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 United States 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.”