Trees Continue Absorbing Carbon Long After They Stop Growing, Study Finds

Trees Keep Absorbing Carbon Even After Growth Seasons End

A groundbreaking new study has revealed that oak trees continue drawing carbon dioxide from the atmosphere long after their annual growth has stopped — a finding that could fundamentally change how scientists model carbon storage in forests around the world.

Published in Science Advances, the research challenges a widely held assumption in climate science: that photosynthesis and tree growth go hand in hand. According to the findings, the two processes are far more independent than previously understood, and that distinction carries serious implications for how we predict forests' role in combating climate change.

The Long-Held Assumption That No Longer Holds

Forests have long been recognized as one of Earth's most powerful natural defenses against rising temperatures. Trees absorb CO₂ through photosynthesis and lock much of that carbon into their trunks, branches, and root systems — sometimes for hundreds or even thousands of years. This carbon sequestration has led scientists and climate modelers to assume that higher levels of atmospheric CO₂ would drive faster photosynthesis, which in turn would accelerate tree growth and increase long-term carbon storage.

The new research suggests that chain of logic may be flawed.

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

How Photosynthesis and Growth Actually Differ

During photosynthesis, trees use sunlight to convert CO₂ and water into sugars, releasing oxygen in the process. That captured carbon stays inside the tree — but not all of it becomes wood.

Some carbon is built into the woody tissue of trunks, branches, and roots, where it can remain stored for centuries. The rest goes toward producing leaves and fruit, fueling metabolic processes, or being released into the surrounding soil to support microbial life, aid nutrient absorption, and help the tree fend off disease.

Because woody biomass stores carbon over such extended timescales, knowing how much photosynthesized carbon actually becomes wood is essential for accurately estimating how much forests can help offset greenhouse gas emissions.

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

How the Study Was Conducted

To explore the relationship between carbon uptake and tree growth, Rao and his team combined multiple streams of data collected across 137 oak forest sites in the eastern United States and California.

The researchers analyzed satellite imagery capable of detecting photosynthetic activity, hourly CO₂ measurements taken from tree canopies, and trunk sensors that tracked subtle daily changes in tree diameter. They also incorporated tree ring records and temperature data stretching back to 1950.

Together, these sources gave the team a detailed, daily picture of how photosynthesis, carbon absorption, and physical growth related to one another across different seasons and climate conditions.

A Clear Divide Between Growth and Carbon Capture

Eastern U.S. Oak Forests

The data revealed a striking gap between when trees grow and when they continue to absorb carbon. In eastern U.S. forests, oak trees typically grew between May and July but kept photosynthesizing all the way through October. Approximately 36 percent of their total annual carbon uptake occurred after growth had already halted for the season.

California Oak Forests

California oaks followed a different seasonal schedule but showed the same underlying pattern. Growth generally took place from December through April, tapering off by mid-summer and stopping entirely by August — yet photosynthesis continued well beyond that point. Around 26 percent of their yearly carbon absorption happened after growth had ceased.

Why Do Trees Stop Growing While Still Photosynthesizing?

The explanation, according to Rao, comes down to water. Tree growth depends on internal water pressure within cells, and that pressure drops sharply when conditions turn hot and dry.

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

The carbon that trees capture after growth ends doesn't simply disappear. Some of it is stored temporarily and used to fuel growth when the next season begins. The rest goes toward developing new roots and leaves or is metabolized to sustain the tree's living cells through winter dormancy.

What remains unclear is how much of that post-growth carbon eventually becomes permanent woody biomass versus how much is released back into the atmosphere over shorter timeframes.

What This Means for Climate Forecasting

The implications of this research extend well beyond forest ecology. Climate models that project how much carbon forests will store in a warming world may be significantly overestimating that capacity if they assume photosynthesis reliably translates into wood production.

The study also found that the gap between photosynthesis and growth widened during years with greater swings between wet and dry weather conditions. Since climate change is expected to increase weather variability across many regions, this disconnect could become even more pronounced in the decades ahead — further reducing the reliability of current carbon storage projections.

Questions That Still Remain

Rao and his colleagues are now turning their attention to other tree species, forest types, and climates to determine whether the same patterns hold more broadly. He expects the degree of separation between photosynthesis and growth will differ depending on the ecosystem, but acknowledges that many fundamental questions are still unanswered.

"I don't really have answers yet," he admitted. "There are many questions still left to address."

What is clear, however, is that the relationship between trees and carbon is far more nuanced than scientists once believed — and that understanding those nuances will be critical for building accurate predictions of Earth's climate future.