Firebombs in the Forest: Untreated Slash Piles Fueled Angora Fire Destruction
The Angora Fire near Lake Tahoe destroyed hundreds of homes and other structures in the community of Meyers, California. There have been several post mortem analyses of the cause and effects of the wildfire disaster, yet there remains a grossly understated issue that has so far failed to generate the attention from journalists or policymakers it deserves: the presence of hundreds of unburned slash piles left over from thinning operations nearly three years ago that helped fuel the wildfire’s destructive power.
Photo Caption: The biggest cluster of destroyed homes was located next to the fuels reduction unit that had untreated slash piles left over from thinning operations completed nearly three years ago. The intense heat from the flaming slash piles lofted large burning embers that were carried by the wind and fell into the residential zone, igniting and destroying homes. Photo by FUSEE.
[For a larger image click here]
What is “Slash”?
“Slash” is the slang term for the needles, limbs, and small-diameter tree trunks left over from commercial logging or non-commercial thinning operations. As it dries out and cures in the sun, slash can be one of the most flammable fuels in the forest because it is easily ignited and burns intensely. So-called fuels reduction treatments that remove large-diameter tree trunks but leave the slash strewn across the ground are more aptly called fuels relocation rather than fuels “reduction” treatments, for they have merely relocated the fuel hazard from the tops of the trees where only the rarest and most extreme kinds of fire behavior—crownfire--can ignite them, down to the ground surface where they immediately become available fuel for the most common form of fire behavior: surface fire.
There are several methods agencies like the Forest Service use to “treat” slash. Some of the most popular methods like “compaction” (e.g. crushing the slash by running it over with bulldozers and log-skidding machines) and “lop and scatter” (e.g. cutting it up with chainsaws into smaller pieces and spreading it all over the forest floor) have been thoroughly discredited by fire scientists as increasing fire risks and fuel hazards, not decreasing them. Another popular method is to pile the slash either by handcrews or by machines. This decreases the horizontal continuity of fuel across the forest floor, but unless the slash piles are burned or otherwise physically removed, they can actually increase the rate of spread, intensity, and severity of a wildfire.
Anyone who has ever burned slash piles knows the kind of intense heat that is emitted—enough to burn the flesh of your face through radiant heat alone. Slash can burn for hours, and embers can smolder for days. Slash piles that are burned too close to standing trees can kill them through “cooking” the roots or “heat girdling” the trees, and ember-filled smoke columns can severely scorch or even ignite the canopy of overstory trees even if the flames from the slash piles do not come close to reaching the tree canopy.
Photo Caption: White ash-covered soil indicates extreme intensity and high severity. Black-topped stumps show that this stand had been thinned before the fire, but this did not prevent high fire severity. Blue paint on the largest remaining trees indicates this stand is a potential salvage timber sale. Photo by FUSEE.
[For a larger image click here]
Slash Caused Spotfires That Ignited Homes
This is precisely what happened on the Angora Fire: several hundred untreated slash piles ignited by the wildfire created such intense heat that it killed nearly all of the remaining overstory trees in the thinned units. In fact, according to the Forest Service’s analysis, the severity of the wildfire burning through the slashpiles matched the severity of nearby untreated stands, calling into question whether the thinning treatments was a complete waste of money and labor. Additionally, large burning embers were sent aloft from the slash piles and were transported by the wind into the nearby residential area where they ignited dozens of homes.
The biggest cluster of destroyed homes was located next to fuels reduction unit number 20 that had contained hundreds of untreated slash piles. The Forest Service claims that the thinning operations successfully reduced the amount of spotfires that occurred in the residential area because the reduction of trees reduced the number of tree crowns that could have sent aloft burning embers. However, the agency failed to acknowledge at all that the slash piles produced embers and spotfires.
Photo Caption: Each white dot is an ash mound that marks the spot where an untreated slash pile was left from thinning operations. There were hundreds of 10 x 10 foot slash piles spread approximately 20 feet apart in fuels reduction unit number 20 that were set ablaze by the Angora Fire. Photo by FUSEE.
[For a larger image click here]
Embers created by burning tree crowns can be picked up by the wind and carried relatively long distances. However, these embers also tend to be smaller in size, and therefore, have a shorter “residence time” (i.e. the time they are combusting before they burn out). If the ember burns out before it reaches the ground, it falls as ash and cannot ignite a spotfire.
