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 2,000 to 3,000 years old, and they are among several ancient Western species, including redwoods and bristlecone pines, that face an uncertain 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, warming weather. Their seedlings and saplings are susceptible to fires, which are likely to increase, especially at high elevations. And if drought persists, the lack of melting snow may keep 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,” and within a century “most of the big trees could be gone.”

Sequoias live in only one place on Earth: California’s Sierra Nevada Mountains. About 70 groves dot a narrow 70-mile band on the west side of the range at 5,000 to 8,000 feet. They include one tree 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 climate changes, so do conditions in which big trees grow. The coastal redwoods of California, for example, are fog drinkers, taking in as much as 40 percent of their water through their needles. In the past half-century, the number of days in which the trees are shrouded by fog has dropped 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. But on the redwood range’s southern and eastern edges, which are warmer and drier, “the crowns are beginning to thin out, and they are dropping needles,” Dawson said.

Stresses pile up

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, the pines have also been hit by blister rust, an invasive fungus from Asia.

“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.”

A program by Sierra Pacific Industries, a lumber producer, has gathered cones from old-growth sequoia groves. Foresters planted seeds in 16 locations with different soil types, elevations and precipitation. Some 130,000 seedlings, the tallest three to four feet, are growing from the ancient seeds, with a goal of 1.4 million.

“The goal is to conserve the genetic diversity of the native groves” should the old trees die, said coordinator Glenn Lunak.

Big trees are on the front lines of climate change. A 2012 study in the journal Science found 100- to 300-year-old trees were dying at high rates around the world, in part because of hotter and drier weather.

“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.”