Science & Environment

How wildfires spread is a mystery Oregon engineers are solving

By Jes Burns (OPB)
July 6, 2021 1 p.m.

A rooster might be the last thing you’d expect to hear when there’s a scientific experiment underway, but at David Blunck’s farm near Albany, Oregon, the soundtrack is positively pastoral.

His roosters crow. His dog barks. Birds fill the air with their song.

And members of his church honk their greetings as they drive by on the country road by his house.

In a small clearing next to a freshly-planted field of vegetables tended to by Blunck’s kids, the Oregon State University engineer and his team are setting out frames of white fire-proof cloth. They’re spaced on the ground in growing semicircles, like lawn chairs at an outdoor concert.

“For how much of a tree or shrub burns, what’s the number of firebrands that’ll be released? That’s the big value we’re trying to back out,” he explains.

The cloth is there to catch those firebrands — or flying embers — when they land.

Once wildfires start, one of the ways they spread is through firebrands that blow into unburned areas. Despite their importance in spreading fires, relatively little is known about how many firebrands different kinds of vegetation generate during wildfires.

“If you want to understand how to control (wildfire) or contain it or prevent it, it’s helpful to know how it spreads,” he says.

To do this, Blunck is taking a vicarious tour of Oregon — by burning a range of the state’s prominent trees and shrubs. They’ve burned ponderosa pine, grand fir, Douglas fir, juniper and sagebrush in experiments already.

On this day, they’re burning chamise – a shrub that’s common in chaparral ecosystems, which can be found in Southern Oregon and California.

A man stands outside holding a white square and points to one of several charred marks on the square.

OSU engineer David Blunck points out scorch marks left by firebrands. He's working to quantify how many embers different types and sizes of Oregon vegetation throw during wildfire.

Jes Burns / OPB

The Burn

The experiment itself is pretty straightforward.

OSU postdoctoral researcher Sampath Adusumilli, who’s leading the day’s burns, works with OSU student Will Heffernan to weigh the branches they’re going to burn. They use bricks to prop them upright at the focal point of cloth frames and then spread straw at the base to emulate flammable groundcover.

Adusumilli drops a few pieces of the straw into the breeze and watches their behavior.

“This is our scientific wind direction testing,” he laughs while keeping an eye on a wind gauge nearby. “We don’t want more than one meter per second, so we’re trying to hit that sweet spot.”

The straw drifts toward the cloth frames on the ground — and it’s go time.

Heffernan uses a BBQ lighter to set fire to the straw, and the flames start to creep toward the chamise.

Suddenly, the bushy branch goes up like a torch, swirling threads of firebrands up into the heat-distorted air.


Then almost as quickly as the experimental burn started, the fire dies.

“It’s just smoldering like a cigarette,” Blunck says, examining the growing ashy tip of a branch that a smoker would be hard-pressed to resist tapping away.

Adusumilli weighs the black skeletal branches that remain to figure out how much of the chamise burned.

“If the fire intensity was higher, I think the whole thing would disappear,” Adusumilli says. “If you look at pictures of chaparral landscape after fire has gone through, it’s just barren. There’s nothing left.”

But even without a complete burn of the chamise, embers have fallen onto the cloth frames, leaving scorch marks behind. Blunck says each mark will be counted and used to determine how many firebrands were produced per kilogram of material that burned.

“Now because we have a number of firebrands per mass that’s burned. That gives the way we could compare between different species,” he says.

Two men use long-handled hand tools to tend to a fire burning outdoors.

OSU student Will Heffernan (left) and researcher Sampath Adusumilli clear away debris around a burned branch of chamise.

Jes Burns / OPB

Understanding the wildland-urban interface

In an actual wildfire, knowing how many firebrands are being produced could eventually be used to help firefighters understand how the fire will behave.

“When the embers are approaching the surface … those are the embers that are responsible for potential ignition of something,” says Jiann Yang, an engineer with the National Institute of Standards and Technology’s Wildland-Urban Interface Fire Group.

The institute is a federal laboratory that partially funds Blunck’s research.

Yang says figuring out something as fundamental and straight-forward as how many embers touch down is useful.

“(Ignition) depends on how many embers land on the surface and it depends on the characteristics of the fuel,” he said. “If an ember lands on mulch, if the mulch is very moist, you’re not going to ignite the mulch. But if you have a lot of embers, you probably still can.”

But it’s moving into the areas where dwellings are built on or adjacent to wildfire-prone landscapes — known as the wildland-urban interface —where the real value could come.

“It’s just not how fire spreads through the forest, but it’s what happens when it gets into the community,” Yang says

Blunck’s findings feed directly into this. They could bolster fire prediction models that help communities plan for wildfire better.

“We can use (the models) without ever having to burn anything,” Blunck said. “So you could say, all right … I want to put some trees or I want to put a house on this location. Where will the firebrands go and what’s the likelihood they’ll start a new fire?”

And because Blunck and his team are burning different sized trees and shrubs, they’re able to start to compare how their ages affect how many firebrands are generated.

Their preliminary findings have been unexpected. The early data suggest that for each kilogram burned, larger trees produce fewer firebrands than smaller ones.

“We’re scratching our heads about that one,” he says. “We’re not sure if those trees happen to have more moisture content. Or maybe the structure of the tree — maybe there’s just fewer of the fine twigs or needles to break off.”

As the world keeps burning fossil fuels, contributing to climate change, the Pacific Northwest will continue to see longer, drier summers. That has state leaders and forest managers doing a little head scratching as well, asking themselves: How do we best prepare communities for the ever-increasing risk of wildfire?

Blunck says once his team finishes burning forest fuels to determine firebrand production, he wants to dig even deeper into questions around how fires spread in communities.

And if that new research moves forward, his neighbors won’t see a burning bush or flaming Douglas fir in his yard as they drive past.

They may see an entire (small) building on fire.