How little I know of that arbor vitae when I have heard only what science can tell me. It is but a word, it is not a tree of life. But there are twenty words for the tree and its different parts which the Indian gave, which are not in our botanies, which imply a more practical and vital science. He used it every day. He was well acquainted with its wood, its bark and its leaves.1
Until twenty years ago, eastern white cedar (also, or more properly known as arbor vitae) was regarded as a tree species inhabiting lowland areas such as swamps, stream banks and lakeshores as well as limestone pavements or rock outcrops. It was also considered by many to be a short-lived pioneer tree species of disturbed land. Age limits had previously been reported as 400 years. We now know that eastern white cedar growing on cliff faces can reach ages surpassing 1,500 years and that it is one of only a handful of species that exhibit such longevity under adverse conditions.
While uncommon in most trees, maximum ages approaching one thousand years are more common in the Cupressaceae, the taxonomic family that includes eastern white cedar, giant arbour-vitae or Thuja occidentalis. Most true cedars, cypresses and junipers are also members of the Cupressaceae. The only other member of the genus Thuja in North America is the western red cedar or Thuja plicata. In the temperate rain forests of the Pacific Northwest western red cedar can reach heights up to sixty mettes and surpass five metres in diameter. At high altitudes, however, it may be only a small tree or shrub. The maximum lifespan of western red cedar is thought to be over a thousand years.
Eastern white cedar on the other hand rarely attains sizes greater than a metre in diameter or more than twenty metres in height. It grows at altitudes between sea level and 1,300 metres, although it only grows at these higher elevations in upstate New York on sites where water flows directly over rock. It commonly grows with eastern hemlock, eastern white pine, yellow birch and silver or red maple or in pure stands on dry sites. In level-ground habitats, eastern white cedars have a distinctive conical shape. In open areas, living branches extend to the ground, but, in forests, the bottom branches are either absent or dead. Living branches tend to dip downwards and then grow skywards near the tip. The leaves are unusual – scale-like, flat, lance-shaped and yellowish-green in colour – and like humans they become more bronze as the growing season progresses.
Cedars grow almost exclusively on alkaline soils enriched with calcium. Lateral movement of water is also critical for eastern white cedar stands, and the water needs to be enriched with high concentrations of oxygen and essential nutrients. A good flow rate is found in many typical eastern white cedar habitats including lakeshores, wetlands, sand dunes, rocky outcrops or cliffs. They are often found growing on shallow soils because cedar is very tolerant of both short-term droughts and flooding. Like other trees it avoids significant drought injury by taking advantage of water when it is plentiful, but closing its stomata (small openings in the leaves through which gases pass) for extended periods of time in response to water stress.
The sex life of the white cedar is probably more interesting than you might think. Cedars are monoecious plants, which means they have both the male and female structures required for reproduction on the same plant. If they were human, they would all be hermaphrodites. Sexual reproduction may start in trees as young as five or six years, but often doesn’t start until trees are much older. Peak seed production occurs after seventy-five years. The oldest known living cedars have full complements of seed cones and these seeds are as viable as seeds from the youngest trees.
Cedar cones mature in the fall during which time their seeds are dispersed by the wind.
The male is represented on the tree by pollen cones. These cones, approximately four millimetres in length, develop near the base of branchlets. The pollen is released in the spring, when it hopes to find its way to female or seed cones at branchlet tips. The pollen cones and seed cones are usually on different branches. Seeds develop over the summer and the cones ripen between August and September. In autumn, the cones dry, seeds are released and then scattered by the wind. A typical eastern white cedar can yield up to 260,000 seeds. That’s a big family! The seeds tend to fall within fifty to sixty metres of the mother tree although seeds produced on cliff-face trees may be carried much further.
