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Eugene Weekly : Viewpoint : 12.06.07

The Climate Factor

Forest thinning won't deter the coming large fires


Whether thinning is an effective strategy to reduce wildfire has been the subject of some debate in Eugene Weekly (Viewpoints 11/08 and 11/29). Increasingly I'm convinced that thinning is ineffective under climatic conditions that are responsible for our largest fires such as the Biscuit Fire that burned across southwest Oregon in 2002.

Indeed, climatic conditions drive all big fires — not fuels. All substantial fires occur only if there is extended drought, low humidity, high temperatures and, most importantly, high winds. Wind, in particular, is critical. Wind increases fire spread exponentially.

When conditions are "ripe" for a large blaze, fires will burn through all kinds of fuel loads. By contrast if the forest is wet like Oregon's coastal forests, you can have all the fuel in the world, and it won't burn.

For this reason, most fires go out without burning more than a few acres. By contrast, when you have drought, low humidity, high temperatures and wind, a few blazes will grow into huge fires. For this reason, approximately 1 percent of all fires are responsible for about 95 to 99 percent of the acreage burned.

Even if thinning works to slow or reduce tree mortality under low and moderate fire conditions, what is becoming increasingly clear is that thinning doesn't stop the very largest blazes that occur under severe fire conditions. If you subtract out the acreage burned by these few large blazes, the total land area affected by all other wildfire that can be influenced by thinning is relatively small.

However, when severe fire conditions exist, nothing can stop a blaze. Under severe conditions, fires burn through all kinds of fuel loads including thinned/logged forests and even natural lightly stocked tree stands. For instance, under the severe conditions that dominated the Biscuit Fire, many of the low-density, widely spaced Jeffrey pine growing on serpentine burned up even though their natural stand density is much lower than what you are left with under even aggressive thinning.

There is growing evidence that thinning can actually acerbate fire spread and mortality — at least under severe fire conditions. Thinning increases solar radiation, leading to greater drying of fuels, and also contributes to greater moisture stress in trees. Thinning also allows wind to penetrate a forest stand with greater velocity, which in turn increases fire spread.

We may be trying to fix something that "ain't broken." Keep in mind that large stand-replacement fires have always occurred under the right conditions, long before we could have altered conditions by logging and fire suppression. The gigantic 1910 Burn raced across 3.5 million acres of Idaho and Montana and charred many acres of low elevation pine and dry Douglas fir forests. And while nearly nine million acres burned nationwide last year, this is dwarfed by the "Dust Bowl Years" of the 1930s when an average of 39 million acres burned annually. So the idea that we have some kind of crisis in our forests may be nothing more than a consequence of short-term memory loss and failure to comprehend how much climatic conditions affects flammability.


There is, however, no doubt that more acres are burning today than in the recent past. I'm inclined to think this is a consequence of global climate change rather than fuel build up. Global warming is lengthening the drying season — by several months — increasing summer temperatures which favors fire spread and may also be increasing average wind speed — another factor critical to large blazes. Therefore, even if thinning/logging were effective in the past — which as I suggest is questionable at least under severe fire conditions responsible for the majority of acres burned — they might not be now.

Given global warming, we are going to experience larger fires and more insect outbreaks as forests seek to balance themselves to changing climate. This is a sign of forest health, not of unhealthy forests as many assert. The fundamental ecological processes that control forest ecosystem are working.

This doesn't mean we have to let fires burn up people's homes. Reducing the flammability of the home — not thinning the forest — is the most sensible response. Installing metal roofs and removing flammable materials from the immediate area surrounding homes has been shown over and over again to be critical to home survivability in large blazes.

We are going to see more large fires in the future, and ultimately this will reset the forest fire regime to the new climatic conditions. Trying to restore some physical historic condition that may or may not have existed in the past makes no sense, especially when there are many negatives associated with logging. Rather we should rejoice that ecological processes like wildfire that shape our forests appear to be working — and working well.

George Wuerthner is an ecologist and editor of Wildfire: A Century of Failed Forest Policy.