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Crown shyness (also ''canopy disengagement'', ''canopy shyness'', or ''inter-crown spacing'') is a phenomenon observed in some tree species, in which the crowns of fully stocked trees do not touch each other, forming a
canopy Canopy may refer to: Plants * Canopy (biology), aboveground portion of plant community or crop (including forests) * Canopy (grape), aboveground portion of grapes Religion and ceremonies * Baldachin or canopy of state, typically placed over an ...
with channel-like gaps. The phenomenon is most prevalent among trees of the same species, but also occurs between trees of different species. There exist many hypotheses as to why crown shyness is an
adaptive behavior Adaptive behavior is behavior that enables a person (usually used in the context of children) to cope in their environment with greatest success and least conflict with others. This is a term used in the areas of psychology and special education. ...
, and research suggests that it might inhibit spread of
leaf-eating In zoology, a folivore is a herbivore that specializes in eating leaves. Mature leaves contain a high proportion of hard-to-digest cellulose, less energy than other types of foods, and often toxic compounds.Jones, S., Martin, R., & Pilbeam, D. (1 ...
insect
larvae A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. T ...
.


Possible physiological explanations

The exact physiological basis of crown shyness is not certain. The phenomenon has been discussed in scientific literature since the 1920s. The variety of hypotheses and experimental results might suggest that there are multiple mechanisms across different species, an example of
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
. Some hypotheses contend that the interdigitation of canopy branches leads to “reciprocal pruning” of adjacent trees. Trees in windy areas suffer physical damage as they collide with each other during
wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ...
s. As the result of abrasions and collisions, there is an induced crown shyness response. Studies suggest that lateral branch growth is largely uninfluenced by neighbours until disturbed by mechanical abrasion. If the crowns are artificially prevented from colliding in the winds, they gradually fill the canopy gaps. This explains instances of crown shyness between branches of the same organism. Proponents of this idea cite that shyness is particularly seen in conditions conducive to this pruning, including windy forests, stands of flexible trees, and early succession forests where branches are flexible and limited in lateral movement. By this explanation, variable flexibility in lateral branches has a large bearing on degree of crown shyness. Similarly, some research suggests that constant abrasion at growth nodules disrupts bud tissue such that it is unable to continue with lateral growth. Australian forester M.R. Jacobs, who studied the crown shyness patterns in
eucalyptus ''Eucalyptus'' () is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae. Along with several other genera in the tribe Eucalypteae, including '' Corymbia'', they are commonly known as ...
in 1955, believed that the trees' growing tips were sensitive to abrasion, resulting in canopy gaps. Miguel Franco (1986) observed that the branches of ''
Picea sitchensis ''Picea sitchensis'', the Sitka spruce, is a large, coniferous, evergreen tree growing to almost tall, with a trunk diameter at breast height that can exceed 5 m (16 ft). It is by far the largest species of spruce and the fifth-larg ...
'' (Sitka spruce) and ''
Larix kaempferi ''Larix kaempferi'', the Japanese larch or karamatsu () in Japanese, is a species of larch native to Japan, in the mountains of Chūbu and Kantō regions in central Honshū.Farjon, A. (1990). ''Pinaceae. Drawings and Descriptions of the Genera ...
'' (Japanese larch) suffered physical damage due to abrasion, which killed the leading shoots. A prominent hypothesis is that canopy shyness has to do with mutual light sensing by adjacent plants. The photoreceptor-mediated
shade avoidance Shade avoidance is a set of responses that plants display when they are subjected to the shade of another plant. It often includes elongation, altered flowering time, increased apical dominance and altered partitioning of resources. This set of r ...
response is a well-documented behavior in a variety of plant species. Neighbor detection is thought to be a function of several unique photoreceptors. Plants are able to sense the proximity of neighbors by sensing backscattered far-red light, a task largely thought to be accomplished by the activity of the
phytochrome Phytochromes are a class of photoreceptor in plants, bacteria and fungi used to detect light. They are sensitive to light in the red and far-red region of the visible spectrum and can be classed as either Type I, which are activated by far-re ...
photoreceptors. Many species of plant respond to an increase in far-red light (and, by extension, encroaching neighbors) by directing growth away from the far-red stimulus and by increasing the rate of elongation. Similarly, blue light is used by plants to induce the shade-avoidance response, likely playing a role in the recognition of neighboring plants, though this modality was just beginning to be characterized . The characterization of these behaviors might suggest that crown shyness is simply the result of mutual shading based on well-understood shade avoidance responses. Malaysian scholar Francis S.P. Ng, who studied ''
Dryobalanops aromatica ''Dryobalanops aromatica'', commonly known as Borneo camphor, camphor tree, Malay camphor, or Sumatran camphor, is a species of critically endangered plant in the family Dipterocarpaceae. The species name ''aromatica'' is derived from Latin (''ar ...
'', suggested that the growing tips were sensitive to light levels and stopped growing when nearing the adjacent foliage due to the induced shade. A 2015 study has suggested that ''
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter ...
'' shows different leaf placement strategies when grown amongst kin and unrelated conspecifics, shading dissimilar neighbors and avoiding kin. This response was shown to be contingent on the proper functioning of multiple photosensory modalities. A 1998 study proposed similar systems of photoreceptor-mediated inhibition of growth as explanations of crown shyness, though a causal link between photoreceptors and crown asymmetry had yet to be experimentally proven. This might explain instances of intercrown spacing that are only exhibited between conspecifics.


