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Sustainable Horticulture

Nature's Masterpiece: Sequoiadendron giganteum (Lindl.) Buccholz

 

Angela Cahill

" 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 . "

~ John Muir

Species Information

Sequoiadendron giganteum (Lindl.) Buchholz, previously known as Sequoia gigantea, Wellingtonia gigantea, and Sequoia wellingtonia , is commonly known as Wellingtonia, sequoia, giant sequoia, giant redwood, Sierra redwood, mammoth-tree, or simply "big tree" (Weatherspoon, 1990; Martin, 1998). It is a member of the Taxodiaceae, recognizable by its spiral (or occasionally fascicled) leaves, spiral cone scales (usually shaped like human lips), and flat or, more commonly, peltate scales which lack distinct bracts. Some taxonomists argue that, due to their similar characteristics, the families Taxodiaceae and Cupressaceae should be combined; if this happens, the priority principle of taxonomy dictates that the family be called Cupressaceae (the older of the names). This explains why some sources (e.g. Thomas Kimmerer, online) actually place Sequoiadendron giganteum in the Cupressaceae. Due to its close resemblance to the coast redwood (Sequoia sempervirens), the giant redwood was once included in the genus Sequoia. However, after careful study, taxonomists decided there were enough differences between the two that Sequoiadendron giganteum deserved its own genus, Sequoiadendron . (Martin, 1998). Sequoiadendron giganteum is distinguishable from other species by its small, scale-like, appressed leaves, which are obovate-ovate-lanceolate in shape and blue-green in color. Cones in this species are approximately 5-8 centimeters long, with 25-40 peltate (attached to the stalk inside the leaf margin) scales (Hamilton, 1997). Giant sequoia has fourteen known horticultural forms, of which only two are common. No hybrids have been confirmed for the species (Weatherspoon, 1990). Giant sequoias are native only to an area of the western Sierra Nevada Mountains near Yosemite National Park. Their natural range comprises an area 260 miles long and 15 miles wide, for a total area of roughly 40,000 acres (Willard, 1994). Sierra Redwoods also grow throughout much of the United States (except in areas with very cold winters) and Europe.

