Thinning forests isn’t the issue

George Wuerthner
Wednesday, July 14, 2021
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One frequently hears from proponents of thinning that active forest management can reduce fire intensity and thus is a beneficial policy to reduce large blazes. However, most of the scientific support for thinning is based on modeling of fuel loading, not real-life experiences. For instance, a recent letter to the editor in the Missoulian made such a claim. The amount of fuel is often the least important factor in fire spread.

First, what drives large fires is drought, low humidity, high temperatures, and lastly, and most importantly is wind. Wind’s effect on wildfire spread is not linear but exponential. A 20 mph wind does not double fire spread over a 10 mph wind but quadruples it. One can imagine how rapidly a blaze spreads when winds are blowing 50-70 mph, as often is the case when large acreages burn.

When a forest is thinned, there is greater wind penetration, and the wind is the most crucial factor in wildfire spread. Thinning also results in more fine fuels on the ground that will carry a fire. And opening up the canopy allows greater solar penetration meaning fuels and soils dry out quicker.

What burns in a forest fire is primarily “fine fuels” like grass, shrubs, and small trees.

Forest Service “thinning projects,” remove the larger trees which provides little benefit.

A review of 1500 wildfires in dry and mixed conifer forests, the very forests that supposedly have excess fuel buildup, found that stands with “active forest management, burned at higher severity than forests in protected areas like parks and wilderness where presumably there is more fuel.”

Finally, even if thinning worked to slow blazes under ordinary fire weather conditions, under the extreme conditions that drive all large wildfires, the wind effectively negates thinning benefits by blowing embers miles ahead of a fire front. Thus, embers are tossed over and around any fuel breaks or thinned stands.

Add to all these factors is the very low probability that any thinned forest stand will encounter a fire. The likelihood is around 1-2%.

The majority of thinned areas are never burned during the period when they “might” be somewhat effective. Thinning by reducing competition for water and sun promotes the rapid growth of more fine fuels—so in 5-10 years, you have more grass, shrubs, small trees, which are the very things that burn readily in a forest fire.

However, thinning is not benign. Thinning removes carbon stored in trees. The disturbance from logging roads and activities spreads weeds, including cheatgrass, which is highly flammable. Logging can displace sensitive wildlife like elk. Sediment from logging roads can fill streams harming aquatic life.

So we trade inevitable negative consequences of active forest management (thinning) today and get only a tiny chance that any thinning will influence a wildfire.


EDITOR’S NOTE: George Wuerthner has written numerous articles and books on wildfire. He lives in Park County.