Species

Trees that display crown shyness patterns include: * Species of ''
Dryobalanops ''Dryobalanops'' is a genus of flowering plants and the genus of family Dipterocarpaceae. The name ''Dryobalanops'' is derived from Greek (''dryas'' = a nymph associated with oaks and ''balanops'' = acorn) and describes the acorn-like nut. The ge ...
'', including ''
Dryobalanops lanceolata ''Dryobalanops lanceolata'' is a species of plant in the family Dipterocarpaceae. The species name is derived from Latin (' = shaped like the head of a spear) and refers to the shape of the leaf. This species is endemic to Borneo. It is common in ...
'' and ''
Dryobalanops aromatica ''Dryobalanops aromatica'', commonly known as Borneo camphor, camphor tree, Malay camphor, or Sumatran camphor, is a species of critically endangered plant in the family Dipterocarpaceae. The species name ''aromatica'' is derived from Latin (''ar ...
'' (kapur) * Some species of
eucalypt Eucalypt is a descriptive name for woody plants with capsule fruiting bodies belonging to seven closely related genera (of the tribe Eucalypteae) found across Australasia: ''Eucalyptus'', ''Corymbia'', ''Angophora'', '' Stockwellia'', ''Allosyn ...
* ''
Pinus contorta ''Pinus contorta'', with the common names lodgepole pine and shore pine, and also known as twisted pine, and contorta pine, is a common tree in western North America. It is common near the ocean shore and in dry montane forests to the subalpine ...
'' or lodgepole pine * ''
Avicennia germinans ''Avicennia germinans'', the black mangrove, is a shrub or small tree growing up to 12 meters (39 feet) in the acanthus family, Acanthaceae. It grows in tropical and subtropical regions of the Americas, on both the Atlantic and Pacific Coasts, ...
'' or black mangrove * ''
Schefflera ''Schefflera'' is a genus of flowering plants in the family Araliaceae. With an estimated 600–900 species, the genus represents about half of its family. The plants are trees, shrubs or lianas, growing tall, with woody stems, the absence of a ...
pittieri'' * ''
Clusia alata ''Clusia alata'' is a species of flowering plant in the family Clusiaceae The Clusiaceae or Guttiferae Juss. (1789) (''nom. alt. et cons.'' = alternative and valid name) are a family (biology), family of plants including 13 genera and ca 750 s ...
'' * K. Paijmans observed crown shyness in a multi-species group of trees, comprising ''
Celtis spinosa ''Celtis spinosa'' is a species of flowering plant in the family Cannabaceae Cannabaceae is a small family of flowering plants, known as the hemp family. As now circumscribed, the family includes about 170 species grouped in about 11 genera ...
'' and ''
Pterocymbium beccarii ''Pterocymbium'' is a genus in the family Malvaceae: in the subfamily Sterculioideae and previously placed in the Sterculiaceae. In Indonesia, ''P. tinctorium'' (''Kelumbuk'') is a significant timber tree. Species ''Plants of the World Online ...
''


References


External links

* {{Commons category-inline Plant morphology Forest ecology