Crown shyness is a phenomenon in which the crowns of 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. One theory as to why this occurs is that the tall and slender trees in windy areas may suffer physical damage, as they collide with each other during winds. To prevent abrasions and collisions, they respond with crown shyness. Another explanation is that the crown shyness inhibits spread of leaf-eating insect larvae.
A prominent hypothesis is that canopy shyness has to do with mutual light sensing by adjacent plants. The photoreceptor-mediated shade avoidance response is a well-documented behavior in a variety of plant species. Neighbor detection is thought to be a function of several unique photoreceptors. Plants are able to sense the proximity of neighbors by sensing back scattered far-red (FR) light, a task largely thought to be accomplished by the activity of the phytochrome photoreceptors. Many species of plant respond to an increase in FR light (and, by extension, encroaching neighbors) by directing growth away from the FR stimulus and by increasing the rate of elongation. Similarly, blue (B) light is used by plants to induce the shade-avoidance response, likely playing a role in the recognition of neighboring plants, though this modality was just beginning to be characterized as of 1998.
The characterization of these behaviors might suggest that crown shyness is simply the result of mutual shading based on well-understood shade avoidance responses.
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