Crown shyness: even trees need personal space

Trees can seem utterly alien in some ways, but in others they are not so different from us. While most are happy to rub branches with their neighbours, some are a little more touchy.

These trees, including some eucalypts and several species of Dryobalanops found in Malaysia and Indonesia, exhibit a phenomenon known as “crown shyness”.

Among full-grown trees, the crown of one keeps a safe distance from the crowns of others, meaning that the forest canopy seen from below contains empty lines and channels (as seen in the photo above).

Why do they do it? Arborists and botanists have studied crown shyness for almost a century but answers are elusive.

Some research suggests it is simply a physical process, whereby wind makes branches of neighbouring trees rub together vigorously enough to cause a sort of mutual pruning. A similar idea proposes that it is not quite pruning but an abrasion from wind-blown branches that prevents the growing rips of branches from sprouting forth.

Still another hypothesis involves the leaves of the tree detecting far-red light backscattered from the leaves of the neighbouring trees. Perhaps the simplest overarching proposal – though the mechanisms of shade-avoidance are complex – is that the trees detect shade caused by other trees, and try to avoid the shade.

This too seems familiar: not only do these trees like to have their own space, they also like to have their time in the sun.