Thuja plicata

Thuja plicata, known commonly as western redcedar despite not being a true cedar, is a conifer from the cypress family native to the Pacific Northwest. It is the biggest member of the Thuja genus, capable of reaching 70 meters in height and 7 meters in trunk diameter, and can live for more than a thousand years. For Indigenous peoples of the region, the wood is used to construct canoes, totem poles, and tools, while the bark is turned into fiber for rope, baskets, clothing, and rain hats—making this tree culturally significant.

The tree’s trunk swells at the base and has shallow roots, with thin, gray-brown bark split into vertical bands. Open-grown specimens often have crowns that reach the ground, but in dense stands, foliage is limited to the top where light penetrates. As it ages, wind damages the crown, which is then replaced by less vigorous branches. The oldest verified individual lived 1,460 years. Its foliage forms flat sprays of scale-like leaves arranged in opposite pairs, each pair rotated 90 degrees from the last. These sprays are green with whitish stomatal bands underneath and give off a strong pineapple-like scent when crushed. Leaves range from 1 to 4 millimeters long on most sprays, though lead shoots can have leaves up to 12 millimeters. Branchlets turn orange-brown before dropping in autumn. Sunlit branches grow denser, overlapping foliage, while shaded ones spread more horizontally with less overlap.

Cones are slender, 10 to 18 millimeters long and 4 to 5 millimeters wide, with 8 to 12 thin, overlapping scales. They start green to yellow-green, ripen to brown about six months after pollination, and open to release seeds. Each seed is 4 to 5 millimeters long and 1 millimeter wide, with a narrow papery wing on each side. Pollen cones are 3 to 4 millimeters long, initially red or purple, and shed yellow pollen in spring.

The heartwood contains chemicals like plicatic acid, thujaplicatin methyl ether, hinokitiol, other thujaplicins, β-thujaplicinol, thujic acid, methyl thujate, 1,4-cineole, and γ-eudesmol. Plicatic acid is thought to be the main irritant causing allergic reactions and asthma in woodworkers exposed to the dust. Thujaplicins act as natural fungicides, preventing rot for about a century after the tree is cut. However, only older trees produce these compounds; saplings lack them and often develop rot early, sometimes leading to hollow trunks as the tree heals. Because of their fungicidal and anti-browning properties, thujaplicins are used in agriculture against fungal diseases and to prevent post-harvest decay. They are also potent chelating agents that bind divalent metal ions. Basic and animal studies suggest they may have antibacterial, antiviral, and antioxidant effects, but reliable evidence for these uses is still lacking.

Taxonomically, T. plicata is one of two North American Thuja species, the other being T. occidentalis. Nuclear genome analysis indicates a closer relationship to T. koraiensis from Korea, with the two likely diverging in the Miocene. A 2008 study found that coastal populations in Washington and Oregon are more genetically variable than those in the Rocky Mountains and coastal British Columbia, suggesting the species spread from a single southern refugium after the Last Glacial Maximum. Some studies proposed an inland refugium in central Idaho, but more recent genomic work supports only one refugium near the southern edge of its current range, with expansion northward and inland since then. Refugial populations may have experienced repeated bottlenecks during Pleistocene glacial maxima. Pollen cores show that T. plicata only became common in southern coastal British Columbia starting around that time.