Crown shyness

The exact physiological basis of crown shyness is uncertain.{{Cite FTP |volume=20 |issue=4 |pages=338–339 |title=Crown shyness in a tropical cloud forest |year=1988 |first=Alan J |last=Rebertus |issn=0006-3606 |url=ftp://169.158.189.34/pub/Biotropica/1980s/1988/20-4/Biotropica-1988-20-4-p338.pdf |doi=10.2307/2388326 |jstor=2388326 |server=Biotropica |url-status=dead |bibcode=1988Biotr..20..338R }} The phenomenon has been discussed in scientific literature since the 1920s.{{Cite web|url=https://www.for.gov.bc.ca/hfd/library/FIA/2008/FSP_Y083088.pdf|title=TASS III: Simulating the management, growth and yield of complex stands}} The variety of hypotheses and experimental results might suggest that there are multiple mechanisms across different species, an example of convergent evolution.{{cn|date=February 2023}}

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 winds; the abrasions and collisions induce a crown shyness response. Studies suggest that lateral branch growth is largely uninfluenced by neighbours until disturbed by mechanical abrasion.{{cite journal|last1=Franco|first1=M|title=The influences of neighbours on the growth of modular organisms with an example from trees|journal=Philosophical Transactions of the Royal Society of London. B, Biological Sciences|date=14 August 1986|volume=313|issue=1159|pages=313, 209–225|doi=10.1098/rstb.1986.0034|bibcode=1986RSPTB.313..209F|doi-access=}} If the crowns are artificially prevented from colliding in the winds, they gradually fill the canopy gaps.{{cite journal |url=http://www.sfmn.ales.ualberta.ca/en/Publications/~/media/sfmn/Publications/ResearchNotes/Documents/RN_E36_CrownShyness_low.ashx |title=Crown shyness in maturing boreal forest stands |author=Victor Lieffers |journal=SFM Network Research Note Series |issn=1715-0981 |volume=36 |access-date=2015-08-23 |archive-url=https://web.archive.org/web/20150925144427/http://www.sfmn.ales.ualberta.ca/en/Publications/~/media/sfmn/Publications/ResearchNotes/Documents/RN_E36_CrownShyness_low.ashx |archive-date=2015-09-25 |url-status=dead }} 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.{{cite journal |title=The vegetation of the Monteverde Cloud Forest Reserve |first1=RO |last1=Lawton |first2=Francis E |last2=Putz |journal=Brenesia |volume=18 |pages=101–116 }} According to this theory, variable flexibility in lateral branches greatly influences the 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 in 1955, believed that the trees' growing tips were sensitive to abrasion, resulting in canopy gaps.{{cite book |author=Maxwell Ralph Jacobs |title=Growth Habits of the Eucalypts |url=http://trove.nla.gov.au/version/28637821 |year=1955 |publisher=Forestry and Timber Bureau }} Miguel Franco (1986) observed that the branches of Picea sitchensis (Sitka spruce) and Larix kaempferi (Japanese larch) suffered physical damage due to abrasion, which killed the leading shoots.{{cite book |chapter-url=https://www.cambridge.org/core/books/abs/nature-of-plant-communities/interactions-between-species/5B1A7C7580CBFC055BC5FEDB05724301 |title=The Nature of Plant Communities |chapter=2: Interactions between Species |first1=J. Bastow |last1=Wilson |first2=Andrew D.Q. |last2=Agnew | first3= Stephen H. |last3=Roxburgh |year=2019 |pages=24–65 |publisher=Cambridge: Cambridge University Press|doi=10.1017/9781108612265.004 |isbn=9781108612265 }}

