Napa and Sonoma are burning again. Here’s why scorched areas can remain vulnerable - San Francisco Chronicle

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Some parts of Northern California are suffering a repeat nightmare of wildfire destruction.

This weekend in Wine Country and Butte County, thousands of residents were once again forced to flee as flames raced toward their homes. In Santa Rosa, some places that burned in the Tubbs and Nuns fires three years ago were evacuated as the Glass Fire advanced. In Butte County, the communities of Paradise, Magalia and Concow — all three devastated by the 2018 Camp Fire — were threatened by the North Complex Fire.

It would make sense to think those burned areas might be safe from future wildfires, at least for some time. But can such areas actually develop some sort of immunity or resistance after a fire?

In reality, experts say, it's not that simple.

Once an area has burned, its vulnerability to future wildfires depends on a host of variables. Those include the type of vegetation, the intensity and behavior of the fire, and the overall climate conditions.

And in fact, experts say that the ecological aftermath of past wildfires may have actually primed areas like Napa and Sonoma to ignite this year — and, in a horrible cycle of destruction, the current fires may be setting the region up for more painful fire seasons to come.

The experts add that in an era of increasing climate extremes due to global warming, achieving a fire-resistant landscape is not a realistic goal. Rather, they say, Californians should aim to co-exist with wildfires so that the blazes can play their natural role without wreaking exponentially greater devastation each year.

To accomplish that, they say, we must not only improve the fire resistance of homes and other structures, but manage forests and vegetation around areas where people interface with nature, including communities, parks and trails.

“The term we like to use is resilience,” said Lenya Quinn-Davidson, fire adviser for the UC Cooperative Extension. “Resistance implies a fire won’t enter an area again. What we’re looking to achieve is resilience in a forest, landscape or community structure, where fire can be part of the system but won’t remarkably change it … and actually preserve what’s there.”

The type of vegetation matters

When determining a burned area’s vulnerability to wildfires in subsequent seasons, one of the most critical questions is: What will feed the flames?

Sasha Berleman is director of the Fire Forward program at Audubon Canyon Ranch in Stinson Beach, which aims to help North Bay communities become better adapted to living with wildfires. She said a burned area’s resilience to a future wildfire “depends quite a bit on the fuel type and the way a fire burns through it.”

In California, primary fuels include grasses, shrubs and forests.

Grasses are the most vulnerable to wildfire. They light easily and spread quickly — and in the Bay Area, they die annually and grow back each year, so a burned grassy area could reburn again the following year.

“Burns sometimes increase growth of grasses due to reduced competition and nutrient recycling,” said Brandon Collins, research scientist for UC Berkeley and the U.S. Forest Service. “Areas that are predominantly grass is what you think of as a main driver of fire behavior.”

Shrub-dominated areas in California are predominantly made up of chaparral, which covers the coastal foothills and interior mountain slopes. Shrub areas burned by wildfires could take about five to 10 years to be susceptible again, Collins said. Some types of shrubs are well-adapted to fire and germinate from seed cured by the smoke itself, so they can recolonize quickly and continue to spread fires, he explained.

Forests generally exhibit the most resilience after wildfires, and could see a reburn in 10 to 15 years, Collins said. But there are some caveats: Resilience depends on what type of trees have been burned, how they were burned, and whether trees were killed.

Northern California’s forests are diverse, with species including both resilient redwoods and more easily burnable eucalyptus trees and conifers.

Eucalyptus are common in the Oakland hills. A devastating 1991 wildfire there spread only 1,500 acres, but killed 25 people and destroyed 3,500 homes.

Despite the continued risk from eucalyptus, a push by fire management officials for their removal has been met with opposition. Coupled with continued droughts that lead to drier fuels, the Oakland hills are once again very susceptible to another fire.

Fire’s intensity also a crucial factor

The intensity of a wildfire matters. Very hot, extreme fires can kill off trees in a forest environment and convert the system into a shrub or grass system, Collins said. This is known as type conversion, and it has become a problem in California from both a biodiversity and fire management standpoint: The vegetation grows back more quickly, and can fuel another major fire sooner.

Berleman said this is happening right now in the North Bay, as well as other parts of the greater Bay Area with woodlands that are made up primarily of oak and bay laurel, and sometimes douglas fir and madrone trees. When a wildfire comes through and the main stem of the tree burns severely enough that it will inevitably die, it can re-sprout from the base and grow in a big, brushy mass that can get 6 feet high, Berleman explained.

“A whole bunch of fire-cured logs all start to come down on each other into a thick mesh...and it all kind of mixes together, and you end up with a really flammable brushy layer of re-sprouts with sticks and logs going up over the sides,” she said. “It ends up becoming a perfect scenario for another high severity fire.”

So if a normally healthy forest system is converted into this less than ideal system, it can make the area even more vulnerable to continued and extreme wildfires.

Berleman recently visited an area near Geyserville that was burned by the 2019 Kincade Fire, and saw these “hazardous” effects starting just a year later.

“These are not shrub ecosystems, they’re woodland ecosystems that are basically pushed by high severity fires turning them into a novel structure that doesn’t benefit them or us,” she added.

Quinn-Davidson uses the 1999 Megram Fire in the Shasta-Trinity National Forest and Six Rivers National Forest as an example of where this has happened. The fire burned at high intensity in a wooded area, destroying trees in its path.

“A lot of the dead trees are still there today, and what grew back was a bunch of brush,” she said. “It’s a shrub field with downed logs, and a totally different fuel type than it was 21 years ago. The Red Salmon Complex is burning right now in the Megram Fire footprint.”

She said those fuel types are cured and very dry, and so now are burning intensely — creating very hazardous conditions that prevent firefighters from entering those areas.