Morphology

The giant sequoia has many small, sharply pointed, scale-like leaves, which are appressed, or grown close to the stem. The leaves are usually blue-green in color, although they are occasionally either gray-green or darker green. When crushed, the leaves smell strongly of aniseed. Young branches are usually green, but become brown as they mature. Bark on this species is initially gray, maturing to a cinnamon-brown color when the tree is about twenty years old. The bark is soft, thick (usually 12 to 14 inches), fibrous, and furrowed or ridged (Habeck, 1992). Giant sequoias usually have only one trunk, which is often buttressed at the base, with the extra width providing support for the tree. Sequoiadendron giganteum tends to grow in the "typical" Christmas tree, or conical, shape. After fifty to one hundred years, the tree begins to gradually develop away from the conical shape towards a more rounded, or dome-shaped crown (Willard, 1994). The ends of its branches turn upwards and reach towards the sky. Sequoiadendron giganteum , while neither the tallest nor the widest species, is the world's largest tree according to total volume. Growth of this species, as in any species, varies according to local conditions such as available water and nutrients, space between trees, access to sunlight (a necessity for this shade-intolerant species), and climate. A typical growth rate for a healthy tree is two feet a year (although this slows with age) and growth in width or girth of 3 or 4 inches (Plants for a Future, 1996; Weatherspoon, 1990). Under optimal conditions, an average tree will reach a height of 250-275 feet (76-84m) and a dbh (diameter at breast height, measured at 4.5 feet above the high point of the land) of 15 to 20 feet (5-7m) (Habeck, 1992). The buttressed base of this species' trunk makes its dbh misleadingly large. An optimal spread for a mature tree is anywhere from thirty (Weatherspoon, 1990) to sixty feet (Mohlenbrock, 1985). This variance likely depends on the trees studied, as well as how the authors define an average or optimal spread. The sources I studied claimed that giant sequoias may reach heights of up to 350 feet (Habeck, 1992) dbh's of up to 35 feet (11m), ages of more than 3,000 years, and root spread of 100 to 200 feet from the base of the tree (Willard, 1994; Weatherspoon, 1990). These differences can be accounted for by the natural variations within any species, and by growth differences in diverse regions of the world. The roots of giant sequoia develop rather rapidly, which can make transplantation difficult, but can also increase the stability of the tree. During its first few years of growth, the root system of Sequoiadendron giganteum consists of a taproot with a few small laterals, which helps the tree survive summer droughts. After six to eight years, lateral roots take over and begin spreading rapidly. In a mature tree, the roots typically spread out 100-150 feet from the central stem, occupying seven-tenths of an acre or more of land. Generally, the largest lateral roots are no more than one foot in diameter. The roots are concentrated in the uppermost twelve to eighteen inches of the soil, and rarely extend deeper than three to six feet (Shirley, 1940; Weatherspoon, 1990). A giant sequoia's roots have a symbiotic relationship with endomycorrhizal fungi in the soil. Vesicular arbuscular mycorrhizae (VAM) are a group of fungi which penetrate the plant's cortical cell walls and form small, branched structures called arbuscules. These arbuscules help transfer mineral nutrients from the fungi to the giant sequoias, and sugars from the giant sequoias to the fungi, benefiting both. The mycorrhizae also have organs called vesicles, which store the nutrients and sugars. Sierra redwoods receive enhanced uptake of such nutrients as manganese, zinc, and calcium, and are protected from excessive uptake of salts and toxic metals from soils. The VAM also allow the plant roots to uptake phosphorous and nitrogen that are not otherwise available to them. Inoculating S. giganteum seedlings with VAM can help them grow 2-3 times larger than non-inoculated seedlings. Tilling soil or removing vegetation for extensive time periods may cause the VAM to leave the soil, so researchers are currently studying ways to apply disease-free VAM to soils prior to adding new plants or to seedlings prior to transplantation (Brady & Weil, 1996; Afek, et al., 1994).

Reproduction

Sequoiadendron giganteum must be at least twenty years old before it will produce cones. Male and female cone buds in Sierra redwoods both appear on the tree in late summer or early fall (often around October). The male cones gradually mature until they turn yellow and shed their pollen in April or May (Habeck, 1992). Fertilization takes place in August, and the embryo begins to grow. Cones take two years to develop, and reach maturity by the end of the second growing season, when they are 5 to 9 centimeters long and bear about 200 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 Phymatodes nitidus , a wood-boring beetle, whose larvae sever vascular connections in cone scales, causing the scales to dry and release seeds. Secondly, chickarees, or Douglas squirrels (Tamiasciurus douglasi), eat cone scales, but discard seeds, which may then germinate and form new trees. Finally, and perhaps most importantly, the heat from fire may dry cones, causing 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 with thick layers of organic matter. Naturally occurring fires are essential to clear soils, remove excessive trees, and provide a healthy habitat in which seedlings will grow. Sequoias must also have at least a partially open forest canopy (which allows sunlight in) in order to grow and reach maturity, and this may also be provided by some form of disturbance (Weatherspoon, 1990). The viability of S. giganteum seeds tends to vary based on a variety of factors. For instance, seeds of giant sequoia cones younger than two years old tend not to be very viable, with an average germination rate of twenty to forty percent, but seeds from older cones tend to be more viable. Larger seeds and seeds from cones that are two to five years old also germinate better than seeds that are smaller or from older or younger cones (Weatherspoon, 1990). Trees growing in rocky areas produce more viable seeds than trees growing on lower land with deeper soil where seeds might be buried too deeply to germinate. Little can be done to artificially increase seed viability. A two-month long cold stratification may help propagated seeds germinate faster, but does not increase germination percentages. (Martin, 1998; Hartmann, et al., 1997). There are no known negative impacts, such as allergies or bad odors, caused by the reproduction of Sequoiadendron giganteum . However, having thousands of seeds released in a small area could create a minor mess for people walking or traveling through the area, even if the seeds are fairly small.

