Embers start spot fires: The real and the imagined stories
Bay Nature recently published an article about the 1991 fire in the East Bay Hills and the closely related belief that such a fire can be prevented in the future by destroying all non-native trees. To Bay Nature’s credit, it was a more balanced article than most. Although the article was heavily weighted in favor of those who want to destroy all non-native trees in the hills, several defenders of our urban forest were also interviewed.
However, the article contains a fantasy about future fires that feeds into the fear of fire that has been fostered by those who advocate for removing all non-native trees:
“A strong wind begins blowing over the hills from the east. And then somehow—maybe a spark from a car, maybe a tossed cigarette—the whole dry, airy mess catches fire. Now the flames on the ground are 30 feet high and even higher off the boughs, roaring like a jet engine. At the fire’s edges, trees appear to explode as the volatile oils in their leaves reach their boiling point and vaporize. The heat of the fire forms a convection column, with 60-mile-per-hour winds that rip burning strips of bark from the trees and toss them upward. This is another of blue gums’ talents—its bark makes ideal braziers. Tucked away inside a rolled-up strip of bark, a fire might live for close to an hour and fly 20 miles.” (1)
Although we have read many times in the plans to destroy trees that eucalyptus casts embers starting spot fires, we have never seen such an extreme description of how far embers could travel while still on fire and capable of starting a spot fire. So, we tracked down the source of this theoretical scenario with the help of the author who cited this as the source of the theoretical scenario: “The potential for an internally convoluted cylinder of bark to be transported tens of kilometres in a continuously flaming state is indicated by the sample that maintained flaming combustion for the entire experiment…This would correspond to a flameout time of almost 2000 s for a sample 2700 mm long, a lofted height of 9600 m and a spotting distance of ~37 km.” (2)
First let’s translate that quote into measurements we commonly use to appreciate how extreme this particular test was: “This would correspond to a flameout time of almost 33 minutes for a sample 9 feet long, a lofted height of 6 miles and a spotting distance of 23 miles, traveling at 41 miles per hour.” That is a very long ember, lofted a great distance at a great speed (but NOT 60 mph), staying lit for a long time (but NOT “close to an hour”).
Theory vs. Reality
The study that was the source of the extreme prediction in Bay Nature about the distance that burning embers can travel was conducted on samples of Eucalyptus viminalis bark (NOT Blue Gum Eucalyptus, E. globulus) “tethered in a vertical wind tunnel.” These are not real-world conditions. So, how does this theoretical study compare to real-world conditions?
The FEMA Technical Report about the 1991 fire in the East Bay Hills contains a map of the full extent of the 1991 fire. As you can see on this map, the maximum distance from the northern-most edge of the fire to the southern edge of the fire is less than 3 miles…not remotely close to 20 miles. In other words, embers could not have started fires 20 miles away because the fire wasn’t even close to 20 miles long.
The FEMA Technical Report doesn’t tell us what the wind speeds were during the 1991 fire, although they describe the wind as being strong at several times during the fire. If there is any evidence that winds were as much as 60 miles per hour, it’s not evidence we have been able to find. We found a source of wind speeds measured on the Bay Bridge, including historical records. This website says the strongest wind measured since 2010 was 31 miles per hour in April 2013. That suggests that 60 mph winds are probably unusual in the San Francisco Bay Area.
The FEMA Technical Report doesn’t report any observations of firebrands or burning embers from eucalyptus. The report mentions embers twelve times, but identifies the source of those embers only once. In that one case, the source of embers was “a growth of brush”….not a eucalyptus tree or any tree, for that matter. There are anecdotal reports of finding debris from the fire as far as San Francisco, but no reports that the debris was still on fire or that it started another fire.
US Forest Service study of embers in actual fires
US Forest Service participated in a comprehensive study of “spotting ignition by lofted firebrands” based on actual wildfires all over the world, including the 1991 fire in the East Bay Hills. (3) There is nothing in that study that corroborates the claim that eucalyptus bark embers are capable of travelling 20 miles while remaining lit and therefore capable of starting spot fires:
- “In the wildland-urban interface fires in California—Berkeley in 1923, Bel-Air in 1961, Oakland 1991—wooden shingles which were popular in California as roof material, assisted fire spread. Wooden shingles increase fire hazard owing to both ease of ignition and subsequent firebrand production.”
