An assumption baked into nearly every climate model

Climate scientists trying to predict how much carbon the world's forests can absorb in a warmer future have long relied on a simple, intuitive link: more photosynthesis means more tree growth, and more tree growth means more carbon locked away in wood for decades or centuries. That assumption underpins a huge share of existing forest carbon models, which generally predict that rising atmospheric CO2 will boost photosynthesis and, in turn, stimulate additional woody growth — effectively treating forests as a carbon sink that scales up automatically as CO2 levels rise.

A new study led by researchers at Columbia University's Lamont-Doherty Earth Observatory, published in Science Advances, found that this core assumption doesn't hold up under close observation. Oak trees, it turns out, keep photosynthesizing and absorbing carbon dioxide for months after their annual wood growth has already stopped entirely. "Right now, most models assume that if you have photosynthesis, you have growth," said Mukund Palat Rao, an ecoclimatologist at Lamont-Doherty and the study's lead author. "We find that's not the case."

Watching two processes that were assumed to move together

To test whether photosynthesis and growth actually track each other as closely as models assume, Rao's team monitored oak trees at multiple sites across the eastern United States and California, generating daily records of photosynthesis, carbon uptake, and actual stem growth over an extended period. That kind of continuous, side-by-side daily tracking — rather than relying on seasonal snapshots or growth-ring data alone — is what allowed the researchers to catch a gap between the two processes that coarser measurement approaches would likely have missed.

The pattern that emerged was consistent across both regions, despite very different climates and growing seasons. In the eastern U.S. sites, oak trees generally added new wood growth from May through July, but their leaves kept actively photosynthesizing well into October — months after the trees had stopped expanding physically. Roughly 36% of the eastern oaks' total annual carbon assimilation through photosynthesis occurred after their growth had already ceased for the season.

California's oaks told the same story, on a different calendar

The California sites offered a useful test of whether this pattern was specific to one climate or growing season, since oaks there operate on an almost entirely different annual schedule. California oaks generally grew from December through April, with growth slowing through the summer and stopping entirely by August. Despite that considerably different timing, the underlying disconnect showed up again: photosynthesis continued even after growth had ceased, with roughly 26% of the California oaks' annual carbon uptake occurring after wood production had already stopped for the year.

That consistency across two regions with fundamentally different climates and growing calendars is what gives the finding real weight. If the decoupling between photosynthesis and growth had shown up in only one region, it might have been dismissed as a local quirk tied to specific soil or weather conditions. Finding a similar pattern, even at different magnitudes, in both an eastern deciduous forest system and a California oak woodland suggests something more fundamental about how oak trees function physiologically, regardless of their specific climate.

Why water, not carbon, turns out to be the real bottleneck

The explanation the researchers arrived at makes intuitive biological sense once laid out, even though it inverts the usual carbon-centric framing of tree growth. Growth — meaning the physical expansion of a tree's cells to build new wood — depends on internal water pressure sufficient to push those cells outward. When conditions turn hot and dry, that internal water pressure drops quickly, and cell expansion essentially shuts down almost immediately, even if the tree still has plenty of carbon available from photosynthesis to use.

Photosynthesis, by contrast, doesn't require the same water pressure mechanism to continue operating, even if it slows somewhat under dry conditions. That means a tree can keep pulling carbon out of the atmosphere through its leaves long after the specific mechanical process required to convert that carbon into new wood has stalled entirely. Water availability, not carbon supply, turns out to be the bottleneck governing when growth actually happens — a distinction that most existing climate models don't appear to account for with much precision.

Where all that extra absorbed carbon actually goes

If a meaningful share of the carbon oak trees absorb late in the season isn't going into new wood, the obvious question is where it ends up instead. The researchers suggest that carbon absorbed after growth has stopped is more likely being used to produce foliage, support ongoing metabolic processes, or otherwise get allocated to shorter-lived plant tissues and functions — carbon pools that don't lock the element away for the long timescales that woody biomass does. Wood, once formed, can store carbon for decades or centuries; leaves and short-lived metabolic products generally cycle back into the atmosphere on a much faster timeline, whether through decomposition or the tree's own respiration.

That distinction matters enormously for climate projections specifically because forest carbon storage estimates are typically built around the assumption that absorbed carbon converts efficiently into long-term wood storage. If a substantial fraction of what forests absorb is instead flowing into shorter-lived carbon pools, existing models may be systematically overestimating how much additional carbon storage forests can deliver as atmospheric CO2 concentrations continue climbing.

What this means for how much forests can actually help

Rao framed the practical stakes of the finding directly: understanding exactly how photosynthesis and growth relate to each other matters enormously for predicting how forests will store carbon over long time scales — precisely the kind of long-range prediction that underpins national and international climate policy built around forests as a carbon offset mechanism. If real-world oak forests convert absorbed carbon into permanent wood storage less efficiently than current models assume, then policies and carbon-offset programs that lean heavily on projected future forest carbon sequestration may be building on numbers that don't hold up as precisely as intended.

None of this means forests stop functioning as meaningful carbon sinks — oaks in this study were still absorbing substantial amounts of carbon dioxide throughout the year, including the months after growth had stopped. What the finding changes is the confidence with which scientists can translate rising photosynthetic activity into a proportional increase in long-term wood carbon storage. That's a more modest, more carefully qualified version of the "forests will save us" narrative that's often embedded in climate modeling — one grounded in what oak trees are actually doing, rather than in the assumption that carbon absorbed automatically becomes carbon stored.

*This article was researched using publicly available reporting from Columbia University's Lamont-Doherty Earth Observatory, ScienceDaily, SciTechDaily, Phys.org, EurekAlert, and the peer-reviewed study published in Science Advances. It is intended for informational purposes.*