The Giant Sequoias of California
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BECAUSE of the large tannin content of its wood, the giant sequoia is practically immune to fatal attack by either fungous diseases or insects, although both attack the tree, the short longitudinal galleries that may be seen beneath the bark on felled or dying sequoias are due to the work of the sequoia bark beetle, Phloeosinus rubicundulus Sw. The insect known as the sequoia scale, Aonidia shastae, has been known to attack and discolor giant sequoia reproduction foliage, although no fatal attacks have been observed.

The only real threats to the life of sequoias are fire, wind, undermining by water erosion, and man's destructive acts.

Although fire is deadly to young sequoias, older trees are less susceptible to total destruction; in fact, few very large trees are known to have been killed by a single fire, even those that must have burned slowly for months. The effects of past fires are to be seen everywhere in the groves. Many of the large visible scars must have been produced centuries ago, since the nearby large but much younger pines and firs often do not show fire scars, and therefore could not have been standing at the time of the fire. Most of the giant sequoias more than 10 feet in diameter exhibit fire scars which vary in size from small basal burns to vast areas of trunk and top. There are several trees in which the entire heart has been burned out and within which one can stand and look up through the shell, as in a giant chimney, and view the sky; yet the trees continue to support a few living branches. Other giant sequoias have but a small section of the trunk alive, all the rest having been burned away from base to top. Most of the larger scars are at the base and are frequently found on opposing faces of two trees standing close together where reflected heat from one to the other was able to keep the temperature above the kindling point and sustain the fire. The same factors account for the continuation of fire in the heart of a tree.

No other species of the plant kingdom is able to survive the intense heat of long-continued fires as does the giant sequoia. Its phenomenal resistance to fire may be attributed to the thick, asbestos-like bark, which does not burn readily even under intense heat. A single fire seldom enters the trunk unless there is already an opening through the bark. However, the first fire may kill the cambium (the living, growing tissue immediately beneath the outer bark) as a result of sustained heat generated in the large accumulations of debris often found at the bases of large trees. Later the bark cracks and falls off, thus exposing the dead sapwood, which is easily ignited by a second fire.

The recuperative power of the giant sequoia is also great. Soon after the bark is burned away the cambium begins to grow around the scarred areas and slowly attempts to close the wounds. With a relatively short life span, the average tree would find difficulty in closing a large wound, but in the case of the sequoia, with centuries in which to effect recovery, the attempt often succeeds.

The sequoias have withstood the ravages of wind and storm for centuries and the few that are known to have fallen during windstorms have done so because of the weakening of one side by fire, erosion, or softening of the ground by a change in moisture condition.

So far as known, lightning is not a usual cause of death to these trees. It is true, however, that many sequoias are struck and that some occasionally are set afire far up in the top and the branches sometimes broken off. One medium-sized tree in the Giant Forest of Sequoia National Park shows the effects of a very severe lightning stroke by its shattered crown and the large cracks that extend far down the trunk—but the tree still lives. Probably the fires of centuries ago, which are still evidenced by the burned trunks, were caused by lightning.

Changes in ground level and soil condition are probably the most common contributing causes of the death of large sequoias. Through the centuries it is inevitable that some change in local physiographic condition would occur. Streams gradually erode away the supporting ground on one side of a tree, weakening the support so that the sequoia may fall across the stream. The dam thus created backs up the water and eventually a marshy meadow is created about the roots of trees upstream from the fallen giant. Through the soaking of the ground some of these trees may be killed, while others may be weakened to such an extent that wind finally topples them.

Strangely enough, some of the large trees fall with a terrific roar on quiet days of late summer or in the winter. The reason for this is not known, but it may be due to internal stresses set up by a great difference in the moisture content of the outer and inner wood at the base of the tree.

FIGURE 10.—The Grizzly Giant in winter, Mariposa Grove, Yosemite National Park.

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Last Updated: 02-Feb-2007