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{{Short description|Genus of conifers}}
{{Automatic taxobox
|image = Thuja standishii.jpg
|image_caption = ''[[Thuja standishii]]'' foliage and cones
|taxon = Thuja
|authority = [[Carl Linnaeus|L.]]
|type_species = ''[[Thuja occidentalis]]''
|type_species_authority=L.
}}

'''''Thuja''''' ({{IPAc-en|ˈ|θj|uː|j|ə}} {{respell|THEW|yə}})<ref>''Sunset Western Garden Book'', 1995:606–607<br/>{{OED|Thuja}}</ref> is a genus of [[conifer]]ous [[tree]] or [[shrub]] in the [[Cupressaceae]] (cypress family). There are five species in the genus, two native to [[North America]] and three native to eastern [[Asia]].<ref name=w>{{cite POWO |id=30076319-2 |title=Thuja |access-date=21 August 2024}}</ref><ref name=farjon>{{cite book |last=Farjon |first=A. |date=2005 |title=Monograph of Cupressaceae and Sciadopitys |publisher=Royal Botanic Gardens |location=Kew |isbn=978-1-84246-068-9}}</ref><ref name=gd>{{Gymnosperm Database |family=Cupressaceae |genus=Thuja}}</ref><ref>{{eFloras|2|132902|Thuja |family=Cupressaceae |first1=Liguo |last1=Fu |first2=Yong-fu |last2=Yu |first3=Robert P. |last3=Adams |first4=Aljos |last4=Farjon}}</ref> The genus is [[monophyletic]] and sister to ''[[Thujopsis]]''. Members are commonly known as '''arborvitaes''' (from the [[Latin]] term for 'tree of life'), '''thujas''', or [[New World]] [[List of plants known as cedar|'''false cedars''']].<ref>{{PLANTS|id=THUJA |taxon=Thuja |access-date=9 December 2015}}</ref><ref>{{cite web|url=https://treespnw.forestry.oregonstate.edu/conifer_genera/false_cedars.html|title=False Cedars (Calocedrus, Thuja, Chamaecyparis)|publisher=Oregon State University}}</ref>

==Description==
[[File:Thuja 'Rheingold' in Autumn.JPG|thumb|Foliage of the cultivar 'Rheingold']]
''Thuja'' are [[evergreen]] [[tree]]s growing from {{convert|10|to|200|ft|m|0|abbr=off}} tall, with stringy-textured reddish-brown [[Bark (botany)|bark]]. The shoots are flat, with side shoots only in a single plane. The leaves are scale-like and {{convert|1|to|10|mm|abbr=on}} long, except young seedlings in their first year, which have needle-like leaves. The scale leaves are arranged in alternating decussate pairs in four rows along the twigs. The male cones are small, inconspicuous, and are located at the tips of the twigs. The female [[conifer cone|cones]] start out similarly inconspicuous, but grow to about {{convert|1|to|2|cm|abbr=on}} long at maturity when 6–8 months old; they have 6-12 overlapping, thin, leathery scales, each scale bearing 1–2 small [[seed]]s with a pair of narrow lateral wings.<ref name=farjon/>

The five species in the genus ''Thuja'' are small to large evergreen trees with flattened [[branchlet]]s. The leaves are arranged in flattened fan shaped groupings with resin-glands, and oppositely grouped in 4 ranks. The mature leaves are different from younger leaves, with those on larger branchlets having sharp, erect, free apices. The leaves on flattened lateral branchlets are crowded into appressed groups and scale-like and the lateral pairs are keeled. With the exception of ''T. plicata'', the lateral leaves are shorter than the facial leaves (Li ''et al.'' 2005). The solitary flowers are produced terminally. Pollen cones with 2-6 pairs of 2-4 pollen sacked sporophylls. Seed cones are ellipsoid, typically {{convert|9|to|14|mm|abbr=on}} long, and mature and open the first year. The thin woody cone scales number from 4-6 pairs and are persistent and overlapping, with an oblong shape, they are also basifixed. The central 2-3 pairs of cone scales are fertile. The seed cones produce 1 to 3 seeds per scale, the seeds are lenticular in shape and equally 2 winged. Seedlings produce 2 cotyledons.<ref>{{eFloras |1 |family=Cupressaceae |first=Kenton L. |last=Chambers}}</ref><ref>{{cite encyclopedia |first=Henry A. |last=Gleason |encyclopedia=New Britton and Brown Illustrated Flora of Eastern North America and Adjacent Canada |title=Coniferae |url=https://archive.org/details/newbrittonbrowni01glea/page/58/mode/1up |volume=1: The Pteridophya, Gymnospermae and Monocotyledoneae |publisher=Hafner Press |pages=58–67}}</ref>

