For most of the United States, current fire frequencies are longer than what historically occurred prior to European settlement. In part, this can be attributed to fire suppression and exclusion efforts. This is particularly true for the eastern 2/3 of the nation. There are some areas of the western United States that are experiencing more frequent fire due to exotic annual grass invasion. For further information about specific areas of the country visit http://www.landfire.gov/.
Historically, fires in North America came from two primary sources, lightning and humans. See full length article for more detailed information.
Fire regime is a term used to characterize the frequency, extent, intensity, severity and seasonality of fires within an ecosystem over an extended period of time. For example, historically, ponderosa pine forests in the Southwest were generally characterized by a high frequency (2-12 years), low intensity, and low severity spring & summer fire regime.