In a level-ground forest, the successful germination of cedar seedlings is dependent on the availability of logs and gaps in the forest canopy associated with them. They are also dependent on temperatures above 18°C to germinate. Thankfully, for the sake of earth’s other species, only a small percentage of seeds actually germinate! Mortality is high amongst cedar seedlings as they typically germinate on substrates such as logs and moss that are prone to drying out as the growing season progresses. Even fewer will survive more than a couple years. On the cliff face, where competition from other plants is minimal, successful germination is dependent on the seed finding a suitable crack or crevice.
This moss-covered rock, which rises above the leaf litter on the forest floor, provides a suitable substrate for the germination of cedar seeds.
This mature cedar was once just a seedling on a stump.
Surprisingly, this is only one way that eastern white cedars can propogate themselves. Eastern white cedar (and a few related species in North America) can simply skip sexual reproduction altogether. How do they do that? They simply produce clones of themselves. This is known as layering. Stems or branches of eastern white cedar are capable of producing roots when moisture conditions are favourable and the tree is in contact with the ground. Sometimes the side branches of a tree that is blown over grows upward. The increased weight causes the original stem to descend to the soil surface. Once contact is made, a new set of roots may develop and the tree can function as if nothing had happened. Except that the cedar looks more like a pretzel now than a tree. This trick also allows cedars thrown by the wind or tipped over by an unstable substrate to establish new sets of roots.
Asexual reproduction through layering is a handy mechanism that allows cedars to persist in swamps where their shallow roots make them vulnerable to falling or tipping over. This process is especially prevalent along cliff edges where shallow soils make the trees vulnerable to wind damage. It also allows cedars to adjust to environmental change and reproduce even after conditions are no longer ideal for sexual reproduction. If there is a habitat shift (perhaps precipitated by industrious beavers!) from swamp to bog (an atypical environment for cedars), then cedar stands may persevere even after the new habitat is invaded by another tree species such as black spruce. Layering also enables cedars to reproduce under dense shade where seedlings wouldn’t survive. It provides them with reproductive flexibility. Layering allows shade-tolerance in a normally shade-intolerant species. On the cliff face, layering is less common because trees become established on small ledges, and in cracks and crevices. An unstable tree is apt to hang upside-down or fall completely into the talus rather than find a suitable neighbouring site on which to produce a new set of roots.
Cedars are at home in swamps, along lake margins, such as here at Crawford Lake north of Burlington, and on cliffs hut only on cliffs do they reach ages surpassing 500 years.
Compared with many of its companion species in central North America, eastern white cedar has relatively few enemies. Cedar is affected by few serious diseases and they are relatively free of insect damage. The main pests are ants and Arborvitae leaf miners. Ants such as Formica exsectoides may kill small cedars, while carpenter ants work within the heartwood of living cedars causing instability in the tree. Arborvitae leaf miners (Argyresthia thuiella being the most common species in Ontario) are small caterpillars that feed within the leaves causing the tips of the branches to turn brown. They start on the outside of the tree, eventually working their way towards the trunk. Cedars can survive losing up to 80% of their leaves to leaf miners. The Cedar-tree Borer [Semanotus ligneus) and the Northern Cedar Bark Beetle (Phloeosinus canadensis) will tunnel into wood, but they feed primarily on dying cedar trees and are not considered a serious threat to healthy trees. The Japanese Cedar Longhorned Beetle is a pest that was accidentally introduced into North America in 1997 and has been found on eastern white cedar. Adults colonize weak cedars and the larvae bore into wood, but apparently cause only minimal damage.
Other insects sometimes found on white cedar include the aphid (Cinara cupressi), the bagworm (Thyridopteryx ephemeraeformis), sawflies (Monoctenus sp.) and several species of loopers, but epidemics are unknown and they are not thought to cause serious injury to the trees. Rare diseases such as leaf blight (Fabrella thujina) and juniper blight (Phomopsis juniperovora) can cause premature leaf browning and shedding. Phytophtora root rot can also cause roots to decay in moist environments while various butt-rot fungi can infect the cedars. None of these pests or diseases are widespread enough to pose a significant threat to white cedar populations.