- “Unlike the flying brush brands which are often consumed before rising to great heights, the flat wood roofing materials soared to higher altitudes carried by strong vertical drafts…”
- The only firebrand found in the 1991 Oakland Hills fire was found approximately 1 km (.6 mile) west from the perimeter of the fire. It was a cedar shingle. Here is a photograph of that shingle:
- Cylinder shaped embers do not travel as far as flat particles. Firebrands in the shape of cylinders were found to have a maximum spotting distance of 2050 meters, because “cylinders always fall tumbling.”
- “The increased burning time inherent in larger firebrands was cancelled out by an increased time of flight because larger firebrands move more slowly.”
- In a study of 245 extinguished fires, experiments and simulations, and observing 48 wildfires, “The longest spotting distance was observed as 2.4 km.”
This comprehensive study of actual wildfires all over the world finds no evidence of embers capable of travelling 20 miles while still burning and starting spot fires. It reports that wooden shingles were the only observed burning embers in the 1991 fire and that wooden shingles are particularly vulnerable to being lofted as embers in a wildfire. There are countless houses in the East Bay Hills covered in wooden shingles, yet instead of addressing that obvious source of embers, we are destroying blameless trees.
Developing the Cover Story
Claims about the extreme flammability of eucalyptus have escalated in the past 15 years as opposition to destroying trees and associated pesticide use has escalated. Nativists have become increasingly dependent on flogging the fear factor as their other storylines have been dismantled by empirical studies and reality:
The “invasiveness” of eucalyptus has been downgraded by the California Invasive Plant Council from “moderate” to “limited,” their lowest rating. There is little evidence that eucalyptus is invasive unless planted along streams and swales that carry their seeds.
- There are many empirical studies that find that all forms of wildlife—such as insects and birds—are served equally well by both native and non-native plants. Some iconic species—such as Monarch butterflies, bees, hummingbirds, hawks, owls—are dependent upon eucalyptus for winter nectar and safe nesting habitat.
- Huge global studies of biodiversity report that the introduction of non-native species has resulted in no net loss of biodiversity. This is particularly true of introduced plants. There is not a single instance of extinction caused by a non-native plant in the continental United States.
- Climate change is making nativism increasingly irrelevant. California’s native conifers, oaks, and redwoods are dying by the millions. Unless we want a treeless landscape, we must plant tree species that are capable of tolerating changed climate conditions.
These studies have left nativists with few tools to justify the eradication of non-native plants. We can see the development of the FIRE!! cover story in the archives of the conferences of the California Invasive Plant Council. In 2004 Cal-IPC held a workshop regarding exotic trees and shrubs. Over 30 representatives of major managers of public lands attended, such as National Park Service, San Francisco’s Natural Areas Program, Marin County Open Space, etc. The record of this meeting reflects the dependence upon fire to justify the eradication of non-native shrubs and trees: “Golden Gate National Recreation Area: ‘inform public ahead of time; use threat of fire danger to help build support for invasive plant removal projects.’” The Golden Gate National Recreation Area—a National Park–advises other land managers to frighten the public into accepting the loss of their trees.
Subterfuge is also recommended to land managers to hide the eradication of shrubs and trees from the public: “To avoid public upset, drilling around into tree buttress roots and injecting 25% glyphosate…Trees die slow and branches fall slowly, so won’t pose an immediate hazard.” In other words, land managers were advised to kill trees using a method that won’t be visible to the public.
Perhaps most disturbing of all is that those who attended this workshop admit that they don’t really know if eucalyptus trees are more flammable than native vegetation and some doubt that they are: “People are afraid of fire. Help them understand Eucalyptus trees and other invasive plants are very fire hazardous. Is there any solid research about Eucalyptus and fire? Are Eucalyptus and brooms any greater fire danger than native chaparral?” In other words, even those who wish to destroy non-native shrubs and trees seem to understand that fire is a cover story for which no supporting evidence exists. The evidence has been fabricated to support the cover story.
We now seem to live in a fact-free world in which various interests can make things up and distribute them on the internet with impunity. The mainstream press is dying and is being replaced by fact-free social media. If we are to protect ourselves from such manipulation, we must drill down into these storylines. In the case of eucalyptus, we have debunked the myth that it is more dangerous than the replacement landscape. Now it’s up to us to disseminate that information far and wide as an antidote to fear-driven nativism.
- Zach St George, “Burning Question in the East Bay Hills: Eucalyptus is flammable compared to what? Bay Nature, October-December 2016
- James Hall, et. al., “Long-distance spotting potential of bark strips of a ribbon gum (Eucalyptus viminalis), International Journal of Wildland Fire, 2015, 24, 1109-1117
- Eunmo Koo, et. al., “Firebrands and spotting ignition in large-scale fires,” International Journal of Wildland Fire, 2010, 19, 818-843