Study solves mystery of uneven tree growth in a ‘carbon-enriched’ world

Photosynthesis [image credit: Nefronus @ Wikipedia]

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Researchers found, by ‘using an engineer’s approach’, that trees in warming and drying conditions may absorb less CO2 than before, due to their stomata (the leaves’ pores) getting smaller to reduce water loss from evaporation. One for ‘plant millions of trees’ advocates to ponder.
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The basics of photosynthesis are something that every student learns in school: carbon dioxide, water and light in; oxygen and sugar for growth out, says Eurekalert.

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 December 1 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.”
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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.

Looking at the interplay between stomatal opening, carbon levels and water loss as an optimization problem is a new approach to complement physiological theories, Katul explained. But it has proven accurate in describing results from the benchmark experiments at Duke and ETH Zurich.

Full news release here.
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Image: Photosynthesis [credit: Nefronus @ Wikipedia]