We know that shyness affects both humans and animals, but what about plants? It turns out that yes, certain species of trees develop what is botanically referred to as shyness, which consists of an unusual growth of their branches that avoids touching those of neighboring trees. When this happens between specimens that are very close and form a tree canopy, you can observe the gaps they leave between themselves to avoid contact. The explanation for this phenomenon is unclear, and there are several hypotheses.
A tree canopy is the kind of forest ceiling formed when the treetops come together, creating a continuous vegetative dome. However, sometimes cases are found where adjacent trees limit the growth of their branches in each other’s direction, creating a kind of fissure between the two canopies.
This is what is known as crown shyness, a term coined in the 1950s by Australian botanist Maxwell Ralph Jacobs, and it’s particularly interesting that it usually occurs among specimens of the same species, though it can sometimes affect different species as well.
In this sense, it’s not a generalized behavior among trees but rather a characteristic of certain species that, apparently, have nothing in common. Examples include the Pinus contorta from North America, the tropical black mangrove, the Japanese larch, and certain types of eucalyptus, but especially the Dryobalanops genus, native to places like Sumatra, the Malaysian Peninsula, and Borneo. You can see that, except for the mangroves, the others are trees that tend to grow taller rather than wider.
As mentioned, many possible explanations have been proposed to explain tree shyness, and experts have not reached any consensus despite debating the topic for a century. Today, it’s thought that there isn’t a single cause, but that each species has developed this adaptive mechanism for different reasons. This is known as convergent evolution: the development of two unrelated structures through similar means, as can happen with the flight of birds and bats, for example, or the swimming ability of fish and seals.
Specifically, one theory suggests an original type of symbiosis: in windy areas, branches tend to collide, damaging each other and creating those gaps in the forest canopy. To avoid or minimize this, the trees develop shyness. This idea arose from the study of these branches, which shows that they grow normally until abrasions occur, at which point their growth stops, creating a safe distance. Likewise, when measures are taken to prevent these collisions (such as pruning), the branches grow back and cover the gap between the two canopies.
Related to this, it has also been observed that the halting of growth in cases of abrasions is due to traumatic damage to the tissues in the branch nodes, which inhibits their growth—something particularly interesting because it has been documented in distant locations. By the way, the danger of damage doesn’t just come from the wind but also from various parasites, such as leaf miners, insect larvae that live in leaf tissues, feeding only on layers with less cellulose and leaving the others as protection. Shyness would help slow their spread.
Other explanations come from very different assumptions. For instance, some highlight the importance of access to light in a forested habitat, where thick foliage can make it difficult. We know that plants have phytochromes (photoreceptor proteins capable of perceiving red light), which trigger a response in the plant based on the signal; flowering, germination, and metabolic regulation based on day or night are some of these responses, and among them is also an evasive reaction to shade.
This would manifest by stopping leaf growth when too much proximity to the neighboring canopy is detected, with the resulting risk of being shaded, thus creating gaps in the forest canopy. As the word implies, light is crucial for photosynthesis (the chemical process by which chlorophyll-containing plants convert inorganic substances into organic ones to obtain energy), which is why other species have developed alternative methods to seek it in shady environments, such as Socratea exorrhiza, a type of Central and South American palm that moves in search of clearings where sunlight penetrates.
These ideas are complemented by another equally fascinating one: that trees maintain a kind of communication among themselves through the emission of volatile organic compounds, chemical substances released into the air by their leaves, which help coordinate some of their physiological processes with other trees. This ability is known as allelopathy, the capacity of some plants to influence others through chemical compounds. However, a cause-effect relationship, if any, has yet to be established, as there’s no scientific evidence proving that it leads to shyness.
All of this may be surprising, but it’s even more so when we look at the case of Arabidopsis, a genus of herbs native to Europe that may sound familiar to some readers because one of its species, Arabidopsis thaliana, was the first plant to have its genome fully sequenced back in December 2000 (or perhaps it’s more familiar because NASA announced plans to grow it on the Moon and Mars). The interesting thing is that Arabidopsis also exhibits shyness in certain situations, but with a fascinating twist: it restrains the growth of its leaves when they are about to contact those of a fellow species member but intensifies growth when they can cast shade over neighbors of another species.
Perhaps the plant world isn’t so different from the human one after all…
This article was first published on our Spanish Edition on November 8, 2018: El curioso fenómeno de la timidez de los árboles
SOURCES
Kevin O’Hara, Multiaged Silviculture: Managing for Complex Forest Stand Structures
Francis E. Putz, Geoffrey G. Parker, Ruth M. Archibald, Mechanical Abrasion and Intercrown Spacing
R. G. Florence, Ecology and Silviculture of Eucalypt Forests
James W. Goudie, Kenneth R. Polsson, Peter K. Ott , The effects of including a model of crown shyness for lodgepole pine in a spatially explicit crown grouth model
Wikipedia, Timidez (botánica)
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