Plant leaves reveal how forests respond to rising carbon dioxide

Plants absorb carbon dioxide and water, use sunlight to make sugars, and release oxygen. So if the air contains more CO₂, shouldn’t forests grow faster, store more carbon, and help cool the planet?

It’s plausible and often repeated, but it hasn’t matched what long-term measurements in real forests actually show.

As atmospheric CO2 has climbed, tree growth and long-term carbon storage have bounced around from “up a bit” to “flat” to “sometimes down.”

This leaves scientists pondering how much of that patchwork is really about carbon in the air, and how much is everything else?

A new lens on a biological puzzle

A new study led by researchers from Duke University and Wuhan University argues that you can’t answer those questions by looking at carbon alone. You have to account for water. 

The team built a model that treats a tree’s daily decision – open leaf pores to grab carbon or close them to conserve water – as a dynamic optimization problem.

That framing, borrowed from engineering, turns out to reproduce decades of observations and explain why forests don’t simply ramp up growth in lockstep with CO2.

“There used to be a common assumption that higher levels of carbon dioxide will cause trees to grow more and store more carbon,” said Gaby Katul, a professor of civil and environmental engineering at Duke. 

“But benchmark experiments showed that while this may be true in isolation, other environmental factors also play a large role. We have now uncovered some of the underlying mechanisms at work.”

Lessons from two long experiments

Those “benchmarks” come from rare, painstaking field experiments. At Duke, a forest plot spent 16 years bathed in extra CO2.

At ETH Zurich, researchers dialed up local humidity. Both sites tracked growth, carbon uptake, leaf physiology, and a raft of environmental variables. 

The outcome was sobering. Trees did not sequester as much extra carbon as earlier, simple models predicted. The surprise wasn’t the result but why this happened.

Rising CO2 and forest growth

To find the mechanism, Katul’s team focused on the valves that govern a tree’s economy: stomata, the tiny pores on leaves that open to let in CO2 but simultaneously leak water vapor. 

In carbon-rich air, stomata don’t need to open as wide to pull in the same amount of CO2, which should, in theory, boost water use efficiency and growth.  

But warming and drying flip the calculus. Hotter, drier air ramps up evaporation through open pores. To protect their internal plumbing, trees constrict those pores, throttling water loss and, inevitably, carbon intake.

“Stomata are like valves that control how much water is drawn up into the leaves and released into the air,” Katul said.

That “valve” manages a precarious tension that spans roots, trunk, and canopy. Lose too much water too fast and the transport columns inside xylem can snap like a straw, a risk that grows with tree height and heat stress.

Turning physiology into math

The team built a model that formalizes that trade-off: maximize carbon gain while keeping water loss within safe limits. They calibrated it with unusually rich data from the Duke and ETH Zurich sites.

At these sites, individual leaves were enclosed and exposed to tightly controlled shifts in temperature, humidity, and CO2 while researchers tracked stomatal behavior in real time.

When they ran the model forward, it did two things well. First, it reproduced the muted carbon gains seen at Duke under elevated CO2.

Second, it captured the humidity experiment’s signal: when air is moist, stomata can stay open longer with less hydraulic risk, allowing higher carbon intake. In other words, CO2 matters, but so does the atmosphere’s thirst.

Making sense of messy global records

Armed with that mechanistic backbone, the team turned to a patchwork of tropical forest studies spanning the past fifty years.

The records show head-scratching diversity: some plots greened faster, others barely changed, some even slowed. Using the new framework, those contradictions look less mysterious.

In places where warmth and vapor pressure deficit climbed (air’s “drying power”), trees protected their plumbing by closing stomata more often – muting or canceling any CO2-driven growth bump.

Where moisture buffered the heat, gains were more likely to show up.

That reconciliation doesn’t mean carbon enrichment never boosts growth. It means the size, and even the direction, of that boost depends on the local balance of carbon supply and water demand.

The effects are mediated through the microscopic leaf valves and the hydraulics behind them.

Beyond a single lever

No single model can capture the full sprawl of a forest’s constraints. Nutrient limits, soil water storage, species mix, pests, shifting seasons, and stand age all shape growth and carbon storage. 

The authors are clear that their framework is a foundation, not a finish line. It explains a big, stubborn piece of the puzzle – the coupling of CO2 gain to water loss – and does so at the leaf-to-tree scale where decisions are made. 

The next challenge is scaling those decisions up into regional and global climate models without smoothing away the very dynamics that matter.

“There is a lot of value in looking at these environmental and biological questions from an engineering perspective,” Katul said.

“Figuring out how best to ameliorate climate change using nature-based green technology in the decades to come is going to take contributions from many disciplines.”

Forest growth as CO2 rises

The takeaway isn’t that forests won’t help, or that CO2 fertilization is a myth. It’s that nature’s response is conditional. 

On hotter, drier days trees “choose” survival over speed. They pull back on the very intake that would make them grow faster.

For policymakers and modelers, that argues for humility about banking on automatic forest gains from rising CO2. 

It also argues for sharpening strategies that protect the water side of the ledger, such as conserving soil moisture, limiting heat stress, and reducing other pressures that force trees into hydraulic triage.

In short: more carbon in the air does not guarantee more carbon in the wood. Between those two lies a living network of valves, vessels, and trade-offs, tuned by evolution to keep trees alive. If we want forests to store more for us, we have to keep them thirsty less.

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NOTE – This article was originally published in Earth and can be viewed here