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.