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- Growth & Development

Photo:  view looking up at the tops  of a group of sierra redwood trees  at Calaveras Big Trees State Park.The Root System
Giant sequoias normally develop an extensive root system very early in their careers. In a region marked by a prolonged summer dry season this kind of root system is essential in all but streamside or other unusually moist sites. Even with an extensive root system the giant sequoia is still sharply limited in its natural range by soil moisture conditions. As we have seen, it is the germination and early seedling stages that are most vulnerable to environmental problems such as drought. Once giant sequoias have germinated and developed into strong young seedlings with good root systems, they are quite capable of surviving in many parts of the world.

Within the first two years of growth, the root system begins to branch out more and more thickly, and as the tree grows larger, it is this lateral development just beneath the soil surface that continues most strongly. Eventually the roots of the larger trees reach out one hundred to one hundred and fifty feet, and in some cases may reach out more than two hundred feet. This means that some large sequoias extend their area of influence throughout some four square acres of forest land.

Most of the giant sequoia's root system is made up of tiny, threadlike feeders that spread out from the larger roots near the base of the tree. None of the large roots is likely to be more than two and one half or three feet in diameter, and most of them are much smaller. The entire root system is likely to be within four or five feet of the soil surface. This is an astonishingly delicate foundation for an above the ground structure that may tower upward two hundred fifty to three hundred feet (twenty to twenty-five stories) and weigh twelve million pounds (as much as a small ocean going freighter).

Despite its immense need for moisture and nutrients, the giant sequoia does not use up or over exploit the resources available to it. On the contrary, it tends to establish a very stable, long-term relationship with the soil around it. Systematic analysis and measurement of soil conditions around sequoias of varying ages show constant and reliable amounts of various chemicals necessary to a sequoia's well being.

The Crown
As soon as a young giant sequoia has an adequate year-round supply of moisture and sunlight it begins to grow quite rapidly. Under optimal conditions its main stem leads the way upward, and the tree becomes conical in shape. The upper part of the crown will retain this shape for many years although if the tree is not growing in full sunlight it may begin to lose its lower branches at it gets taller and as the shade deepens around its base. Eventually, however, sexual maturity is achieved and thereafter the crown of the tree gradually loses its sharply spired appearance and takes on a more rounded, dome shape. The crown of a mature giant sequoia takes on a gracefully rounded look with great cloud like billows of greenery that stand in marked contrast to the more pointed tops of other conifers around it.

The top of the crown becomes more and more rounded as great age and great height combine to inhibit further growth upward. Massive branches are also typical of the old veterans. Natural pruning may eventually result in main trunks that are branchless for one hundred to one hundred fifty feet above the ground. Above that level the older trees may keep one or more branches that come horizontally out of the main trunk and then turn upward, reaching another one hundred to one hundred fifty feet into the sunlight.

Many old giant sequoias are snag-topped in appearance. That is, they have dead wood at the top of their crowns, indicating that they have been taller and then died back somewhat even though they otherwise appear to be healthy and robust. The largest single cause of snag-tops is fire damage. Eighty-five percent of sequoias that have snag-tops also have serious burn scars in their trunks.

Rate of Growth
Coast redwoods may put on six, eight or even more feet of height in a single season whereas the giant sequoia is more likely to grow about two feet in height per year throughout its first fifty to one hundred years. On the other hand, the massive trunk of the giant sequoia continues to grow - increasing its overall volume - at a rate far surpassing that of any other tree. Growth rings one half inch in thickness are common in young giant sequoias under optimal conditions. This amounts to an increase of one inch of diameter per year. And rapid growth is likely to continue even when the trunk has become one hundred or more feet in circumference. By then the annual growth rings may have become narrower, but the overall volume of growth may be continuing at the same or an increased rate.

As part of their work for the National Park Service, the Hartesveldt research team measured both the size and rate of growth of many giant sequoias. They calculated that the General Sherman Tree (in the Giant Forest) - widely considered to be the world's largest tree - may also be the fastest growing. They found that the diameter of the tree had increased about three inches during the forty years years since careful measurements were made in 1931. Three inches of diameter in forty years may not sound like rapid growth until one remembers that the General Sherman Tree is 272 feet high and more than thirty feet in diameter - well over one hundred feet in circumference. Layers of new wood one millimeter thick spread over a surface this broad and this high, means that during the last forty years the General Sherman Tree has added enough new wood each year to construct a five or six room house. The Hartesveldt team also described the General Sherman's rapid growth in another way. Based on forty years of known growth, they calculated that the addition of new wood during this time was about forty cubic feet per year, or in other words approximately the same volume of wood as might be found in a tree twelve inches in diameter at breast height, and fifty feet high. Thus in order for a seedling tree to match the growth rate of the General Sherman it would have to start from nothing and reach fifty feet in height and one foot in diameter within a single year.

Theoretically it would appear that a giant sequoia could go on living and growing forever. Death comes to them only by means of fire or through some other external physical event such as undermining by erosion or overthrow by the wind. Although they are no longer considered the oldest living things in the world - they could conceivably regain the title at some time in the future. Today, the oldest known giant sequoia is some 3,300 years of age.

Excerpted from "The Enduring Giants" by Joseph H. Engbeck Jr., published by the California State Parks.