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Crown Shyness

Ralff Nestor Nacor
Wikimedia Commons

Imagine that you’re in a forest on a windless, still day. You lie on your back and look up at the treetops high above you. Notice that none of the individual tree crowns touch each other, so that tracks of blue sky show in between them.

That phenomenon is what botanists call “crown shyness.” It occurs in forests around the world where trees are all the same age. But the type of tree doesn’t seem to make a difference; those open tracks can appear between different species, the same species or even within the same tree.

For over a century, scientists have posed different theories.

Some suggest that competition for light is the key. Tree leaves contain photoreceptors, proteins that can detect and interpret the quality of light that arrives on leaf surfaces.

When receptors detect an increase in the far-red light that bounces off the leaves of their neighbors — rather than direct light that originates from the sun — a tree can shift its growth away from its neighbor’s light. The resulting gaps minimize direct competition for light capture. So, crown shyness might be all about plant physiology.

But there’s another hypothesis that makes sense if you returned to that same forest on a windy day. You’d see that the air movement in the canopy makes one tree’s crown hit up against its neighbor’s, scraping against its buds and twigs — creating wind-driven pruning!

In 2008, researchers photographed the crown-shy canopy of a Canadian pine forest, and then roped individual trees to prevent their crowns from colliding in the wind. Six years later, the crowns had grown toward each other, filling in the empty spaces between them, evidence that crown shyness might instead be about the physical effects of those crown collisions.

So, is it physiology? Bud-bumping? Or both?

That’s part of the fun of learning about trees: scientists see patterns in nature and arrive at different conclusions. It’s the searching for answers that makes it interesting.

Dr. Nalini Nadkarni is an emeritus professor of both The Evergreen State College and the University of Utah, one of the world’s leading ecologists and a popular science communicator. Dr. Nadkarni’s research and public engagement work is supported by the National Geographic Society and the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. @nalininadkarni
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