The largest tree in the world is as tall as a 27-story building. It has a branch that is thicker than an average person is tall (6 feet, 9-1/2 inches). And if you turned the wood in its trunk into 12-inch-wide planks, they would stretch for more than 100 miles end to end.
The tree is the General Sherman Tree, a 275-foot-tall giant sequoia in Sequoia National Park in California. It was named in the 1870s for the Civil War general, William Tecumseh Sherman. It is not the tallest tree in the world, or the oldest. But based on the volume of its trunk, it is the biggest.
"This is the most massive tree on earth," says Bill Libby, a retired professor of forestry and genetics at the University of California, Berkeley. It also may be the largest living thing on earth and one of the fastest growing organisms. Trees keep growing and adding a ring of wood underneath the bark each year.
Those growth rings provide scientists much information about trees. "They act like these incredible books for reading about the past," says Nate Stephenson, a research ecologist with the US Geological Survey in Sequoia National Park. Scientists can count the rings to tell how old a tree is and when droughts slowed the tree's growth.
The General Sherman Tree is so big that no one has been able to count all its rings. So no one knows exactly how old it is. People once thought the tree might be as old as 6,000 years. But Dr. Stephenson did a study a few years ago and determined that it is about 2,150 years old. "It is still only an estimate, and it can be off by centuries," he says.
Sequoias come from tiny seeds that look like flakes of oatmeal. The seeds form in cones that can stay on the tree for 20 years. Sometimes squirrels help spread the seeds around by eating the cones. The heat from fires also makes the cones open and release their seeds.
That's just one way that fire helps sequoias grow. Fire also kills fungus in the soil that can attack sequoia seedlings. And it clears the ground so the new seedlings can grow. The giant trees are protected from most fires by their bark, which can be more than a foot thick, says Ronald Lanner, author of "Conifers of California."
Even so, very few seeds actually grow into giant trees. Some seeds are eaten by birds or small animals. Some seedlings don't get enough light. Others get too much. The seedlings that survive grow taller and taller for hundreds of years. Most stop getting taller before they reach 300 feet, although they keep getting bigger around.
Sometimes sequoias get so big that they fall over. In fact, this is the main way giant sequoias die, says William Tweed, who retired as chief park naturalist in Sequoia and Kings Canyon National Parks.
Mr. Tweed once heard a giant sequoia fall. He was giving a nature walk on a cloudless day when all of sudden he heard what sounded like "thunder in a canyon that echoes around," he says. "It was just so powerful that you stopped paying attention to anything else. It was like being buzzed by a jet aircraft at 100 feet when you didn't know it was there."
Tweed reached the fallen sequoia about 15 minutes after it had collapsed. The freshly broken wood was wet and cold, he says. The inside of a giant sequoia is "just like being down in a cave or a deep basement."