A wildfire threatening the largest grove of giant sequoias in Yosemite National Park 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 as the U.S. weathers another very active year for fires.

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.

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