CLIMBING THE REDWOODS.

The coast redwood tree is an evergreen conifer, a member of the cypress family, which grows in valleys and on slopes of mountains along the coast of Central and Northern California, mostly within ten miles of the sea. The scientific name of the tree, which is usually simply called a redwood, is Sequoia sempervirens. A coast redwood has fibrous, furrowed bark, flat needles, and small seed-bearing cones the size of olives. Its heartwood is the color of old claret and is extremely resistant to rot. It has a lemony scent. Redwoods flourish in wet, rainy, foggy habitats. The realm of the redwoods starts in Big Sur and runs northward along the coast to Oregon; fourteen and a half miles up the Oregon coast, the redwoods abruptly stop.

The main trunk of a coast redwood can be up to twenty-five feet in diameter near its base, and in some cases it can extend upward from the ground for more than two hundred and fifty feet before the first strong branches emerge and the crown of the tree begins to flare. The crown of a tall coast redwood is typically an irregular spire that can look like the plume of a rocket taking off. Very few trees of any species today other than redwoods are more than three hundred feet tall. The tallest living coast redwoods are between three hundred and fifty and three hundred and seventy feet high--the equivalent of a thirty-five-to-thirty-seven-story building.

In its first fifty years of life, a coast redwood can grow from a seed into a tree that's a hundred feet tall. Redwoods grow largest and tallest in silty floodplains near creeks, in spots that are called alluvial flats. There, a redwood can suck up huge amounts of water and nutrients, and it is protected from wind, which can throw a redwood down. In its next thousand years, it grows faster, adding mass at an accelerating rate. The living portion of a tree is a layer of tissue called the cambium, which exists underneath the bark. If the cambium of one of the bigger coast redwoods were to be spread out into a flat sheet, it would be nearly half the size of a football field. Each year, a coast redwood can add one millimetre of new growth to its cambium, or the equivalent of one ton of new wood.

As a young redwood reaches maturity, it typically loses its top. The top either breaks off in a storm or dies and falls off. A redwood reacts to the loss by sending out new trunks, which typically appear in the crown, high up in the tree, and point at the sky like the fingers of an upraised hand. The new trunks shoot upward from larger limbs, travelling parallel to the main trunk, or they emerge directly out of the main trunk and run alongside it. The new trunks send out their own branches, which eventually spit out more trunks through the crown. The resulting structure is what mathematicians call a fractal; botanists say that the tree is forming reiterations. The redwood is repeating its shape again and again.

With the passing of centuries, the reiterated trunks begin to touch one another here and there. The trunks fuse and flow together at these spots, like Silly Putty melting into itself. The bases of the extra trunks bloat out, and become gnarled masses of living wood called buttresses. In the crowns of the largest redwoods, bridges of living redwood are flung horizontally from branch to branch and from trunk to trunk, cross-linking the crown with a natural system of struts and cantilevers. This strengthens the crown and may help it to grow bigger, until it can look like a thunderhead coming to a boil. There are often blackened chambers and holes in the trunks--fire caves, caused by big forest fires. The tree survives and regrows its burned parts, and it continues to thrust out new trunks.

Botanists judge the size of a tree by the amount of wood it contains, not by its height. By that measure, the largest species of tree is the giant sequoia, a type of cypress that is closely related to the coast redwood. The biggest living giant sequoia trees have fatter and more massive trunks than the coast redwoods. But the coast redwood is the tallest species of tree on earth.

Extremely large coast redwoods are referred to as redwood giants. The very biggest are called titans. Currently, about a hundred and twenty coast redwoods are known to be more than three hundred and fifty feet tall. Eighty per cent of them live in Humboldt Redwoods State Park, along the Eel River, in Humboldt County, in Northern California. No one knows exactly how old the biggest coast redwoods are, because nobody has ever drilled into one of them to count its annual growth rings. Botanists think that the oldest redwoods may be somewhere between two thousand and three thousand years old. They seem to be roughly the age of the Parthenon.