From: Allison Winter, ENN
Published January 14, 2013 10:15 AM

Tree height and leaf size dependent on internal physics

The tallest trees in the world can grow up around 100 meters (think of a tree climbing the length of an entire football field!) but if a tree has all the necessary sunlight, water, and space what actually stops a tree from growing even taller?

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According to researchers at Harvard University and the University of California, Davis, the answers lie in the physics of a tree's internal plumbing.

"It all comes down to the leaf size and tree height that provide for the optimal flow of sap and energy throughout the tree," said Maciej Zwieniecki, a biologist in the UC Davis Department of Plant Sciences, who collaborated on the study with biophysicist Kaare Jensen of Harvard University.

Their findings offer a new explanation for the lack of tall trees in environments with limited water and the prevalence of the tallest trees in the world's wettest environments like tropical rain forests.

After analyzing data on 1,925 angiosperm tree species (the group of flowering plants like oaks and sycamores), researchers found that, overall, tree leaves range in size all the way from less than one inch to more than four feet long. The tallest of these trees, however, have leaves that range from 4 inches to 8 inches long.

Jensen and Zwieniecki suggest that this leaf size range that accompanies increasing tree height is due to the fluid dynamics within the tree.

The sugar-rich fluid produced through photosynthesis in the leaves travels to other parts of the tree via a system of channels called phloem. The researchers modeled the tree phloem system and saw that as the fluid passed through the leaf phloem toward the stem, it gathered speed as more and more water was pulled in from the leaf through osmosis. Consequently, the longer the leaf, the faster the fluid flowed.

The research proves that there is an advantage for a tree to have larger leaves because it could produce more nutrient-rich fluid that would flow more quickly toward the trunk and roots. The research also shows that taller trees would increase the length and thus flow resistance of the trunk phloem, slowing the movement of nutrients toward the roots.

"So there comes a point where the optimal limits on leaf size and tree height intersect, indicating the point at which it is no longer advantageous for the tree to become taller or producer larger leaves," Zwieniecki said.

The findings can be found in this week's issue of the journal Physical Review Letters.

Read more at UC Newsroom.

Tree image via Shutterstock.

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