The phenomenon of crown shyness has been discussed in scientific literature since the 1920s, but what causes it is not fully understood. One theory suggests that this empty space around the crown might be caused by breakage of
twigs and branches from violent collision that happens during storms and high winds. Experiments show that if trees with crown shyness are artificially prevented from swaying and colliding in the wind, they gradually fill in the
empty space in the canopy.
Crown shyness in Dryobalanops aromatica (Kapur trees) in Forest Research Center - Kuala Lumpur.
Researchers have also found that crown shyness is absent when the trees are young and short, but develop later once the trees grow in height and are able to sway in the wind. Trees with slender trunks have relatively small crowns because of their lesser ability to resist deflection in wind. They therefore sway widely in wind and are more likely to collide with neighbors.
One Malaysian scholar who studied Dryobalanops aromatica, however, found no evidence of abrasions due to contact in that tree. He suggested that the growing tips were sensitive to light levels and stopped growing when nearing the adjacent foliage.
Some suggest that trees exhibit this phenomenon to prevent leaf-eating insect larvae from spreading.
One of the handful of places where you can see the crown shyness phenomenon is the Forest Research Institute of Malaysia, in Kuala Lumpur. All the pictures accompanying this article were taken there.