A New Wedge in the Carbon Equation

Research published in New Phytologist in January 2026 finds that photosynthesis does not reliably translate into wood growth — a gap that matters directly for how much carbon forests will actually store as the climate shifts.

The finding challenges a cornerstone assumption in climate strategy: that more CO₂ in the air drives faster photosynthesis, which drives more woody tissue accumulation, which locks away carbon for decades. The study questions that chain at its crucial middle step. Trees take in carbon through photosynthesis, but much of that carbon gets spent on respiration (the energy cost of living), reproduction, root exudates, defence compounds, and stored starches. Only the surplus becomes structural wood — the part that actually stores carbon long-term. The New Phytologist paper formalizes what tree physiologists have suspected for years: the link between what a canopy captures and what a trunk adds is far messier than most large-scale models assume.

William Anderegg's group at the University of Utah had already flagged this tension, noting that tree growth can stall even when photosynthesis runs normally — limited instead by cold, drought, or lacking nutrients. The 2026 findings extend and sharpen that insight.

A 2023 study in Nature Climate Change estimated that rising CO₂ has boosted global terrestrial photosynthesis by roughly 13.5% — or about 15.9 billion tonnes of carbon — since industrialisation began. If a substantial portion of that extra photosynthetic activity is not becoming durable wood, then projections of how much carbon forests will absorb may be inflated.

The Carbon Credit Problem

Forest carbon credits are financial instruments: companies and governments pay to plant or protect trees on the promise that those trees will soak up a measurable amount of CO₂ from the air. The logic rests on being able to calculate — reliably — how much carbon a forest will store. If photosynthesis and wood growth are not tightly coupled, that calculation becomes shakier, especially as regulators scrutinise voluntary carbon markets more closely.

A Harvard Forest study examining how management and disturbance shape forest carbon found that the carbon trajectory of any forest depends heavily on the choices foresters make, not simply on the presence of trees and rising atmospheric CO₂.

Tree Planting Dominates the Narrative

Research published in Nature Communications Earth & Environment in April 2025 found that tree planting has become so culturally prominent as a climate fix that it may be crowding out other carbon-removal methods — what researchers call the "flagship effect." This matters because durable carbon removal almost certainly needs a portfolio of strategies, not reliance on a single approach.

Urban trees occupy a somewhat different position. A 2025 Scientific Reports study documented that trees planted along roads absorb CO₂ while also cooling neighbourhoods, improving air quality, and managing stormwater — benefits that carbon accounting does not capture but that remain genuinely valuable. For urban trees, the gap between photosynthesis and wood growth may matter less because the case for planting rests on multiple gains anyway.

The Necessary Revision

What emerges is not a refutation of forest carbon storage but a refinement. Land vegetation continues to absorb large amounts of CO₂. A 2020 Nature Communications study traced much of that absorption to forest growth. Forests remain essential in any credible net-zero pathway. But the New Phytologist work adds an important qualification: the amount of carbon stored in future forests may be lower than photosynthesis figures alone would suggest, and the uncertainty ranges around those projections should probably be wider.

For government officials building net-zero targets that depend on forest carbon removals, and for the carbon credit registries trying to audit forest sequestration claims, that is a meaningful adjustment to integrate into planning.