How trees decide between growing and breathing

Trees begin their day by opening tiny pores under their leaves called stomata. These pores allow them to absorb carbon dioxide (CO₂) from the air – a key ingredient for photosynthesis.

But there’s a tradeoff. As they “breathe” in CO₂, trees also lose water. A single tree can lose several hundred liters of water per day this way.

To protect themselves, plants close these pores when water becomes scarce. This behavior isn’t new to scientists. But what has remained a mystery – until now – is when exactly this happens and what drives the decision to shut down.

Researchers in the Department of Environmental Sciences at the University of Basel have discovered a surprising answer.

It’s not all about photosynthesis

At first glance, photosynthesis seems to be the priority. That’s long been the belief. After all, plants need sunlight and CO₂ to make food. But the team from the University of Basel, using data from their forest lab in Hölstein, found that trees don’t always follow this rule.

“When it comes to plants, researchers have traditionally focused on photosynthesis. So it was previously assumed that trees treated this process as a priority and therefore kept the stomata open for as long as possible to absorb CO₂, only closing them when there was no other option,” explained study co-author Professor Ansgar Kahmen.

This thinking makes sense. The act of absorbing CO₂ is tied to water loss through the stomata. So it seemed reasonable that plants would risk losing water to keep photosynthesis going, only shutting down when things got extreme. But the new research says otherwise.

Trees grow at night – but only with water

Water doesn’t just help with photosynthesis – it keeps the tree alive and growing. During the day, trees lose water through evaporation. At night, they work to replenish what was lost. The stomata stay closed, and water flows upward from the roots to refill the cells.

This refill process creates pressure in the cells, known as turgor pressure. Without it, trees can’t grow. That’s why most tree growth happens at night.

If the soil is dry, though, trees can’t draw in enough water. Their cells stay under-filled. Without that pressure, they can’t expand or grow – even if they have all the sugars they need from photosynthesis.

And it gets worse. As drought intensifies, the water pathways inside the tree can break down. This collapse can create air bubbles – called embolisms – that block water movement.

“When this happens, irreparable damage occurs, the water transport system collapses and the plant eventually dies,” explained Kahmen.

When water refills stop

The big surprise from the team was that trees are much more cautious than expected. They don’t wait for a crisis to close their stomata. If they can’t get enough water during the night, they simply don’t open the stomata the next morning.

“For the first time, we were able to show that a tree does not even open its stomata in the morning if it cannot absorb enough water overnight,” said Kahmen.

In other words, they skip photosynthesis to protect their ability to grow. This may sound counterintuitive, but it’s a smart survival strategy. Growth is what allows a tree to build new tissue, repair damage, and keep going. If it can’t grow, making more sugars doesn’t help.

“So the aim is not to optimize photosynthesis and maintain it for as long as possible, but to use the products of photosynthesis as efficiently as possible for growth,” said the plant physiologist.

Predicting forest carbon capture

The new findings could have a major impact on how we predict forest carbon capture. Trees take in less carbon than expected during dry periods. That’s because their stomata close earlier and stay shut for longer than previously thought.

Study lead author Richard L. Peters is a former postdoc at the University of Basel and now professor at the Technical University of Munich.

“Climate models that assume a certain growth in carbon storage volume would therefore have to be adapted,” said Peters.

Tree decisions shape their growth

The findings are especially relevant as summers become hotter and drier in many places, including Switzerland. If trees shut down photosynthesis earlier, forests might store less carbon than climate models currently assume.

“What is remarkable is that our early stomatal closure observations apply to all tree species, whether deciduous or coniferous. How well a tree species will cope with drought, therefore, cannot be solely determined by the process of stomata closure,” noted Peters.

Trees might seem passive, but they’re making active decisions every day. And thanks to new research, we’re finally starting to understand how those decisions shape their growth – and our planet.

The full study was published in the journal Nature Plants.

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