Not too hot.
Not too cool.
Not too often.
Not too rarely.
Turns out, restoring forest health through controlled burns is a lot more complicated than foresters first assumed.
That’s the conclusion that emerges from a sweeping review of decades of tinkering with the formula for controlled burns in the ponderosa pine forests of the Southwest, published jointly by a host of fire research centers operated by the Forest Service.
The future of the forest and every community in its long shadow hangs on the conclusions emerging from efforts to restore prescribed burns and managed wildfires to their natural role in the forest ecology over the past 20 years.
Everyone now agrees that tree densities pose an existential threat to the forest and forested communities, with an ecosystem adapted to 50 trees per acre now smothered under the weight of 1,000 trees per acre. Crown fires have not only consumed whole towns, they’ve so changed the ecosystem that the ponderosa pine forests don’t come back.
A century of fire suppression and grazing dramatically reduced the number of fires across millions of acres. This eventually replaced an open, grassy forest with a sickly, overcrowded forest — prone to soil-sterilizing, town-destroying crown fires.
The controlled burns on thousands of acres across Arizona right now attests to the new focus on prescribed burns. However, the proliferation of towns and subdivisions together in a forest burdened with an average of 34 tons of dead and downed wood on every acre means that a random return of fire to the landscape will work devastating changes.
“Naturally occurring fires will likely never play a role in most ecosystems again because of changes that have occurred with Euro-American settlement,” concluded the study authors, which included Stephen Sackett, Sally Haase and Michale Harrington.
However, studies of prescribed fires stretching back 50 years suggests managed fires combined with large scale thinning projects could restore healthy forest conditions. However, every fire also has the potential to get out of control and cause disaster.
The study validates the effort by the Forest Service to use prescribed fires on thousands of acres in the cooler weeks of fall, before winter’s rains and snow make such burns impossible.
However, the researchers concluded “using fire in current forest conditions is much different than fire thinning that occurred naturally years ago.”
Many stands have reached 2,000 trees per acre. Stunted from overcrowding and competition for water, many of the little trees are 80 years old with thick bark that resists low-intensity flames. By contrast, in pre-settlement forest the saplings were 20 years old and thin-barked, which means they burned readily in a low-intensity fire. Few ever survived to become fire-resistant, old-growth trees – widely spaced at 50 trees per acre and living to 600 or 800 years.
As a result, “one fire does not correct problems associated with more than 100 years of fire exclusion, especially as it applies to thinning,” the researchers concluded.
The researchers noted that although “much has been learned within the last 50 years about the use of fire in the Southwest. Many fire experts have developed their skills, learning from failures as well as success. This type of knowledge is difficult to pass on to less experienced individuals.”
The researchers said the best time to burn remains mid-September on into December, although this poses problems with smoke. The cold, dry air can create a cap that traps smoke close to the ground. The most effective fires occur on days with a temperature between 50 and 75 degrees and with fuels containing 9 to 12 percent moisture and humidity below 40 percent.
Ideally, wind speeds should be 3-8 miles an hour – but if they rise to 10 miles an hour the fire becomes erratic. Most controlled burns need to move uphill to burn hot enough to do any good – but that again poses a control problem.
Moreover, even a successful controlled burn will require follow-up burns. Initially, those burns should come every three years. But once the decades of downed wood has burned off and tree densities have been dramatically reduced, the “maintenance burns” can come every seven years or so.
The bottom line: We’re must learn to live with fire. It will take years of calculated risks, irritating smoke and the annual possibility of disaster to fix a problem a century in the making.