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Thuya occidentalis L. Eastern white-cedar

Developing seed cones

Habitat

Eastern white-cedar is a shade-tolerant species that grows on a variety of sites. It may occur in pure stands in swamps or over limestone on better-drained sites, but it also occurs in association with balsam fir, eastern hemlock, spruces, red maple, and yellow birch on moist, moderately drained sites.

Mature tree

Form

Eastern white-cedar grows to heights of up to 16 m and to stem diameters of up to 45 cm. In a stand, the crown is relatively open and narrowly conical, but in more open conditions, the crown is dense, conical to columnar, and often long. The stem is noticeably tapered and sometimes irregular in cross section. In poorer conditions, where growth is stunted, several curved and twisted stems are common.

Morphology

The tiny yellowish-green leaves are of two scale-like forms borne in pairs, with each pair alternating in direction from the previous pair. Leaves on the upper and lower surfaces of the shoots are shield shaped and each has a small gland at its center. They overlap the base of the next pair of side leaves, which are keel shaped.

End of spray of shoots with many scale-like leaves

Preformed over wintering pollen cones at tips of shoots

Together, the paired leaves cover the small shoots, which branch often to form flattened sprays. Older sprays of foliated shoots senesce, turn yellowish then orange brown by October, and fall from the trees. The stronger shoots, from which the sprays branch, tend to bear leaves in pairs that are more pointed. As they age, these turn first yellowish, then pale brown, and form the “bark” of the twigs or branches until they split off exposing the true dark reddish-brown bark developing beneath them.

As indicated, there is frequent branching among the shoots, but this is accomplished from axils of leaves without the formation of scaly buds.

Open, mature seed cone

Growth, and development of new pairs of leaves at shoot tips simply stops at the end of the growing season. The apical growing points responsible (apical meristems) are protected over winter by their latest tiny products, the last pairs of leaves. The next spring, growth simply picks up from where it had stopped. The latest leaves of the previous season grow a little, and new leaves and shoots start to form. Because there are no marks left on the shoots (no bud scars), it is not possible to determine how much of the shoot system grew in a given year. Eastern white-cedar, does not produce buds, so all branching is neoformed (a type of branching known as sylleptic branching).

Upper crown of a young tree bearing developing seed cones

At times, darkened tips to some of the tiny shoots look as though they may be buds, but these are either preformed seed cones or preformed pollen cones. Very early in the spring, these extend either to expose ovule tips for pollination or to shed pollen. The pollen cones then shrivel up and remain as deadened tips on the shoots for many months. The seed cones continue growing, become green upright structures through the summer, and then change through greenish yellow to orange brown by October when the opposite scales of the small (7–15 mm long), still upright cones spread apart a little to permit seed to be shed. The seeds are not winged at one end as are those of most other conifer trees, but have flat lateral wings spread out on either side of the elongated seed.

Bark of stem, 25 cm in diameter

The bark is reddish brown, and shiny when young, and quickly becomes fibrous. With increasing age, it becomes separated into narrow, flat, greyish- brown, longitudinal strips. Sometimes the strips spiral slightly up the stem.

Notes

Eastern white-cedar is a member of the cypress family (Cupressaceae), not the pine family (Pinaceae), as are our other coniferous trees. The name “cedar” is not really appropriate, as that is the name for trees of the Cedrus genus (in the Pinaceae), like “cedar of Lebanon” (Cedrus libani A. Rich.). Eastern white-cedar has many other common names: among these, eastern thuja or eastern arbor-vitae would be the preferred names.

Because the heartwood of eastern white-cedar is extremely durable, tree stems are used for posts, poles, and fences. The wood is also used for fence boards, decks, shingles, siding, and boat building and, because of its aroma and moth-deterring properties, for cedar chests and closets.

The foliage of eastern white-cedar is a favorite winter food for deer, so overwintering deer yards are frequently found in or near areas where the species is common.