Soil

Giant sequoias generally grow in "granitic-based residual and alluvial soil" (Habeck, 1992) or in glacial outwash. Schistose, andesitic, and dioritic rocks are common parent materials; common soil series include Holland, Dome, Chaix, and Shaver. This species does best when grown in deep, well-drained sandy loams but also survives in a variety of other soils. The soils are mostly mesic, meaning they have mean annual temperatures of 8-15° C, though occasionally they are frigid, or have mean annual temperatures lower than 8 ° C (Brady & Weil, 1996; Weatherspoon, 1990). Sequoiadendron giganteum generally grows in soils with pH ranging from 5.5 to 7.5, but does best when the pH is 6.5. The soil needs to be moist year round and retain enough water to get the tree through periods of summer aridity. Until recently, it was believed that Sierra redwoods had few or no specific nutrient requirements from soil. However, researchers are looking into the possibility that certain nutrients may be limiting factors to the growth of giant sequoias (Willard, 1994). It is known that these trees need nitrogen, phosphorus, calcium, magnesium, potassium, iron, manganese, and zinc (Zinke & Stangenberger, 1994). These nutrients are obtained by the giant sequoias from the soil, used, and recycled back into the soil through dropped foliage and decomposing trees. Areas heavily impacted by humans (through logging, development, or other activities) tend to have excessive amounts of nitrogen, zinc, and sodium. Harvesting S. giganteum tends to remove phosphorus, zinc, potassium, and nitrogen from the soil, so small quantities of these nutrients may need to be added prior to reforestation (Ibid.). Giant sequoias need well-drained soils with good aeration. Standing or stagnating water may drown the roots and kill the trees. Sequoiadendron giganteum likes loose soils and cannot withstand heavy compaction, though moderate compaction may be acceptable. Little research has been done into the affects of salinity on giant sequoias, but it is theorized that they do not do well in soils with high salinity (Knigge, 1994). Vesicular arbuscular mycorrhizae are believed to prevent some uptake of heavy metals and other pollutants, but excessive pollution levels may injure or kill a Sierra redwood. The extent and type of injury varies among individual trees and stands of trees (Brady & Weil, 1996; Zinke & Stangenberger, 1994). Long-term occupation of sites by giant sequoias may greatly improve their soils. According to Zinke and Stangenberger (1994), soils under S. giganteum , when compared to those under other mature conifers, have "lower bulk densities, higher carbon and calcium contents, much higher base saturation of the soil exchange capacity due to calcium, a resultant higher soil pH, and higher nitrogen contents" (p.71). Weatherspoon claims that "except for its moisture content, soil apparently plays only a minor role in influencing the distribution of the species, as evidenced by… the fact that giant sequoia grows vigorously when planted in diverse soils around the world" (1990, p. 552).

Water

Giant sequoias are generally found in humid climates with dry summers. They typically live in regions with mean annual precipitation of 35 to 55 inches (88 to 138 cm), most of which comes in the form of snow (Habeck, 1992). These trees usually grow in areas where the soil retains enough moisture to withstand summer droughts. While seedlings cannot survive in very wet soils, giant sequoias of all ages need soils that retain enough water to help them last through dry summer weather. Seedlings and 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 does not usually kill the trees directly (Plants for a Future, 1996). According to Willard (1994), occasional flooding is good for S. giganteum . Overflowing streams may be especially beneficial, as the floodwaters add nutrients to the soil and also clear an area for seedling generation. However, if the flooding is caused by a sudden, heavy rainfall, surrounding soil may be loosened too much, harming the plants' roots. According to a recent study, fog is an important source of moisture for Sierra redwoods. When fog rolls into a stand of giant sequoias, water suspended in the fog is slowly captured by the tree and drips down the tree's needles, limbs and trunk. The study's authors claim that this fog drip can provide as much as half the water entering a forest in a year, and during some dry seasons sword ferns ( Polystichum spp.) are completely dependent on the water provided by redwoods. Researchers are currently studying whether this fog drip increases stream flows, which could benefit threatened and endangered salmon species (Yoon, 1998).

