The key to protecting Yosemite’s sequoias from wildfires? More fire

For decades, the park has used prescribed fire to make its forests healthier—and it’s paying off right now to protect the famous Mariposa Grove

Yosemite ecologist Garret Dickman has seen a lot of scary, intense fire in his career.

So when the Washburn fire started last week just around the corner from the national park’s famous, beloved Mariposa Grove—a patch of about 500 mature giant sequoias, some of which are more than 2,000 years old­—the first word out of his mouth was an expletive.

Not again, he thought.

California’s sequoias have experienced unprecedented losses in recent years. In just 15 months between 2020 and 2021, one explosive fire after another ripped through their range, killing an estimated 13 to 19 percent of all mature trees. Many researchers and tree-lovers like Dickman get a bad feeling in the pit of their stomach at the news of any new fire, fearing it will spin up into another disaster.

But then he took a breath. Yosemite National Park, and the Mariposa Grove in particular, has one of the longest-running and most thorough “prescribed fire” programs in the United States. By treating the grove with more than 20 low-level, carefully managed fires since 1971—one every two to three years on average—the park service has cleared out combustible brush and dead wood. That has given the sequoias a layer of protection from catastrophic wildfire—not a guarantee against all damage, but a much stronger chance of escaping fire unscathed.

“This is what we’ve been preparing for 50 years,” Dickman says. “We know what to do; we know how to save these trees.”

As of July 14, the fire had burned about 4,400 acres and was 23 percent contained. The cause is still unknown, but in a recent public meeting the park superintendent said that it was human-started.

In the donut hole

So far, the decades of preparation in the grove seem to be working.

The ring of forest around the Mariposa Grove, dubbed the donut by local fire teams, hasn’t been thinned or burned by prescription lately. So it was “loaded with fuel,” says Kristen Shive, a fire ecologist soon to be affiliated with UC Berkeley's Cooperative Extension program. Encountering such overabundance, the Washburn fire burned hot and intense, turning into the kind of fire that’s dangerous and difficult for firefighters to control.

But once it hit the edge of the carefully managed grove, “the fire dropped right down to the ground,” says Dickman. Unlike in the donut, which was packed with small trees, dead branches, and other fuel, the grove’s floor was relatively open. Without much material to burn, the fire became manageable. Crews were able to wet down the base of Grizzly Giant, one of the world’s top 25 biggest sequoias, and to clear small fallen branches, while the fire was just a few dozen yards away.

Forest scientist Nate Stephenson saw similar behavior during last year’s KNP Complex fire, in Sequoia/Kings Canyon National Park. At the Redwood Mountain Grove there, in “areas that hadn’t [recently seen prescribed fire], things got nuked. In areas that had had a recent prescribed fire, most of it did really, really well,” Stephenson says, burning at the kind of low or moderate severity that actually benefits sequoias, which require fire for their seeds to germinate.

Nonetheless, the best forest management in the world “doesn’t guarantee a good outcome,” says Stephenson. Bad luck and bad weather conditions can spin up intense fires even in carefully treated areas; in Redwood Mountain Grove, some treated areas still burned severely. Overall, as many as 3,700 mature sequoias died in the KNP Complex and adjacent Windy fire last year, about 3 to 5 percent of the remaining population.

But “these fuels treatments can improve odds of a good outcome, and greatly,” Stephenson says.

It’s critical to understand that distinction, says Matt Hurteau, a fire ecologist at the University of New Mexico. “Forest fuels treatments do not stop fire,” he says. “They change the way fire interacts with the vegetation. It’s releasing fuel on landscape in smaller bites—and decreasing the chance of those big, hot, fast-moving fires.”

Errors of history

Most of California’s forest isn’t anywhere near as thoroughly managed as the famous sequoia groves like Mariposa. Not even all the known sequoia groves have seen such thorough care; another grove in Yosemite, the Merced Grove, is thickly overgrown and therefore highly at risk during any fire that may sweep through. (It’s not in the expected path of the Washburn fire.)

