The Big Tree is nature’s finest masterpiece…the greatest of all living things, it belongs to an ancient stock and has a strange air of another day about it, a thoroughbred look inherited from long ago—the Auld Lang Syne of Trees.
One of the most majestic of the West’s signature trees is giant sequoia (Sequoiadendron giganteum) from the western slopes of the Sierra Nevada, where temperatures and soil moisture are conducive to its great growth. Previously known as Sequoia gigantea, Wellingtonia gigantea, and Sequoia wellingtonia, it has been called commonly Wellingtonia, giant redwood, Sierra redwood, giant sequoia, and big tree. Their home range is only about 260 miles long and fifteen miles wide, smaller in extent than that of its close relative, coast redwood (Sequoia sempervirens). Yet, because of its greater hardiness, giant sequoia has been cultivated in parks and gardens throughout much of the United States and Europe since its discovery in the early 1850s.
The giant sequoia has appressed, pointed, scale-like leaves that are usually blue green or gray green in color. The bark is initially gray, maturing to a cinnamon-brown color when the tree is about twenty years old; it is soft, approximately twelve to fourteen inches thick, fibrous, and furrowed on mature trees. These trees tend to grow in a conical shape in their youth, but after fifty to one hundred years, they gradually develop a more rounded crown.
While neither the tallest nor the widest tree known, giant sequoia is the world’s largest tree in total mass. Under optimal conditions, a tree will reach a height of 250-275 feet and a diameter at breast height (four and a half feet above ground) of fifteen to twenty feet. The largest giant sequoias may reach heights of 350 feet, diameters of thirty-five feet, root spreads of 400 feet in diameter, and ages of more than 3,000 years.
The roots of giant sequoia develop rapidly, which can increase the tree’s stability but makes transplanting difficult. In a mature tree, the roots typically spread out 100-150 feet from the trunk, occupying seven-tenths of an acre or more of land. The largest lateral roots are rarely more than one foot in diameter, and all roots are concentrated in the uppermost twelve to eighteen inches of soil.
A giant sequoia’s roots have a symbiotic relationship with endomycorrhizal fungi in the soil. Vesicular arbuscular mycorrhizae (VAM) are fungal-root associations wherein fungi penetrate the roots’ outer cell walls and form small, branched structures called arbuscules. These arbuscules help transfer the soil’s mineral nutrients from the fungi to the giant sequoias, and sugars produced by the trees to the fungi, benefiting both. The mycorrhizae also have organs called vesicles, which store nutrients and sugars. Through these associations, giant sequoias are able to take in phosphorous and nitrogen that are not otherwise readily available to them. The trees also gain protection against excessive uptake of salts and toxic metals from the soils. Inoculating seedlings with VAM can help them grow two to three times larger than non-inoculated seedlings. Tilling soil or removing vegetation for extensive periods may cause the VAM to disappear from the soil, though new methods of applying disease-free VAM to soils prior to adding plants, or to seedlings prior to transplanting, may eventually alleviate this problem.
Giant sequoia must be at least twenty years old before it will produce cones. Male and female cone buds appear around October. Male cones shed their pollen in April or May. Fertilization takes place in August. Seeds reach maturity by the end of the second growing season, when cones are two to four inches long and bear roughly two hundred seeds each. The seeds may remain in the cone (and remain viable) for twenty years.
In order to germinate, seeds first need to be released from their cones, which usually is facilitated by one of three agents. The first is a wood-boring beetle (Phymatodes nitidus) whose larvae sever vascular connections in the cone scales, causing the scales to dry and release their seeds. Secondly, chickarees, or Douglas squirrels (Tamiasciurus douglasi), eat cone scales, but discard the seeds. Finally, and perhaps most importantly, the heat from fire will dry the cones, causing a large-scale release of seeds. The seeds have a better chance of germinating when they are buried in mineral soils in areas cleared by fire or other disturbances, and rarely germinate in undisturbed areas or under thick layers of organic matter. Naturally occurring fires are essential to clear soils, remove competing trees, and provide a healthy habitat for seedlings.
