Leaves’ Pores Explain Longstanding Mystery of Uneven Tree Growth in a Carbon-Enriched World

The basics of photosynthesis are something that every student learns in school: carbon dioxide, water and light in; oxygen and sugar for growth out. In a world where atmospheric carbon dioxide levels are rising, it is plausible to think that trees and other plant life growth will rise in lockstep.   

But that is not what observations have borne out. As global levels of carbon dioxide have risen, measurements of tree growth—and how much carbon they are storing for the long-term—have varied greatly. How much of that variance can be attributed to carbon dioxide levels has long been unknown.

In a paper published online on November 24 in the journal Nature Climate Change, researchers led by Duke University and Wuhan University describe a model that answers many of these questions. By looking at the tradeoffs between taking in more carbon dioxide to grow and losing water to evaporation, they show how an engineer’s view of this delicate balance in the pores of a tree’s leaves can explain and predict its growth over decades and centuries.

“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, the George Pearsall Distinguished 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.”

The benchmark experiments Katul is referring to took place at Duke University and ETH Zurich to investigate how much carbon the world’s forests might capture in a future carbon-rich atmosphere. Over the course of 16 years, the Duke site fed groups of trees excess carbon dioxide while the ETH Zurich site increased their local humidity levels. By closely measuring tree growth and carbon sequestration, and monitoring many other variables, researchers showed that trees in general would not take in nearly as much carbon as previously conjectured.

But the reasons why were still not fully understood. To help explain these results, and dozens of others from around the world, Katul and his collaborators turned to an engineer’s view of water movement in a tree.

For a tree to take in carbon dioxide, it must open pores on its leaves called stomata. With more carbon dioxide in the atmosphere, the working assumption has been that proportionally more carbon dioxide would enter these pores.

However, in warmer and drier environments, water evaporates from these pores into the atmosphere more quickly. To keep their internal water systems balanced, trees compensate by making their stomatal pores smaller, which in turn leads to them absorbing less carbon dioxide.

This dynamic causes a direct tradeoff between gathering more carbon dioxide to grow and losing water needed to survive. And to complicate matters further, there is a delicate balance of water tension held throughout a tree’s roots, trunk and limbs that risks disruption if too much water is lost too quickly, especially as trees reach their mature heights.

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