Biology And Ecology Of Sequoias
The two living species of sequoias are the redwood or coast redwood (Sequoia sempervirens) and the giant sequoia, big tree, or Sierra redwood (S. gigantea, sometimes placed in another genus, Sequoiadendron). Both of these species can be giants, reaching an enormous height and girth. However, the tallest individuals are redwoods, while the widest ones are giant sequoias.
The redwood occurs in foggy rainforest of the Coast Range from sea level to about 3,300 ft (1,000 m) in elevation. The range of the redwood extends from just south of San Francisco, through northern California, to southern Oregon. This tree has evergreen, flattened, needle-like foliage that superficially resembles that of yews (Taxus spp., family Taxodiaceae) and has two whitish stripes underneath. The seed-bearing female cones are as long as 1 in (2.5 cm) and have 15-20 scales. The seeds tend not to germinate prolifically. If cut down, redwoods will regenerate well by vegetative sprouts from the stump and roots, an unusual characteristic among the conifers. Redwoods have a thick, reddish, fibrous bark as much as 10 in (25 cm) deep. Redwood trees commonly achieve a height of 200-280 ft (60-85 m). Exceptional trees are as tall as 360 ft (110 m), can have a basal diameter of 22 ft (6.7 m), and can be older than 1,400 years. No other living trees have achieved such lofty heights.
The giant redwood has a somewhat more inland distribution in northern California. This species occurs in groves on the western side of the Sierra Nevada Mountains, at elevations of 4,000-8,000 ft (1,200-2,400 m) with fairly abundant precipitation and soil moisture. The giant redwood has scale-like, awl-shaped foliage, very different in form from that of the redwood. The female cones are rounder and larger than those of the redwood, up to 3.5 in (9 cm) long and containing 24-40 wedge-shaped scales. The bark is fibrous and thick and can be as much as 24 in (60 cm) thick at the base of large trees. One of the largest known individuals is known as the General Sherman Tree, which is 274 ft (83 m) tall, has a basal diameter of 31 ft (9.4 m), and is estimated to be a venerable 3,800 years old. In terms of known longevity of any organism, the giant redwood is marginally second only to individuals of the bristlecone pine (Pinus aristata) of subalpine habitat of the southwestern United States.
Other living relatives of sequoias are the bald cypress (Taxodium distichum) of the southeastern United States and the Montezuma bald cypress (T. mucronatum) of parts of Mexico. Asian relatives, sometimes cultivated as unusual ornamentals in North America, include the metasequoia or dawn redwood (Metasequoia glyptostroboides) and the Japanese cedar or sugi (Cryptomeria japonica). The dawn redwood of central China was described in the fossil record prior to being observed as living plants by astonished western botanists in the 1940s. For this reason, the dawn redwood is sometimes referred to as a "living fossil."
Wildfire is important in the ecology of redwood forests, but especially in groves of giant redwood. Young seedlings and trees of giant redwood are vulnerable to fire, but older, larger trees are resistant to ground-level fires because of their thick bark. In addition, older redwoods tend to have lengthy expanses of clear trunk between the ground and their first live branches so that devastating crown fires are not easily ignited. Some of the competitor trees of the giant redwood are not so tolerant of fire, so this disturbance helps to maintain the redwood groves.
The development of lower- and mid-height canopies of other species of conifer trees in an old-growth stand of giant redwoods could potentially provide a "ladder" of flammable biomass that could allow a devastating crown fire to develop, which might kill the large redwood trees. Because giant redwoods do not sprout from their stump after their above-ground biomass is killed, they could end up being replaced by other species after a grove is badly damaged by a crown fire. This could result in the loss of a precious natural stand of giant redwoods, representing a tragic loss of the special biodiversity values of this type of rare natural ecosystem.
To prevent the development of a vigorous understory of other species of trees in old-growth groves of giant redwoods, these stands are sometimes managed using prescribed burns. Fire allows open stands of redwoods to occur, while preventing the development of a potentially threatening, vigorous population of other species of trees, such as white fir (Abies concolor) and Douglas-fir (Pseudotsuga menziesii). Fire may also be important in the preparation of a seedbed suitable for the occasional establishment of seedlings of giant redwood.