t212 Cedar

Libocedrus/Calocedrus is a genus of 2 to 3 species of coniferous trees in the family Cupressaceae (Cypress). The genus is related to the genus Thuja and has similar overlapping scale-leaves.

The incense-cedar tree is native to western North America, with the bulk of the habitat being in the USA: western Oregon, Nevada, and California. The tree also occurs in Mexico and in Western and Central Europe. The incense-cedar is cultivated worldwide as an ornamental tree.

The incense-cedar is a resinous, aromatic tree, generally 18 to 45 m tall but able to reach 60 m. The tree has an irregularly angled trunk with a diameter of up to 3 m, and a columnar crown. The bark is light- or reddish-brown and deeply and irregularly furrowed into ridges. The foliage is produced in flattened sprays with scale-like leaves 2 to 15 mm long; they are arranged in opposite decussate pairs, with the successive pairs closely and then distantly spaced, forming apparent whorls of 4; the facial pairs are flat, with the lateral pairs folded over their bases. The leaves are evergreen, shiny, with only inconspicuous stomata, and are aromatic when crushed. The wood is soft, with a strong spicy-resinous fragrance.

Incense-cedar is monoecious: both male and female flowers may be borne on the same tree. The flowers are 6 mm, yellow-green strobili, borne on the ends of twigs from early September. The pollen is shed from late winter to early spring.

The seed cones are inconspicuous in spring, becoming pendent and red-brown to golden-brown in colour when they mature in late summer. The seed cones are 20 to 35 mm long, with 4 (rarely, 6) scales arranged in opposite decussate pairs; each outer pair of scales bears 2 winged seeds, the inner pair(s) usually being sterile and fused together in a flat plate. The pollen cones are 6 to 8 mm long. Seed dispersal begins in late August and lasts until October. Seeds of incense-cedar are carried great distances by wind.

Although the large ‘Cedars’ (actually, species of Calocedrus, Chamaecyparis, Cupressus, and Thuja) of western North America tend to look alike at first, the incense-cedar is distinguished by the flattened vertical sprays of its foliage.

Incense-cedar heartwood is ideal for exterior use where moisture is present, and may be found as mud sills, window sashes, sheathing under stucco or brick veneer construction, greenhouse benches, fencing, poles, and trellises. Incense-cedar is also used as the primary material in the manufacture of pencils, because it is soft and tends to sharpen easily without forming splinters.

It is a popular ornamental tree, grown particularly in cool-summer climates (notably eastern Britain and elsewhere in northern Europe, and in parts of the northern Pacific Northwest of North America), and admired for its very narrow columnar crown. This tree has become popular as an ornamental tree in Northern Italy, where the pollen season is the winter (January and February). (1)

IgE-mediated reactions

Anecdotal evidence suggests that Incense-cedar tree pollen can induce asthma, allergic rhinitis and allergic conjunctivitis; however, few studies have been reported to date. (1,2)

A 40-year-old woman reported having experienced rhinitis and conjunctivitis since the age of 12 years. She experienced intense allergic symptoms while in close proximity to incense-cedar trees. A skin-prick test and serum IgE antibody test for cypress tree were both positive. A crude extract prepared from the cones of Incense cedar were used for a skin-prick test which resulted in a very strong positive reaction. The IgE antibody concentration to incense-cedar was 5.2 RAST arbitrary units. An incense-cedar pollen challenge resulted in the immediate onset of sneezing, rhinorrhoea, nasal obstruction, redness of the conjunctiva, and tearing and itching of the eyes. A challenge with cypress pollen extract was negative. (1)

Other reactions

Cases of dermatitis from cedar-wood pencils have been described; (6) reactions may be due to the presence of thymoquinone, thymoquinol and carvacrol in the wood. (3)

No allergens have been characterised.

Cross Reactivity

Cross-reactions with other members of this family are possible. (4) A high degree of cross-reactivity also occurs among the Cupressaceae and Taxodiaceae families. (5)

Last reviewed : June 2022