Temperature

Sequoiadendron giganteum can withstand temperatures ranging from -12 to 104 degrees Fahrenheit (24 to 40° Celsius), but it typically is found in areas with January minimums of 21 to 34° F and July maximums of 75 to 84 ° F (Weatherspoon, 1990). According to Time Life books (online), this species is native to USDA zone 9, with average annual minimum temperatures of 20 to 30 ° Fahrenheit. However, the plant is hardy to zone 6, with average annual minimum temperature of -10 to 0° Fahrenheit. The USDA system has been criticized for using only minimum temperatures to predict plant survival, neglecting other important factors such as elevation and water balance. For this reason, many people prefer the Sunset system, which takes more factors into account. According to the Sunset Western Garden Book (1995), S. giganteum can be found in any of the twenty-four zones of the Western United States and several in the southern and eastern U.S., though generally not in the northernmost states. Both extremely high temperatures and sudden freezes will damage or kill this tree.

Light

Giant sequoias are not shade tolerant and need large amounts of sunlight both for initial seedling or plant development and for later plant growth. This species is said to "suffer badly" from all types of shade (Knigge, 1994). In general, relatively small openings in forest canopies allow enough full or moderately filtered sunlight into an area for development and growth of these trees. Developing Sequoiadendron giganteum seedlings, like most seedlings, should be kept out of direct sunlight until they are hardened off. Stohlgren, et al. (1998) and Stark (1968) studied the effects of light and photosynthetically active radiation on giant sequoias. Unfortunately, I was unable to access the full text of these or other articles pertaining to light, and the authors did not discuss their findings in their abstracts. Biotic

Stresses

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, through weakening the trees' roots and stems. In addition, trees grown in nurseries or outside the natural range of the species are more vulnerable to biotic stresses than those found within the natural range. Shirley (1940) believed that giant sequoias were unusually resistant to insect and fungi attacks because of their high concentrations of tannin. Tannin is a substance that gives the bark of giant sequoias a reddish color, is also abundant in cones, and was used to treat burns on humans. If a tree is wounded, tannin covers the wound, protecting the tree from decay, and possibly from insects and diseases as well. In seedlings, desiccation is evidently a much more common cause of death than are insects. Parmeter (1986) reports that in studies on seedling death, only 6-30 percent of the losses were caused by insects. The camel cricket (Pristocauthophilus pacificus), two geometrids (Sabuloides caberata and Pero behresarius), and cutworms (Noctuidae spp.) are the organisms most commonly responsible for seedling death, although any general grazers can apparently kill seedlings. Researchers are currently looking into the role of damping-off and root rot fungi (Phytophora citrophthora) in seedling mortality (Martin, 1998). In nurseries, the main pest that harms Sequoiadendron giganteum is grey mold (Botrytis cinerea). The mold causes death of both foliage and root tissue, and may cause cankers in the main stem that could eventually kill the plant (Martin, 1998). Nursery stock is also killed by Macrophomina phaseoli , which causes charcoal root disease (Parmeter, 1986). Nine fungi have been found to be associated with decayed giant sequoia wood. Armillaria mellea, Heterobasidion annosum, Poria incrassata, and P. albipellucida are probably the most significant of these fungi, and the first two are also root pathogens (Weatherspoon, 1990). No other diseases are known to cause significant problems within the natural range of this species. However, when planted outside its natural range, the giant sequoia is more susceptible to a several other pests and diseases, including a canker fungus (Botryosphaeria spp.), the 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 past fire damage) and with the aphids found on white firs, a species commonly associated with giant sequoias (Parmeter, 1986).

