Edward Willett

The science of tall trees


tall treeSometimes science is focused on really big questions: where did life come from? How did the universe begin?

But sometimes, the focus is much smaller. Sometimes, researchers set out to answer a simple question, one that many people have perhaps asked, but no one has ever set out systematically to answer.

A question, for example, about trees.

Trees are everywhere. You’d think there’d be very little to learn about them at this late date. But there are still questions to be asked and answered.

For example…why do the tallest trees all top out at about the same height? And why are the leaves of those trees all pretty much the same size?

That was the question that Maciej Zwieniecki, a biologist in the UC Davis Department of Plant Sciences, and biophysicist Kaare Jensen of Harvard University set out to answer. And though the question itself falls firmly in the realm of biology, they found the answer lies within physics.

Also, plumbing.

According to Zwieniecki, “It all comes down to the leaf size and tree height that provide for the optimal flow of sap and energy throughout the tree.”

The researchers chose to focus on angiosperm trees (angiosperms are the flowering plants; angiosperm tree species include things like oaks and sycamores, as opposed to the gymnosperm trees like pine and spruce).

They analyzed data on 1,925 angiosperm tree species and found leaf lengths ranging from less than one inch all the way up to more than four feet. But there was nowhere near that much variation in leaf size in the tallest angiosperm species, all of which had leaves between about four inches and eight inches long.

The reason, Zwieniecki (boy, is that a hard name to type) and Jensen say, lies in the tree’s fluid dynamics: that is, the way fluids move through it.

Plants, of course, feed themselves through photosynthesis, a process which uses the energy from sunlight to convert water, carbon dioxide and minerals into carbohydrates: plant food.

This plant food takes the form of a sugar-rich fluid, which is transferred from the leaves to other parts of the tree via a system of channels, called phloem.

The researchers simplified the problem by creating a computer model of the phloem, treating it as if it were composed of permeable, cylindrical tubes. In the model, as in a tree, the leaf phloem collected the sugar-rich fluid and generated energy to transport it down to the much longer tube running through the trunk down to the roots.

What they found was that the fluid gathered speed as it passed through the leaf phloem as more and more water was pulled into it from the leaf through osmosis, just as tiny rivulets high on a mountain flow together to create a rushing river further down.  The longer the leaf, the faster the fluid flowed.

Once the liquid reached the trunk, no more sugar was being pulled into it from the surrounding tissue: instead, it drew only water. The much longer trunk phloem also presented much more resistance to the fluid’s flow.

Larger leaves can be beneficial because they produce more nutrient-rich fluid, which flows more quickly toward the trunk and the roots. Trunk height can also be beneficial, because it provides better access to sunlight—but trunk height also increases the resistance faced by the nutrients. A very tall trunk provides enough resistance that there is no benefit in a larger leaf size.

Within the model, those two things—optimal leaf size and optimal trunk size—intersect at a point where the trunk is about 100 metres tall and the leaves are no more than eight inches long: just like in the tallest angiosperms in nature.

These findings also help explain why there are very few tall trees in environments with limited water; or, to put it another way, why the tallest trees are found in the world’s wettest environments, such as tropical rain forests or foggy river ravines. To grow tall, a tree has to have a substantial flow of liquid coming from its leaves and tissue in order to overcome the resistance of its trunk. In a dry environment, that liquid just isn’t available.

An earth-shaking discovery? No. But it’s a little too easy to focus on the big discoveries in science. Most are of a much smaller scale…but of such small pieces is the mosaic of our understanding of the universe constructed: and you never know when one small piece will suddenly make a much larger section become clear.

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