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SEQUOIA NATIONAL PARK, Calif. – High in the Sierras, biologists are struggling to find ways to protect some of the world’s oldest and most storied trees from drought, forest fires and climate change.

The trees are the giant sequoias, some of them 2,000 to 3,000 years old, and they are just one of several ancient Western species, including redwoods and bristlecone pines, that face a daunting future.

Although the sequoias are not at immediate risk, even from California’s current drought, scientists say they were not built to withstand decades of dry and warming weather. Their seedlings and saplings are susceptible to fires, which are likely to increase, especially at higher elevations. And if the drought persists, the lack of melting snow may keep the seedlings from developing a robust root system.

“If there’s long-term drought, within 25 years we could see seedlings in trouble,” said Nathan Stephenson, an ecologist with the U.S. Geological Survey. “In 50 years, the whole population could be in trouble,” he went on, and within a century “most of the big trees could be gone.”

Sequoias are found in only one place on Earth: the Sierra Nevada Mountains in California. There are 65 to 70 groves, most in a narrow 70-mile band on the west side of the range at 5,000 to 8,000 feet. They include one tree here called the General Sherman, the world’s largest by volume.

Preservation efforts are hampered by the fact that so little is known about big trees, from their root systems to how they die.

As the climate changes, so do conditions in which sequoias and other big trees grow. The coastal redwoods of California, for example, are fog drinkers, taking as much as 40 percent of their water in through their needles. In the past half-century, the number of days in which the trees are shrouded by fog has declined by 30 percent.

In some places, that appears, paradoxically, to be contributing to increased growth: With less fog cover there is more light, said Todd E. Dawson, a biologist at the University of California, Berkeley.

Climate change is one of a host of complex factors in the health of big trees. Many forests have been weakened from previous stresses, including habitat fragmentation, air pollution and selective breeding for timber. Warmer temperatures can also usher in disease and insects, along with fire.

Bristlecone pines – at 4,000 years or more the oldest trees in the world – have adapted to some of the harshest conditions on the continent. But rapid warming atop mountain ranges in Colorado, New Mexico and elsewhere has allowed bark beetles to gain a foothold. Once the bristlecones are infected, nothing can be done to save them. And although the beetle threat has waned of late, the pines have also been hit by blister rust, an invasive fungus from Asia.

Protecting such venerable trees is a challenge.

“We might start irrigating the sequoias,” Stephenson said, “or we might build a giant fuel break around the giant sequoias, so if a fire came toward the grove, we could defend it. These things are getting hard discussion.”

Sequoias are resilient. They have no disease or serious insect enemies, and their spongy bark, eight inches thick, is highly resistant to fires. One can lose 95 percent of its crown in a fire and recover. Still, no one knows what the climate future holds.

A novel program by Sierra Pacific Industries, a lumber producer and the largest landowner in California, has gathered cones from old-growth sequoia groves. In 2012, foresters started to plant seeds in 16 locations with different soil types, elevations and precipitation levels.

Some 130,000 seedlings, the tallest three to four feet, are now growing from the ancient seeds. The company program is aiming to grow 1.4 million, even though sequoias have little timber value.

“It’s a very, very disturbing trend,” said an author of the paper, Bill Laurance, an environmental scientist at James Cook University in Australia. “We are talking about the loss of the biggest living organisms on the planet, of organisms that play a key role in regulating and enriching our world.

“A world where a child can’t stare up in wonder at a giant cathedral-like crown is a very real possibility.”