[[File:Riesenlebensbaumstmrp.jpg|thumb|right|''T. plicata'' bark, foliage]]
[[File:The Kalaloch Cedar - Flickr - rachel thecat.jpg|thumb|right|[[Old growth]] ''T. plicata'', [[Olympic Peninsula]], USA]]
[[File:Podlaskie - Suprasl - Kopna Gora - Arboretum - Thuja occidentalis 'Smaragd' - plant.JPG|thumb|right|Cultivars of ''T. occidentalis'' in an arboretum]]
[[File:Thuja occidentalis 'EuropeGold'-201601-JAPAN.jpg|thumb|''Thuja occidentalis'' cultivar 'EuropeGold']]

A [[Hybrid (biology)|hybrid]] between ''T. standishi'' and ''T. plicata'' has been named as the [[cultivar]] ''Thuja'' 'Green Giant'.{{cn|date=July 2025}}

Another very distinct and only distantly related species, formerly treated as ''Thuja orientalis'', is now treated in a genus of its own, as ''[[Platycladus|Platycladus orientalis]]''. The closest relatives of ''Thuja'' are ''[[Thujopsis|Thujopsis dolabrata]]'', distinct in its thicker foliage and stouter cones, and ''[[Tetraclinis articulata]]'' ([[Ancient Greek]] {{lang|grc|θυία}} or {{lang|grc|θύα}}, formerly classed in the genus and after which ''Thuja'' is named), distinct in its quadrangular foliage (not flattened) and cones with four thick, woody scales.

The genus ''Thuja'', like many other forms of conifers, is represented by ancestral forms in [[Cretaceous]] rocks of northern Europe, and with the advance of time is found to migrate from northerly to more southerly regions, until during the [[Pliocene]] period, when it disappeared from Europe. ''Thuja'' is also known in the [[Miocene]] beds of [[the Dakotas]].<ref>{{Cite NIE|wstitle=Arbor Vitæ}}</ref>

==Taxonomy==
===Phylogeny===
{| class="wikitable"
|-
! colspan=1 |Stull et al. 2021<ref>{{cite journal |last1=Stull |first1=Gregory W. |last2=Qu |first2=Xiao-Jian |last3=Parins-Fukuchi |first3=Caroline |last4=Yang |first4=Ying-Ying |last5=Yang |first5=Jun-Bo |last6=Yang |first6=Zhi-Yun |last7=Hu |first7=Yi  |last8=Ma |first8=Hong |last9=Soltis |first9=Pamela S. |last10=Soltis |first10=Douglas E. |last11=Li |first11=De-Zhu |last12=Smith |first12=Stephen A. |last13=Yi |first13=Ting-Shuang |display-authors=et al. |year=2021 |title=Gene duplications and phylogenomic conflict underlie major pulses of phenotypic evolution in gymnosperms |journal=Nature Plants |url=https://www.nature.com/articles/s41477-021-00964-4 |volume=7 |issue= 8|pages=1015–1025 |doi=10.1038/s41477-021-00964-4|biorxiv=10.1101/2021.03.13.435279 |pmid= 34282286|pmc= |bibcode= 2021NatPl...7.1015S|s2cid=232282918 |doi-access=|url-access=subscription }}</ref><ref>{{cite web|last1=Stull |first1=Gregory W. |display-authors=et al. |year=2021 |title=main.dated.supermatrix.tree.T9.tre |publisher=Figshare |doi=10.6084/m9.figshare.14547354.v1 |url=https://figshare.com/articles/dataset/Gene_duplications_and_genomic_conflict_underlie_major_pulses_of_phenotypic_evolution_in_gymnosperms/14547354 |doi-access=}}</ref>
|- 
| style="vertical-align:top|
{{clade|style=font-size:90%;line-height:80%
|label1=''Thuja''
|1={{clade
  |1=''[[Thuja plicata|T. plicata]]''
  |2={{clade
    |1=''[[Thuja koraiensis|T. koraiensis]]''
    |2={{clade
      |1=''[[Thuja sutchuenensis|T. sutchuenensis]]''
      |2={{clade
        |1=''[[Thuja occidentalis|T. occidentalis]]''
        |2=''[[Thuja standishii|T. standishii]]''
         }}
       }}
     }}
   }}
}}
|}