Edaphic and Climatic Characteristics
Giant sequoias generally grow in granitic-based residual and alluvial soils or in glacial outwash. They do best in deep, sandy loams, but survive in a variety of other soils, suggesting that, aside from moisture content, soils play only a minor role in influencing the tree’s distribution pattern. They generally grow in soils with a pH between 5.5 and 7.5, performing best at pH 6.5. They require well-drained soils with good aeration; standing or stagnating water may drown the roots and kill the trees. The species cannot withstand heavy compaction, though low to moderate compaction may be acceptable.
Long-term occupation of sites by giant sequoias may greatly improve their soils. Soils under these trees, compared to those under other mature conifers, have lower bulk densities, an increased pH, and higher nitrogen, calcium, and carbon contents.
Giant sequoias grow naturally in a region with a mean annual precipitation of thirty-five to fifty-five inches, most of which falls in the form of snow. Summers are dry with only occasional, and widely scattered, brief showers.
Giant sequoias of all ages need soils that retain enough water to help them persist through the summer dry period. Young plants are not drought resistant, but older, established plants can usually withstand aridity and drought. A severe or prolonged drought may weaken a giant sequoia’s resistance to insects or disease, but usually does not kill the trees directly. Occasional flooding can be beneficial, unless the flooding is severe and sudden, loosening soil and damaging a tree’s roots.
Giant sequoia can withstand temperatures ranging from –12° to 104° F, but it typically is found in areas with January minimums of 21° to 34∞ F and July maximums of 75° to 84∞ F. Its natural range falls within USDA zone 9; however, the tree is hardy to zone 6. Both extremely high temperatures and sudden freezes can damage or kill giant sequoias.
Giant sequoia is intolerant of shade and benefits from plenty of sunlight both for initial seedling development and for later growth. In general, relatively small openings in forest canopies allow enough full or moderately filtered sunlight into an area for tree development and growth.
Opinions vary on the susceptibility of giant sequoias to insects, bacteria, and other biotic stresses or pests. In general, such organisms and stresses do not kill the trees directly, but rather indirectly, by weakening a tree’s roots and trunk. Trees grown in nurseries or outside the natural range of the species may be more vulnerable to biotic stresses than those growing within their natural range.
Some believe that giant sequoias are unusually resistant to insect and fungal attacks because of their high concentrations of tannin, a substance that gives the bark its reddish color. If a tree is wounded, tannin covers the wound, protecting the tree from decay, and possibly from insects and diseases as well.
With seedlings, desiccation is evidently a much more common cause of death than are insects. The camel cricket (Pristocauthophilus pacificus), two geometrids (Sabuloides caberata and Pero behresarius), and cutworms (Noctuidae spp.) are the organisms most responsible for seedling death, although any general grazers could kill seedlings. Damping-off and root rot fungi (Phytophora citrophthora) may also play a role in seedling mortality.
In nurseries, the main pest that harms giant sequoia is gray mold (Botrytis cinerea). The mold causes the death of both leaf and root tissue, and may cause cankers in the main stem that eventually prove fatal. Nursery stock is also killed by Macrophomina phaseoli, which causes charcoal root disease.
Nine fungi are associated with decayed giant sequoia wood. Armillaria mellea, Heterobasidion annosum, Poria incrassata, and P. albipellucida are the most significant of these fungi; the first two are also root pathogens. When planted outside its natural range, the giant sequoia is more susceptible to several other pests and diseases, including a canker fungus (Botryosphaeria spp.), a wood borer (Trachykele opulenta), and carpenter ants (Camponotus spp.). Carpenter ants are possibly the most serious pests because they are associated with both fire scars (common in older trees and stands with previous fire damage) and with aphids found on white firs, a species commonly associated with giant sequoias.
