Why two California wildfires had very different outcomes

The two fires started just 17 miles apart on rough terrain in California’s western Sierra Nevada – but their results couldn’t have been more different.

The Washburn Fire, which burned July 7 along a forest trail in Yosemite National Park, is mostly under control, causing no damage to structures or to the Sequoias Forest. famous giant.

But the Oak fire, which broke out nearly two weeks later in the foothills near Midpines, confused firefighters as it exploded four times the size of Washburn and forced thousands to flee as it destroyed at least 106 houses. Sometimes, the smoke of wildfires can be seen from space.

Why is a fire more destructive than that?

Experts attribute the difference to changes in weather, vegetation and topography. The management history of each landscape also plays an important role: Yosemite boasts decades of active management, including regulatory burns, while areas outside the park have a legacy of mining. industrial logging and extinguishing fires.

The Washburn Fire started along a trail at the edge of Mariposa Grove, just below a road used by shuttle buses to carry tourists from a parking lot.

Because the vegetation along the trail was dense and the fire moved faster uphill, officials were concerned that the blaze would grow hotter, speeding up and slamming into the Sequoia grove, Yosemite Sheriff, Dan Buckley, said. That could lead to a high-severity crown fire similar to those that have wiped out about 20 percent of the world’s population of ancient giants since 2020, he said.

But luck was on the side of the fire officials: Two Yosemite battalion commanders were teaching a chainsaw class to a sizable contingent of firefighters in the nearby Wawona area and were able to react quickly, with with a water contractor, two engines and the park’s water. -helicopter.

As the engines spewed burning solids, rangers enlisted civilian tour bus drivers to evacuate more than 450 visitors from the forest.

“They really had to drive through the flames on both sides of the road when they got these people out,” Buckley said. The rescue was completed in about 90 minutes, allowing firefighters to focus on the blaze, he said.

As the fire spread to Mariposa Grove, it encountered a prescribed burn area that teams addressed in 2017, reducing the amount of broom and mulch, or rotting vegetation, which should help the fire. move faster. In addition, the winds also blew gently and pushed the flames away from the heart of the forest and downhill towards Wawona.

Those elements allowed firefighters to build side roads and control the blaze around the forest, said Mike Theune, fire information officer for the Western Pacific region of the National Park Service.

When the Oak Fire started on the afternoon of July 22 in a field of oak grass, temperatures hit the 90s and humidity was in the single digits. It took just 16 hours for the blaze to overtake the Washburn fire in scale as it blazed through steeper, less accessible, more populated terrain.

“The Oak fire started at a much lower altitude. It’s much drier,” said Adrienne Freeman, fire information officer for the US Forest Service. “And the important thing is that it’s a different kind of fuel. Has a heavy brush component. “

The brush and sliced ​​grass, says Freeman, helped spread the flames into the dense wooden supports with drought-stricken conifers and bark beetles.

Scott Stephens, a professor of fire science at UC Berkeley, said drought in California, which has been exacerbated by climate change, has killed more than 100 million trees in the central and southern Sierra. Dead trees, falling overgrown shrubs are extremely effective. When there’s enough fire at the same time, they suck up oxygen, increasing the rate of combustion, says Stephens. Researchers call these “mass fires,” a term coined to describe the impact of incendiary bomb attacks during World War II, he said.

“It really creates a local low-pressure system,” he said.

Stephens was part of a team of researchers who documented this behavior during the 2020 Creek Fire for a study published in May in the journal Forest Ecology and Management. They found dead biomass, followed by live tree density, to be the two most important variables when predicting fire severity.

“Someone showed me a photo of the Oak fire, and it had a cloud of pyrocumulus created above it that was at least 20,000 feet high,” Stephens said. “That makes me wonder if we’re seeing similar behavior, at least in parts of that fire.”

The density of vegetation is closely tied to the history of the land, which is recorded starting in the mid-1800s.

“The Oak Fire is burning in areas that already have large trees cut down and lots of small and medium trees, and homogeneity is going to make it worse,” Stephens said.

Additionally, Mariposa County is the ancestral home of the Southern Sierra Miwuk, who regularly burned controlled fires – that is, until white settlers criminalized cultural burning. Miwuk and fire scientists say its absence has upset the balance of ecosystems. According to the fire history map, the area where the Oak fire started hasn’t burned in nearly 100 years.

“The landscape here is especially in need of a healthy fire,” said Clay River, director of the Miwumati Healing Center, which serves as a hub for Miwuk health and social services.

After the Miwuks were driven out of Yosemite Valley to establish the national park, they were forced to relocate to the foothills along Highway 140, where the fire started, River said. Many people are now seeing their homes and cultural sites burned down, she said.

Acknowledging the benefits cultural burning once had on forest resilience, park officials have sought to bring low-intensity fire back to the landscape for the past 50 years. They have been managing rural fires for the benefit of resources since 1972, resulting in a mosaic of small fire footprints that limit themselves to new beginnings, Buckley said. . They also performed a series of prescribed burns, including 22 in Mariposa Grove alone.

However, in the days following the initial advance of the Washburn fire, officials fear it could return and threaten the forest. The area is littered with dead trees and shrubbery due to a January 2020 wind that blew down 28 mature giant Sequoias and thousands of others.

But firefighters reported a major victory about three days after the fight, when they were able to keep the blaze at Wawona Road to the west.

Officials noted a 2020 biomass removal project led by Garrett Dickman, a park forest ecologist, and the Mariposa County Resource Preserve, with funding from California Climate Investments. Dickman said teams have removed nearly 9,000 tons of vegetation along the three miles of road between Wawona and Mariposa Grove, which they hope will follow a series of regulated fires.

When the flames reached the project site, they dimmed and in some places went out, says Dickman. Firefighters were able to stand on the road and clear hot spots, freeing up personnel to actively protect Wawona and the forest.

Some models indicate that if the fire released embers on the road, it could burn into Wawona, Fish Farm, Ponderosa Basin or Lushmeadows, Dickman said.

“It’s going to be a very, very, very different fire,” he said.

https://www.latimes.com/california/story/2022-07-30/why-two-california-wildfires-had-very-different-outcomes Why two California wildfires had very different outcomes

Edmund DeMarche

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