Embers produced by slash piles generally have less chance to be picked up by winds because they start off at the ground surface, however, this does not apply during high wind speeds, and fuels reduction units that have excessively thinned trees enable high winds to blow right through to the ground surface. The intense and prolonged heat output from a burning slash pile can produce a convective column of hot air and smoke that can readily send embers high aloft. Additionally, embers produced by slash piles tend to have larger particle size with a much longer residence time, thereby increasing the probability that a burning ember can reach the ground and ignite a spotfire.
This appears to be exactly what happened when the winds shifted and pushed the Angora Fire into the residential neighborhood in Meyers. The intense heat and ember wash from hundreds of well-cured slash piles ignited homes located downwind, and killed all the trees in the thinned units. The Forest Service determined that the fire severity was identical between the treated stand full of slash in Unit 20 and adjacent untreated stands that had not been thinned.
Photo Caption: Another view shows the size of Unit 20 and the scale of untreated slash piles that sent the wildfire spotting into the tree-covered residential zone where the large white spots reveal the sites of completely destroyed homes. Photo by FUSEE.
[For a larger image click here]
Thinning Without Treating Slash is Not a "Completed Fuels Reduction Treatment”
The Forest Service has some legitimate reasons why it failed to effectively treat the slash in fuels reduction units burned by the Angora Fire, namely, the opposition by air quality regulators and local residents to the smoke that would be produced by slash burning. However, in its report on the effects of fuels treatments, the agency made an illegitimate excuse that claimed the piles were left untreated because they needed a minimum of one to two years to dry out prior to burning. This is simply not true—slash piles can be ready for burning in just a couple months after cutting, especially in the warm-dry climate around Lake Tahoe. And even a “green” pile of slash can burn if you pour enough burning fuel from your driptorch on it!
[September 25, 2007 Update: In response to the statement above, Forest Service managers have disclosed that the slashpiles contained larger logs that required one to two years to cure before burning. This would be a legitimate excuse for delaying up to two years to burn the piles, however, it has yet to be explained why these piles remained unburned after that two year curing time had passed. A September 5, 2007 story in the local newspaper, the Tahoe Bonanza, revealed that over 3,000 acres within the Tahoe basin have untreated slashpiles from past thinning projects, and local residents are concerned about the fire hazard presented by these piles. To the best of our knowledge, the Forest Service has not publicly disclosed a timetable for treating these unburned slashpiles.]
In reporting its accomplishments to Congress, the Forest Service regularly counts as “fuels reduction completed” the number of acres where it has thinned trees, and then can count these same acres again as accomplishments in fuels reduction if and when it later treats the slash created by thinning operations. Thus, for example, the agency claims a total of 80 acres of fuels reduction completed when it thins trees and later burns the slash on a 40 acre unit. In any other accounting system, this would be rightfully condemned as “double dipping” to cook the books. No fuels reduction treatment or forest restoration project should be considered properly “completed” unless and until it has BOTH effectively reduced the slash fuel AND completed understory broadcast prescribed burning to deal with remaining surface fuels.
The presence of unburned slash piles, their role in spreading the Angora Fire, and their location next to the biggest cluster of destroyed homes is worthy of more thorough examination by the press, but what is even more critical is for policymakers to deal with the wider, generalized issue of unburned slash piles—because thousands of slash piles are currently littering the forest floor throughout the Lake Tahoe Basin and hundreds of others areas across the West where logging and thinning operations have occurred.
From the evidence on the ground it is clear that in the Lake Tahoe Basin the pace of tree thinning is far outracing the ability of land management agencies to effectively deal with the slash left behind. The slash piles are so numerous around Lake Tahoe communities that in some areas there is barely any horizontal separation between piles. Slash piles are located right next to busy highways were a single carelessly discarded cigarette butt could set them ablaze. In many cases, piles are located dangerously close to remaining trees such that if and when the piles are burned by managers or by wildfire, they will likely damage or kill the trees. Thinning units littered with slash piles offer as much fire hazard reduction as covering up oil spills with a bunch of rags.