Giant sequoia was believed to be one of the species most resistant to smog damage. However, it is suffering from increasing levels of ozone in its native habitat. The ambient levels of ozone, as well as sulfur dioxide and smog, are killing the trees’ cells and may be causing such negative effects as premature needle loss, decreased growth and vigor, and increased susceptibility to attacks by bark beetles. Some theorize that older trees are less susceptible to ozone damage then younger trees. Older trees generally have lower levels of stomatal conductance, and therefore allow less ozone into the leaves, reducing the risk of injury. Plants receiving less than optimal exposure to direct sunlight may experience a greater risk of injury from ozone or other air pollutants. With lowered light exposure, the plants are less able to compensate for the decreased photosynthesis and increased respiration levels caused by pollutants, so they absorb more pollutants and suffer more injuries.
Since giant sequoia is a shade intolerant species, it performs poorly in many urban areas, where buildings create large shadows and cloud cover may be greater than in rural regions. The higher temperatures common to urban areas are usually not a serious problem unless accompanied by extreme water shortages. Increased precipitation levels caused by urbanization may benefit giant sequoias. Soil compaction hurts them by decreasing the aeration and changing the drainage patterns of the soil, leaving the soil excessively wet or anaerobic.
Giant sequoia is generally propagated by seeds, which are subjected to cold stratification for about two months before being planted. Seeds should be planted approximately one-eighth-inch deep, and seedlings should not be exposed to direct sunlight for at least sixty days following germination. This species is occasionally propagated through cuttings, which should be taken from young trees (preferably under age twenty). The cuttings should be treated with the growth hormone IBA (indolebutyric acid) and placed on a mist bench to keep them moist.
Giant Sequoias in the Landscape
Due to its majestic stature, giant sequoia should only be planted in areas with abundant space, such as in parks, on campuses, or in large gardens. In such situations, a mature tree can be a strong focal point. Used in mass, giant sequoias can provide privacy, screening, or wind shelter near a building or home. Their shade can help lower energy use in nearby buildings.
Giant sequoias, because of their great potential size, can also create problems in the landscape. Falling limbs can damage buildings and other vegetation or injure people. The shallow roots may cause damage to sidewalks, home foundations, sewers, roads, and driveways, or could trip pedestrians. Mature plants may grow into power lines or buildings if not given adequate clearances to allow them to reach their full natural heights.
A variety of wildlife species utilize giant sequoias for food and habitat. Chickarees, beetles, and other animals eat the trees’ cone scales, but few species eat the small seeds, which provide little energy or nutrition. Groves provide ideal habitat for such threatened or endangered species such as the spotted owl, pine martens, and fishers. Birds commonly found among giant sequoias include western tanagers, Stellar’s jays, and several species of woodpeckers. Groves also provide shelter and homes for mule deer, Douglas squirrels, black bears, and, in summer, bats. In groves at lower elevations, western fence lizards, alligator lizards, and western rattlesnakes are commonly found on or under giant sequoia.
Since nursery-grown trees are more susceptible to diseases and insects than naturally grown trees, they should be carefully inspected before purchase. A tree should show no signs of damage to its roots or shoots from insects, fungi, or other biotic stresses. The growth habit should be normal, and the trunk should be straight, of a good caliper, able to withstand wind, and have only one leader.
As with any plant, make sure that this is the right species for you and that you have the right conditions for the tree before planting it. Giant sequoia is a magnificent tree that needs a vast amount of space, both for its roots and for its aboveground portions. The soil must hold enough moisture for the tree to last through dry months, and should contain the endomycorrhizae on which the trees depend. The roots are rather shallow, so care should be taken not to damage the roots through construction or gardening activities. The planting area should receive large amounts of direct or moderately filtered sunlight. While this tree does not require much attention to pruning or fertilizing, gardeners may want to irrigate occasionally during dry periods, and allow a disturbance such as fire to occur in a large grove of trees in order to aid seedling growth and reduce competition from other trees.