Crown Shyness

Crown shyness is a naturally occurring phenomenon in some tree species where the upper most branches in a forest canopy avoid touching one another. The visual effect is striking as it creates clearly defined borders akin to cracks or rivers in the sky when viewed from below. Although the phenomenon was first observed in the 1920s, scientists have yet to reach a consensus on what causes it. According to Wikipedia it might simply be caused by the trees rubbing against one another, although signs also point to more active causes such as a preventative measure against shading (optimizing light exposure for photosynthesis) or even as a deterrent for the spread of harmful insects.

(via Kottke, Robert Macfarlane)