Fuel Matters: Why Wildfire Behavior Depends on What's Burning

In his latest attack on California’s forest management policies, President Donald Trump claimed on Tuesday that the solution to the state’s punishing wildfire season should be a no-brainer.

“We’re tired of giving California hundreds and hundreds of millions of dollars, all the time for their forest fires,” Trump told the White House State Leadership Day Conference, “when you wouldn’t have them if they managed their forests properly. They don’t.”

Leaving aside the fact that nearly 60 percent of California’s forestland is managed by the federal government, most scientists agree that the buildup of fuels is a factor in fire severity.

But it’s not nearly that simple.

Firefighters on the ground know that fire behaves very differently depending on whether the flames are spreading in grass, chaparral, forest, or a mix. And according to fire scientists, property owners and policymakers should be paying more attention to these differences, too.

Building the Campfire

If you’ve ever built a campfire, you know that you need different kinds of fuel to get a fire going: paper, small twigs, or pine needles to ignite the fire, then small sticks to keep it going, and finally big logs that keep the fire going and produce the big flames that keep you warm all night.

Wildfires work the same way.

Firefighters categorize plants based on their size and how quickly they dry out — and consequently, how easily they will ignite and burn. Grasses are 1-hour fuels, sometimes called light fuels, or flashy fuels. If the weather becomes hot and dry, they become just as dry as the surrounding atmosphere in about an hour. Trees and dead logs and are usually 100 or 1000-hour fuels; it takes much longer before they’re ready to burn, but when they get going they can give off bigger flames, more intense heat, and can burn for a long time.

A Multiplying Threat

Because of this range of fuel types, you get different kinds of fires at different times of year, according to Jonathan Cox, a battalion chief with Cal Fire. Earlier in the season, blazes tend to be grass fires that are easier to get under control, though extreme weather can make it harder. The County Fire, which started at the end of June in Yolo County, burned mostly in grasses and shrubs.

But as the summer wears on, the big stuff dries out, too. The Carr Fire, which began in late July and has driven the destruction around Redding, has burned through a lot of heavily forested land.

In mid-July, Cox noted that heavier fuels were already starting to be receptive to fire.

“That’s a problem,” he told KQED, “because it takes more resources, and it takes more time to suppress those types of fuels. That’s why, as the fire season progresses, it’s kind of a continuing threat that kind of multiplies as those heavier fuels dry out.”

The Trouble with Grasses

It might seem like light fuels — like grasses — are preferable to heavy forest fuels or dense shrublands. In general, they are easier to manage because the flames are smaller and it’s easier for firefighters to maneuver. But grass fires have their own challenges.

The easier it is for fuels to dry out, the faster the rate of spread. According to Cox, fire spreads twice as fast in grass as it does in brush, and twice as fast in brush as it does in timber.

Because of their faster spread, grass fires are often the deadliest. Cox says a common denominator among some of the most destructive fires is that they started in light, flashy fuels.

“It is a very big kind of watch-out factor for firefighters as far as their safety is concerned,” he adds.

No One Size Fits All
It’s not just firefighters who need to be aware of fuel types. There are implications for land managers and homeowners, too.

For one thing, it offers clues to future fire patterns in any given area, points out Alexandra Syphard, a fire specialist at the Conservation Biology Institute.

In the Bay Area and coastal Southern California, shrublands, grasslands, and forests come together in a patchwork of fuel types. Managing these lands for wildfire hazard, ecology, and resource value can be a challenge. When it comes to managing fire in the coast ranges, “there’s no one-size-fits-all,” said UC Berkeley fire scientist Scott Stephens at a symposium in May.

Because fire behaves differently in different types of fuels, if you change the fuel type, you change the fire you get there. The reverse is also true: if you change the amount of fire, you can change the type of fuel.

For example, native grasslands and woodlands once flourished in the Bay Area, maintained by regular cultural burns by Native Americans and by grazing. A reduction in grazing, as well as the end of these traditional burns, led to the conversion of grassland to shrubland and an increase in heavy fuels in some areas. Indigenous groups and conservation organizations are working to return fire to the land in Northern California.

Grass Versus Shrub In Southern California

On the other hand, coastal Southern California and some chaparral areas in other parts of the state actually have as much or more frequent fire today than they did historically, explains Syphard. The result is a widespread conversion of chaparral to non-native grassland. This process may be accelerated by climate change.

Syphard and other researchers who focus on chaparral caution against the indiscriminate application of fire management practices that work for forests or other regions — like prescribed fire or cutting fuel breaks — to Southern California shrublands. Research shows that these tools aren’t that effective during big chaparral fires driven by Santa Ana winds, like the Thomas Fire.

And those light flashy fuels might make it easier to fight the fire, but they also increase the risk of ignition in the first place. Syphard’s research has shown that fuel breaks are mostly effective as an access point for firefighters and not so much as a hazard reduction, because exotic grasses fill in the gaps. The same conclusion might apply to management around homes.

“If you’re removing shrubland or woody vegetation that has pretty high fuel moisture and replacing it with grasslands that you’re not going to irrigate, you could unknowingly be putting yourself in an even worse position than you were before, ” Syphard cautions.

It’s not an easy story, says Syphard, and managing fuels means balancing safety, cultural, and ecological concerns.

“One of the most important things to understand is that it’s very geographically variable and the relationships that might be true in one region are likely to be really different than others,” says Syphard.

Jared Dahl Aldern, an environmental historian, agrees.

“You know the whole southern California region is a distinct place with its own characteristics as compared to, for instance, the Sierra Nevada, ” he says. “So the key is to collaborate with local experts, including tribes who have some experience and some idea with how to proceed.”