Sometimes trees can be a little too respectful of another tree's boundaries. Or maybe they just don't like each other and don't want to hold hands.
The phenomenon is called crown shyness — when the tops of individual trees don't touch in the tree canopy, creating separation lines and boundaries that offer a defined line in the sky.
The reason why 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. 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.”
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.
The Smithsonian describes it as "a giant, backlit jigsaw puzzle. A thin, bright outline of light isolates each tree from the others." Steve Yanoviak, a researcher at the Smithsonian Tropical Research Institute (STRI) in Panama, says to think of each tree as an individual island in the forest. At the same time, these "islands" are still connected via a network of woody vines known as lianas, the Smithsonian says, that act like telephone lines.