This 600-year-old cedar is still alive despite the erosion of the rock surface upon which it was growing.
Eastern white cedar is a favourite browse food for some mammal species, especially in winter. Deer favour cedars by a wide margin over other commonly available wintertime browse species. When given the option of browsing cedar over twenty other browse species, deer made cedar 90% of their diet. Deer can account for significant mortality of the youngest trees because eastern white cedar is such a slow-growing species. Deer also frequent white cedar stands because they impose important winter thermal cover. Porcupines also feed on cedar foliage in the winter particularly in the tree crowns. In some cases, this can kill the main axis of the tree. Rodents can account for significant losses of seedlings, while the red squirrel harvests branchlets with cones from mature trees in autumn. Snowshoe hares are also heavy browsers and the northern pileated woodpecker bores large holes in mature cedars in their quest for carpenter ants. Humans pose an indirect hazard to white cedars because of roadway construction. White cedar is negatively affected by road salt.
While some northern tree species are relatively resistant to fire (some are dependent on fire for reproduction!), eastern white cedar is not. A ground fire easily damages their shallow root systems and the shaggy oil-rich bark is flammable. Excessive heat alone is fatal to these trees and large cedars may survive only if ground cover is relatively sparse. The oldest cedar trees persist where fire is excluded. Eastern white cedar is one of the first species to colonize sites razed by fire and many cedar forests became established after major fires. An intense fire that destroys the surface organic horizon, however, may delay colonization for some time.
Although popular as an ornamental tree, eastern white cedar occurs naturally throughout central and eastern Canada and throughout the northern United States. It occurs as far west as Manitoba, including isolated populations along the shores of Cedar Lake (not surprisingly!) and the northern shores of Lake Winnipeg and Lake Winnepegosis. Its distribution is continuous between Winnipeg and southern Nova Scotia, as far north as James Bay and as far south as Chicago, Illinois; Flint, Michigan; the Hudson River Valley in New York and southern Maine. Below this range, eastern white cedar persists as far south as Virginia and Tennessee in small isolated populations. It has not always been this way.
Some remarkable details on the recent history of this species have been uncovered by Dr. Gary Walker of Appalachian State University, who has studied cedar populations in North Carolina and Tennessee. Walker found high levels of genetic diversity in populations of white cedar on the Eastern Highland Rim of Tennessee but low genetic diversity at other sites with a history of disturbance. This led him to conclude that the Highland Rim populations were relict stands from a once-continuous distribution at the height of the last glacial maximum 20,000 years ago. When the continent warmed some 14,000 years ago, the glaciers receded, and the eastern white cedars migrated northwards along river valleys and back into present-day Canada. These pioneers became the forefathers of populations now prevalent throughout the Great Lakes region. By 12,500 years ago white cedar had moved northwards into southwestern Ontario and cedar occupied the shores of the Champlain Sea – a postglacial body of water that covered much of northeastern North America 12,000 to 10,000 years ago. A leafy twig collected from sediments on Manitoulin Island reveals that cedar had migrated there by at least 10,000 years ago. The southern populations fragmented about 12,000 years ago and became isolated stands by 8,000 years ago.
Today, white cedar is abundant within its range where it is perhaps the most recognizable and easily identifiable tree species. Ironically, cedar in Tennessee is now at the extreme southern extension of its range where it only occupies steep north-facing limestone slopes or cliffs. These cliffs provided white cedar with a refuge at the height of the last glaciation and possibly saved the species from extinction. Here, isolated glacial relict plant communities on cliffs are tangible reminders of the widespread havoc that ice sheets wreaked on the distribution of the continent’s flora, including eastern white cedar.
The four colours and medicines of importance to southern Ontario’s First Nations People (from a display at a reconstructed 15th Century Iroquoian village at Crawford Lake Conservation Area).
