Yosemite > Library > Yosemite Sequoias > Enemies >
Next: Map • Contents • Previous: Age
Since trees renew their vital organs of wood, leaf, and root each year it would seem that they could live forever. It is true that trees do not get old in the same sense that we do. Were it not for the enemies of the giant sequoias they would indeed be “immortal.” In spite of natural enemies, fire, insects, fungi, storms and erosion, they are able to survive to great old age.
Because of the large tannin content of the wood (the sap is 74 per cent tannin), it is practically immune to attack by either fungous diseases or insects. The chipmunks , robins and juncos, as possible evidence of the insecticidal qualities of even the bark, can frequently be seen taking sequoia bark baths in the depressions about seven feet above the ground in the buttressed base of the Lafayette Tree. In these depressions is a quantity of finely powdered bark. And yet there are several insects that attack the giant sequoia. The short longitudinal galleries that may be seen beneath the bark on fallen branches are due to the work of the sequoia bark beetle (Phloeosinus rubicundulus). The large horizontal branches in the crowns of the more mature trees are sometimes ringed by this insect, killing the branch. Because of the consequent danger to persons below, they have acquired the name of “widow makers.” Also the insect known as the sequoia scale (Aonidia shastae), has been known to attack and discolor the foliage of young giant sequoias, although no fatal attacks have been observed. Termites seldom attack even the fallen trees. However, termites have been at work many years on the uprooted base of one large tree which has fallen across Rattlesnake Creek just below the Mariposa Grove Museum.
Attacks by fungi are chiefly limited to a small amount of “heart” rot, but since heartwood is non-living tissue the life of the tree is not imperiled. “Heart” rot could easily have been a contributing cause of the fall of the Massachusetts Tree in the Mariposa Grove, making a fine “punk” in which fire could tunnel up through its trunk. Further weakened by soil erosion, storm, and mutilation of its supporting roots by road builders, it crashed to the ground under a heavy mantle of snow in the spring of 1927.
The real threat to the life of giant sequoias is man’s destructive acts. Fire is deadly to the young and has been the single greatest source of destruction of the mature trees. The effects of past fires are to be seen everywhere in the groves. Many of the large black 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. By the use of the increment borer a study of the more recent fire scars in the larger living trees has been made. Thus far the dates of the occurrence of 14 fires in the Mariposa Grove have been determined: 450 A. D., 1622, 1652, 1690, 1710, 1734, 1742, 1752, 1760, 1775, 1803, 1807, 1809, 1842 and 1862. Before the coming of the white man to these groves, most of the fires were started by lightning. Now 50 per cent are started by lightning—the remaining 50 per cent are man made.
Giant sequoias have a most astonishing resistance to fire. A single fire has perhaps never killed a mature tree. It would take weeks of burning, fed by branches of other trees, for a fire to penetrate the great thickness of its asbestos-like bark and reach the living tissues beneath. It is only by a succession of fires that many such as the Haverford, Corridor, and Telescope Trees in Mariposa Grove have acquired great cavernous fire burns which have eaten into the heartwood and weakened the bases of their massive trunks. It is only then that the factors of lop-sided growth, soil erosion about the roots and the winter storms may cause their downfall. Thus fire is at least a contributing factor to the untimely end of many of these forest giants.
Fortunately time and efficient fire control has prevented the occurrence of a destructive fire among these giant trees since the devastating fire of 1862.
The recuperative power of the giant sequoia is remarkable. Soon after the bark is burned away new wood and bark begin to form around the scarred areas and slowly, at the rate of about one-fourth inch a year, close the wounds. The average, short-lived tree would find difficulty in closing a large wound even though it were not killed by insect and fungi attack; but the giant sequoia, with centuries in which to effect recovery, often succeeds.
Lightning is seldom a cause of death to these trees. The only known instance is an individual 14 feet in diameter located just across the road from the Telescope Tree in Mariposa Grove. The lightning bolt struck about 100 feet above the ground, beheading it and the larger tree against which it is now leaning. The second tree is still alive, as it was hit above some of the lowest branches. In its cratered crown a large vine of California wild grape has grown. Hanging down the golden brown trunk, it waves in each passing breeze.
With increase of travel to the groves, man has become an unnatural enemy of the giant sequoias. Not only does he start 50 per cent of the fires, but the soil covering the surface roots of the trees has been trampled and the bark and the base of the trunks have been injured. The continued trampling of several hundred thousand feet around the bases of some of the more famous trees is a cause of concern to those who have made a careful study of its effect. That such damage may be kept to a minimum a great deal of thought, effort and money has been spent on the area surrounding the Grizzly Giant in Mariposa Grove.
In the 1890’s grazing and the sawmill came to the giant sequoia groves. John Muir, speaking from his experience as a sheepherder, terms the sheep which were herded in droves up the quiet valleys among the giant trees, “hoofed locusts.” And surely they earned the name!
Five sawmills were already at work, wreaking havoc among the giant sequoias by 1894. Fortunately the groves in Yosemite National Park had been reserved by the public for a higher purpose.
Next: Map • Contents • Previous: Age