If you’ve ever looked up at a forest canopy and noticed that neighboring tree crowns don’t quite touch, you’ve seen a phenomenon known as crown shyness. These narrow, river-like gaps of light separate the upper branches of mature trees and occur in forests around the world.
Where is crown shyness found?
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.
Crown shyness has been observed in eucalyptus forests of Australia, Sitka spruce forests in North America, mangroves, lodgepole pines, and many tropical rainforest species. While it is most common among trees of the same species and similar age, it can also occur in mixed forests.
From below, crown shyness appears as dark, branching channels of sky separating neighboring tree crowns. The gaps resemble a network of rivers or cracks weaving through the forest canopy and are easiest to notice when looking straight up.
While crown shyness is most noticeable from the ground, aerial photography, drones, and high-resolution satellite imagery can also reveal the distinctive gaps between neighboring tree crowns in some forests. These overhead views help researchers study forest structure and canopy dynamics across entire landscapes.
What is canopy closure?
The opposite of crown shyness is canopy closure, where neighboring tree crowns overlap to form a continuous layer of leaves and branches. In forests with canopy closure, much less sunlight reaches the forest floor than in forests exhibiting crown shyness.
Why does crown shyness happen in trees?
Crown shyness has been studied extensively, but researchers still don’t know exactly why it occurs. Three leading explanations have emerged (MacDonald, 2018).
Mechanical Damage Theory
One explanation is that neighboring branches repeatedly brush against one another in the wind. Over time, these collisions wear away young twigs and shoots, maintaining the narrow gaps between adjacent tree crowns (Hastings et al., 2020).
Light Optimization Theory
Another hypothesis suggests that trees detect nearby vegetation through light-sensitive buds and slow branch growth before their crowns come into contact.
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
A third hypothesis suggests that crown shyness helps limit the spread of insects, parasites, and plant diseases. By avoiding direct contact between neighboring crowns, trees may reduce opportunities for pests and pathogens to move from one tree to another.
Benefits of crown shyness in forests
The gaps created by crown shyness allow better sunlight penetration down to 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.
The gaps also improve air circulation through the canopy, which may help reduce humidity around leaves and create less favorable conditions for fungal diseases. Increased sunlight reaching the forest floor also supports a greater diversity of understory plants.
Looking at forests differently
The next time you walk beneath a forest canopy, take a moment to look up. Those intricate gaps between neighboring tree crowns are more than a beautiful pattern. They are one of the forest’s enduring mysteries, offering a glimpse into the complex ways trees grow, compete, and coexist with their neighbors.
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 Sciences, 313(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 Sensing, 12(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/
This article was originally written on June 19, 2020 and has since been updated.
