(sĭkwoi'Ә), name for the redwood (Sequoia sempervirens) and for the big tree, or giant sequoia (Sequoiadendron giganteum), both huge, coniferous evergreen trees of the bald cypress family, and for extinct related species. Sequoias probably originated over 100 million years ago. Once widespread in temperate regions of the Northern Hemisphere, the trees were almost exterminated by the ice sheets of the glacial ages. Several species are known only by fossil remains; some such fossils have been found in the Petrified Forest in Arizona.
The two living species survive only in a narrow strip near the Pacific coast of the United States. The redwood occurs along the coast of California and S Oregon, often in easily lumbered, pure stands. Growing 100 to 385 ft (30–117 m) high, it is probably the tallest tree in the world; the tallest known tree is the redwood Hyperion (379.1 ft/115.5 m), in Redwood National Park. The redwood is able to obtain the abundant moisture needed to sustain its towering growth by capturing water from regularly occurring ocean fogs. The water then drips down from the leaves and branches to the soil, where it penetrates to be absorbed by the roots. The redwood's trunk is 20 to 25 ft (6.1–7.6 m) in diameter, and its needlelike leaves are usually bluish green. Some redwoods are believed to be over 2,000 years old. The big tree, 150 to 325 ft (46–99 m) tall and with a trunk 10 to 30 ft (3–9.1 m) in diameter, grows on the western slopes of the Sierra Nevada in California. It reaches an even greater age than the redwood; some individuals are believed to be 3,000 to 4,000 years old. The leaves are small, overlapping scales. Both trees have deeply grooved, reddish bark and soft, straight-grained, reddish heartwood whose resistance to decay makes it especially valuable for outdoor building purposes, e.g., for shingles, siding, and flumes. Although the sequoias are protected in Kings Canyon, Redwood, Sequoia, and Yosemite national parks, Giant Sequoia National Monument, and several California state parks, their existence elsewhere is threatened by exploitation.
China's deciduous dawn redwood tree (Metasequoia) is believed to be a related species and is perhaps an ancestor of the California redwood. This genus was named and described from fossil remains a few years before the few living specimens were discovered during World War II. Some thousand trees were subsequently found, and they were on the verge of extinction by lumbering. The fast-growing dawn redwoods are now propagated elsewhere. The East Indian and South American redwoods are in the unrelated brazilwood genus.
The sequoia is classified in the division Pinophyta, class Pinopsida, order Coniferales, family Taxodiaceae.