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How did the US Forest Service's success in fighting Montana's forest fires in the early 20th century lead to worse fires by century's end, according to Collapse?

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According to Collapse, the US Forest Service stopped every small fire, which increased the "fuel load" because natural small fires which burn off underbrush were always put out. As the fuel load was allowed to increase, it led to larger fires later in the twentieth century. 

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Diamond argues that the spate of forest fires in Montana in the late twentieth century can actually be traced back to the irresponsible logging practices that characterized forest use throughout the 1900s, as well as the US Forest Service’s decision to adopt a policy of complete fire suppression in the 1910s. In the first place, over-logging produces what Diamond calls “a huge pile of kindling.” This means that as logging companies cut through the forest and remove the largest tree trunks, they leave behind massive amounts of debris—branches, twigs, underbrush, etc.—all of which makes for excellent fuel for relatively small fires to blaze out of control.

In the second place, the decision of the US Forest Service to extinguish every and any fire that it could starting at the beginning of the century has destroyed the forest’s natural ability to suppress the spread of fire itself. Diamond argues that a lightning-strike-mediated forest fire occurs in Montana’s wilderness once about every ten years. In the interim period, large Ponderosa Pine trees, with bark about two inches thick and with a relatively clear understory, are able to withstand the effects of small-scale fires that arise from natural causes. Douglas Fir saplings—trees with shoots and branches that are much more susceptible to spreading wildfires—only have a ten-year window to mature until the next forest fire moves through and burns them out. The brevity of this period prevents them from concentrating in too high a degree underneath the Ponderosa undercarriage, maintaining the equilibrium of the forest.

However, because of the persistent actions of the US Forest Service, populations of Douglas Firs have been growing out of control. This is because, as the government extinguishes every single fire (no matter how anodyne), the Douglas Fir population has begun to rapidly expand, adding much fodder at the understories of the Ponderosa trees and enabling a much faster, much less easy to control spread of forest fires. The increase “fuel load” greatly contributes to the amount of material fires have to burn, meaning that smaller fires grow and spread out of control at a much faster rate than they had in the past.

As Diamond concludes,

In an ideal world, the Forest Service would manage and restore the forest, thin them out, and remove the dense understory by cutting or by controlled small fires.

But this is too expensive a program to implement, he argues, providing a clear example of how society (in this case, American society) has contributed to its own decline.

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The US Forest Service's policy of extinguishing all fires as soon as possible failed to take into account the role of smaller, naturally occurring fires in preserving the structure of forests. Diamond argues that the role of fires caused by lightning varies, depending on the nature of the trees in the forest and other factors such as altitude, but uses the example of mature Ponderosa pine forests as a case in point. In such forests, research shows that natural fires occur about once every ten years. These fires burn the smaller Douglas fir saplings and clear out the underbrush, but never become large enough to threaten the big trees. The result is that "fuel load" values after such fires are much lower, and the forest is better protected from catastrophic fires. By suppressing these natural fires, the Forest Service unwittingly improved the chances of a truly big fire breaking out.

Another contributing factor was the Forest Service's logging policies. By removing the big trees, more room was made in the forest for smaller, more flammable saplings. Diamond says that as a result fuel loads in such forests increased by a factor of six. Fires under these conditions can truly kill a forest, not only burning the trees, but killing the seeds in the ground.

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In ecosystems like that of the forests that Diamond describes in Montana, wildfires have always been an important part of the natural process.  In 20th century (and particularly in the years after World War II) the Forest Service’s new methods of fire suppression made it so that fires no longer happened much.  This was good for a while, but it ended up causing fires to get worse overall. 

When forests like those of Montana are left to their own, they develop large Ponderosa Pine trees with very little in the way of undergrowth.  This is because of fire.  When fire hits, it tends to burn the Douglas Fir trees that are still relatively small.  These trees are susceptible to fire.  However, the large Ponderosa Pines are not susceptible.  The fire burns the short Douglas Firs but does not burn the Ponderosas because their bark is very thick and fire resistant.  The fire clears the undergrowth, leaving the Ponderosas growing.

When the Forest Service started to suppress fires, it stopped this process.  Now, Douglas Firs could grow much taller than they used to grow.  When fires start in this environment, the Douglas Firs burn and, because they are so tall, they allow the flames to reach the tops of the Ponderosa Pines (where those trees are vulnerable to fire).  As Diamond says (I can’t give a page number because I only have this on Kindle)

…the dense tall (Douglas Fir) saplings may become a ladder that allows the fire to jump into the crowns.  The outcome is sometimes an unstoppable inferno…

In this way, the well-meaning efforts of the Forest Service have actually made fires worse in Montana. They have allowed the fires to burn hotter and to burn more trees than would previously have happened.

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