Crown Shyness: When Trees Need Personal Space

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If you’ve ever walked among a cluster of trees or in a forest and looked up towards the sky, you may have noticed a phenomenon where the outstretched branches of one tree don’t touch neighboring tree branches. This condition is known as “crown shyness”. With crown shyness, the crowns full-grown trees don’t touch each other, creating river-like gaps in the canopy.

Crown shyness is most commonly found among trees of the same species and age but it can also occur in heterogenous forests. Crown shyness has been studied extensively, but researchers still don’t understand why it happens (MacDonald, 2018). There have been various hypotheses put forward over the decades. One theory is that mechanical abrasion from neighboring trees causes the gaps. As trees growing, colliding branches abrade and more fragile branches are trimmed off (Hastings et al., 2020). As growth continues, the cycle of abrasion and branch removal maintains the gaps.

Crown shyness in trees.  Photo: Felix Mittermeier, CC BY 2.0
Crown shyness in trees. Photo: Felix Mittermeier, CC BY 2.0

A different hypothesis proposed that the buds of trees have light receptors to sense when nearing adjacent vegetation and stop growing to avoid getting too close. The avoidance of growing into neighboring trees is believed to be an adaptive response to maximize access to sunlight while minimizing the harmful effects of competition (Franco, 1988).

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Franco, M. (1986). The influence of neighbours on the growth of modular organisms with an example from trees. Philosophical Transactions of the Royal Society of London. B, Biological Sciences313(1159), 209-225.

Hastings, J. H., Ollinger, S. V., Ouimette, A. P., Sanders-DeMott, R., Palace, M. W., Ducey, M. J., … & Orwig, D. A. (2020). Tree Species Traits Determine the Success of LiDAR-Based Crown Mapping in a Mixed Temperate Forest. Remote Sensing12(2), 309.

MacDonald, J. (2018, August 25). The Mysteries of Crown Shyness. JStor Daily.



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