A study published in Science Advances finds that oak trees can keep photosynthesizing after growth has stopped—meaning some carbon uptake may not translate into long-term storage in wood. Researchers report that heat and aridity curb growth more than photosynt
By mid-summer, oak trees in the eastern United States appear to have stopped growing—even though they keep photosynthesizing well into October.
The disconnect is stark: the carbon absorbed through photosynthesis doesn’t always end up in new woody biomass. the slow. long-term store that makes forests such an important carbon sink. Instead, some of that carbon likely gets redirected toward foliage, starch, short-lived cellular processes, or compounds released into the soil.
A new study, published in the journal Science Advances, argues that this “decoupling” between photosynthesis and growth could reshape how forests respond as the climate warms.
Lead author Mukund Palat Rao, an ecoclimatologist at Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School, puts the point bluntly: right now, most models assume photosynthesis tracks growth. But in the oak sites they studied, that assumption didn’t hold.
“Right now, most models assume that if you have photosynthesis, you have growth. We find that’s not the case,” Rao said. “Just because there is more photosynthesis might not necessarily mean more tree growth in the future.”
In plain terms, it’s intuitive to think that if a tree is using sunlight to power CO2-to-sugar conversions, it must be building more wood. The study’s measurements suggest that the biological link is weaker than many models treat it as.
During photosynthesis. plants absorb CO2 from the air and use sunlight to convert CO2 and water into sugars. releasing oxygen back into the atmosphere while the carbon remains in the plant. In trees, that carbon can become part of the woody biomass of the trunk, branches, and roots. The rest can go into foliage and fruits and be temporarily stored as starch. or be turned into compounds released into the soil to feed microbial communities. make nutrients available for uptake. and defend against pathogens.
That division matters for climate timelines. Carbon stored in woody biomass may take decades. centuries or even millennia to re-enter the atmosphere—depending on conditions—making wood an especially valuable long-term sink. Understanding how photosynthesis and growth are linked. Rao said. is therefore “very important from the perspective of understanding how forests will store carbon over long time scales.”.
The study also sets out why the field has been missing crucial details. Researchers have known for some time that carbon uptake and tree growth don’t have to match perfectly, but detailed measurements have been scarce and the mechanisms unclear.
To probe the relationship directly, Rao and his colleagues combined multiple lines of observation across 137 sites across the eastern United States and California. They used:
— photosynthesis-detecting satellite imagery of trees at those sites;
— readings from instruments providing hour-by-hour measurements of treetop CO2 levels;
— trunk-borne sensors delivering real-time measurements of minute fluctuations in tree size.
Those trunk sensors matter because trees tend to expand at night as roots take up water and shrink slightly in daytime as they transpire water; growth shows up in the long-term trajectory that accumulates over time. The researchers also incorporated growth ring records and temperature data from 1950 to the present.
From all of that, daily recordings emerged of photosynthesis, carbon uptake, and tree growth. The results showed that in eastern sites. oak growth generally occurred from May through July. even while trees continued to photosynthesize into October. Roughly 36 percent of all carbon assimilation through photosynthesis occurred after growth had stopped in late summer.
In California sites, the timing shifted with climate and seasonality: oak growth ran from December through April, but growth slowed in mid-summer and ceased by August, while photosynthesis continued. About 26 percent of those trees’ annual carbon uptake occurred after growth ceased.
The pattern follows a physiological logic tied to water. When conditions are dry and hot, trees lose the internal water pressure they need to grow, Rao said. “The moment you have dry and hot conditions. growth activity stops pretty instantly while photosynthesis seems to continue at a slightly decreased rate. ” he said.
The study also points to what happens to at least some of that post-growth carbon. Some fraction may be used to kick-start growth the following year, Rao said. The rest is used to grow new leaves and roots or is oxidized to keep cells alive through winter.
Exactly how much of the carbon remains locked away long-term in woody biomass, and how much returns on shorter time scales, is still unknown. But the researchers argue that projections of forests growing larger and storing more carbon in a warmer, CO2-saturated world will need to be revisited.
They also found that the decoupling between photosynthesis and growth was especially pronounced in years when local climates swung between wet and dry extremes. Those oscillations are expected to become more common as the climate changes.
Rao and his colleagues are now working to see whether the same pattern appears beyond oaks—studying other tree species, ecosystems, and regions. Rao expects decoupling to show up in varying degrees across different forest types and climates, but he stressed there are still unanswered questions.
“I don’t really have answers yet,” he said. “There are many questions still left to address.”
oak forests photosynthesis tree growth carbon sink carbon uptake aridity climate models Science Advances Lamont-Doherty Earth Observatory carbon storage