Trees may store less planet-heating carbon than hoped, study suggests

A new study challenges assumptions about trees' capacity to sequester atmospheric carbon, finding that photosynthetic activity does not reliably translate into wood accumulation.

Researchers examined 137 forest sites across the United States and documented a critical timing mismatch. Trees halted wood growth months before they ceased photosynthesis, meaning carbon fixed through photosynthesis was not consistently stored as biomass in trunks and branches.

This distinction matters for climate policy. Trees represent a leading nature-based carbon removal strategy, with governments and corporations investing heavily in reforestation and afforestation projects premised on their carbon storage potential. If photosynthesis does not reliably produce wood growth, the actual carbon sequestration rates of forests may fall short of current models and targets.

The research exposes a flaw in how scientists have historically calculated forest carbon storage. Previous assessments often treated photosynthetic output as equivalent to carbon sequestration, failing to account for the lag and disconnect between carbon fixation and structural growth. The carbohydrates produced during extended photosynthetic seasons appear to support other metabolic functions—root growth, reproduction, defense compounds, respiration—rather than accumulating as harvestable or long-term stored carbon in wood.

The findings arrive as governments pursue carbon offset programs that depend on forests as a climate solution. The United Nations Framework Convention on Climate Change and various national pledges rely partly on expanded forest coverage to help meet emissions reduction targets. Overestimating forest carbon storage capacity could lead policymakers to underestimate the emissions reductions needed from other sectors like energy, transportation, and industry.

The research does not suggest forests offer zero climate benefit. Rather, it highlights the need for more precise accounting of how trees actually sequester carbon under varying environmental conditions. Future forest management and carbon credit schemes may need adjustment to reflect these biological realities. Scientists recommend incorporating seasonal growth patterns