But fire is an integral part of the cultural history and ecology of the region. Before European colonizers arrived in California, an estimated 2 to 4 million acres burned across the state each year. Indigenous people used fire to manage the forests in many ways, influencing the growth patterns of shrubs to make them better for basket weaving, opening the understory to encourage game to pass through, and much more. As they were forcibly removed from their homelands, fire often disappeared from the landscape.

From the early 1900s on, federal policy focused on stamping out fires as thoroughly and soon as possible. Places that had previously experienced fire every 10 or 20 years went decades longer without a burn, allowing the understory to fill with dead material and young, densely packed trees—a tinderbox waiting for a spark. Scientists estimate that some forest areas in California have twice as many trees or more than they did before European contact.

“We have a problem of overly dense forests because we have prohibited indigenous burning over the past 250 years,” says Joanna Nelson, science director at the Save the Redwoods League, a nonprofit dedicated to forest preservation in California.

How to go about addressing that overabundance is a major challenge. Prescribed fire is difficult to do both technically and politically; many more treatments are planned each year than can be completed, especially as climate change exacerbates the risks. Earlier this year in New Mexico, a prescribed fire escaped its bounds on a hot and windy day. It merged with another fire and eventually burned more than 340,000 acres and hundreds of homes. In response, the U.S. Forest Service announced they would stop all prescribed fire for 90 days while they reviewed their practices. Overall, the Forest Service reports that less than one percent of all prescribed burns escape.

The question is how to balance the risks from prescribed fire with the risks from an uncontrolled wildfire, says Shive: “There’s no solution going forward that doesn’t involve fire. It’s either the kind of fire we want, or the kind of fire we don’t.” While the New Mexico experience highlights the risks from prescribed fire, “there are even bigger risks from doing nothing,” she says.

Scientists estimate that the state should be using prescribed burns on about a million acres of California forest each year. Currently, the California Air Resources Board says about 125,000 acres see prescribed fire annually.

Scarier days ahead for sequoias?

The natural range of giant sequoias is confined to the western slopes of the Sierra Nevada in California. Since 2015, wildfires have swept across 80 to 85 percent of that range. Not all the fires were harmful; many burned through groves at low or moderate intensity,. But a shocking proportion of them were. The 2020 Castle Fire alone killed an estimated 7,500 to 10,600 mature sequoias, up to 14 percent of the known population at the time.

The risks are not likely to ebb anytime soon.

Fire scientists point to three factors that influence fire behavior: fuel load, weather conditions, and the topography of the landscape.

We can’t do anything about topography, says Shive—fires start where they start.

Climate change is dialing up the likelihood of risky weather: Hotter days, big winds, and ongoing drought all promote fire-friendly conditions. By cutting the emissions we produce from burning fossil fuels, we can slow climate warming, but that takes time. Meanwhile, the fires will intensify.

So the main knob we can turn to control fire risk to sequoias is changing the fuel load around them.

Most sequoia groves in California have felt either prescribed fire or wildfire since 2015 (mostly wildfire). There are 16 groves left (of 73 in all) that have not recently felt fire, prescribed or wild—in some cases for decades. The risks are highest in those heavily overgrown areas. Groups across the state are working together to create a “triage” plan to do at least basic treatments—mechanical removal or prescribed fire or both—in the most at-risk groves.

Simultaneously, scientists like Stephenson are looking back at the places that have burned since 2015. He and others have estimated that fire used to return to one spot in the Sierras roughly every 10 to 20 years. So even in the places where fire is still a painfully recent memory, it’s nearly time to come back in and use fire again to clean it up.

“It’s like mowing your lawn. You don’t go out once and mow your lawn and say you’re done for the rest of your life,” he says.

For now, the Mariposa Grove sequoias seem to be safe. Whether they’ll be safe for the rest of the summer and beyond is far from clear.

But today, “the punchline is, the only reason [the Washburn fire] is not going to kill those giant sequoias is because the park has done so much work around them,” says Hurteau.

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