===Species===
The five extant species are:<ref name=w/><ref name="GRINSpecies">{{cite web |url=http://www.ars-grin.gov/cgi-bin/npgs/html/splist.pl?12106 |title=GRIN Species Records of ''Thuja'' |work=Germplasm Resources Information Network |publisher=United States Department of Agriculture |access-date=2013-01-22}}</ref><ref>{{BONAP|ref |genus=Thuja}}</ref>

{| class="wikitable"
|-
! Image !! Scientific name !! Common name !! Distribution 
|-
|[[File:Thuja koraiensis PAN foliage.JPG|120px]] || ''[[Thuja koraiensis]]'' <small>Nakai</small> || Korean thuja || [[Jilin]], [[Korea]]
|-
|[[File:Thuja occidentalis 003.JPG|120px]] || ''[[Thuja occidentalis]]'' <small>L.</small> || eastern arborvitae, northern whitecedar || E Canada ([[Manitoba]] to [[Nova Scotia]]), E United States (primarily [[Northeastern United States|Northeast]], [[Great Lakes region|Great Lakes]], [[Appalachians]])
|-
|[[File:Thuja plicata 43569.JPG|120px]] || ''[[Thuja plicata]]'' <small>Donn ex D.Don</small> || western redcedar || from [[Alaska]] to [[Mendocino County]] in [[California]]
|-
|[[File:Thuja standishii Brno2.JPG|120px]] || ''[[Thuja standishii]]'' <small>(Gordon) Carrière</small> || Japanese thuja || [[Honshu]], [[Shikoku]]
|-
| || ''[[Thuja sutchuenensis]]'' <small>(Gordon) Carrière</small> || Sichuan thuja || [[Sichuan]], [[Chongqing]] China almost extinct in the wild
|-
|}

Species formerly placed in ''Thuja'' include:<ref>{{cite web |url=http://wcsp.science.kew.org/namedetail.do?name_id=377490|date=2022-10-20|title=WCSP, the last web-archive snapshot before migration to POWO (Archive copy)|archive-url=https://web.archive.org/web/20221020000834/http://wcsp.science.kew.org/namedetail.do?name_id=377490 |archive-date=2022-10-20 }}</ref>{{citation needed|date=August 2024}}
* ''[[Austrocedrus chilensis]]'' <small>(D.Don) Pic.Serm. & Bizzarri</small> (as ''T. chilensis'' <small>D.Don</small>)
* ''[[Callitris rhomboidea]]'' <small>R.Br. ex Rich.</small> (as ''T. australis'' <small>Poir.</small>)
* ''[[Cupressus nootkatensis]]'' <small>D.Don</small> (as ''T. excelsa'' <small>Bong.</small>)
* ''[[Dacrycarpus imbricatus]]'' <small>(Blume) de Laub</small> (as ''T. javanica'' <small>Burm.f.</small>)
* ''[[Glyptostrobus pensilis]]'' <small>(Staunton ex D.Don) K.Koch</small> (as ''T. pensilis'' <small>Staunton ex D.Don</small>)
* ''[[Libocedrus plumosa]]'' <small>(D.Don) Sarg.</small> (as ''T. doniana'' <small>Hook.</small>)
* ''[[Platycladus orientalis]]'' <small>(L.) Franco</small> (as ''T. orientalis'' <small>L.</small>)
* ''[[Tamarix aphylla]]'' <small>(L.) H.Karst.</small> (as ''T. aphylla'' <small>L.</small>)
* ''[[Tetraclinis articulata]]'' <small>(Vahl) Mast.</small> (as ''T. articulata'' <small>Vahl</small>)
* ''[[Thujopsis dolabrata]]'' <small>(Thunb. ex L.f.) Siebold & Zucc.</small> (as ''T. dolabrata'' <small>Thunb. ex L.f.</small>)
* ''[[Widdringtonia nodiflora]]'' <small>(L.) Powrie</small> (as ''T. cupressoides'' <small>L.</small>)
and many more

