By Steve Gorman
July 31 (Reuters) - Firefighters tightened their grip Thursday on a blaze roaring for a sixth day through drought-parched timber and brush on the western edge of Yosemite National Park as they took special measures to protect a famed grove of giant Sequoia trees.
The El Portal fire has blackened more than 3,900 acres (1,580 hectares) since it erupted on Saturday, destroying a vacation home and duplex, prompting the evacuation of dozens of other dwellings and forcing the closure of three campgrounds, fire officials said.
No serious injuries have been reported.
By Thursday at midday, a firefighting force of at least 1,000 personnel had carved containment lines around more than a third of the blaze's perimeter, up from a containment level of just 5 percent reported at the start of the week.
Wednesday night, park officials reopened a stretch of Highway 120, a key route across Yosemite, after halting the advance of flames encroaching on the road, said fire command spokesman Mark Savage.
However, the blaze continued to creep in the direction of a prized cluster of Sequoias called the Merced Grove, the smallest of three towering stands of giant redwoods in Yosemite, about 200 miles (320 km) east of San Francisco.
As of Thursday afternoon, the flames were burning within 2 miles (3.2 km) of the grove, home to 20 to 30 specimens of Sequoia, trees that can live 3,000 years or more and are among the largest living things on Earth.
"We know it's a very special resource, and we're doing everything we can to protect it," park ranger and spokeswoman Kari Cobb said.
A special sprinkler system was being installed around the grove and crews were using bulldozers to clear road-sized buffer zones of vegetation on its perimeter, officials said.
Residents forced to flee the tiny community of Old El Portal were permitted to return early on Tuesday, but 45 homes in the neighboring enclave of Foresta, just inside the park, remained under evacuation, Savage said.
The blaze was burning in a corner of Yosemite and the adjacent Stanislaus National Forest south of the 250,000-plus acres (100,000 ha) scorched last summer by the so-called Rim Fire, which ranked as the third-largest California wildfire on record.
Cobb said that except for the three closed campgrounds, Yosemite remained opened to the public, with visitation appearing to be down only slightly from usual levels.
The park averages nearly 4 million visitors a year, most of them in summer. (Reporting by Steve Gorman from Los Angeles; Editing by Eric Walsh)