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 in the U.S. or western red cedar in the UK, and it is also called pacific red cedar, giant arborvitae, western arborvitae, just cedar, giant cedar, or shinglewood. It is not a true cedar of the genus Cedrus. T. plicata is the largest species in the genus Thuja, growing up to tall and 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 tall and 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 above and green marked with whitish stomatal bands below; they emit a strong aroma reminiscent of pineapple when crushed. The individual leaves are long and broad on most foliage sprays but up to 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 self-overlap.
The cones are slender, long, and 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 long and broad, with a narrow papery wing down each side. The pollen cones are long, red or purple at first, 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 exaggeration. 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.
Etymology
The species name plicata derives from the Latin word and means 'folded in plaits' or 'braided,' a reference to the pattern of its small leaves.