Claim That Mature Trees Stop Locking Carbon Into Wood Is Partially False

The claim holds that once a tree stops growing taller, the carbon it absorbs is redirected toward short-lived uses — foliage turnover and metabolic respiration — rather than being locked into wood for decades or centuries. The verdict is partially false. There is a kernel of truth buried in the framing, but the conclusion it drives is directly contradicted by multiple large-scale empirical studies.

The most decisive evidence comes from Stephenson et al. (2014, Nature), who analyzed 403 tropical and temperate tree species and found that the absolute rate of carbon accumulation into woody biomass increases continuously with tree size and age. A very large tree (over 100 cm diameter) locks in roughly 103 kg of carbon per year on average, compared to just 1.2 kg for a small tree under 10 cm diameter. Larger, older trees are not retreating from wood production — they are the most prolific per-tree carbon accumulators in the forest. Separately, Luyssaert et al. (2008, Nature) measured 519 old-growth forest stands globally and found they remain net carbon sinks averaging 2.4 Mg C per hectare per year, with carbon still flowing into woody biomass and soil. These are not young, fast-growing stands — they are the oldest forests on Earth, and they are still building wood.

The claim does have a legitimate scientific basis worth acknowledging. Ryan et al. (1997, Advances in Ecological Research) and Gower et al. (1996, Trends in Ecology and Evolution) both document that maintenance respiration rises with stand age, consuming a larger fraction of gross photosynthesis and reducing the efficiency with which carbon is converted into new growth. Chapin et al. (2011, Principles of Terrestrial Ecosystem Ecology) confirm that autotrophic respiration consumes roughly 50% of gross primary production across biomes. So the respiration cost is real, and the ratio of wood production to total photosynthesis does decline in older trees.

But the claim crosses from that partial truth into a false conclusion. Körner (2003, Journal of Ecology) argues that mature trees are not primarily carbon-limited, and that excess photosynthate is stored as non-structural carbohydrates or respired — but a substantial fraction still flows to wood increment even in slow-growing trees. Fatichi et al. (2014, New Phytologist) reinforce this: in mature trees, carbon allocation is sink-limited, meaning wood, respiration, and storage all compete, but wood allocation does not simply cease. The shift toward respiration is partial, not a wholesale redirection away from long-lived pools, as the claim implies.

The manipulation pattern here is a missing denominator combined with a false endpoint. Saying 'respiration takes a bigger share' is true but incomplete without noting that the total carbon budget of a large old tree is itself enormous — so even a smaller percentage going to wood still means more absolute kilograms of carbon locked away per year than a young tree achieves. The claim strips out the denominator and presents the declining efficiency ratio as if it were a declining absolute flow, which it is not.

Watch for this pattern in arguments that old-growth forests should be logged and replaced with young plantations on climate grounds. The underlying science — that older stands have lower net primary productivity per unit leaf area — is real but narrow. It does not support the conclusion that old trees are carbon-neutral or that their wood is no longer a meaningful long-term carbon store. When someone cites 'age-related productivity decline,' ask whether they mean relative efficiency or absolute accumulation. Those are very different things, and conflating them is where this claim goes wrong.