One of the assumptions behind federal legislation like the Resilient Federal Forest Act is that more thinning of our forests will halt or significantly reduce large wildfires. Yet the scientific evidence for such a conclusion is ambiguous at best.

Any number of studies have find that thinning usually fails under severe fire conditions.

First, the bulk of fires in the Rockies are burning through forests of lodgepole pine, subalpine fir, Douglas fir, and other forest types that tend to burn at long rotations of 50-500 years or more.

These forest types commonly burn as “mixed severity” to “high severity” fires. Large fires are the norm in these forest types, not a consequence of “fire suppression” as falsely asserted.

However, most fuel reductions have limited value under mixed to high severity fire conditions.

Because most fuel reductions are primarily designed to facilitate commercial timber cutting, they are frequently less than effective in halting blazes.

Even if done correctly, fuel reductions have a short effective life–usually not more than 5-20 years depending on the vegetation. That is because once you reduce the competition by removing vegetation, the remaining trees, shrubs and grasses, grow quickly, creating new fuels, negating any fuel reductions.

A further complication is that the chances that a fire will encounter a fuel reduction during the time when it is effective is very small (Rhodes and Baker 2008)

Under severe fire conditions which includes low humidity, high temps, drought, and high winds you usually cannot stop a blaze. If you have those ingredients, very few thinned fuel reductions work. According to a number of “review studies” as well as lots of anedotal evidence, most of these fuel reductions fail—again under severe fire weather conditions.

Why is that important? Nearly all of the acreage burned annually is the result of a few large fires burning under severe weather/climate conditions. So even if fuel reductions were effective–a questionable assumption–they do little to halt the very large fires that pose the greatest threat to homes and communities. Most fires burning less than severe conditions are relatively easy to stop or put out–in fact, some research suggest most will go out on their own without any suppression if left alone. They self-extinguish because the conditions for fire spread are moderate or simply not good.

There are many variables that affect fire intensity and rate of burn. Without knowing the exact situation at the time when the fire encountered the thinned forest, you can’t really draw conclusions.

For example, I have read other assertions that fuel reductions appeared to slow or halt a fire, but if you dig deeper you find the wind stopped blowing at that time, or the fire began burning down a hill (fires slow significantly burning downhill) or the humidity rose significantly or it encountered the fuel reduction at night when temps are lower. These kinds of things can change rapidly in as little as an hour. So unless you know the exact conditions at the time when a fire entered a fuel reduction, you can’t just assert it worked.

The second qualifier is that a number of review articles have all concluded that fuel reductions do not work under severe fire conditions. Review articles look at the published literature and try to find common threads in the conclusions. What they find is that is that when the wind blows burning embers fly miles in front of a fire front starting new spot fires, and the wind fans flames to greater rates of burn (heat release). That is why fires regularly burn through clearcuts, jump across 16 lane freeways and other situations where there is virtually no fuels.

So here’s a few quotes:

Wildfire Cost Management

“Finally by current standards, even our best fuel reduction do not appear to be adequate to provide much assistance in the control of high intensity wind-driven fires. If fuel treatment is the answer, it will need to be done on a level that is far more extensive (area) and intensive (fuel reduction than we are now accomplishing—even on our best fuel breaks.”

Gedalof el al 2008

“fuel treatments ….cannot realistically be expected to eliminate large area burned in severe fire weather years.”

Objectives and considerations for wildland fuel treatment in forested

ecosystems of the interior western United States Elizabeth D. Reinhardt *, Robert E. Keane, David E. Calkin, Jack D. Cohen

“Extreme environmental conditions…overwhelmed most fuel treatment effects. . . This included almost all treatment methods including prescribed burning and thinning. . .. Suppression efforts had little benefit from fuel modifications.”

“It may not be necessary or effective to treat fuels in adjacent areas in order to suppress fires before they reach homes; rather, it is the treatment of the fuels immediately proximate to the residences, and the degree to which the residential structures themselves can ignite that determine if the residences are vulnerable.”

Learning to coexist with wildfire

Max A. Moritz, Enric Batllori†, Ross A. Bradstock, A. Malcolm Gill, John Handmer, Paul F. Hessburg.

Justin Leonard6, Sarah McCaffrey5, Dennis C. Odion7, Tania Schoennagel8 & Alexandra D. Syphard9

Moreover, opening up the overstory canopy and increasing sunlight penetration can increase growth of highly flammable understory vegetation.

Although there are many examples of fuel treatments reducing fire behaviour when conditions are not extreme, recently treated forests can experience a stand-replacing crown fire when wind speeds exceed 30 km h−1 and when fuel moisture is low102. When the probability of fire occurring in a particular area is relatively low, the odds of a fuel treatment influencing the behaviour of a wildfire there, within the time frame that treatments are effective, is also low.

Finally Jack Cohen at the Missoula Fire Lab concludes:“ Wildland fuel reduction may be inefficient and ineffective for reducing home losses, for extensive wildland fuel reduction on public lands does not effectively reduce home ignitability on private lands”

The one thing that almost all researchers agree upon is that if you want to protect structures, start with the home ignition zone around the houses and work outward. Second, stop building homes in the woods.

I might also emphasize that even if we could stop these large fires, we should not as they are critical to healthy forest ecosystems. Dead trees are very important as biological legacy and large wildfires are the primary means of input into forest ecosystems.

Keep in mind there is always damage from logging, including the spread of weeds, reduction in biomass, sedimentation from roads that degrades aquatic ecosystems, reduction in hiding cover for big game, loss of carbon storage, and so forth.

So when you consider all these factors that most fuel reductions do not work, and in some instances actually increase fire spread; that there is a low probability that any fire will encounter a fuel reduction at all, and that even in the face of a large severe fire; if you take the proper precautions of reducing home ignitability, it doesn’t matter. Finally we need large fires to create important snag and down wood habitat critical for healthy forest function.