Urban Stresses

According to Paul Miller, giant sequoia is "one of the tree species most resistant to 'smog' damage" (qtd. in Parmeter, 1986, p. 12). However, research by Evans and Leonard (1991) shows that Sequoiadendron giganteum is being subjected to increasing levels of ozone in its native habitat, including Sequoia and Kings Canyon National Park. Their research shows that the ambient levels of ozone (as well as SO2 and smog) are killing cells in the trees, and may be causing such negative effects as premature needle loss, decreased growth and vigor, and increased susceptibility to attacks by bark beetles. Kolb et al (1997) and Miller et al (1994) theorize that older trees are less susceptible to ozone damage then younger trees. The older trees generally have lower levels of stomatal conductance, and therefore allow less ozone into the leaves, reducing injury. Miller et al (1994) also believe that plants receiving shorter durations of exposure to direct sunlight 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 S. giganteum is a shade intolerant species, it won't do well in many urban areas, where buildings create large shadows, and cloud cover is greater than in more rural regions. Giant sequoias have been found in areas with temperatures as high as 104° F, so the high temperatures common to urban areas are not a serious problem. However, if the temperature gets extremely high, or if the heat is accompanied by a serious shortage of water, this can harm the trees (Weatherspoon, 1990). Increased precipitation levels caused by urbanization may benefit Sierra redwoods by providing them with ample water. However, if the water is mostly lost to surface runoff, or runs into streams or ditches, thereby undercutting plant roots, any benefits are lost, and the trees may instead be harmed. Soil compaction hurts giant sequoias by decreasing the aeration and changing the drainage patterns of the soil, leaving the soil too wet or devoid of air (Knigge, 1994).

Practical information

This species is fairly commonly sold in the nursery business. Many companies sell plants or seeds in both printed and online catalogs. Sequoiadendron giganteum is generally propagated through seeds, which are subjected to cold stratification for approximately two months before being planted. Seeds should be planted approximately one-eighth of an inch deep, and seedlings should not be exposed to direct sunlight for at least sixty days following propagation (Hartmann, et al., 1997). In nature, giant sequoia seedlings often sprout from the boles of dead or removed trees. Sometimes, this species is also propagated through cuttings, which should be taken from young trees (preferably under age 20). The cuttings should be treated with the growth hormone IBA (indolebutyric acid) and placed under a mist bench to keep them moist (Ibid.). No other methods of propagation have proven very successful for this species. Several concerns arise when a giant sequoia is transplanted. The fast growing roots could become kinked, circled or girdled in a container, or could easily be damaged in transport, possibly killing the tree. If the tree is very large, it may be wise to prune the roots a year or so before transplanting it, allowing the roots time to recover prior to transplantation. Without sufficient moisture before, during, and after transplantation, the giant sequoia could dry out and be injured or killed. Transplanting in the dormant season, at night, and during periods of precipitation will reduce the risks of the tree becoming too dry. Finally, mishandling during transplantation could damage the roots or shoots of the tree.

Landscape Concerns

Sequoiadendron giganteum has several positive features in landscapes. This is a beautiful species that is quite aesthetically pleasing. Mature trees may provide a visual centerpiece for a large landscaped area. These trees could provide privacy or a wind shelter near a building or home, and the shade the trees produce could help lower energy usage in nearby buildings. This tree also provides habitat for a variety of animal species. Unfortunately, a tree as large as a giant sequoia also creates a few drawbacks or hazards in a landscape. Falling limbs (or even whole trees) can damage buildings and other vegetation or injure people. The large size of this tree, particularly its extensive root system, could be considered a negative feature. The large roots may cause damage to sidewalks, home foundations, sewers, roads, and driveways, or could be tripped over by passersby. Mature plants may grow into power lines or buildings if not given enough clearance to reach their full heights. Most people do not have large enough open spaces to provide ample growing room for this species. In addition, when the tree releases large numbers of seed-bearing cones, or big quantities of pollen, things can get messy. Sequoiadendron giganteum is mainly used as an ornamental species that supplies habitat as well as beauty. It is occasionally used as timber, although the brittle wood of older trees makes them unpopular among some loggers. In addition, public resistance makes logging giant sequoias of any age politically unpopular if not impossible. Some people grow giant sequoias on tree farms for use as Christmas trees. A variety of wildlife species utilize giant sequoias for food and habitat. Chickarees, beetles, and other animals eat the cone scales of this species. Few species eat the tree seeds because the seeds are small and provide little energy or nutrition for animals (Weatherspoon, 1990). Giant sequoia groves provide ideal habitat for such threatened or endangered species 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. The shelter provided by tree groves also provides homes for mule deer, Douglas squirrels, black bears, and, in summer, bats. Finally, in hotter groves at lower elevations, western fence lizards, alligator lizards, and western rattlesnakes are commonly found on or under S. giganteum (Willard, 1994). This species poses no known risk of invasion.