The extant species ''Thuja sutchuenensis'' was believed to be extinct until 1999, when a small population was discovered in southeast China.<ref>{{Cite journal|last1=Qiaoping|first1=Xiang|last2=Fajon|first2=A.|last3=Zhenyu|first3=Li|last4=Likuo|first4=Fu|last5=Zhengyu|first5=Liu|date=2002-07-01|title=Thuja sutchuenensis: a rediscovered species of the Cupressaceae|journal=Botanical Journal of the Linnean Society|language=en|volume=139|issue=3|pages=305–310|doi=10.1046/j.1095-8339.2002.00055.x|issn=0024-4074|doi-access=free}}</ref>

==Ecology==
''Thuja'' species are used as food plants by the [[larva]]e of some [[Lepidoptera]] species including [[autumnal moth]], the [[Engrailed (moth)|engrailed]] and [[juniper pug]]. The foliage is also readily eaten by [[deer]], and where deer population density is high, can adversely affect the growth of young trees and the establishment of seedlings.<ref>{{cite journal |last=Stein |first=W. I. |year=1997 |title=Ten-year survival and growth of planted Douglas-fir and western redcedar after seven site-preparation treatments |journal=Western Journal of Applied Forestry |volume=12 |issue=3 |pages=74–80 |doi= 10.1093/wjaf/12.3.74|doi-access=free }}</ref>

== Distribution ==
The genus ''Thuja'' has current populations in both North America and East Asia. ''T. plicata'' has wide distribution in the Pacific Northwest from Northern California to Alaska, reaching East into Idaho and central British Columbia. ''T. occidentalis'' has populations in the Northeastern United States, reaching north into Ontario and Quebec, with some distribution as far south as Tennessee.<ref>{{Cite journal|last1=Cowley|first1=Jill|last2=Zheng-yi|first2=Wu|last3=Raven|first3=Peter H.|date=1997|title=Flora of China Volume 15, Myrsinaceae through Loganiaceae|journal=Kew Bulletin|volume=52|issue=3|page=765|doi=10.2307/4110318|issn=0075-5974|jstor=4110318|bibcode=1997KewBu..52..765C }}</ref>

''T. standishii'' has populations in mountainous regions of Honshu and Shikoku islands in Japan, with no recorded population in the north of the country. ''T. koraiensis'' is native to both North and South Korea and has a small population in the Northern Chinese province of Jilin.<ref name="Cui e0138544">{{Cite journal|last1=Cui|first1=Yi-Ming|last2=Sun|first2=Bin|last3=Wang|first3=Hai-Feng|last4=Ferguson|first4=David Kay|last5=Wang|first5=Yu-Fei|last6=Li|first6=Cheng-Sen|last7=Yang|first7=Jian|last8=Ma|first8=Qing-Wen|date=2015-09-22|title=Exploring the Formation of a Disjunctive Pattern between Eastern Asia and North America Based on Fossil Evidence from ''Thuja'' (Cupressaceae)|journal=PLOS ONE|language=en|volume=10|issue=9|article-number=e0138544|doi=10.1371/journal.pone.0138544|issn=1932-6203|pmc=4579098|pmid=26393513|bibcode=2015PLoSO..1038544C|doi-access=free}}</ref> The newly rediscovered species ''T. sutchuenensis'' has extremely limited distribution in the mountains of Chengkou county in southeastern China.