In another type of species expansion resulting from California wildfires, some systems are converting to knobcone pines. This is happening in the Shasta-Trinity National Forest and starting to occur in the Bay Area, Berleman said. Entire mountains have converted from chaparral to knobcone pines, which leads to a decline in biodiversity.

“Not only are these Bay Area woodland ecosystems undergoing a structure shift, but also a species composition shift as a result of wildfires,” Berleman said. “The knobcone pine tree population is expanding rapidly as a result of wildfires...They have adapted to high intensity fires, and require high intensity fires to go through their full ecological cycle.”

In this conversion, long-lived woodland systems that were adapted to low intensity fires are transformed into ones that encourage high intensity fire behavior. And when this happens near communities, it has a direct impact on fire safety for residents, in addition to many other ecosystem services that are lost, Berleman said.

Berleman works mostly in the North Bay and has seen the devastating effects of recent fires, including the Nuns and Tubbs fires in Wine Country. She said woodland areas that have burned at moderate to high severity can buy some time, but not for long due to the types of vegetation.

“I am concerned about the possibility of a high intensity reburn in the areas of the Nuns and Tubbs footprints,” she said. “The combination of dead fire-cured wood with the volatile dense resprouts from the base creates a continuous fuel load across the surface and into whatever canopy remains.”

In fact, Santa Rosa fire chief Tony Gossner on Monday said such a reburn had occurred during the Glass Fire, which also charred one of the last remaining areas in Sonoma County previously untouched by wildfires.

“(The Glass Fire) did also burn into the Nuns and the Tubbs scars and it burned just as well there because that had been filled in with fuels like grass and some new brush,” he said. “Every year brings a new challenge.”

When some fires are good fires

Sometimes, experts say, wildfires burn in a way that mimics prescribed fires — which could help protect an area in the short term.

Prescribed fires, also known as controlled or prescribed burns, are an important part of modern fire management. They are often used to help reduce vegetation — the fuel that feeds the flames — with the goal of preventing a very destructive fire, according to the National Park Service.

Prescribed burns lower the fuel load, and can reduce the severity of a wildfire moving through an area, said Roger Bales, an environmental engineering professor at UC Merced. Without them, the fire may get into the crowns — the branches and leaves — and result in a higher intensity fire that could kill the trees.

Berleman recently visited Armstrong Redwoods State Natural Reserve, which was affected by the Walbridge Fire. That blaze was part of the LNU Lightning Complex, which ignited in mid-August from multiple lightning strikes and spread across Lake, Napa, Sonoma, Solano, and Yolo counties.

The Walbridge Fire, in Sonoma County, has burned more than 55,000 acres and taken more than a month to achieve near-containment. Berleman said the portions that she examined in the state reserve’s redwoods produced “beautiful fire effects” that helped clean up the underbrush.

“It ate the dead fuel on the ground but left the canopies green,” she said. “Looking at the fire effects, I could not have achieved objectives any better with a planned prescribed burn.”

Berleman said that doesn’t necessarily mean a wildfire won’t burn in the area again, but that it would be very difficult for a fire to burn through the area at high severity anytime soon. That situation is similar to what a prescribed fire aims to accomplish, she said.

“It leaves the canopy intact with a nice shade cover still, but removes the dead material or fuel from across the surface that is generally needed to sustain any significant fire behavior,” she added.

Such situations create a great starting point for future maintenance: Because the fuel load is lighter, it’s less of a lift for fire managers to deal with, Berleman said. They can follow up in a few years and then do repeat prescribed burns on a four- to five-year basis.

The August Complex fire is now the largest in state history, burning nearly 860,000 acres mostly in remote areas in Mendocino, Shasta-Trinity and Six Rivers national forests. Collins said the fire never really had “explosive growth” and hasn’t created a lot of risk to communities, so firefighting agencies have been more hands-off compared to other wildfires.

“I think that when we see the effects of it, it might be doing ecological good and translate into that resistance idea,” he said. “If it doesn’t cause type conversion, it would be well set up for tolerating and resisting future fires.”

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Learning to live with the wildfires

The state’s largest, deadliest and most destructive wildfires have occurred in just the past few years. Paltry rainy seasons have resulted in longer, hotter and drier summers, which scientists attribute to climate change. They warn these mega blazes will continue to be a reality.

Hundreds of thousands of California residents live in wildland areas next to forests and grasslands, putting them at high risk from wildfires. Collins said fuel management should be a key tactic in those places, including towns like Paradise.

“And not just right there — that is a mistake some people make by just managing fuel around their home — but also a buffer around the city,” he said. “When the (Camp Fire) got up and going, it was well away from Paradise… There needs to be an understanding that they are living in a setting with a much larger contact threat that is very well out of the immediate circle.”

The priority of fire management is to protect the most strategic places where nature abuts humans, including communities, parks, trails and viewpoints. This should be done with regular vegetation and forest maintenance, and prescribed burns, Berleman said.

“Each piece we’re able to accomplish does make a difference in not only creating pockets of resilience for high value areas, but also creating long-term climate, fire and drought resistance as well,” Berleman said.

Retrofitting structures and rebuilding with fire resistant materials is another key step for communities under fire risk, Collins said. According to a blog published by the University of California Cooperative Extension Forestry, those communities should also think strategically about the best areas and configurations for new buildings and rebuilt structures to go to reduce wildfire risk.

“Communities need to be set up so fire has a natural role, but is not going to destroy a bunch of houses, cause damage and destroy a community,” said Quinn-Davidson. “We’re not going to be able to create a fire resistant landscape, or take fire out of the landscape. It’s increasingly part of our future.”

Kellie Hwang is a San Francisco Chronicle staff writer. Email: kellie.hwang@sfchronicle.com Twitter: @KellieHwang

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