Yosemite wildfire continues to grow as it pushes east into Sierra national forest

The wildfire sweeping through Yosemite national park swelled to more than 4,375 acres (1,770 hectares) by Thursday morning, and is now pushing east into the Sierra national forest.

Raging across steep and rugged terrain, firefighters have faced challenges battling the blaze, which has exhibited extreme fire behavior, officials said. Warm and dry conditions as well as dried out vegetation have upped the intensity, spurring flames that, in some areas, stretched high into the canopies of the tall trees and produced large plumes of smoke that billowed into the sky.

Winds have, however, remained light in the area where the so-called Washburn fire continues to burn. Temperatures lingered just below 100F (38C) on Thursday and relative humidity remained in double digits, aiding firefighters as they achieved roughly 23% containment on the fire.

But the blaze continues to grow, and officials said it will probably smolder through the rest of the year, until rains and snows terminate the risk completely.

The sequoias are adapted to fire and rely on it to survive. But more than a century of aggressive fire suppression has left forests choked with dense vegetation and downed timber that has provided fuel for large wildfires that have grown more intense during acontinuing drought and exacerbated by the climate crisis.

So-called prescribed burns most recently conducted in the grove in 2018 mimic low intensity fires that help sequoias by clearing out downed branches, flammable needles and smaller trees that could compete with them for light and water. The heat from fires also helps cones open up to spread their seeds.

The treatments also help mitigate the risks that turn healthier fires into infernos. High-intensity burns and extreme fire behavior have increasingly threatened the massive trees. Once thought to be almost fireproof, up to 20% of all giant sequoias, native only in the Sierra Nevada range, have been killed in the past two years during intense wildfires.

Without a reprieve in dry weather, the Washburn fire will probably burn through the season and into January, Bercovitz said. Meanwhile, firefighting preparations had already been under way in the national forest.

The cause of the blaze is still under investigation, though it is believed to be human-caused. After igniting on 7 July, hundreds of visitors and residents from the small town of Wawona were evacuated and the southern entrance of Yosemite remains closed. One firefighter suffered a heat injury and recovered, but no structures have been damaged.

The rest of the park has remained open to visitors, even as the air quality declined.

The Associated Press contributed to this story.