According to Collapse, how did the US Forest Service's success in improving its ability to stop Montana's forest fires during the first half of the 20th century lead to worse forest fires by the end of the 20th century?
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.