Crown Shyness: When Trees Need Personal Space

Caitlin Dempsey

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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 upper branches that form the crowns full-grown trees don’t touch the upper branches of neighboring trees, creating river-like gaps of light in the canopy where the sun shines through.

Crown shyness is most commonly found among trees of the same species and age but it can also occur in heterogenous forests. Crown shyness is also known as canopy disengagement. Canopy disengagement is a natural phenomenon where the canopies of individual trees avoid overlapping leaves and branches with those of adjacent trees. This phenomenon can be found in forests globally.

The opposite of crown shyness in a forest is known as canopy closure.


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Two side by side photos - one on the left shows overlapping branches in a forest, the one of the left shows branches that don't meet from neighboring trees.
The forest on the left is not exhibiting crown shyness. The branches of neighboring trees overlapping. The forest on the right shows tress exhibiting crown shyness with gaps of light between the branches. Photos: (L) Caitlin Dempsey, (R) NPS.

Why does crown shyness happen in trees?

Crown shyness has been studied extensively, but researchers still don’t understand why it happens (MacDonald, 2018).

There have been various theories put forward over the decades as to why crown shy exists in some forests.

Mechanical Damage Theory

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. Gaps between the tree canopies on Fire Island.  Photo: NPS, public domain.
Crown shyness in trees. Gaps between the tree canopies on Fire Island. Photo: NPS, public domain.

Light Optimization Theory

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). This theory suggests that crown shyness is a phototropic response, where trees grow towards the most abundant light source to maximize photosynthesis. The phenomenon is seen as a strategy for individual trees to capture and utilize light most efficiently.

Insect and Disease Avoidance

The third prevailing theory argues that crown shyness serves as a defense mechanism against the spread of disease and pests. By avoiding physical contact, trees minimize the risk of having pathogens or parasites transfer to that tree fromadjacent trees.

Benefits of crown shyness in forests

The gaps created by crown shyness allows better sunlight penetration down tot the forest floor. The greater amount of sunlight enhances the forest understory’s ability to support various plant species through photosynthesis, the process by which plants convert sunlight to sugars. This can lead to higher levels of biodiversity within the forest, creating a more resilient ecosystem.

Watch: Crown Shyness – When Trees Need Personal Space

References

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. https://www.jstor.org/stable/2396899

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. https://doi.org/10.3390/rs12020309

MacDonald, J. (2018, August 25). The Mysteries of Crown Shyness. JStor Daily. https://daily.jstor.org/the-mysteries-of-crown-shyness/

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.