Conclusion and Recommendations for Use in Landscaping

Due to its large size and majestic stature, I would only recommend planting a giant sequoia in areas with abundant horizontal and vertical space, such as parks, campuses, or large yards. The Time Life Plant Encyclopedia (online) suggests growing the plant in a small area for about twenty years, or until it gets too big, then removing it. However, I cannot imagine removing such an exquisite tree just as it is reaching its prime, especially if it's healthy and thriving. It would be better to plant it in a large space and let the tree grow as long as it can, providing beauty all throughout its life. This tree could even be used as a natural, living Christmas tree. Since nursery-grown trees are more susceptible to diseases and insects than naturally grown trees, they should be carefully inspected before purchase. The 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 for this species, and the trunk should be straight, able to withstand wind, have only one leader, and should be of a nice caliper. In addition, the roots should be inspected to make sure they are healthy and are not kinked, circling, or girdling the trunk and no "knees" are protruding from the surface of the soil.

As with any plant, you should make sure this is the right species for you, and that you have the right conditions for the tree, before planting it. Sequoiadendron giganteum is a large tree that needs a vast amount of space, both for its roots and for its aboveground portions. The soil must hold enough moisture for this plant to last through any dry months, and should contain endomycorrhizea to benefit the roots and the plant as a whole. This species' roots are rather shallow, considering the great height the trees reach, so care should be taken to not damage the roots and to give them sufficient space to grow. The planting area will need to receive large amounts of direct or moderately filtered sunlight. S. giganteum should be cold hardy to the area it is planted in. Giant sequoias should also work well with other species in the area, without any species receiving inadequate light, moisture, space, or nutrients. While this trees does not require much caretaking such as pruning or fertilizing, humans may want to add water during dry periods, or allow a disturbance such as fire to occur in a large grove of trees in order to allow seedling growth. Finally, people wishing to provide food and habitat for birds, squirrels, and other organisms could help several species by planting a giant sequoia.

Works Cited

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Evans, L.C. & Leonard, M.R. (1991). Histological determination of ozone injury symptoms of primary needles of giant sequoia ( Sequoiadendron giganteum Bucch.). New Phytologist, 117. 557-564.

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Hartman, H.T., Kester, D.E., Davies, F.T. Jr., & Geneve, R.L. (1997). Plant Propagation: Principles and Practices (6 th ed.). Upper Saddle River, NJ: Prentice Hall.

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Willard, D. (1994). Giant Se quoia Groves of the Sierra Nevada: A Reference Guide . Berkeley, CA: Self-published.

Yoon, C.K. (1998, November 26). Does fog spur on the mighty redwoods? Theory may hold water. Seattle Post-Intelligencer. p. A25.

Zinke, P. & Stangenberger A.G. (1994). Soil and nutrient element aspects of Sequoiadendron giganteum . In Proceedings of the Symposium on Giant Sequoias: Their Place in the Ecosystem and Society. (pp. 69-77). Albany, CA: USDA Forest Service Pacific Southwest Research Station.

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