The Angora Fire offers a wake up call: untreated slash piles function like “firebombs” in the forest, increasing the spread, intensity, and severity of wildfires, and can be a major agent of home ignitions. The number, location, and extent of unburned slash piles scattered throughout the Lake Tahoe Basin and elsewhere across the West constitutes a real and present danger to residents living near similar so-called “fuels reduction” units.
--FUSEE Staff
Photo Caption: The biggest cluster of destroyed homes was located next to the fuels reduction unit that had untreated slash piles left over from thinning operations completed nearly three years ago. The intense heat from the flaming slash piles lofted large burning embers that were carried by the wind and fell into the residential zone, igniting and destroying homes. Photo by FUSEE.
[For a larger image click here]
What is “Slash”?
“Slash” is the slang term for the needles, limbs, and small-diameter tree trunks left over from commercial logging or non-commercial thinning operations. As it dries out and cures in the sun, slash can be one of the most flammable fuels in the forest because it is easily ignited and burns intensely. So-called fuels reduction treatments that remove large-diameter tree trunks but leave the slash strewn across the ground are more aptly called fuels relocation rather than fuels “reduction” treatments, for they have merely relocated the fuel hazard from the tops of the trees where only the rarest and most extreme kinds of fire behavior—crownfire--can ignite them, down to the ground surface where they immediately become available fuel for the most common form of fire behavior: surface fire.
There are several methods agencies like the Forest Service use to “treat” slash. Some of the most popular methods like “compaction” (e.g. crushing the slash by running it over with bulldozers and log-skidding machines) and “lop and scatter” (e.g. cutting it up with chainsaws into smaller pieces and spreading it all over the forest floor) have been thoroughly discredited by fire scientists as increasing fire risks and fuel hazards, not decreasing them. Another popular method is to pile the slash either by handcrews or by machines. This decreases the horizontal continuity of fuel across the forest floor, but unless the slash piles are burned or otherwise physically removed, they can actually increase the rate of spread, intensity, and severity of a wildfire.
Anyone who has ever burned slash piles knows the kind of intense heat that is emitted—enough to burn the flesh of your face through radiant heat alone. Slash can burn for hours, and embers can smolder for days. Slash piles that are burned too close to standing trees can kill them through “cooking” the roots or “heat girdling” the trees, and ember-filled smoke columns can severely scorch or even ignite the canopy of overstory trees even if the flames from the slash piles do not come close to reaching the tree canopy.
Photo Caption: White ash-covered soil indicates extreme intensity and high severity. Black-topped stumps show that this stand had been thinned before the fire, but this did not prevent high fire severity. Blue paint on the largest remaining trees indicates this stand is a potential salvage timber sale. Photo by FUSEE.
[For a larger image click here]
Slash Caused Spotfires That Ignited Homes
This is precisely what happened on the Angora Fire: several hundred untreated slash piles ignited by the wildfire created such intense heat that it killed nearly all of the remaining overstory trees in the thinned units. In fact, according to the Forest Service’s analysis, the severity of the wildfire burning through the slashpiles matched the severity of nearby untreated stands, calling into question whether the thinning treatments was a complete waste of money and labor. Additionally, large burning embers were sent aloft from the slash piles and were transported by the wind into the nearby residential area where they ignited dozens of homes.
The biggest cluster of destroyed homes was located next to fuels reduction unit number 20 that had contained hundreds of untreated slash piles. The Forest Service claims that the thinning operations successfully reduced the amount of spotfires that occurred in the residential area because the reduction of trees reduced the number of tree crowns that could have sent aloft burning embers. However, the agency failed to acknowledge at all that the slash piles produced embers and spotfires.
Photo Caption: Each white dot is an ash mound that marks the spot where an untreated slash pile was left from thinning operations. There were hundreds of 10 x 10 foot slash piles spread approximately 20 feet apart in fuels reduction unit number 20 that were set ablaze by the Angora Fire. Photo by FUSEE.
[For a larger image click here]
Embers created by burning tree crowns can be picked up by the wind and carried relatively long distances. However, these embers also tend to be smaller in size, and therefore, have a shorter “residence time” (i.e. the time they are combusting before they burn out). If the ember burns out before it reaches the ground, it falls as ash and cannot ignite a spotfire.
