So various bunnies have been wondering why the areas burnt in US forest fires decreased sharply starting in the 1930s
There have been various doubts expressed about this data, but the data looks ok when cross checked against the Historical Statistics of the United States: Colonial Times to 1970 (should be on everybunny's bookshelf. Ms Rabett made Eli's day years ago when it appeared under the carrot bush).
Turns out that the answer is pretty simple
National forest management from 1900 up to the Second World War was mostly custodial in nature. An early focus was to establish the boundaries of the national forests and to prevent, or respond to, unauthorized uses (such as illegal timber felling, unauthorized mining, agricultural encroachment).The government did it. More to the point, New Deal public employment programs such as the Civilian Construction Corp provided the manpower to better manage the national and state forests and fight the fires. Then everybody saw that this was a good thing and got together to manage the problem.
Another main focus of Forest Service efforts was reducing uncontrolled wildfires that were common prior to the 1930s. Curtailing the 8 to 20 million hectares that consistently burned annually, mostly on private lands, was considered a prerequisite for the long-term management of forests and grasslands — both public and private.
The focus of these efforts was on protecting all lands from wildfire, regardless of their ownership; but systematic control became effective only during the 1930s, when large public employment programmes were established. By the 1960s, the area burned by wildfire had declined by 90 percent compared to the 1930s (Figure 2). This was accomplished through highly successful federal, state and private landowner cooperation. Within the Forest Service, the State and Private Forestry Division was responsible for this coordination.