New research published in New Phytologist in January 2026 finds that photosynthesis does not reliably translate into wood growth — a decoupling that carries direct consequences for how much carbon forests can actually sequester under future climate conditions.

The finding cuts at a foundational assumption embedded in many climate mitigation strategies: that higher atmospheric CO₂ drives greater photosynthesis, which in turn drives greater woody biomass accumulation and, by extension, long-term carbon storage. The New Phytologist study challenges that chain of inference at its second link.

The Gap Between Photosynthesis and Wood Growth

Carbon fixed by photosynthesis — gross primary production — does not automatically flow into structural biomass. Trees allocate photosynthate to respiration, reproduction, root exudates, defence compounds, and non-structural carbohydrate reserves before any surplus reaches the woody tissue that constitutes durable carbon storage. The New Phytologist paper formalises what ecophysiologists have long suspected: the relationship between canopy-level carbon uptake and stem radial growth is far more contingent than broad-scale models typically represent.

Work from the University of Utah, involving researcher William Anderegg, had already examined this tension — noting that tree growth can be limited by factors other than carbon supply, including temperature, water availability, and nutrient constraints, even when photosynthesis is proceeding normally. The 2026 findings extend that line of inquiry and sharpen its implications for carbon accounting.

The stakes are considerable. A 2023 study in Nature Climate Change estimated that CO₂ fertilization has increased global annual terrestrial photosynthesis by 13.5 ± 3.5% — or roughly 15.9 ± 2.9 PgC — since pre-industrial times. If a meaningful fraction of that additional photosynthetic activity is not being converted into stable woody carbon, projections of the terrestrial carbon sink may be systematically overstated.

What This Means for the Carbon Credit Market

The commercial and policy architecture built around forest-based carbon removal rests on the credibility of sequestration estimates. Forest carbon credits are instruments through which companies, individuals, and governments pay to plant or protect trees on the premise that doing so offsets a calculable volume of emissions. Carbon offsetting more broadly is premised on a legible equivalence between investment and atmospheric effect.

If wood growth is less tightly coupled to photosynthesis than models assume, the carbon accounting underlying many of those instruments becomes harder to defend — particularly in a regulatory environment already scrutinising the integrity of voluntary carbon markets.

A Harvard Forest study examining alternative land-use scenarios and their effects on forest carbon storage provides relevant regional context. Its findings reinforce that the carbon trajectory of any given forest depends heavily on management choices and disturbance regimes, not simply on the presence of trees and rising CO₂.

The Wider Political Economy of Tree Planting

There is a separate but related problem on the demand side. Research published in Nature Communications Earth & Environment in April 2025 found that the cultural prominence of tree planting as a climate solution may be crowding out support for other carbon removal approaches — a phenomenon sometimes described as the "flagship effect," where one popular intervention reduces enthusiasm for less visible alternatives. That dynamic matters because durable carbon removal almost certainly requires a portfolio of methods, not a monoculture of tree-planting schemes.

Urban forestry sits at a slightly different point in this landscape. A 2025 study in Scientific Reports documented that road-adjacent urban trees absorb CO₂ through photosynthesis while delivering co-benefits — air quality improvement, heat island mitigation, stormwater management — that are not captured in carbon accounting but remain real. The photosynthesis-to-wood-growth gap may be proportionally less consequential for urban trees, where the case for planting is already multifactorial.

The broader picture is one of accumulating scientific qualification rather than refutation. Land vegetation is absorbing large quantities of atmospheric CO₂ — a 2020 Nature Communications study attributed much of that uptake to tree growth stimulation — and forests remain indispensable in any realistic mitigation pathway. What the New Phytologist findings add is precision: the magnitude of forest carbon storage under future conditions may be smaller than headline photosynthesis figures suggest, and the confidence intervals around sequestration projections should probably be wider than current models reflect.

For policymakers calibrating net-zero commitments against land-sector carbon removals, and for the registries and auditors trying to put credible numbers on forest carbon credits, that is an important revision to absorb.