YOSEMITE NATIONAL PARK, California (AP) — A wildfire threatening the largest grove of giant sequoias in Yosemite National Park in the United States (US) more than doubled in size in a day, and firefighters were working in difficult terrain Sunday to protect the iconic trees and a small mountain town.

Campers and residents near the blaze were evacuated but the rest of the sprawling park in California remained open, though heavy smoke obscured scenic vistas and created unhealthy air quality.

“Today it’s actually the smokiest that we’ve seen,” Nancy Phillipe, a Yosemite fire information spokesperson, said Sunday. “Up until this morning, the park has not been in that unhealthy category, but that is where we are now.”

More than 500 mature sequoias were threatened in the famed Mariposa Grove but there were no reports of severe damage to any named trees, including the 3,000-year-old Grizzly Giant.

A sprinkler system set up within the grove kept the tree trunks moist and officials were hopeful that the steady spray of water along with previous prescribed burns would be enough to keep flames at bay, Phillipe said.

The cause of the Washburn Fire was under investigation. It had grown to nearly 2.5 square miles (6.7 square kilometres) by Sunday morning, with no containment.

Beyond the trees, the community of Wawona, which is surrounded by parkland, was under threat, with people ordered to leave late Friday. In addition to residents, about 600 to 700 campers who were staying at the Wawona campground in tents, cabins and a historic hotel were ordered to leave.

Temperatures were expected to rise and reach the lower 90s in the coming days, but fire crews working in steep terrain were not contending with intense winds, said Jeffrey Barlow, senior meteorologist with the National Weather Service in Hanford.

The giant sequoias, native in only about 70 groves spread along the western slope of California’s Sierra Nevada range, were once considered impervious to flames but have become increasingly vulnerable as wildfires fueled by a buildup of undergrowth from a century of fire suppression and drought exacerbated by climate change have become more intense and destructive.

Phillipe, the park spokesperson, previously said some of the massive trunks had been wrapped in fire-resistant foil for protection, but she corrected herself on Sunday and said that was not the case for this fire. However, crews have wrapped a historic cabin in the protective foil, she said.

Lightning-sparked wildfires over the past two years have killed up to a fifth of the estimated 75,000 large sequoias, which are the biggest trees by volume and a major draw for tourists to the national park that’s the size of the state of Rhode Island.

There was no obvious natural spark for the fire that broke out Thursday next to the park’s Washburn Trail, Phillipe said. Smoke was reported by visitors walking in the grove that reopened in 2018 after a $40 million renovation that took three years.

A fierce windstorm ripped through the grove over a year ago and toppled 15 giant sequoias, along with countless other trees.

The downed trees, along with massive numbers of pines killed by bark beetles, provided ample fuel for the flames.

Meanwhile, most evacuation orders were lifted Saturday in the Sierra foothills about 80 miles (128 kilometres) to the northwest of the Yosemite fire, where a fire broke out on July 4. The Electra Fire that began near Jackson was mostly contained, and only areas directly within the fire’s perimeter remained under evacuation orders, according to the California Department of Forestry and Fire Protection.

So far in 2022, over 35,000 wildfires have burned nearly 4.7 million acres in the US, according to the National Interagency Fire Centre, well above average for both wildfires and acres burned.