One hypothesis is that the gap forms when trees collide with each other in the wind, and to prevent damage, the crowns stop growing. Experiments support that hypothesis. When researchers physically restrain trees from colliding in the wind, the crowns will grow to touch each other.
Malaysian scholar F.S.P. Ng offers competing hypothesis. According to Wikipedia, he “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. In Betula pendula (silver birch), fewer buds develop in parts of the crown that are already dense or where the crowns of different trees start meeting, possibly because of less light.”
Another reason tree crowns are shy might be to slow the spread of leaf-eating insect larvae.
Image: Canopy of Dryobalanops aromatica in Forest Research Center – Kuala Lumpur
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