GIANT SEQUOIA'S NATURAL ENEMIES
Accidents happen to trees, and the older a tree the more occasion there has been for damage. Lightning, fire and other injuries have left their marks in broken tops, black scars, and temporary loss of symmetry.
Doubtless many Sequoias have been struck by lightning. Crowns have been damaged, branches broken away, and sections split from the trunks, but occasions when a giant sequoia has been killed outright or severely damaged by lightning are extremely rare. The greatest damage has not been from lightning but by fires set elsewhere in the forest by lightning. The abundant blackened scars, and charcoal-lined cavities and hollow trees, as well as many snag tops are eloquent evidence of this fact.
It was not a single fire, blazing and roaring through the forest, that produced the numerous black scars and cavities, but numerous fires which burned during the remote past. No fire of consequence has occurred in the Giant Forest or Grant Grove during white man's regime, and the most recent destructive fires were from one to three hundred years ago. How is this known? The conclusion is partly from direct evidence of tree rings, and partly from inference.
To begin with, we must keep in mind that the thick, fibrous, non-resinous bark is very resistant to fire. It will burn when shredded, but the thick bark at the base of a tree merely chars on the outside when subjected to a hot fire. If there is sufficient fuel piled up at the basedead branches, fallen trees, etcto keep a fire going for several days, the bark may char through, or heat may penetrate to the cambium layer and kill this portion of the tree. Years later a second fire may take advantage of this weakened place, and here find an entry into the wood itself, burning out a deep cavity, or hollowing the entire trunk.
Fires which were hot enough and sufficiently long burning to thus damage the mature sequoias, must have been hot enough to destroy completely the firs, pines, cedars, or young sequoias of the same forest. Today, when we see a one hundred fifty year old sugar pine growing beside a deeply fire-scarred sequoia, we conclude that the latest fire occurred before that pine began to grow.
There is more direct evidence, however. We had opportunity to examine the cross section of a fallen sequoia in Giant Forest in 1950. A comparatively small fire scar was fully exposed, but that one fire scar represented the total damage of at least four separate fires.
After each fire, new wood grew outward to cover part of the scar. Each successive fire destroyed part of the new growth, and burned deeper into the tree. Thus, a cross section of the scar today shows layers of charred wood alternating with new growth, and reveals a fairly complete record of the fires. There well may have been more fires for some may have destroyed completely the record of earlier ones. This tree records the following fires:
The first fire occurred 489 years ago, about 1465.
The ability of the tree to persist in spite of severe damage, and to cover fire scars and injuries with new bark and new wood is truly remarkable. No matter how severely the tree may be damaged by lightning or fire, as long as a thin band of live bark remains to provide communication between roots and foliage, the tree continues to live and to grow. A most remarkable example of this is the Black Chamber on the Crescent Meadow road in Giant Forest. This very large tree, severely damaged by fire, has only a few feet of live bark left, yet it not only supports live foliage and branches, but is growing new bark to cover the edges of the burn. One marvels at such persistence, and wonders if in time this tree might heal itself completely.
Such evidence is seen throughout the groves, and the new bark is easily recognized around the border of most fire scars by its lighter color, smoother texture, and a rather silvery sheen in contrast to the rough and duller old bark. The rate of healing over must vary a great deal, but is often sufficiently rapid to be noticeable within a few years. Comparisons of General Sherman tree today with early photos show that its fire scars have been very much reduced in size within the past 60 years. We know of one tree in Giant Forest where the lateral growth of new bark to cover a burn has been about an inch a year for the past 25 years, but the most spectacular examples are found in the stumps of trees logged off in the early days. In these stumps there are often masses of bark, and areas of charred wood, entirely enclosed within the completely restored trunks. Some stumps in Big Stump Basin or Converse Basin show evidence of a dozen or more fire injuries, each followed by complete healing and full restoration of the trunks. Before these trees were cut down, there could have been no sign of fire damage, so complete was the self-restoration of the tree. Usually one or more branches will turn upward to replace a damaged crown.
