Biogenic carbon & underlying assumptions

I came across this report from Arup last night:

Within it makes a claim that managed forests increase carbon sequestration over natural forests, and makes reference to another Arup report “Rethinking Timber Buildings”, 2019 (available here). Wherein I can find one reference to a paper that asserts that modelling finds that a managed forest sequesters more carbon than a natural one - Greenhouse Gas Balance of Native Forests in New South Wales, Australia. This paper, upon which the whole house of cards seems to rest, was produced by the NSW Department of Primary Industries (i.e., the logging department), and when you look at the forecasts it becomes immediately clear that their estimation of maximum carbon in a forest is wildly underestimated at around 130 tCO2/ha.

More recent (and dare I say more rigorous) data is available from this summary by Keith, Lackey and Lindenmayer wherein the lowest value for above ground biomass in NSW forests is 182 tC/ha, and 252 tC/ha when you include the below ground biomass. In some NSW forests the values are as high as 447 tC/ha above ground and 615 tC/ha total. Converting to tCO2/ha yields a minimum above ground value of ~670 tCO2/ha.

Below I have included the summary box from the report - obviously there are some methodological flaws, and it is difficult to reconcile what the true impact is, but I’m curious what the implications are when the evidence does not really support the underlying premise. @raphael.sperry and @frances.yang I’m especially curious to hear your thoughts.

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Hi Will! Thanks for diving into this in such detail! Whenever I’ve tried to look at forest stored carbon previously I’ve ended up having to assume that those static carbon values e.g. tC/ha, are for a mature steady state forest. Then trying to find the growth rate curves for that forest to get it in tCO2/ha/yr.
So when you have a steady state forest it isn’t necessarily increasing its store of carbon but through forestry you can constantly keep the forest in the growth phase.
I’m approaching this purely from a carbon lens here but I would also hate for us to start managing every forest on earth to counter our fossil fuel addiction, particularly based on flawed carbon accounting!

Except that there is no evidence that forestry can constantly keep the forest in growth phase.

Trees produce carbohydrates that are then dispersed into the soil, larger trees sequester more carbon than smaller trees, and by cutting down a tree it takes decades to catch up to the carbon sequestration rates, during which time the carbon emitted is warming the planet. You may want to read this report from the WRI: Wood Is Not the Climate-friendly Building Material Some Claim it to Be

Great, thanks for the link. I don’t claim to be an expert in this but I will endeavour to educate myself