Giant sequoias (Sequoiadendron giganteum) are famous for being some of the largest organisms on the planet. Known colloquially as big trees, Sierra redwoods, Sequoiadendrons, or simply sequoias, these trees have been one of the most important species in Northern California for millions of years.
Although they are among the most robust and resilient macro-organisms the world has ever seen, their population is in decline, and their long-term future is uncertain.
Giant with a Capital “G”
Although they do not climb as high as their relatives the coastal redwoods (Sequoia sempervirens) do, giant sequoias are the world’s largest trees with respect to volume. The largest specimen currently known — a 275-foot-tall tree living in Sequoia National Park that bears the name “General Sherman” — occupies over 50,000 cubic feet of space and weighs about 2.7 million pounds. By contrast, the volume of the tallest coastal redwood – “Hyperion”, a coastal redwood — is only about 18,000 cubic feet.
To provide some context, if General Sherman were planted next to the Statue of Liberty, the top of the canopy would be about 15 lower than the top of the torch.
Ancient Trees
Many of the giant sequoias living in the Sierra Nevadas are among the oldest organisms on the planet. While the oldest documented specimen was only about 3,200 years old, it is likely that these trees can reach ages of 4,000 to 5,000 years. Some botanists have even speculated that a few specimens may reach twice these ages.
However, unlike some other trees that live very long lives, giant sequoias mature relatively quickly. Most specimens begin producing cones by 20 years of age. This allows the trees to produce a ridiculous number of seeds over their lifetimes. Because each tree holds approximately 11,000 cones, and each cone contains about 230 seeds, giant sequoias often hold about 2.5 million seeds at any point in time.
However, the cones must dry out to open and release the seeds, so many cones lay dormant for several years. This high fecundity is necessary because the seeds require extremely specific conditions in order to germinate. Therefore, only a small percentage of the released seeds become new trees.
Fire Resistance
The only way that trees can reach great ages in the western United States is by developing methods to survive periodic fires. To that end, giant sequoias have evolved one of the thickest barks known in the world, which may be 2 feet thick in places. Additionally, the bark has very little resin, which makes it further resistant to fire.
When fires move through sequoia-dominated forests, the giants usually survive, while thin-barked trees (including sub-mature sequoias) perish. This provides new ecological space in which young sequoias can grow, so the seed-bearing cones usually open in response to fire. Forest-grown specimens tend to shed their lower branches, thereby offering further protection against fires, as it reduces the chances that fire can climb into the delicate canopy.
Mortality
Part of the reproductive strategy for giant sequoias relies on mature trees living very long lives. This gives them the time to produce an unthinkable number of seeds over their lifetimes. Accordingly, the trees have evolved a number of adaptations that allow them to survive for thousands of years.
Part of their success precipitates from the fact that mature giant sequoias only face a few threats. Chainsaw-wielding humans have killed more sequoias than any other cause over the last 100 years, but they need not fear many other animals, including insects or other pests. The mighty trees have few problems with disease and are they relatively immune to fire. As gigantic trees, they often serve as lightning rods, but even those that are struck often manage to survive.
The primary non-human causes of their mortality appear to be wind, fire and erosive forces. For example, over hundreds of years, a river or run-off channel may undercut the foundation of the tree, causing it to topple.
Extinct Trees
Giant sequoias are not the only member of the genus Sequoiadendron – an extinct species, called Sequoiadendron chaneyi, is known from the fossil record. This extinct species inhabited the Colorado Plateau until sometime between 5 and 15 million years ago. Many paleobotanists believe that S. chaneyi was the direct ancestor from which S. giganteum evolved.