Just because there is more photosynthesis might not mean more tree growth
For generations, forests have stood in the human imagination as patient allies against our excesses—vast living archives slowly locking away the carbon we release. A new study of 137 American forest sites now complicates that faith, revealing that trees cease growing months before they stop photosynthesizing, meaning a significant portion of the carbon they draw from the air never becomes the durable wood we have counted on. The finding, emerging from Columbia University's Lamont-Doherty Earth Observatory, suggests that the computational models guiding global climate strategy may have been telling us a more hopeful story than the forests themselves are living.
- Trees across the eastern United States absorbed 36% of their yearly carbon after growth had already stopped—meaning that carbon never became permanent wood and cycled back into the atmosphere far sooner than models assume.
- Heat and drought shut down tree growth almost instantly while photosynthesis quietly continues, a dangerous decoupling that will only widen as climate change drives more frequent and severe extreme weather.
- Earth system models treat photosynthesis and wood growth as inseparable twins, but this research exposes that assumption as dangerously optimistic, potentially inflating projected forest carbon storage across the entire 21st century.
- With land-based carbon removal—predominantly forests—accounting for 99.9% of current global CO2 removal efforts, an overestimate of forest storage capacity could quietly hollow out the foundation of natural climate solutions.
- Researchers are now racing to determine whether this photosynthesis-growth decoupling holds across other species and regions, knowing that the answer could force a fundamental recalibration of how humanity plans to survive its own emissions.
The mathematical foundation of climate science's relationship with forests may be cracking. A study led by Mukund Palat Rao at Columbia University's Lamont-Doherty Earth Observatory examined 137 forest sites across the United States and uncovered a stubborn gap between what trees do and what actually results from it: trees stop growing months before they stop photosynthesizing.
This distinction carries enormous weight. Only carbon stored in wood—solid, structural material—remains out of the atmosphere for decades or centuries. Carbon absorbed after growth has ceased gets used for other biological purposes and returns to the atmosphere on a far shorter timescale. In eastern US forests, roughly 36 percent of a tree's yearly carbon intake arrived after growth had already ended. In California, the figure was around 26 percent.
The mechanism is stress. When heat rises and moisture disappears, trees shut down their growth machinery almost immediately—but photosynthesis, driven by light at the leaf level, continues at a reduced rate. As climate change makes droughts and heatwaves more common, forests will photosynthesize more while growing less wood to show for it.
The problem is that most Earth system models assume photosynthesis and wood growth move in lockstep. More photosynthesis automatically means more carbon stored. Rao's findings suggest this assumption is dangerously optimistic, and the gap between model and reality could be substantial.
The stakes extend well beyond academic modeling. Forests currently account for the overwhelming majority of global carbon removal efforts. A separate report published the same week warned that humanity must deploy new carbon-removal technologies at extraordinary speed just to meet existing climate targets. If natural carbon sinks are storing meaningfully less than predicted, the entire architecture of natural climate solutions may need to be rebuilt from the ground up.
The math that climate scientists use to predict how much carbon forests will absorb over the coming century may be fundamentally broken. A new study of 137 forest sites across the United States has found that trees stop growing months before they stop photosynthesizing—a gap that suggests the planet's forests are far less efficient at locking away carbon than most climate models assume.
The research, led by Mukund Palat Rao at Columbia University's Lamont-Doherty Earth Observatory, reveals a stubborn disconnect between what trees do and what happens as a result. Photosynthesis, the process by which trees convert sunlight and carbon dioxide into energy, does not automatically translate into wood growth. This matters enormously because only carbon stored in wood—solid, structural material—stays out of the atmosphere for decades or centuries. Carbon used for other purposes, like producing leaves or fueling internal processes, cycles back much faster.
In forests across the eastern United States, the researchers found that roughly 36 percent of the carbon trees absorbed in a given year came after the trees had already stopped growing, typically in late summer. In California forests, the figure was about 26 percent. This means that a substantial portion of the carbon dioxide trees pull from the air never becomes part of their permanent structure. It gets used and released back into the atmosphere on a much shorter timescale.
The culprit, Rao's team discovered, is stress. When temperatures rise and moisture becomes scarce—conditions that are becoming more frequent as the climate warms—trees simply shut down their growth machinery. But photosynthesis, the light-driven chemical reaction at the leaf level, continues 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," Rao explained. This is a problem because it means that as climate change makes heatwaves and droughts more common, forests will photosynthesize more carbon but grow less wood to store it.
Most Earth system models—the vast computational frameworks that climate scientists use to project the future—assume a tight coupling between photosynthesis and growth. They essentially treat the two as inseparable. If a model shows more photosynthesis, it automatically predicts more wood accumulation. The new research suggests this assumption is dangerously optimistic. "Just because there is more photosynthesis might not necessarily mean more tree growth in the future," Rao said.
The implications ripple outward. Forests are supposed to be a crucial buffer against climate breakdown, absorbing carbon dioxide that would otherwise accumulate in the atmosphere. Climate scientists have long expected that as atmospheric CO2 levels rise, forests will absorb more of it, keeping the global carbon sink stable or even growing through the 21st century. But if the models are overestimating how much of that absorbed carbon actually gets locked into wood, then the real carbon-storage capacity of forests is lower than predicted. The gap between model and reality could be substantial.
Rao's team is now investigating whether this decoupling of photosynthesis and growth appears in other tree species and regions beyond their initial study sites. The stakes are high. Last week, a separate report concluded that humanity must deploy new carbon-removal technologies far faster than it has deployed solar panels just to meet climate targets. Land-based actions like tree planting currently account for the vast majority of global carbon removal efforts—about 99.9 percent of the 2.2 billion tonnes of CO2 removed annually. If forests are storing less carbon than models predict, the entire strategy for natural climate solutions may need recalibration.
Citações Notáveis
Just because there is more photosynthesis might not necessarily mean more tree growth in the future.— Mukund Palat Rao, carbon cycle scientist at Columbia University
The moment you have dry and hot conditions, growth activity stops pretty instantly, while photosynthesis seems to continue at a slightly decreased rate.— Mukund Palat Rao
A Conversa do Hearth Outra perspectiva sobre a história
So the trees are still photosynthesizing, still pulling carbon from the air. Why does it matter if they're not growing?
Because photosynthesis is just the first step. The carbon has to go somewhere. If it goes into wood, it stays locked away for centuries. If it goes into leaves or gets burned for energy, it cycles back to the atmosphere in months or years.
And the study found that a lot of the carbon trees absorb in a year happens after they've already stopped growing?
Yes. In the eastern US, about 36 percent of yearly carbon uptake happened after growth had stopped. The trees were still photosynthesizing, but they weren't building anything permanent with it.
What's stopping the growth if photosynthesis is still happening?
Heat and drought. When conditions get dry and hot, trees essentially shut down their growth machinery to conserve water and energy. But the leaves keep working, still pulling in CO2, just at a reduced rate.
And that's going to get worse?
That's the worry. As climate change makes heatwaves and droughts more common, we'll see more of this pattern—more photosynthesis happening during stress periods when trees can't actually grow. The carbon gets absorbed but not stored.
So the climate models are too optimistic?
They assume that more photosynthesis automatically means more wood growth. This study suggests that assumption breaks down under the exact conditions we're heading toward.