For decades, the climate science of trees and carbon logic felt somewhat simple: More carbon dioxide in the air means more photosynthesis, which means more tree growth, which means more carbon locked safely away in wood.

But a new study on oak trees just poked a hole in that thinking, and it could mean forests aren't the climate safety net we've been counting on.

Oaks keep pulling carbon dioxide out of the air long after they've stopped physically growing for the year, researchers led by ecoclimatologist Mukund Palat Rao at Columbia's Lamont-Doherty Earth Observatory found. In other words, a tree can be busy photosynthesizing while adding no new wood at all.

"Right now, most models assume that if you have photosynthesis, you have growth," Rao said. "We find that's not the case."

Without new wood growth, the tree doesn't store that carbon for the long haul.

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How the oaks were studied

The team studied 137 oak forest sites across the eastern U.S. and California. They used satellite images to track photosynthesis; instruments that measured carbon dioxide in the canopy hour by hour; sensors strapped to trunks that caught tiny daily changes in size; and tree rings and temperature records that stretched back to 1950.

In the eastern U.S., they found that oaks did most of their growing from May to July, but they kept photosynthesizing into October. About 36% of their annual carbon uptake happened after growth had already stopped. In California, where oaks grow on a different calendar, about 26% of the yearly total came in after growth ceased.

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So where does all that late-season carbon go, if not into wood? It goes into leaves, roots, stored starches, basic cell maintenance and compounds in the soil, and some of it cycles back into the atmosphere.

That's the crux of the problem, because wood is the long-term vault. Carbon that is stored in a tree trunk can stay put for decades or even centuries. Carbon that goes elsewhere may not.

Absorbing isn't the same as storing carbon

If a tree soaking up extra carbon sounds like good news, there’s one thing to consider: Absorbing carbon isn't the same as storing it.

Scientists had assumed that when a tree takes in more carbon, more of it automatically gets locked away long-term in wood. This research suggests otherwise.

A lot of that extra carbon is instead spent on growing leaves and roots and fueling the tree's everyday functions. Much of it is short-lived and able to leak back into the air instead of getting banked in wood.

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Growth depends on internal water pressure inside the tree, and when it turns hot and dry, that pressure drops fast.

"The moment you have dry and hot conditions, growth activity stops pretty instantly," Rao said. And photosynthesis barely slows down.

The disconnect was worst in years that swung between wet and dry extremes. 

This challenges one of the more hopeful ideas in climate science: that a warmer, CO2-rich world will supercharge forests into ever-bigger carbon sponges. If less of a tree's absorbed carbon actually becomes wood, forests might bank less carbon than today's climate models predict.

This matters a great deal for carbon budgets and net-zero plans that lean on forests to soak up our emissions.

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Keep in mind that this is a study of oaks in two regions, and research is still ongoing about whether or not other species and ecosystems behave the same way. 

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

None of this means forests aren't valuable, or that planting trees doesn't help. It means the payoff may be smaller, and less certain, than the optimistic math assumes.

As Rao put it: "Just because there is more photosynthesis might not necessarily mean more tree growth in the future."

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