Passage IV

NATURE SCIENCE: This passage is adapted from the article Ancient Trees: Trees That Live for a Thousand Years by Anna Lewington and Edward Parker (@1999 by Anna Lewington and Edward Parker).

Giant redwoods and their close ancestors have been on the earth for at least 200 million years and, like their coastal cousins, once formed massive forests across the northern hemisphere. Today they are a truly reflect species, occurring only in isolated groves on the western slopes of the Sierra Nevada in California.

Although the largest giant redwood in existence does not hold any records for being the oldest, tallest or broadest tree in the world, nothing can match its sheer volume. The tree known as the General Sherman, after the famous general in the American Civil War, contains an estimated 50,000 cu ft/1,415 cu m of wood. The largest trunk of any redwood is found on a tree in Alder Creek, which averages 53 ft/16 m around its base. Giant redwoods are virtually indestructible because they have fire-resistant bark. In fact, the trees need the clearing effect of forest fires to establish new seedlings successfully. They are also resistant to fungi and wood-boring insects. The main cause of death is being blown over.

Today there are just seventy-five groves of giant redwoods left, concentrated in the King’s Canyon and Sequoia National Parks, with a further three groves located in Yosemite. Where they do appear, the redwood forests seem to be healthy and reproducing well. Giant redwood groves are found only on the slopes of upland ridges between major river canyons at 4,000-8,000 ft/1,220-2,440 m above sea-level. Here the climate is characterized by warm, dry summers and sunny winters. The latter are, however, interrupted by infrequent snowstorms, which may last from a few says to a week, leaving several feet of snow. The groves are generally tucked away from areas of high wind, but lightning strikes and thunderbolts are relatively common. Just why the giant redwoods are confined to isolated groves remains a mystery. Not a single tree is found outside these groves, even where the same climatic conditions prevail.

The giant redwood has cinnamon-coloured bark, which may grow to a massive 18 in/45 cm thick. Unlike the slim, tapering trunk of the coast redwood, the giant’s trunk is conical in shape and has a broad base that can reach over 40 ft/12 m across. Even at 200 ft/60 m above the ground, it can still be more than 20 ft/ 6 m in diameter. Its main branches may grow to more than 8 ft/2.4 m wide, making the tree’s biggest limbs the size of large trees themselves.

The tree stands on a shallow but widely spreading root-pad, which can be enormous, radiating from the trunk for up to 300 ft/91 m, but seldom reaching down into the earth more than 6 ft/ 1.8 m. Giant redwoods reach their maximum height in the first 500 years of their life, after which lateral expansion goes on for at least 3000 years. It is not clear when a redwood stops expanding, because there is no definite record of any giant redwood dying of old age and they continue to grow indefinitely until a natural disaster, such as a lightning strike or storm- force wind, occurs, the greatest age that has been verified from a stump by its tree rings is 3200 years. However, naturalist John Muir claimed to have discovered a stump containing 4,000 rings.

The giant redwoods start to produce seeds after only a few years of life. Mature trees generate about 600 new cones every year and, since each cone contains several hundred healthy seeds, a tree can produce more than 100,000 seeds annually. The largest giant redwood produces some 10,000 cones and as many as two million seeds each year. The cones do not simply fall, however, and release their seeds. This function is often performed unwittingly by the chickaree, or Douglas squirrel, which finds the fleshy scales of the cones delicious. As it feeds on them, the seeds are scattered onto the forest floor. Once the seeds reach the ground, they will germinate only under exactly the right conditions.

The sugar pines and yellow pines that, along with the redwoods, form the mixed conifer forests of the Sierra Nevada, all rely on fire to create gaps, in the overhead canopy and clear the forest floor. The fires also dry the redwood cones on higher branches, which then release their seeds onto the cooling ashes below. The action of the fires allows the seeds to fall on areas of bare mineral soil, where the sunlight is able to filter through. Where fires have been prevented, the forest floor rapidly becomes colonized by shade tolerant white firs and incense cedars, hindering redwood regeneration.