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An oak trunk does most of its growing in the dark. Through the small hours, when the air is cool and damp and the leaves are doing nothing at all, water creeps back into the stem, pressure builds in the living cells of the cambium, and the tree quietly thickens by a few micrometers. Come midday, with the sun blazing and photosynthesis running flat out, that growth has all but stopped. The factory is humming. The building work has knocked off for the day.
That mismatch, it turns out, is not just a daily quirk. It runs through the whole year, and it may force a rethink of how much carbon the world’s forests can really lock away.
The intuition most of us carry, and the one baked into most climate models, is simple enough: a tree that is photosynthesizing is a tree that is growing. Sunlight in, sugars made, wood laid down, carbon stashed for decades or centuries inside the trunk. It is a tidy story. It is also, according to a new study of oaks published in Science Advances, not quite right.
“Right now, most models assume that if you have photosynthesis, you have growth. We find that’s not the case,” says Mukund Palat Rao, an ecoclimatologist at the Lamont-Doherty Earth Observatory, part of the Columbia Climate School, and the study’s lead author.
Rao and his colleagues went after the question with an almost obsessive thoroughness, watching eight oak species at scales from the cellular to the satellite. They strapped sensors to trunks that could detect changes of a few millionths of a meter, the tree swelling overnight as the roots drank, shrinking by day as it lost water through its leaves. They mounted cameras on the canopy, ran flux towers to sniff carbon dioxide above the treetops, and pulled in growth-ring records and satellite readings of photosynthesis across 137 sites, from the woods of the eastern United States to the oak savannahs of California.
What the numbers showed was a tree leading something of a double life.
In the eastern sites, the oaks grew mostly from May through July, then downed tools, even as they carried on photosynthesizing well into October. Roughly 36 percent of the year’s carbon uptake happened after growth had already stopped. In California, where the rhythm runs to a wetter winter, growth ran from December to April and petered out by August, with about a quarter of the annual carbon haul coming in after the building work was done. And at the daily scale the split was starker still: at one semi-arid Californian site, more than 80% of all growth happened before seven in the morning, while photosynthesis peaked around noon. The reason, Rao thinks, comes down to water and pressure. Growing cells need to be turgid, swollen tight, to divide and expand, and that takes water the tree simply cannot hold onto when the air turns hot and thirsty. “The moment you have dry and hot conditions, growth activity stops pretty instantly while photosynthesis seems to continue at a slightly decreased rate,” he says.
Where does all that extra carbon go?
So the leaves keep feeding the tree long after the trunk has stopped putting on weight. The obvious question is what happens to the surplus. Some of it, Rao says, is squirreled away to kick-start growth the following spring; some goes into new leaves and roots, or is simply burned to keep cells ticking over through winter. Not much of it, on this picture, ends up as the long-lived wood that makes forests such a useful carbon sink in the first place.
That distinction matters more than it might sound. Carbon in a trunk can sit there for decades, centuries, sometimes longer. Carbon shunted into leaves or fine roots or metabolic housekeeping comes back out again rather quickly. If a warming, carbon-rich world really does crank up photosynthesis, as many models assume, but that extra sugar does not become wood, then the rosy projections of forests growing fatter and storing ever more carbon start to look a bit shaky.
A wobble that gets worse as the climate swings
There is a sting in the tail. The decoupling was sharpest, the researchers found, in years when the local climate lurched between wet and dry, the kind of whiplash that climate change is expected to make more common. In other words, the very conditions coming down the line are the ones most likely to drive growth and photosynthesis further apart. Earth system models that assume the two stay locked together, the team argues, may be overestimating how much carbon forests will salt away as the air grows hotter and drier.
For now, the work covers oaks, an ecologically mighty group sometimes called the most important woody genus in the Northern Hemisphere, but only oaks. Rao and his colleagues are already chasing the same pattern across other species and ecosystems, expecting to find it in varying degrees. “I don’t really have answers yet,” he says. “There are many questions still left to address.”
Frequently Asked Questions
If a tree is photosynthesizing, isn’t it automatically growing?
Not necessarily, and that is the heart of this study. Photosynthesis makes the sugars, but laying down new wood is a separate process that needs the right physical conditions, chiefly enough water pressure inside the cells to let them divide and expand. The oaks in this research kept photosynthesizing for months after their trunks had stopped thickening for the year.
Why does it matter where the carbon ends up inside a tree?
Because different parts of a tree hold carbon for wildly different lengths of time. Carbon locked into trunk wood can stay put for decades or centuries, which is what makes forests valuable for offsetting emissions. Carbon that goes into leaves, fine roots or everyday metabolism is released again far sooner, so it does little for long-term storage.
Does this mean forests aren’t useful carbon sinks anymore?
No. Forests still absorb a large share of human carbon emissions and remain one of our best natural defenses. The finding is narrower but important: the amount of carbon stored over the long haul may be lower than optimistic models predict, because more sunlight does not reliably translate into more wood.
Why would hot, dry weather stop growth but not photosynthesis?
Growth depends on turgor, the internal water pressure that lets cells swell and divide, and that pressure collapses quickly when the air is hot and dry and the tree loses water faster than it can replace it. Photosynthesis is more resilient, slowing only gradually as the leaves’ pores tighten. So growth shuts off almost instantly while the leaves keep working.
Where can I read the original research?
The study, “Decoupled carbon assimilation and growth responses to aridity in temperate deciduous oaks,” appears in Science Advances and is open access at https://doi.org/10.1126/sciadv.ady7139.
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