From a tiny sapling can grow an enormous tree, but as Earth’s botanical behemoths literally branch out they can easily invade each other's personal space. While in some parts of the globe this results in a dense canopy that restricts the forest floor of sunlight, elsewhere trees get the ick when it comes to touching leaves, exhibiting what’s known as crown shyness.
What is crown shyness?
One of the earliest papers surrounding crown shyness was led by Professor Francis E Putz, who made a curious observation whilst standing beneath the canopy of a mangrove forest. Here, he could see that as the trees swayed in the breeze their tips collided, snapping off foliage and branches.
“The crowns of neighboring trees of a similar height to not interdigitate but rather are generally separated by spaces called “crown shyness” gaps,” reads the 1984 paper. “In a black mangrove (Avicennia germinans) forest in Costa Rica, the width of crown shyness gaps was positively correlated with the distance pairs of trees or branches adjacent to the gap swayed in the wind.”
Why does crown shyness happen?
There are several theories as to how crown shyness can emerge in the canopy, but two key theories center around abrasion and phytochromes.
Abrasion hypothesis
As in Putz’s observation of the mangrove forest branches colliding tips, some believe that crown shyness is the end result of tree tips catching and snapping on those of their neighbors. The constant abrasion of growth nodules and bud tissue would explain why the trees' ability to grow laterally is clipped, and it’s thought that a tree's sensitivity to this mechanism of growth control is dependent on the species. The wind is also a significant factor in this instance, as heightened weather conditions will increase the likelihood of trees touching tips as they sway.
This mechanism for crown shyness was supported in research led by Mark Rudnicki of Michigan Technological University who looked at the influence of wind-induced jostling among lodgepole pines in Canada. Investigations revealed that not only did the trees most likely to butt heads (indicated by their slim trunks and similar canopy heights) exhibit higher rates of crown shyness, but the research also found that when trees were held in place by ropes their crowns increased in size, interlocking with neighboring trees.
Phytochromes and shade-avoidance
Plants and trees contain proteins that are sensitive to light known as photoreceptors, and one of the most common is phytochrome. This photoreceptor is able to detect red light which sunlight is abundant in, and research into many species has found that plants are capable of altering their growth as a means of shade avoidance. Through this mechanism, it would stand to reason that gaps could appear in the canopy as when crowding occurs the trees' outermost branches will receive less light as the branches of other trees cast shade.
Shade could also be crucial in the role it plays not just on the trees’ tips but their trunks, too. A densely matted canopy would block out sunlight from the forest floor, limiting what could grow here. It’s feasible that by exhibiting crown shyness the trees bolster their own growth by improving the health of the forest floor.
Convergent evolution and crown shyness
While the concept of crown shyness has been circulating among the scientific community since around the 1920s, there remains much to be uncovered about the specifics of this phenomenon. While both theories could account for crown shyness, it’s not known if one mechanism applies more steadfastly to specific species than the other. If different botanical species have adapted to practice crown shyness but with differing motivations at its onset, this would be an example of convergent evolution where shared traits emerge among distantly related species.
Whatever the cause, it’s likely crown shyness carries many benefits for the trees who practice it, giving them the space to exist without snapping twigs and potentially keeping their tippy-toes happy too. Plus, it looks really, really cool.
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