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A "prescribed fire" illuminates the giant sequoia trees. Hot enough to burn sur face duff but not so hot that it kills large trees, these controlled fires are what California's giant sequoias need to open cones and clear space for seedlin gs.

A controlled wildfire in California illuminates giant sequoia trees.

  Photograph by Raymond Gehman, National Geographic Stock

Brian Clark Howard

National Geographic

Published August 26, 2013

As the Rim Fire rages in and around Yosemite National Park, biologists have taken steps to protect two groves of giant sequoia trees. One tree expert suspects, however, that the measures have more to do with public relations than with sound forest policy.

The Rim Fire has burned for nine days, razing 134,000 acres, which makes it one of the biggest blazes in California's history. The flames have largely been confined to Yosemite's remote northwestern section, about 20 miles (32 kilometers) from the iconic Yosemite Valley.

The fire has not yet reached the two groves of giant sequoias in the park, small enclaves called the Tuolumne Grove and Merced Grove. But rangers have taken the precaution of placing sprinklers around those big trees. (Read about giant sequoias in National Geographic magazine.)

"These two groves are precious resources that the public is concerned about, and rightly so, because they are amazing," Stephen C. Sillett told National Geographic. Sillett is an ecologist with Humboldt State University who specializes in tall trees. He has received grants from the National Geographic Society to study the giants in Sequoia National Park.

But Sillett said placing sprinklers around the sequoias in Yosemite isn't really necessary. "The main thing they are doing with sprinklers is appeasing the public, who are worrying about how ugly the area will look when they visit later and that some trees are going to die," he said.

"The big trees are going to be fine," Sillett explained. "Smaller, weaker, non-giant sequoias will die, but it's not so much that they are protecting the trees."

Sillett said full-grown sequoias are adapted to survive even the hottest wildfires. They have fibrous, fire-resistant bark that can grow up to two feet thick. Although fires can damage the biggest trees, they usually don't kill them.

Giant sequoias are the world's largest single trees by volume. They reach an average height of 160 to 279 feet (50 to 85 meters) and average diameter of 20 to 26 feet (6 to 8 meters). Record trees have been identified at 311 feet (94.8 meters) and 56 feet (17 meters) in diameter. The oldest known giant sequoia is estimated at 3,500 years old. (See "Giant Sequoias Grow Faster With Age.")

Uneven Management

Sequoia management is all over the map, says Sillett. The giant trees live in about 70 groves that are spread along the western slopes of the Sierra Nevada.

Some of those trees live in national parks, where the Park Service often conducts occasional controlled burns to reduce the buildup of dead material that can lead to bigger fires later (see video). In some areas, forest managers also practice selective logging to thin out the surrounding forest.

On other land, the forest is not actively managed, and dead wood is allowed to pile up for years. "In those cases, when it does burn it's going to be a hotter fire that will damage the trees even more than it normally does, although I don't think it will kill them," Sillett said.

Forest managers have not been able to conduct controlled burns in places where too many people live nearby, such as Calaveras Big Trees State Park and Mountain Home Demonstration State Forest. Instead, scientists take a combined approach. They selectively prune and log dead and dying trees and then stack up the wood and burn it in controlled piles.

"Giant sequoias are flourishing in response. They love it," said Sillett.

(See pictures of scaling sequoias.)

How Fire Benefits Sequoias

Schoolchildren often learn that sequoias benefit from fire because the heat makes their cones open up, the flames eliminate competing plants, and the ash serves as fertilizer. The relationship is actually a bit more complex.

"It's not like sequoias need fire to spread their seeds, but they live in a fire-prone environment, and they know what they're doing," he said.

Sillett said fires can reduce competition for sequoia seedlings and provide fertilizer in the form of ash, but the cones will also open up on their own.

A single sequoia can play host to more than 100,000 cones, said Sillett. The cones are green when living, meaning they carry out photosynthesis, producing some of the sugars that they need for their own growth. Each cone can live 10 to 20 years. It then dies, opens up, and drops its seeds.

When there is a fire, it kills a large percentage of the cones, causing them to drop their seeds en masse. Normally, it takes some time for the cones to open up. By the time the seeds hit the forest floor, the fire has moved on, and the earth has cooled.

To Sillett, what happens to the small number of sequoias in Yosemite's two groves is only part of the picture of the species' success. Sequoias are well adapted to survive fires, he said. "They are incredible trees."

Follow Brian Clark Howard on Twitter and Google+.

2 comments
Lindsey Nitta
Lindsey Nitta

Scott Stephens from UC Berkeley was quoted in SF Chron saying that High-intensity fires generally kill the big trees.  

Reports say fires are burn hotter and faster than ever before. Couldn't be they were taking precaution to help ensure the trees survival? 

Michael Wysocki
Michael Wysocki

Sequoias, the largest living thing on earth. How special 

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