The question of old growth forests’ carbon storage capacity and climate change

January 22, 2024
Barbara Steinhoff

Earthroots has been pursuing old growth forest protection on the grounds of carbon sequestration and climate change for a long time and have heard all the arguments. Gord Miller, Chair of Earthroots explains the science and the arguments used to obfuscate this issue.

The confusion on the topic of old growth forest carbon sequestration is cultivated on the difference between the rate of sequestration of carbon (per unit area) of different forest stages of development and the total standing inventory of sequestered carbon (per unit area). The ecological concept involved is something called the P/R ratio of ecosystems. The P is photosynthesis which is a measure of the amount of carbon fixed from the atmosphere in molecules (initially glucose) and forms the energy source to build and power the ecosystem. The R stands for respiration which is a measure of the carbon released by the ecosystem when it utilizes that energy by oxidizing the carbon compounds. The ecological concept is that as ecosystems develop and become more complex the community P/R ratio increases and approaches 1 (which would mean no more net carbon is being fixed). 

It is true that in early stages of development forest stands fix much more carbon in the living tissue (in wood and foliage) through photosynthesis per unit time than they release as they respire. P/R >> than one. And, in old growth stands the community respiration is much higher in the decaying accumulating forest residues so the net accumulation of carbon is less so P/R approaches one. But it is important to note that measurements show that while the P/R ratio of old growth is higher, it is never equal to 1. Old growth continues to sequester carbon in the system, it’s just trapped as organic soil and woody debris as well as the plethora of birds, insects, crustaceans, fungi, mosses, algae, and microbes that form the total forest biomass.

So in the first instance the forestry industry uses this concept to form the argument that cutting the old growth and replacing it with young commercial trees is climate positive because the P/R ratio of the young growth is so much more favourable to carbon sequestration than old growth. This argument is fatally flawed, however, because it ignores the immense difference in assembled community biomass between young and old growth systems. When you cut (euphemistically ‘harvest’) old growth systems you open to oxidation the organic soils and massive biomass accumulated over centuries, thus releasing vast amounts of carbon dioxide. New forests have little of such community biomass. And, replacement amounts will take decades to begin to accumulate int he new forest stands and centuries to truly be ‘replaced’. And, the one thing we don’t have in the global warming situation is many decades of time. Releases of forest carbon in the near term substantially undermine societies' current efforts to rein in fossil fuel emissions by 2050.

The forestry industry also argues that this biomass oxidation is limited because the wood they extract goes into dimensioned lumber which in turn becomes structures and furniture. Surely, they contend, such long lived uses of this material amounts to some level of sequestration. This argument is equivalently fallacious. It ignores the fact that bark, sawdust and planning wastes exist in lumber production and it again totally discounts the magnitude of ecosystem damage involved in extraction of the wood.

The essential point with respect to carbon management is not to focus on the fact that forests sequester carbon (which they do at various rates at various stages of development). The need is to be concerned about the standing biomass of carbon sequestered in old growth stands which is carbon dioxide that has been out of the atmospheric system for centuries and can and should remain so. In this respect they are like fossil fuel deposits.




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