Giant Sequoia (Sequoiadendron giganteum)

The giant sequoia, which is also called the sequoia or Sierra redwood, has inspired generations of Californians by its ability to grow to massive size. In fact, giant sequoias are the largest living things to ever inhabit the earth and are among the tallest. Individual trees are up to 3,200 years old and 310 feet tall! The sequoia seems to reach such heights because of its ability to grow rapidly and continue growing into old age.

Range

The current natural range of the sequoia is very limited. It occurs only in the mixed conifer zone on the west slope of the Sierra Nevada in Central California. The tree grows in about 75 groups, known as groves, scattered over a 260 mile long strip which is less than 15 miles wide. 

The current groves are considered a relic of a past time when the climate was wetter and giant sequoias were widespread as far east as Colorado, Wyoming and Nevada.

As a relative of the moisture loving coast redwood, the giant sequoia is the most moisture demanding of the mixed conifer species. It requires a year round soil moisture of at least 15-20 percent. 

In the Mediterranean climate of the interior of California, the majority of the precipitation falls in fall, winter, and spring, leaving a summer dry season. The sequoia's current range is the product of moisture and temperature gradients that shift with elevation. Low temperatures seem to limit the giant sequoia from growing at higher elevations, while hot dry summers limit it from growing at lower elevations.

Sequoias are found associated with a variety of different soils but they grow best in deep, well-drained sandy loams. Within groves, trees are densest in moist areas, such as drainage bottoms and meadow edges. Nevertheless, large vigorous individuals do grow in shallow and rocky soils when underground water is available.

 Regeneration

Sequoias regenerate by seed, by stump sprouting when injured, and artificially by cuttings. Seeds are borne in serotinous cones, meaning the cones may remain attached to their stems without opening for up to 20 years. Cones are opened to release seeds through the work of three agents, without which the sequoia would not be able to reproduce.

The first agent is the long-horned wood-boring beetle (Phymatodes nitidus). The beetle's larvae dig into the cone which causes the cones to dry and shrink, allowing the seeds to release over a period of six months to a year.

The second agent is the chickaree, or Douglas squirrel (Tamiasciurus douglasi). The fleshy green scales of the young cones are a major food source for the squirrel. As squirrels eat the scales of the cone throughout the year, sequoia seeds are liberated.

The third and most important agent is fire. When fires burn on the forest floor, hot air rises into the tree tops and dries cones, allowing them to open. This releases enormous quantities of seed at once, onto the recently burned soil, which is the most favorable type of seed bed for successful sprouting and growth. Fire burns off the litter layer, the top most layer of organic matter covering the soil, which tends to prevent seed germination and seedling survival.

Giant sequoia groves are found within the Sierra Nevada mixed conifer type, which includes California white fir, red fir, sugar pine, ponderosa pine, incense-cedar, and California black oak. The tree has not been found growing in a pure stand, but it is found in relatively pure clumps.

Giant sequoia is the most intolerant to shading of the mixed conifer species. Successful regeneration in shade is less likely than for other mixed conifer species. This is consistent with its ability to regenerate following a fire. However, when established in open sun with adequate moisture, seedlings can outgrow any associated tree species. Height growth up to 24 inches in one year is not uncommon.

Sequoias have remarkably shallow roots. One estimate is that 95% of these trees have roots no deeper than 3 feet. Despite their shallow roots, sequoias are resistant to toppling because roots spread out over large areas, sometimes up to half an acre in well drained soils. 

 Damaging Agents

Of the mixed conifer species, the giant sequoia is the most resistant to damage from fire. The wood is resistant to termites and other wood-eating organisms. This resistance to fire, insects, and disease accounts for their longevity and ability to grow to such mammoth proportions. However, trees planted in nurseries and outside the giant sequoia's natural range are more susceptible to disease and insect damage.

The wood from huge old-growth giant sequoia trees does not make good lumber, despite the its resistance to decay, because it is brittle and has little strength. Nevertheless, sequoias were logged in the 1870's and their wood was used for fenceposts and shake shingles. Since the 1890's, most of the groves of giant sequoia have been protected by California state parks and Yosemite, King's Canyon, and Sequoia National Parks so that all Californians may enjoy their stately grandeur.

Unfortunately, protection of groves from fire has reduced the regeneration of giant sequoia and pines, and led to an increase in the amount of California white fir. Prescribed fire and mechanical seed bed preparation is one option for regenerating some groves, although such treatment is generally not consistent with National Park policy.

Young growth sequoias, unlike old growth sequoias, have wood properties similar to young growth redwood and is suitable for lumber. There have been successful plantations of the sequoia outside its natural range. In fact, the tree has grown vigorously when planted around the world. On proper sites it outperforms most other species. The best plantations average 1.6-2.3 feet of height growth and 0.5 to 0.8 inches of diameter growth per year.

This text was largely summarized from an article originally by C. Phillip Weatherspoon Research Forester, USFS Pacific Southwest Forest and Range Experiment Station, Berkeley, CA that appears in Burns, Russell M., and Barbara H. Honkala, tech. coords. 1990. Silvics of North America: 1. Conifers. Agriculture Handbook 654. U.S. Department of Agriculture, Forest Service, Washington, DC. vol. 1, 675 p.

Interested readers are referred to the original article for more detailed and technical information and references. Publication of this series was in part funded by the California Department of Forestry and Fire Protection under Contract numbers 8CA96027 and 8CA96028.