable to plantations in the USA. Timber sourced from elsewhere is unlikely to have the same carbon dynamics.