Sometimes trees can be a little too respectful of one another's boundaries. Or maybe they just stop growing when they get too close.

The phenomenon is called crown shyness — when the tops of individual trees avoid touching in the forest canopy, creating separation lines and boundaries in the sky.

Why it happens

This rainforest canopy at the Forestry Research Institute Malaysia shows crown shyness in Kapur trees.
This rainforest canopy at the Forestry Research Institute Malaysia shows crown shyness in Kapur trees. (Photo: Mikenorton/Wikimedia Commons)

Experts aren't exactly sure why the naturally occurring phenomenon happens, but they've been studying it for decades and have a few theories.

The first has to do with competition for resources — especially light, according to Venerable Trees, a conservation nonprofit. Trees have a highly sophisticated system for measuring light and telling time, the organization says. They can tell whether light is coming from the sun or if it's being reflected off leaves. Leaves have been shown to detect far-red light bouncing onto them after hitting trees close by.

When they discern that light is being reflected off leaves, that's a signal: "Hey, there’s another plant nearby, let’s slow down growth in that direction."

It's a way for trees to optimize light exposure for everything under the canopy. As JSTOR Daily reports:

According to this theory, each tree forces its neighbors into a pattern that maximizes resource collection and minimizes harmful competition. Whether by accident or by design, crown shyness functions as a form of truce between competitors with limited options.

Another possible reason for crown shyness is to prevent the spread of harmful insects and their larvae, which could eat the tree's leaves.

Where it happens

https://www.youtube.com/watch?v=nS10ArRV49k

Crown shyness occurs with many species of trees, such as black mangrove trees, camphor trees, eucalyptus, Sitka spruce and Japanese larch. Intercrown spacing can happen between different species, the same species or even within the same tree. You can see this intercrown spacing in action in the video above.

Trees at Plaza San Martín in Buenos Aires, Argentina
Trees at Plaza San Martín in Buenos Aires, Argentina (Photo: refractor/Wikimedia Commons)

Crown shyness doesn't happen all the time, and it can occur in any forest.

You're more likely to see crown shyness in a tropical forest, which tend to have flatter canopies, according to Venerable Trees. For example, the photo above is from a park in Buenos Aires, and the one below is from a research facility in Malaysia; both are tropical climates.

Shy, but still connected

A canopy of <I>Dryobalanops aromatica</I> at the Forestry Research Institute Malaysia.
A canopy of Dryobalanops aromatica at the Forestry Research Institute Malaysia. (Photo: Patrice78500/Wikimedia Commons)

The Smithsonian describes crown shyness as "a giant, backlit jigsaw puzzle. A thin, bright outline of light isolates each tree from the others."

It helps to think of each tree as an individual island in the forest, says Steve Yanoviak, a researcher at the Smithsonian Tropical Research Institute in Panama. These "islands" are still connected via a network of woody vines known as lianas that act like telephone lines.

Generally, bigger islands have more species than smaller islands. Yanoviak's research shows the same is true in trees. For example, trees with lianas had more than 10 species of ants on them, whereas trees without the communication lines were home to 8 or fewer species of ants.

Editor's note: This article has been updated since it was originally published in August 2017.

Angela Nelson ( @bostonangela ) is an exhausted mom of two young daughters and two old cats, and a Pulitzer Prize-winning digital editor with more than 15 years of experience delivering news and information to audiences worldwide.