Thuja plicata is a large evergreen coniferous tree in the family Cupressaceae, native to the Pacific Northwest of North America. Its common name is western redcedar. It is also less commonly called western red cedar, pacific red cedar, giant arborvitae, western arborvitae, just cedar, giant cedar, or shinglewood. It is not to be confused with the genus Cedrus, the true cedars. T. plicata is the largest species in the genus Thuja, growing up to 70 metres (230 ft) tall and 7 m (23 ft) in diameter. It mostly grows in areas that experience a mild climate with plentiful rainfall, although it is sometimes present in drier areas on sites where water is available year-round, such as wet valley bottoms and mountain streamsides. The species is shade-tolerant and able to establish in forest understories and is thus considered a climax species. It is a very long-lived tree, with some specimens reaching ages of well over 1,000 years. Indigenous peoples of the Pacific Northwest use the wood of this species for many purposes, such as building canoes, totem poles, and tools. The bark is harvested by indigenous peoples and processed into a fiber, which they use to make items such as rope, baskets, clothing, and rain hats. Because of its wide range of uses, the species is of great cultural importance to these people. Western redcedar wood is aromatic and rot-resistant and is used for applications such as the construction of shingles and siding. It has been introduced to cool temperate areas in other parts of the world, such as Northern Europe and New Zealand.
Description Thuja plicata is a large to very large tree, ranging up to 45 to 70 metres (150 to 230 feet) tall and 2.4 to 7 m (8 to 23 ft) in trunk diameter, larger than any other species in its genus. The trunk swells at the base and has shallow roots. The bark is thin, gray-brown, and fissured into vertical bands. Trees growing in the open may have a crown that reaches the ground, whereas trees densely spaced together will exhibit a crown only at the top, where light can reach the leaves. As the tree ages, the top is damaged by wind and replaced by inferior branches. The species is long-lived; some trees can live well over a thousand years, with the oldest verified aged 1,460. The foliage forms flat sprays with scale-like leaves in opposite pairs, with successive pairs at 90 degrees to each other. The foliage sprays are green throughout but marked with whitish stomatal bands below; they emit a strong aroma reminiscent of pineapple when crushed. The individual leaves are 1 to 4 millimetres (1⁄32 to 5⁄32 in) long and 1 to 2 mm (1⁄32 to 3⁄32 in) broad on most foliage sprays but up to 12 mm (1⁄2 in) long on strong-growing lead shoots. The foliage of individual branchlets turns orange-brown before falling off in autumn. Branches growing in full sunlight produce denser foliage with more overlap, while shaded branches grow more horizontally, with less overlap. The cones are slender, 10 to 18 mm (3⁄8 to 11⁄16 in) long, and 4 to 5 mm (5⁄32 to 3⁄16 in) broad, with 8 to 12 (rarely 14) thin, overlapping scales. They are green to yellow-green, ripening brown in fall about six months after pollination, and open at maturity to shed the seeds. The seeds are 4 to 5 mm (5⁄32 to 3⁄16 in) long and 1 mm (1⁄32 in) broad, with a narrow papery wing down each side. The pollen cones are 3 to 4 mm (1⁄8 to 5⁄32 in) long, red or purple initially, and shed yellow pollen in spring.
Chemistry The heartwood of western redcedar contains numerous chemical substances, such as plicatic acid, thujaplicatin methyl ether, hinokitiol and other thujaplicins, β-thujaplicinol, thujic acid, methyl thujate, 1,4-cineole, and γ-eudesmol. Plicatic acid is believed to be the main irritant and contact allergen responsible for provoking allergic reactions and asthma exacerbation. This leads to occupational asthma in woodworkers that are exposed to western redcedar wood dust. Thujaplicins serve as natural fungicides which prevent the wood from rotting. This effect lasts around a century even after the tree is felled. However, thujaplicins are only found in older trees. Saplings do not produce the chemical, causing them to often develop rot at an early stage, causing some trees to grow with a somewhat hollow trunk, as the tree moves to heal itself as it grows. Due to their fungicidal and anti-browning properties, thujaplicins are used in agriculture for fungal diseases and to prevent post-harvest decay. Thujaplicins, like other tropolones, are potent chelating agents and bind divalent metal ions. Basic and animal studies have shown that thujaplicins may have other biological properties, including antibacterial, antiviral, and antioxidant activities, however reliable evidence on their effectiveness is still lacking.
Taxonomy Thuja plicata is one of two Thuja species native to North America, the other being T. occidentalis. Nuclear genome analysis indicates that it is likely more closely related to T. koraiensis, which is native to the Korean Peninsula. The two species are hypothesized to have diverged in the Miocene. A 2008 study found that western redcedar populations in coastal areas of Washington and Oregon are more genetically variable than populations in the Rocky Mountains and coastal British Columbia, indicating that the species most likely spread throughout its current range from a single refugium in the southern portion of its range after the Last Glacial Maximum. Some studies have suggested the existence of an inland refugium during the Last Glacial Maximum, likely in central Idaho. However, more recent genomic approaches corroborate the existence of only a single glacial refugium near the south of the current distribution, with subsequent expansion northward and inland since the last glacial maximum. Refugial populations may have undergone repeated population bottlenecks during the glacial maxima of the Pleistocene. Pollen cores indicate that T. plicata only became common in southern coastal British Columbia starting at approximately 6,000 YBP. This coincides with a transition from the warmer, drier climate of the early Holocene to a cooler, wetter climate. The species arrived later in the Canadian Rockies, as recently as 4,000–5,000 YBP. Pollen assemblages at Seeley Lake, on the eastern slope of the Coast Mountains east of Haida Gwaii, indicate that western redcedar became common there at approximately 2,200 YBP.
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