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There are a few forests out there where the trees seem to be especially... polite. Can scientists explain why these species give each other space?

Hosted by: Hank Green

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Sources:
https://www.researchgate.net/publication/237866513_Crown_shyness_in_lodgepole_pine_stands_of_varying_stand_height_density_and_site_index_in_the_upper_foothills_of_Alberta
https://www.sciencedirect.com/science/article/pii/S0378112714002667
https://web.archive.org/web/20150925144427/http://www.sfmn.ales.ualberta.ca/en/Publications/~/media/sfmn/Publications/ResearchNotes/Documents/RN_E36_CrownShyness_low.ashx
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3350169/
https://edis.ifas.ufl.edu/hs186

Image Sources
https://www.flickr.com/photos/7147684@N03/2548928203/
https://commons.wikimedia.org/wiki/File:Lodgepole_pine_in_Washington.JPG
https://commons.wikimedia.org/wiki/File:Crown_Shyness.jpg
https://www.istockphoto.com/photo/the-gap-gm1022850616-274574877
https://www.videoblocks.com/video/beautiful-120-fps-slow-motion-shot-of-leafs-and-trees-blowing-in-the-wind-nature-atmospheric-contemplative-shot-rk35a7ehzj8f0lbtr
https://www.istockphoto.com/photo/winter-view-of-the-dark-hedges-gm1155827147-314801990
https://www.videoblocks.com/video/aerial-forest-after-rain-pb1135i
https://www.istockphoto.com/photo/forest-treetops-wide-angle-black-and-white-gm1019414000-273963805
https://www.videoblocks.com/video/sun-shines-through-tree-leaves-bdn-mwbuiq4xwlvr
https://www.videoblocks.com/video/plant-leaves-budding-tree-buds-timelapse-time-lapse-green-leaves-closeup-on-black-background-rewbo5t5vjuuxjql4
https://www.istockphoto.com/photo/close-up-of-pine-needles-gm480952861-37029146
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Go to Brilliant.org/SciShow to learn more. {♫Intro♫}. There are a few forests out there where the trees are especially… polite.

The tops, or crowns, of these trees don’t touch each other, leaving a few inches of space all around -- creating a bizarre, beautiful spiderweb of gaps in the canopy. Scientists refer to this phenomenon as crown shyness. It’s observed only in specific circumstances -- only in certain species, and often in stands of trees that are all the same species and age.

And while scientists don’t fully understand why it happens, they do have a few different hypotheses. The first thought is that crown shyness might be caused by contact between trees. When wind makes the trees sway back and forth, some that are too close may bump into each other, causing branches to break and leaves to fall, which is costly for the tree.

Trees can combat this by growing taller than their neighbors and out-competing them for sunlight and other resources. But trees can only grow so tall. And lodgepole pines, which are the tree of choice for studying crown shyness, cap out their crown size well before that -- when they are about eight to ten meters high.

When the tree reaches ten meters high, it’s more likely to hit other trees when it’s windy, which causes the branches of the crown to break once more. This damage to the tree’s branches limits the growth of future branches until eventually, they stop growing back, creating a small gap between the trees’ crowns. But when the trees are artificially immobilized so that they don't hit each other, they begin expanding their crowns again.

This may help explain why crown shyness is usually seen in mature stands of certain species. The crowns have reached their maximum width, so the space between them is maintained. Another potential cause of crown shyness could be the result of a physiological response to the shade of other trees.

Leaves contain pigments to absorb light. Chlorophyll, the primary pigment used in photosynthesis, and it absorbs red light. So if boughs of nearby trees are causing shade, the light that comes through is going to have less red light because of shading from the leaves.

Instead, that light will include a relatively large amount of far-red wavelengths, the extreme end of visible red light that isn’t absorbed by leaves. Specific pigments called phytochromes pick up on the ratio of red to far-red wavelengths. If they detect a lot of far-red light, the trees promote upward growth instead of outward growth and try and escape the shade.

But that just means we run into the height cap again. The trees can’t grow taller anymore, and the branches won’t grow outward towards other trees because of the shade, which leaves that space between them. One other possible explanation for crown shyness involves an idea known as allelopathy.

According to some scientists, this is a phenomenon where plants produce specific chemical compounds that influence the growth and behavior of neighboring plants. Some researchers have hypothesized that similar chemical compounds could be released by the leaves of these “shy” trees, and they limit the growth of their neighbors. But such a compound has never been identified, and many ecologists are far from sold on the idea.

There probably isn’t one specific cause for tree politeness. Instead, it’s likely that crown shyness occurs as a result of multiple mechanisms. But whatever the cause, these shy trees are a sight to see.

These trees make incredible patterns. If you want to know more about the math behind this spectacle, you might enjoy Brilliant.org’s Beautiful Geometry course, which uses fascinating patterns to teach you to love math. All of Brilliant’s courses can help you cultivate your math and scientific thinking skills.

From math to computer science, they can help you learn more about the rules that shape our world. Each course is designed to pull you in with interactive quizzes and hands-on guided problems. No dry walls of text here.

Best of all, you can get 20% off an annual Premium subscription if you’re one of the first 200 people to sign up at Brilliant.org/SciShow. {♫Outro♫}.