Fire frequency by ecoregions for the historic period (prior ~1900 CE) and the current period (after ~CE 1900).
The area burned in North America began to decline substantially in the 1800s, and in some areas even before this period. The most detailed information on the timing of the decline comes from tree-ring studies. The graphic below describes the results of select studies organized from cool moist ecoregions on the top to warm/dry ecoregions on the bottom.
Although a cooler and moister climate may be implicated with some of the declining fire area, the most consistant factor was cultural change.