Thuja (arborvitae) description

Thuja

L. (1753)

Common names

Arborvitae, thuya, cédre (Chambers 1993), ya bai shu (Chinese; Fu et al. 1999).

Taxonomic notes

A generally-accepted genus of five species:

Molecular phylogenies have placed Thuja sister to Thujopsis (Xiang and Li 2005, Adelalu et al. 2020, Yang et al. 2022). Within Thuja, nuclear ITS evidence places T. koraiensis sister to T. plicata and T. occidentalis sister to T. standishii, with T. sutchenensis not investigated (Xiang and Li 2005). Conversely, sequencing of the complete plastome places T. koraiensis sister to T. standishii and T. occidentalis sister to T. sutchuenensis, with T. plicata sister to the latter pair (Adelalu et al. 2020). This discrepancy was addressed by Peng and Wang (2008), who reported "High topological discordance was observed between chloroplast and nuclear gene trees, even between different nuclear gene trees, suggesting that Thuja could have a reticulate evolutionary history due to multiple interspecific hybridization events. The eastern Asian species Thuja koraiensis might have obtained its chloroplast genome from the eastern North American species T. occidentalis by chloroplast capture, while the western North American species T. plicata is very likely to have inherited a recombinant cpDNA." The same analysis placed T. standishii sister to T. sutchuenensis, which is at least biogeographically plausible. Given the very different results reported from these molecular studies, it seems premature to reach firm conclusions about the phylogeny of Thuja. It does seem to reflect a common pattern in Cupressus and its related genera of arising in Paleogene high latitudes and, through later Neogene cooling, segregating into Asian and North American species groups.

Description

Evergreen, monoecious trees or shrubs. Branchlets in fan-shaped, flattened sprays. Leaves 4-ranked, scalelike, base decurrent only on leading branchlets, dimorphic along branchlets: facial pairs obovate-rhombic; lateral pairs boat-shaped, ridged abaxially, less than 4 mm, overlapping facial pairs, without conspicuous, white stomatal bands abaxially. Pollen cones with (4-)6-10(-16) microsporophylls, each with 2-4 pollen sacs. Seed cones maturing and opening first year, terminal, solitary, cone scales in 4-6 pairs, decussate, flat, only middle, larger 2 or 3 pairs fertile, each fertile scale bearing 1 or 2 seeds; free bract apex a very short mucro. Seeds 1-3 per scale, lenticular, with 2 lateral, narrow wings. Cotyledons 2 x = 11" (Chambers 1993, Fu et al. 1999). The following key is provided by Fu et al. (1999) and Xiang et al. (2002):

1a.	Leaves obtuse at apex, rarely subacute.
2a.	Facial leaves without a gland; young branchlets not glaucous	T. sutchuenensis
2b.	Facial leaves with a gland; young branchlets ± glaucous	T. koraiensis
1b.	Leaves subacute to acuminate at apex.
3a.	Leaves acuminate at apex; lateral leaves longer than facial leaves, apex straight	T. plicata
3b.	Leaves acute or subacute at apex, lateral leaves slightly shorter than or as long as facial leaves, apex incurved.
4a.	Facial leaves with a conspicuous abaxial gland	T. occidentalis
4b.	Facial leaves without a gland	T. standishii

Distribution and Ecology

North America, E Asia (Chambers 1993).

Remarkable Specimens

The largest trees are recorded in western redcedar, Thuja plicata, which is far larger than the other species. Few or no age data exist for the three Asian species. Of the two North American species, Thuja occidentalis has the greater demonstrated age at 1653 years, but Thuja plicata almost certainly attains much greater ages, likely 2000 to 3000 years. All of the largest trees, however, have lost most of their tree-ring record to severe heartwood decay.

Ethnobotany

The common name arborvitae is Latin for "tree of life" (17th cent.) because resins of these trees were used medicinally in western Europe. Effects include increasing blood pressure and reducing fever. The wood is reddish, light, and durable. Some of the larger species are valuable timber trees. The smaller species are cultivated in gardens and as wind screens for more tender plants.

Thuja plicata is extraordinarily significant to native peoples in its home range. The other species are less so.

Dendrochronological research has used Thuja plicata and, especially, Thuja occidentalis. Thuja standishii has been investigated as well. See the species accounts for details.

Observations

See the species accounts.

Remarks

'Thuya'; was a Greek name for some evergreen, resinous trees (Chambers 1993).

Citations

Adelalu, K. F., X. Zhang, X. Qu, et al. 2020. Plastome phylogenomic and biogeographical study on Thuja (Cupressaceae). Research Article, Hindawi. https://www.hindawi.com/journals/bmri/2020/8426287/.

Chambers, Kenton L. 1993. Thuja. Flora of North America Editorial Committee (eds.): Flora of North America North of Mexico, Vol. 2. Oxford University Press.

Linnaei, Caroli. 1753. Species plantarum: exhibentes plantas rite cognitas, ad genera relatas, cum differentiis specificis, nominibus trivialibus, synonymis selectis, locis natalibus, secundum systema sexuale digestas. Tomus I & II. Stockholm: Impensis Laurentii Salvii; p. 1002. Available: Biodiversity Heritage Library, accessed 2025.12.24.

Peng, D., and X.-Q. Wang. 2008. Reticulate evolution in Thuja inferred from multiple gene sequences: implications for the study of biogeographical disjunction between eastern Asia and North America. Molecular Phylogenetics and Evolution 47:1190–1202. doi.org/10.1016/j.ympev.2008.02.001.

Xiang Qiaoping, Farjon, Aljos, Li Zhenyu, Fu Likuo and Liu Zhengyu. 2002. Thuja sutchuenensis: a rediscovered species of the Cupressaceae. Botanical Journal of the Linnaean Society 139(3):305-310. LINK (accessed 2012.10.20). ABSTRACT: After more than a century, the only conifer species listed as being extinct in the wild (EW) by IUCN-SSC has been rediscovered in the Dabashan Mountains of central China. The history, taxonomy, ecology and conservation of Thuja sutchuenensis are described, and illustrations, based on both the earliest and latest collection of botanical material of this tree, are provided. The taxonomic context of this rare species is discussed.

Yang, Yong, David Kay Ferguson, Bing Liu, Kang-Shan Mao, Lian-Ming Gao, Shou-Zhou Zhang, Tao Wan, Keith Rushforth, and Zhi-Xiang Zhang. 2022. Recent advances on phylogenomics of gymnosperms and a new classification. Plant Diversity 44(4):340-350.