Evidence of fire before written records is found in soil profiles, lake sediments, landslides, and in old-growth trees that have been scarred by fire. It is clear that wildfire has had a role in the dynamics of Yellowstone’s ecosystems for thousands of years.
Although many fires were caused by human activities, most ignitions were natural. The term "natural ignition" usually refers to a lightning strike. Afternoon thunderstorms occur frequently in the northern Rocky Mountains but release little precipitation, a condition known as dry lightning. In a typical season there are thousands of lightning strikes in Yellowstone. Lightning strikes are powerful enough to rip strips of bark off of a tree in a shower of sparks and blow the pieces up to 100 feet (30 m) away. However, most lightning strikes do not result in a wildfire because fuels are not in a combustible state.
Fire ecologists use estimates of fire return intervals to better understand the role of fire in different forest types. Fire return intervals represent the average frequency of fire for an area or plant community type on the landscape. Natural, historical fire return intervals in Yellowstone range from 20 to 25 years for shrub and grasslands in the Northern Range (Houston 1973) to 300 years or more for lodgepole pine forests on the central plateau (Romme 1982, Romme and Despain 1989) and subalpine whitebark pine stands.