"Crown shyness"

Crown shyness is a phenomenon observed in some tree species, in which the crowns of fully stocked trees do not touch each other, forming a canopy with channel-like gaps. It is also known as canopy disengagement, canopy shyness, or intercrown spacing.
The term was new to us, but the concept was not; it's quite readily observable by anyone who spends a significant amount of time in the woods.  What did surprise us was the notion that the cause of this phenomenon is unknown; we had always assumed it was a manifestation of contact inhibition of growth. Apparently it's not so simple.