Yosemite fire: Why giant sequoia trees are surviving now when others died in recent fires

Years of controlled burns have thinned flammable material in Mariposa Grove, paying off during Washburn Fire, experts say

YOSEMITE NATIONAL PARK — Last year and the year before, in an unprecedented environmental disaster, wildfires in California’s Sequoia National Park and nearby national forests roared through treasured sequoia groves in the Southern Sierra Nevada, generating flames hundreds of feet high and killing nearly 20% of all the giant sequoia trees left in the world.

But the Washburn fire burning now in Yosemite National Park, licking around the edges of the roughly 500 giant sequoias in Mariposa Grove — some over 200 feet tall and more than 2,000 years old  — so far hasn’t killed a single one of the massive old-growth trees there.

A big part of the reason, experts say, is that park officials have completed 23 projects at the grove since 1971 to lightly thin brush and set controlled burns to remove dead wood and vegetation that had built up over more than century of fire suppression.

That left less dead material on the forest floor, and fewer shrubs and small trees like firs that can make fires burn hotter. As a result, the forest was restored to a more natural condition, expert say, similar to the way it would have looked centuries ago when lightning strikes and burning from native tribes sent low-impact fires through the Sierra every 10 years or so.

In recent days, firefighting crews also were able to build hand lines around the famed grove, and set up a sprinkler system next to the Grizzly Giant and several other massive sequoias. And so far the outlook for the iconic trees has been good.

“The fire’s not impacting the trees. They’re fine,” said Yosemite National Park spokesman Mark Ruggiero on Tuesday.

Fire officials are optimistic that the grove, first set aside for protection by Abraham Lincoln in 1864, will emerge unscathed, even though pine and fire forests around it could burn for weeks. Still, a change in wind or weather could quickly alter the outlook.

“Under current conditions and forecast conditions and what the fire’s doing, we feel good about the Mariposa Grove, but I couldn’t say that we’re not worried,” Ruggiero said. “We are.”

By Tuesday afternoon, the Washburn Fire had grown to 3,221 acres. It was 22% contained, with 649 firefighters on hand. Crews have cut lines and held the fire back not only from Mariposa Grove, but from the historic Wawona Hotel and the community of Wawona, which were evacuated late last week.

As the drama plays out around the Yosemite grove, the fire is being cited as an example of how other giant sequoias across the Sierra can be saved in an era of hotter, more intense fires made worse by climate change.

“Prescribed fire works,” said Nate Stephenson, a scientist emeritus with the U.S. Geological Survey who has studied giant sequoias for decades. “It greatly improves the odds of a good outcome.”

Giant sequoias evolved over millions of years with low intensity fire. Not only does fire remove debris from the forest floor to allow new plants to grow, it also generates heat that opens Sequoia cones and releases their seeds. Although coast redwoods are the tallest trees on earth, giant sequoias, their biological cousins, are the largest by volume, with bark that can be 2 feet thick and resistant to fire. As a result, individual trees are often 1,000 to 3,000 years old, having survived many fires.

But starting in the late 1800s, settlers began putting out lightning-strike fires. Native tribes who set fires for centuries were removed from the land. And the forests became thicker, more choked with dead wood and smaller, flammable vegetation.

“When you do a prescribed fire under good conditions — no winds, with moderate temperatures  — you burn up the dead stuff, kill a lot of the small trees and return it to the condition it was 150 or 200 years ago,” Stephenson said.

In 2020, the SQF Complex fires, started by lightning, killed 7,500 to 10,600 giant sequoias in Sequoia National Forest in Tulare County. The following year, the KNP Complex and Windy fires killed another 2,261 to 3,637 giant sequoias in Sequoia National Park, Sequoia National Forest and surrounding lands.

Stephenson, who helped document the devastation using satellite imagery and other tools, said those fires burned much hotter in giant sequoia groves that had not had any controlled burns.

“Areas that had not seen a recent prescribed fire did really poorly,” he said. “They were moonscaped. It was awful. Not a living sequoia left. Areas that had prescribed fires mostly did really well.”

There are roughly 78 giant sequoia groves over a 250-mile long range in the Sierra Nevada. They total about 25,000 acres, and exist nowhere else on earth in primeval groves.

Stephenson said about 80% of those groves have burned in recent years, and the remaining lands remain at high risk. Save the Redwoods League, a San Francisco non-profit that has worked for 100 years to preserve coast redwoods and sequoias, estimates it will cost $500 million to treat about 60,000 acres of giant sequoia groves and the forests around them over the next five years with controlled burns and thinning to restore natural conditions and create buffer zones, said Sam Hodder, the group’s president.

“There is an existential threat to the species,” Hodder said Tuesday. “What we have seen in the last five years, in terms of mass mortality events in giant sequoias, is unlike anything we have found anywhere in the tree ring data going back thousands of years.”

Hodder’s group is supporting a bill introduced in the House of Representatives last month, the “Save Our Sequoia Act,” to provide more federal funding and streamline paperwork requirements to allow more controlled burns. The bill has bipartisan supporters, including Rep. Scott Peters, D-San Diego, and Rep. Kevin McCarthy, R-Bakersfield.

Controlled burns can cause controversy because they generate smoke that neighboring communities and state air districts oppose. They also have at times generated opposition from some environmentalists, and they can be expensive, and occasionally burn out of control. Hodder notes that it cost $144 million to suppress the SQF Complex Fire in 2020, a point he makes to show that it can be cheaper to take steps to reduce fire risk among some of America’s most iconic landscapes than battling big fires on them.

“Fire is inevitable in the Sierra,” he said. “Much of that landscape is vulnerable to catastrophic wildfire. Mariposa Grove is showing that the combination of fire and good stewardship to reduce the fuel load can lead to resilience.”

Stephenson said even with climate change, the situation is not hopeless, if people act.

“I really want my grandchildren to walk among living 2,000 to 3,000-year old sequoias and get that sense of awe,” he said. “It’s moved me so much. It has moved people all over the world. They are inspiring.”