SEQUOIA SELDOM DIES ON ITS FEET
No one knows how long a sequoia might live. The oldest tree of authentic record was 3124 years old when cut down in Converse Basin, and John Muir reported a stump with 4,000 annual rings. The General Sherman and General Grant trees are perhaps 3500 years old, but it is entirely possible that they are even older. Yet none of these trees shows evidence of approaching old age; each is vigorous and virile. Insects, disease, fire, and lightning, common enemies of most forests, seldom kill the giant sequoia outright. Why, then, should not the sequoia live for 5000 or 10,000 years, or more? Perhaps it could, except for one thingitself.
In one sense, the giant sequoia is its own worst enemy. The tremendous weight of the mature tree makes it sensitive to any change in the stability of the ground upon which it stands. It has no tap root. Its massive but shallow roots, like its wood of trunk and limbs are brittle and without great strength. Balance, and weight distributed upon a broad base, rather than strong roots, keep the tree erect. As ground conditions and drainage conditions change, as stream erosion cuts away the soil from one side of a tree, as the soil is softened by changes in underground drainage perhaps even as outward growth of the tree expands its base over less stable soil, the balance of the tree is threatened.
Once a giant sequoia loses its balance, rarely is that balance restored. Sometimes the lean of the tree may be arrested, but usually loss of balance is followed by an increasing degree of tilting until finally the giant, its roots unable to hold against the tremendous leverage, crashes to earth. This is not a sudden process, but, in scale with the long life expectancy of the tree, may require several score years. Toward the last, the accelerated rate of lean can be observed, and actual measurements of the tilting give reasonable warning of the impending fall.
Falling sequoias are not a common occurrence by any means. If fall is the only important cause of death in a forest of 3000 trees, and if each can be expected to live 3000 years, then you may expect an average of one to fall a year. This seems to be in accord with events. For example, only ten sequoias of moderate to large size have been known to fall in the Giant Forest area within the past 13 years. The fall of one of these trees was witnessed, and the crash of one other which fell in the Hazelwood area during the summer of 1947 was heard by hundreds of people, but most of the others fell during winter and early spring. At such times the ground is wet from melting snows, the crown may be heavily burdened with snow, and occasional winds may provide the final push that overbalances the tree.
There are leaning trees in some of the groves today. Sometimes we are asked why we do not prop up these trees, and prevent their eventual fall. In the National Parks, one of the things we strive to preserve is the naturalness of the scene. Trees, other than sequoias, eventually die, usually by insect and disease after they have been weakened by old age. It happens that a very common way for a sequoia to die is by falling down. The falling of a giant opens up new ground so that new seedlings may plant themselves, and a new generation come into being. In this, as in most circumstances where the preservation of a natural environment is the objective, nature will better accomplish our ends and her objectives the less we interfere.
Kaweah Indians who inhabited Kaweah valley visited the Giant Forest annually to gather acorns and to hunt. Apparently they had no important traditions associated with the giant sequoias, nor is there any evidence that they set the fires which produced the burn scars.
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An Austrian botanist, Stephen Endlicher, is believed to have named the Sequoia genus in honor of Chief Sequoyah, a Cherokee Indian, who devised a phonetic alphabet that enabled his people to learn to read and to write within a few weeks' time.
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Range of the Sequoia gigantea:
Highest elevation: 8800 feet, Atwell Grove, Sequoia National Park.
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A purple powder is produced with the seeds in the cones. This cone pigment is largely tannin, is soluble in water, and in solution has been used experimentally as ink and wood stain. Tannin is believed to be the factor which gives the giant sequoia such remarkable resistance to attack by insects and disease.
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When we say that the General Sherman Tree is the largest living thing, we must remember that most of the trunk is really dead wood. One living portion of the trunk is light cream in color and lies just beneath the bark. This includes a layer of cells which grow and divide, forming new wood on the inside and new bark on the outside. In the sapwood water and minerals from the soil flow upward to the foliage. Just outside the layer of dividing cells the food materials manufactured in the leaves flow downward, nourishing the growing cells of trunk and roots.
The remainder of the trunk is dead tissue, functioning only to give strength to the trunk. This heartwood is bright pink in color when first exposed but soon turns to a dark brown or black.
Last Updated: 02-Feb-2007