== Evolution and paleobiogeography ==
Current research suggests that ''Thuja'' originated in the Americas and migrated to East Asia via the [[Bering land bridge]] in the [[Miocene]]. Fossil records show that Thuja was significantly more widely distributed during the late [[Cretaceous]] and [[early Tertiary]] than we see today.<ref>{{Cite journal|last1=Peng|first1=Dan|last2=Wang|first2=Xiao-Quan|date=June 2008|title=Reticulate evolution in Thuja inferred from multiple gene sequences: Implications for the study of biogeographical disjunction between eastern Asia and North America|journal=Molecular Phylogenetics and Evolution|volume=47|issue=3|pages=1190–1202|doi=10.1016/j.ympev.2008.02.001|pmid=18346917|bibcode=2008MolPE..47.1190P |issn=1055-7903}}</ref> The oldest known ''Thuja'' fossil is of ''T. polaris'' (an extinct species) from the Paleocene of Ellesmere Island in present-day Nunavut, Canada.<ref name="Cui e0138544" />

Other hypotheses of ''Thuja'' origin involved an East Asian origin, with the genus migrating twice; once east into North-western America and then west to the North-eastern America, but since no reliable fossil records of ''Thuja'' exist in either Western Asia or Europe, the possibility can be eliminated.<ref name="Cui e0138544"/>

== Systematics ==
Thuja is a monophyletic genus that sits within the order Pinales in the Cupressaceae. ''Thuja'' is in the Cupressoid clade and is sister to the genus ''Thujopsis''. The sister relationship between ''Thuja'' and ''Thujopsis'' is supported with 100% bootstrap support and 1.0 posterior probability.<ref name="researchsquare.com">{{Cite journal|date=2019-08-30|journal= BioMed Research International|title=Plastome phylogenomic and biogeographic study on Thuja (Cupressaceae)|language=en|doi=10.1155/2020/8426287|doi-access=free|last1=Adelalu|first1=Kole F.|last2=Zhang|first2=Xu|last3=Qu|first3=Xiaojian|last4=Landis|first4=Jacob B.|last5=Sun|first5=Yanxia|last6=Meng|first6=Aiping|last7=Sun|first7=Hang|last8=Wang|first8=Hengchang|volume= 2020|article-number= 8426287|pmid= 32685531|pmc=7335403}}</ref>

Within the genus the taxonomy is in flux, but most recent research based on molecular analysis of plastomes in the genus ''Thuja'' showed evidence for a new grouping, with two sister clades: ''T. standishii'' and ''T. koraiensis'' together and ''T. occidentalis'' and ''T. sutchuenensis'' together, with ''T. plicata'' sister to ''T. occidentails'' and ''T. sutchuenensis.''<ref name="researchsquare.com"/>  This newest grouping is hypothesized to be the result of reticulate evolution and hybridization within the genus.

==Uses of thuja==
They are widely grown as [[ornamental tree]]s, and extensively used for [[hedge (barrier)|hedges]]. A number of cultivars are grown and used in landscapes.<ref>{{cite web |first=Mark H. |last=Brand |date=2001 |title=Thuja plicata |url=http://www.hort.uconn.edu/plants/t/thupli/thupli1.html |website=UConn Plant Database |publisher=University of Connecticut |archive-url=https://web.archive.org/web/20071029202830/http://www.hort.uconn.edu/plants/t/thupli/thupli1.html |archive-date=2007-10-29 }}</ref><ref>{{cite web |first=Mark H. |last=Brand |date=2001 |title=Thuja occidentalis |url=http://www.hort.uconn.edu/plants/t/thuocc/thuocc1.html |website=UConn Plant Database |publisher=University of Connecticut |archive-url=https://web.archive.org/web/20071021032615/http://www.hort.uconn.edu/plants/t/thuocc/thuocc1.html |archive-date=2007-10-21 }}</ref> Homeowners will sometimes plant them as privacy trees. The cultivar 'Green Giant' is popular as a very vigorous hedging plant, growing up to 80&nbsp;cm/year when young.<ref>{{Cite web |url=http://www.usna.usda.gov/Gardens/faqs/GreenGiant.html |title=U.S. National Arboretum: Gardens: FAQs: 'Green Giant' Arborvitae<!-- Bot generated title --> |access-date=2008-01-23 |archive-date=2013-01-27 |archive-url=https://web.archive.org/web/20130127021253/http://www.usna.usda.gov/Gardens/faqs/GreenGiant.html }}</ref>

