Climate Models May Be Wrong About How Trees Store Carbon

A new study reveals that oak trees can keep photosynthesizing for months after growth ends, challenging assumptions about how effectively forests convert absorbed carbon into long-term storage.

A tree can look busy long after it has stopped building itself. Its leaves may still be absorbing sunlight and pulling carbon dioxide from the air, but deep inside the trunk, the season’s wood production may already be over.

That surprising split is the focus of a new study of oak trees published in Science Advances. The researchers found that oaks can keep photosynthesizing late into the year even after their growth has shut down by mid-summer. The finding challenges a common assumption in climate models: that more photosynthesis usually means more tree growth.

A Carbon Sink With a Complication

Rising atmospheric carbon dioxide (CO₂) has often been expected to boost plant photosynthesis, a response sometimes called the carbon fertilization effect. In theory, more CO₂ could allow trees to absorb more carbon and grow larger, locking away some of that planet-warming gas in wood.

The new findings complicate that picture. The study suggests that carbon uptake and wood production can become separated, especially when environmental conditions are not favorable for growth. Some of the carbon absorbed after growth stops may go into leaves, roots, temporary starch reserves, soil compounds, or basic cellular maintenance rather than long-term wood storage.

That does not mean the carbon is wasted. Trees use carbon for many essential functions. But from a climate perspective, not all carbon use is equal. Carbon stored in leaves, sugars, or short-lived tissues can return to the atmosphere much faster than carbon stored in wood.

Why Climate Models May Need a Rethink

The results have important implications for how scientists estimate the future role of forests in the carbon cycle.

“Right now, most models assume that if you have photosynthesis, you have growth. We find that’s not the case,” says lead author Mukund Palat Rao, an ecoclimatologist at Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School. “Just because there is more photosynthesis might not necessarily mean more tree growth in the future.”

During photosynthesis, plants use sunlight to convert CO₂ and water into sugars, releasing oxygen as a byproduct. In trees, some of that carbon becomes wood in trunks, branches, and roots. Some is used to grow leaves and fruits. Some is stored temporarily as starch. Some is sent belowground in compounds that feed microbes, help trees access nutrients, or defend against pathogens.

Only a portion of that carbon ends up in woody biomass, which is the part most important for long-lasting carbon storage. That makes it essential to understand when photosynthesis actually leads to growth, and when it does not.

“Understanding how photosynthesis and growth are linked is very important from the perspective of understanding how forests will store carbon over long time scales,” says Rao.

Measuring Trees Day by Day

Scientists have suspected for years that carbon uptake and tree growth do not always move in step, but the relationship has been difficult to measure clearly. Tree growth is not a smooth, constant process. A trunk can swell overnight as roots take up water, then shrink during the day as leaves lose water through transpiration. Actual growth emerges from those tiny daily changes over time.

To capture that process, Rao and his colleagues combined several kinds of observations. They used satellite data sensitive to photosynthetic activity at 137 sites across the eastern United States and California. They also analyzed instruments that measured CO₂ exchange near treetops hour by hour, along with trunk-mounted sensors that tracked minute changes in tree size in real time. (Trees tend to expand at night as roots take up water, then shrink slightly in daytime as they transpire water, with the long-term trajectory adding up to growth.) The team also used tree ring records and temperature data from 1950 to the present.

Photosynthesis Continued After Growth Ended

At the eastern U.S. sites, oak trees generally added new growth from May through July. Yet their photosynthetic activity continued well into October. About 36% of their annual carbon assimilation through photosynthesis occurred after late-summer growth had already stopped.

The same general pattern appeared in California, although the seasonal timing was different. There, oak trees grew mainly from December through April. Growth slowed in mid-summer and had stopped by August, but photosynthesis continued. About 26% of annual carbon uptake at those sites occurred after growth had ceased.

The result shows that a tree’s leaves can remain active even after the tissues responsible for expanding wood have largely shut down for the year.

Water Stress May Help Explain the Split

The pattern makes biological sense. Tree growth depends on internal water pressure, which helps cells expand and allows new wood to form. When conditions become hot and dry, that pressure drops. Growth can stop quickly, even if the leaves continue photosynthesizing at a reduced rate.

“The moment you have dry and hot conditions, growth activity stops pretty instantly while photosynthesis seems to continue at a slightly decreased rate,” says Rao.

Where Does the Carbon Go?

Some of the carbon absorbed after growth stops may be stored and used to help trees restart growth the following year, according to Rao. Other portions may support new leaves and roots or be oxidized to keep cells alive through winter.

The researchers also found that the disconnect between photosynthesis and growth was strongest in years when local weather swung sharply between wet and dry extremes. Those kinds of unstable conditions are expected to become more common as the climate changes.

Rao and his colleagues are now investigating whether the same pattern appears in other tree species, ecosystems, and regions. He expects the strength of the disconnect to vary depending on forest type and climate, but the broader question remains open.

“I don’t really have answers yet,” he says. “There are many questions still left to address.”

Reference: “Decoupled carbon assimilation and growth responses to aridity in temperate deciduous oaks” by Mukund Palat Rao, Arturo Pacheco-Solana, Rong Li, Bar Oryan, Johanna E. Jensen, Milagros Rodriguez-Caton, Lily Klinek, Zoe A. Pierrat, Sophie Ruehr, Rose Oelkers, Laura E. Boeschoten, Kevin L. Griffin, M. Luke McCormack, Xi Yang, Joseph Verfaillie, Dennis Baldocchi, Jeremy Hise, Alexander J. Turner, Todd M. Scanlon, Laia Andreu-Hayles, Jan U. H. Eitel, Neil Pederson, Daniel Griffin, David Stahle, Justin T. Maxwell, Steven Voelker, Steven A. Kannenberg, Josep Peñuelas and Troy S. Magney, 12 June 2026, Science Advances.
DOI: 10.1126/sciadv.ady7139

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