Embers produced by slash piles generally have less chance to be picked up by winds because they start off at the ground surface, however, this does not apply during high wind speeds, and fuels reduction units that have excessively thinned trees enable high winds to blow right through to the ground surface. The intense and prolonged heat output from a burning slash pile can produce a convective column of hot air and smoke that can readily send embers high aloft. Additionally, embers produced by slash piles tend to have larger particle size with a much longer residence time, thereby increasing the probability that a burning ember can reach the ground and ignite a spotfire.
This appears to be exactly what happened when the winds shifted and pushed the Angora Fire into the residential neighborhood in Meyers. The intense heat and ember wash from hundreds of well-cured slash piles ignited homes located downwind, and killed all the trees in the thinned units. The Forest Service determined that the fire severity was identical between the treated stand full of slash in Unit 20 and adjacent untreated stands that had not been thinned.
Photo Caption: Another view shows the size of Unit 20 and the scale of untreated slash piles that sent the wildfire spotting into the tree-covered residential zone where the large white spots reveal the sites of completely destroyed homes. Photo by FUSEE.
[For a larger image click here]
Thinning Without Treating Slash is Not a "Completed Fuels Reduction Treatment”
The Forest Service has some legitimate reasons why it failed to effectively treat the slash in fuels reduction units burned by the Angora Fire, namely, the opposition by air quality regulators and local residents to the smoke that would be produced by slash burning. However, in its report on the effects of fuels treatments, the agency made an illegitimate excuse that claimed the piles were left untreated because they needed a minimum of one to two years to dry out prior to burning. This is simply not true—slash piles can be ready for burning in just a couple months after cutting, especially in the warm-dry climate around Lake Tahoe. And even a “green” pile of slash can burn if you pour enough burning fuel from your driptorch on it!
[September 25, 2007 Update: In response to the statement above, Forest Service managers have disclosed that the slashpiles contained larger logs that required one to two years to cure before burning. This would be a legitimate excuse for delaying up to two years to burn the piles, however, it has yet to be explained why these piles remained unburned after that two year curing time had passed. A September 5, 2007 story in the local newspaper, the Tahoe Bonanza, revealed that over 3,000 acres within the Tahoe basin have untreated slashpiles from past thinning projects, and local residents are concerned about the fire hazard presented by these piles. To the best of our knowledge, the Forest Service has not publicly disclosed a timetable for treating these unburned slashpiles.]
In reporting its accomplishments to Congress, the Forest Service regularly counts as “fuels reduction completed” the number of acres where it has thinned trees, and then can count these same acres again as accomplishments in fuels reduction if and when it later treats the slash created by thinning operations. Thus, for example, the agency claims a total of 80 acres of fuels reduction completed when it thins trees and later burns the slash on a 40 acre unit. In any other accounting system, this would be rightfully condemned as “double dipping” to cook the books. No fuels reduction treatment or forest restoration project should be considered properly “completed” unless and until it has BOTH effectively reduced the slash fuel AND completed understory broadcast prescribed burning to deal with remaining surface fuels.
The presence of unburned slash piles, their role in spreading the Angora Fire, and their location next to the biggest cluster of destroyed homes is worthy of more thorough examination by the press, but what is even more critical is for policymakers to deal with the wider, generalized issue of unburned slash piles—because thousands of slash piles are currently littering the forest floor throughout the Lake Tahoe Basin and hundreds of others areas across the West where logging and thinning operations have occurred.
From the evidence on the ground it is clear that in the Lake Tahoe Basin the pace of tree thinning is far outracing the ability of land management agencies to effectively deal with the slash left behind. The slash piles are so numerous around Lake Tahoe communities that in some areas there is barely any horizontal separation between piles. Slash piles are located right next to busy highways were a single carelessly discarded cigarette butt could set them ablaze. In many cases, piles are located dangerously close to remaining trees such that if and when the piles are burned by managers or by wildfire, they will likely damage or kill the trees. Thinning units littered with slash piles offer as much fire hazard reduction as covering up oil spills with a bunch of rags.
The Angora Fire offers a wake up call: untreated slash piles function like “firebombs” in the forest, increasing the spread, intensity, and severity of wildfires, and can be a major agent of home ignitions. The number, location, and extent of unburned slash piles scattered throughout the Lake Tahoe Basin and elsewhere across the West constitutes a real and present danger to residents living near similar so-called “fuels reduction” units.
--FUSEE Staff
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