The [[wood]] is light, soft and aromatic. It can be easily split and resists decay. The wood has been used for many applications from making chests that repel moths to shingles. ''Thuja'' poles are also often used to make fence posts and rails. The wood of ''Thuja plicata'' is commonly used for [[guitar]] [[sound board (music)|sound board]]s.<ref>{{cite book|last=Bucur |first=Voichita |date=1995 |title=Acoustics of wood |place=Boca Raton |publisher=CRC Press |doi=10.1201/9780203710128}}</ref> Its combination of light weight and resistance to decay has also led to ''T. plicata'' being widely used for the construction of bee hives.<ref>{{cite web |last1=Owano |first1=Nacy |title=Flow Hive 2 sees sweet success in offering new features for honey on tap |url=https://techxplore.com/news/2018-03-hive-sweet-success-features-honey.html |website=Tech Xplore |access-date=15 October 2022}}</ref>

''T. plicata'' is an important tree to the First Nations people of the Pacific Northwest and is sometimes called "Canoe Tree" because of its use as a material for Native American canoes.{{citation needed|date=September 2021}}

Oil of ''Thuja'' contains the [[terpene]] [[thujone]] which has been studied for its [[GABA]] receptor antagonizing effects, with potentially lethal properties.<ref name="pmid10725394">{{cite journal |vauthors=Höld KM, Sirisoma NS, Ikeda T, Narahashi T, Casida JE |title=Alpha-thujone (the active component of absinthe): gamma-aminobutyric acid type A receptor modulation and metabolic detoxification |journal=[[Proceedings of the National Academy of Sciences of the United States of America|Proc. Natl. Acad. Sci. U.S.A.]] |volume=97 |issue=8 |pages=3826–31 |year=2000 |pmid=10725394 |doi=10.1073/pnas.070042397 |pmc=18101|bibcode=2000PNAS...97.3826H |doi-access=free }}</ref> Cedarwood oil and cedar leaf oil, which are derived from ''Thuja occidentalis'', have different properties and uses.<ref>{{cite web | url=http://cedarleafcanada.com/faq/cedarwood-oil-vs-cedar-leaf-oil/ | title=Cedarwood Oil Vs Cedar Leaf Oil | publisher=Cedar Leaf Canada | access-date=16 June 2015}}</ref>

The natives of Canada used the scaled leaves of ''Thuja occidentalis'' to make a tea that has been shown to contain 50&nbsp;mg of vitamin C per 100&nbsp;grams; this helped prevent and treat [[scurvy]].<ref name="silvics">{{Silvics |volume=1 |genus=Thuja |species=occidentalis |first=William F. |last=Johnston}}</ref>

In the 19th century, ''Thuja'' was commonly used as an externally applied tincture or ointment for the treatment of [[wart]]s, [[dermatophytosis|ringworm]] and [[candidiasis|thrush]],<ref>{{cite book |first=David |last=Hoffmann |title=Medical Herbalism: Principles and Practices |publisher=Healing Arts Press |year=2003 |page=588 |isbn=978-0-89281-749-8 }}</ref> and a local injection of the tincture was used for treating [[genital wart|venereal warts]].<ref>{{cite book |first=M. |last=Grieve |title=A Modern Herbal |location=London |publisher=Jonathan Cape |year=1931 |page=177 }}</ref>

A 2017 trial showed that its extract effectively killed both gram-positive and gram-negative bacteria.<ref>{{Cite journal|last1=Sah|first1=Shiv Nandan|last2=Regmi|first2=Sunil|last3=Tamang|first3=Man Kumar|date=2017-06-29|title=Antibacterial Effects of Thuja Leaves Extract|journal=International Journal of Applied Sciences and Biotechnology|language=en|volume=5|issue=2|pages=256–260|doi=10.3126/ijasbt.v5i2.17617|issn=2091-2609|doi-access=free}}</ref>

As with many Cupressaceae, ''Thuja'' can induce allergic reactions, including skin, eye and breathing problems.{{citation needed|date=September 2021}}

==References==
{{Reflist}}

==External links==
{{Commons category}}
* {{Cite Americana|wstitle=Arbor Vitæ |short=x}}

{{Acrogymnospermae classification}}
{{Cupressaceae}}
{{Taxonbar|from=Q147255}}
{{Authority control}}

[[Category:Thuja| ]]
[[Category:Conifer genera]]
[[Category:Botanical taxa named by Carl Linnaeus]]
[[Category:Garden plants]]
[[Category:Medicinal plants]]