A prominent hypothesis is that canopy shyness has to do with mutual light sensing by adjacent plants. The photoreceptor-mediated shade avoidance response is a well-documented behavior in a variety of plant species.{{cite journal|last1=Ballaré|first1=CL|last2=Scopel|first2=AL|last3=Sánchez|first3=RA|title=Far-red radiation reflected from adjacent leaves: an early signal of competition in plant canopies|journal=Science|date=19 January 1990|volume=247|issue=4940|pages=329–32|pmid=17735851|doi=10.1126/science.247.4940.329|bibcode=1990Sci...247..329B|s2cid=39622241}} Neighbor detection is thought to be a function of several unique photoreceptors. Plants can sense the proximity of neighbors by sensing backscattered far-red light, a task widely thought to be accomplished by the activity of the phytochrome photoreceptors.{{cite journal|last1=Ballare|first1=C. L.|last2=Sanchez|first2=R. A.|last3=Scopel|first3=Ana L.|last4=Casal|first4=J. J.|last5=Ghersa|first5=C. M.|title=Early detection of neighbour plants by phytochrome perception of spectral changes in reflected sunlight.|journal=Plant, Cell and Environment|date=September 1987|volume=10|issue=7|pages=551–557|doi=10.1111/1365-3040.ep11604091|bibcode=1987PCEnv..10..551B }} 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.{{cite journal|last1=Ballaré|first1=CL|last2=Scopel|first2=AL|last3=Sánchez|first3=RA|title=Foraging for light: photosensory ecology and agricultural implications|journal=Plant, Cell and Environment|date=June 1997|volume=20|issue=6|pages=820–825|doi=10.1046/j.1365-3040.1997.d01-112.x|doi-access=free|bibcode=1997PCEnv..20..820B }} Similarly, plants use blue light to induce the shade-avoidance response, likely playing a role in the recognition of neighboring plants,{{cite journal|last1=Jansen|first1=Marcel AK|last2=Gaba|first2=Victor|last3=Greenberg|first3=Bruce M|title=Higher plants and UV-B radiation: balancing damage, repair and acclimation|journal=Trends in Plant Science|date=April 1998|volume=3|issue=4|pages=131–135|doi=10.1016/S1360-1385(98)01215-1|bibcode=1998TPS.....3..131J }} though this was just starting to be recognised in 1988.{{cite journal|last1=Christie|first1=JM|last2=Reymond|first2=P|last3=Powell|first3=GK|last4=Bernasconi|first4=P|last5=Raibekas|first5=AA|last6=Liscum|first6=E|last7=Briggs|first7=WR|title=Arabidopsis NPH1: a flavoprotein with the properties of a photoreceptor for phototropism.|journal=Science|date=27 November 1998|volume=282|issue=5394|pages=1698–701|pmid=9831559|doi=10.1126/science.282.5394.1698|bibcode=1998Sci...282.1698C}}

The characterization of these behaviors might suggest that crown shyness is simply the result of mutual shading based on well-understood shade avoidance responses.{{cite journal |journal=Nature Malaysiana |title=Shyness in trees |volume=2 |pages=34–37 |year=1997 |author=F.S.P. Ng }} Malaysian scholar Francis S.P. Ng, who studied Dryobalanops aromatica, 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 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.{{cite journal|last1=Crepy|first1=María A.|last2=Casal|first2=Jorge J.|title=Photoreceptor-mediated kin recognition in plants|journal=New Phytologist|date=January 2015|volume=205|issue=1|pages=329–338|doi=10.1111/nph.13040|pmid=25264216|bibcode=2015NewPh.205..329C |s2cid=28093742 |hdl=11336/37860|hdl-access=free}} 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.

Trees that display crown shyness patterns include:

• Species of Dryobalanops, including Dryobalanops lanceolata{{cite book |author1=Margaret Lowman |author2=Soubadra Devy |author3=T. Ganesh |title=Treetops at Risk: Challenges of Global Canopy Ecology and Conservation |url=https://books.google.com/books?id=hVNGAAAAQBAJ&pg=PA34 |date=22 June 2013 |publisher=Springer Science & Business Media |isbn=978-1-4614-7161-5 |page=34 }} and Dryobalanops aromatica (kapur)
• Some species of eucalypt{{cite book |author=R. G. Florence |title=Ecology and Silviculture of Eucalypt Forests |url=https://books.google.com/books?id=aO1Jgc7d17UC&pg=PA182 |date=January 2004 |publisher=Csiro Publishing |isbn=978-0-643-09064-4 |pages=182–}}
• Pinus contorta or lodgepole pine{{cite journal |journal=Forest Ecology and Management |year=2008 |volume=257 |issue=1 |pages=321–331 |doi=10.1016/j.foreco.2008.09.005 |title=An empirical model of crown shyness for lodgepole pine (Pinus contorta var. latifolia [Engl.] Critch.) in British Columbia |first1=James W. |last1=Goudie |first2=Kenneth R. |last2=Polsson |first3=Peter K. |last3=Ott |url=https://books.google.com/books?id=FB3YaZKPoi4C&pg=PA39 |isbn=9781437926163 |url-access=subscription }}
• Avicennia germinans or black mangrove
• Schefflera pittieri
• Clusia alata
• K. Paijmans observed crown shyness in a multi-species group of trees, comprising Celtis spinosa and Pterocymbium beccarii

{{Reflist|30em}}

{{Commons category-inline}}

Category:Plant morphology

Category:Forest ecology