THREE RIVERS, Calif. — Two lightning-sparked wildfires in California merged and made a run to the edge of a grove of ancient sequoias, momentarily driving away firefighters as they try to protect the world's tallest tree by wrapping its base in protective foil.

A shift in the weather led to explosive growth on the fires in the Sequoia National Park in the Sierra Nevada on Friday, the National Park Service said, and the flames reached the westernmost tip of the Giant Forest, where it scorched a grouping of sequoias known as the "Four Guardsmen" that mark the entrance to the grove of 2,000 sequoias.

Firefighters wrapped the base of the General Sherman Tree, along with other trees in the Giant Forest, in a type of aluminum that can withstand high heat. It wasn't immediately known how the Four Guardsmen, which received the same treatment, fared, fire spokeswoman Katy Hooper said.

The General Sherman Tree is the largest in the world by volume, at 52,508 cubic feet (1,487 cubic meters), according to the National Park Service. It towers 275 feet (84 meters) high and has a circumference of 103 feet (31 meters) at ground level.

The fires, known together as the KNP Complex, blackened 28 square miles (72 square kilometers) of forest land. Fire activity increased Friday afternoon when winds picked up and low-hanging smoke that had choked off air and limited the fire's growth in recent days lifted, Hooper said.

Firefighters who were wrapping the base of the sequoias in foil and sweeping leaves and needles from the forest floor around the trees had to flee from the danger, Hooper said. They went back Saturday when conditions improved to continue the work and start a strategic fire along Generals Highway to protect the Giant Forest grove, Hooper said.

The fires forced the evacuation of the park this week, and parts of Three Rivers, a foothill community of about 2,500 people outside the park's main entrance. Crews have been bulldozing a line between the fire and the community.

The National Weather Service issued a red flag warning through Sunday, saying gusts and lower humidity could create conditions for rapid wildfire spread.

However, fire officials weren't expecting the kinds of explosive wind-driven growth that in recent months turned Sierra Nevada blazes into monsters that devoured hundreds of homes.

Giant sequoias are adapted to fire, which can help them thrive by releasing seeds from their cones and creating clearings that allow young sequoias to grow. But the extraordinary intensity of fires — fueled by climate change — can overwhelm the trees.

"Once you get fire burning inside the tree, that will result in mortality," said Jon Wallace, the operations section chief for the KNP Complex.

The fires already have burned into several groves containing trees as tall as 200 feet (61 meters) feet tall and 2,000 years old.

To the south, the Windy Fire grew to 19 square miles (50 square kilometers) on the Tule River Indian Reservation and in Giant Sequoia National Monument, where it has burned into the Peyrone grove of sequoias and threatens others.

The fire also had reached Long Meadow Grove, where two decades ago then-President Clinton signed a proclamation establishing its Trail of 100 Giant Sequoias as a national monument.

Fire officials haven't yet been able to determine how much damage was done to the groves, which are in remote and hard-to-reach areas. They said crews were "doing everything they can" to protect the trail by removing needles, leaves and other fuels from around the base of the trees.

Last year, the Castle Fire killed an estimated 7,500 to 10,600 large sequoias, according to the National Park Service. That was an estimated 10% to 14% of all the sequoias in the world.

The current fires are eating through tinder-dry timber, grass and brush.

In far Northern California, an early season rain was a welcome sign for firefighters battling a cluster of wildfires ignited by lightning in the Klamath National Forest in late July. Fire officials say it won't extinguish the nearly 300-square-mile (772 square-kilometer) blaze, but will help crews reach their goal.

Light rain is expected in the coastal area north of San Francisco over the weekend. But forecasters say conditions are likely to dry out by early next week, prompting a fire weather watch that may lead to power shutoffs in Napa, Sonoma and Solano counties.

Historic drought tied to climate change is making wildfires harder to fight. It has killed millions of trees in California alone. Scientists say climate change has made the West much warmer and drier in the past 30 years and will continue to make weather more extreme and wildfires more frequent and destructive.

More than 7,000 wildfires in California this year have damaged or destroyed more than 3,000 homes and other buildings and torched well over 3,000 square miles (7,770 square kilometers) of land, according to the California Department of Forestry and Fire Protection.

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