The Gymnosperm Database describes thuja trees as coniferous trees in the cypress family; two species are native to North America and three to Asia. Of the three Asian varieties, two are very rare. Also known as arbor vitae, or tree of life, the thuja may grow as shrub or tree, depending on species and growing conditions.
Thuja plicata is also known as giant redcedar, as well as giant arbor vitae and other common names. The tree can grow up to 210 feet tall, though 150 feet is more common. The crown may be conical or irregularly shaped, and the bark is red-brown or grey-brown. This plant is found in western North America from latitudes of about 40 degrees north up to 56 degrees north. The tree grows in mixed evergreen forests and prefers sites with moisture.
Thuja occidentalis is also known as northern white-cedar, eastern whitecedar or American arbor vitae. The trees typically grow to around 45 feet tall; they may grow leaning or stunted where conditions are not optimal. The crown is conical and the bark red-brown or grey-brown. This plant grows in eastern North America, from New England up to latitudes around 50 degrees north. The tree's environment is now somewhat threatened by grazing, and it is not as common as it once was. It is also widely used as an ornamental tree and has at least 120 variants, or cultivars.
The most common Asian aborvitae tree is Thuja standishii, also known as Japanese arbor vitae. The tree can grow up to about 55 feet tall and has red-brown bark with a broad, pyramidal shape. Its native environment is the Honshu and Shikoku mountains of Japan. It prefers subalpine and cool, temperate forests with other conifers. In the United States, this tree is used ornamentally up to U.S. Department of Agriculture plant hardiness zone 7.
Thuja sutchuenensis is one of the two endangered species of thuja. Indeed, it was considered instinct in the wild until 1999, when a specimen was discovered in China. This thuja tree grows up to 60 feet tall in shrub or tree format. Its bark is orange-brown when young but turns grey-brown with age. The tree is currently found in a small area of the Daba Mountain Nature Reserve, near Chongqing. According to the Global Trees Campaign, efforts between the non-profit and the Chinese government are now in place to preserve and increase the population of this tree and its other rare coniferous cousins.
Thuja koraiensis, or Korean arbor vitae, is the smallest of the thuja trees, reaching only about 30 feet high. Its bark is red-brown but turns grey-brown with age. The tree grows in North Korea and China at elevations between about 2,000 and 5,500 feet; in China, the species is "vulnerable." The tree appears on the International Union for Conservation of Nature Red List, but was last assessed in 1998. The greatest threat to this tree is from uncontrolled logging; however, at least some of the trees are safe, because they grow in the Changbaishan Nature Reserve.