Have you ever seen in the woods that blade of light that passes between one hair and another when you raise your head? The crown of the tree grows, but it knows – feels – where the other grows and does not dare to touch it. She’s shy. When the other hair is close it stops growing in her direction. She doesn’t invade her, she doesn’t touch her
If you were under the canopy of trees in a forest of eucalyptus , but also of Sitka spruce or Japanese larch , you would immediately notice the regular spacing lines that are observed between the tallest leaves. The upper branches of these species, but also of others (you can find them on the dedicated Wikipedia page ) avoid touching each other, creating characteristic escape routes between the branches of the foliage.
Crown shyness is a behavior that only some plants assume. It consists in the development of an arboreal vault in which the crowns of the different trees do not touch, composing what can be described from above as a mosaic. This phenomenon, although well documented, however, lacks a definitive explanation.
Mechanical abrasion hypothesis
Side branch growth is thought to be affected by what surrounds the tree canopy. In this case, especially in windy regions, the phenomenon of shyness would be a response induced by the friction that is released between adjacent foliage. Experiments conducted in this direction reveal that if this contact is artificially avoided, there is no inhibition of the growth of the lateral branches.
Photoreceptive hypothesis
According to this vision, the phenomenon of ” crown shyness ” is due to the reciprocal shielding that the hair has on the adjacent ones. In this case the shyness would be due to the phenomenon of “escape from the shadow”, well known to botanists. Plants, through their own photoreceptors (phytochromes), perceive the presence of a nearby tree thanks to the change in quality of light that reaches the photoreceptors themselves. As a consequence, the plant stops growing laterally, preferring instead a vertical extension.
Adaptive hypothesis
It is the most recent explanation for this phenomenon. According to some scientists, the space left between the foliage would serve to reduce the possibility of parasites or herbivorous animals to move through the forest using the foliage. Over time, in fact, those plants that grew keeping a certain distance from the neighboring foliage would have been privileged. This probably allowed these plants to reduce attacks by herbivorous insects (such as moth larvae) or to decrease the frequency with which they were attacked by phytoparasites.
Given the numerous hypotheses and the presence of this phenomenon in phylogenetically distinct groupings of plants, scientists think that the shyness of the foliage is a case of evolutionary convergence. It is the mechanism by which similar behaviors or characteristics develop independently.
Focus Fitocromo
This receptor is sensitive to red (R) and far red (FR) light. In full sunlight R and FR are equally present and once the ray hits the leaf, it mainly absorbs the red component. Therefore, in the shadow of a leaf, the light that will filter will be poor in R. This modification of the quality of light is perceptible by the leaves of other plants which will thus interrupt the growth in that direction, giving rise to the phenomenon of crown shyness.
References
- Ecology of Woodlands and forests – P.A. Thomas, J.R. Packham
- Mechanical Abrasion and Intercrown Spacing – repository.si.edu
- What is a forest? – zoo.org
