I Have Always Associated The Phrase,

'Crown Shyness', 

 with hair loss, as in Male Pattern Baldness, but it appears the term Crown Shyness also applies to some trees, 

it appears it 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 (optimising light exposure for photosynthesis) or even as a deterrent for the spread of harmful insects,

in any event next time I am in a forest I must remember to look up to see if the phenomenon is happening above me.