I have to take issue with the recent diary Firebombs in the Forest: Slashpiles Fueled Angora Fire Destruction by FUSEE. There are some conclusions in the diary which, while they may sound reasonable, simply aren't supported by experience or research.
The tendency in almost any wildfire where property destruction takes place is to place the blame on someone else - someone other than the property owner who suffered the loss. Most often, that someone else is the Forest Service.
In some cases, the Forest Service shares some of the blame, and the deteriorated conditions of western US forests is certainly a result of (among other causes) past Forest Service practice.
However a look at the science of structure ignition in wildfires makes FUSEE's claims seem unlikely.
The diary linked discusses the role of piles of brush and limbs ("slash piles") leftover from thinning operations, for as long as 3 years. The piles were the result of a procedure called "pile and burn", which is often used in reducing fuel loads in overgrown forests. Except in this case, the "burn" part of the operation didn't take place until a wildfire swept the area.
FUSEE's argument is nicely summarized by the concluding paragraph of his diary:
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 structure 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.
Now there is something to this: leaving slash piles from thinning operations in place for three years or so is a fire hazard. The object is to remove fuels from an area being treated, not to simply move them around. And while there probably is some increase in local intensity and spot fire ignitions from the piles, it's important to keep in mind that the fire intensity and spread would have likely been the same or greater if the fuels had been left in place instead of being cut and piled.
One of the main objects of thinning is to break the fuel "ladders" that allow a fire to climb from grasses to shrubs to the lower limbs of trees and finally into the crowns or tree-tops, leading to a crown fire. Crown-fires are considerably more destructive and harder to control than surface fires. According to FUSEE, the slash piles actually led to some torching of trees, which is clearly contrary to the purpose of thinning in the first place, but again, is hard to compare to what would have happened if the fuels were left in place to burn.
However the conclusions I'd take issue with are the claims that the slash piles were a "major agent of structure ignitions" and a "danger to residents". Those statements indicate to me confusion about who ultimately is responsible for the protection of property in forested areas (whether private or public lands), and what the ultimate cause of structure ignitions is.
There are basically two mechanisms by which a wildfire can ignite a structure: radiative transfer of heat and ignition from embers. While FUSEE makes a lot of the intensity with which slash piles burn, their heat energy emissions are nothing compared to a crown fire. In my experience burning slash piles from pile and burn operations, you can easily get within 10 feet of a pile burning at full-bore. Around here, crews don't build piles larger than that, unless they'll be burned immediately. How close you can get relates to the radiative component of the fire.
One of the goals of thinning was likely to reduce the likelihood of crown fire, and a crown fire is slightly more intense than a burning slash pile. An intense crown fire creates winds high enough to suck trees out of the ground, an can actually create it's own weather (hail or rain). The measured heat output of a crown fire is on the order of 45 kilo-watts per square meter - about 45 times more intense than the sun on a beach on a clear, hot day. It's unlikely that you could get within a few hundred feet of a crown fire and survive.
However, the same isn't true for your home. Research conducted by Jack Cohen of the Forest Service's Missoula lab indicates that your home will survive the heat energy radiated by a crown fire without even scorching, as long as the fire is kept about 30 feet (10m) away. Cohen constructed stud walls covered with plywood and placed them 10, 20 and 30 meters from an experimental crownfire in Canada's Northwest Territories. The wall section closest to the fire didn't even scorch, and none of the sections were damaged.
Given that the energy of a crown fire is on the order of 100 times more intense than a burning slash pile, radiation of heat from slash piles was not likely a source of home ignition. FUSEE does do a good job of outlining the other (and most common) cause of structure ignitions: embers thrown from the fire. Here's Cohen on the subject (from Reducing the Wildland Fire Threat to Homes):
(WU-I is "Wildland-Urban Interface")
As previously mentioned, firebrands are also a principal W-UI ignition factor. Highly ignitable homes can ignite during wildland fires without the fire spreading near the structure. This occurs when firebrands are lofted downwind from fires. The firebrands subsequently collect on and ignite flammable home materials and adjacent flammables. Firebrands that result in ignitions can originate from fires that are at a distance of 1 kilometer or more. For example, during the 1980 Panoram Fire (San Bernardino, California), the initial firebrand ignitions to homes occurred when the wildland fire was burning in low shrubs about 1 kilometer from the neighborhood. During severe W-UI fires, firebrand ignitions are particularly evident for homes with flammable roofs. Often these houses ignite and burn without the surrounding vegetation also burning. This suggests that homes can be more flammable than the surrounding vegetation. For example, during the 1991 fires in Spokane, Washington, houses with flammable roofs ignited without the adjacent vegetation already burning. Although firebrands may be lofted over considerable distances to ignite homes, a home’s materials and design and its adjacent flammables largely determine the firebrand ignition potential.
Cohen's paper cites other studies that showed for homes with non-flammable roofs, those clear of dense vegetation for an area of 30 feet or more had a probability of survival of from 86% to 95%. Cohen himself has done considerable post-mortem analysis of homes that survived or failed to survive fires, and his conclusions are the basis for the Firewise program among other things.
Before pointing the finger at the Forest Service or even slash piles in the fire, or claiming the piles were repsonsible for structure losses, it would be imperitive to understand what kinds of structures were involved and what the conditions were around them. Dense vegetation near the homes, shake roofs, firewood stacked next to the house, paths of dry vegetation leading directly to the structure, decks or porches not screened underneath, even a build-up of pine needles on the roof or in the gutters are all invitations to ignition by firebrands, even if the fire remains a long distance away.
Cohen concludes (and anyone who lives in a high fire danger area would agree) that:
The congruence of research findings from different analytical methods suggests that home ignitability is the principal cause of home losses during wildland fires. Any W-UI home fire loss assessment method that does not account for home ignitability will be critically non-specific to the problem. Thus, to be reliable, land classification and mapping related to potential home loss must assess home ignitability. Home ignitability also dictates that effective mitigating actions focus on the home and its immediate surroundings rather than on extensive wildland fuel management. Because homeowners typically assert their authority for the home and its immediate surroundings, the responsibility for effectively reducing home ignitability can only reside with the property owner rather than wildland agencies.
The potential for survival or destruction of a home in a wildfire is largely a result of the choices homeowners make - whether the structure is built with fire survival in mind and what kind of surroundings the homeowner maintains. Government can improve the survivability of structures through things like building codes and regulation - something the homewoners in Lake Tahoe might have opposed strongly - but ultimately, the survival of homes isn't determined by conditions in the nearby forest, and is the responsibility of the property owner. Some of us take that responsibility seriously.
If you put a highly flammable house next to a burning forest, the house is likely to burn as well - it's made of the same stuff as the forest, and kept in a drier condition. If you take steps to prevent the house from catching fire in the first place, the odds that it will survive the fire are extremely high. Whether or not someone irresponsibly left slash piles or other hazardous conditions in the near vicinty.