Nationally, the causes and extent of fire on lands administrated by the United States Forest Service varied significantly from 1940 to 2000, with California experiencing the largest relative annual burned areas. The south-east and California experienced the largest relative area burned by fires from human ignitions. No significant differences were detected in the relative area burned by lightning in California, the upper and central Rocky Mountains, and the south-west, which all experienced the highest levels. The north-west and Rocky Mountains have experienced significant increases in the relative total area burned; the north-east, south-east, California, and coastal Alaska all remained unchanged. The northern Rocky Mountains, south-west, and north-east have all experienced significant increases in the amount of area burned by lightning without significant increases in lightning ignitions. Increasing fuel hazards in these areas probably contributed to the increasing area burned by lightning fires; changing climate could have also contributed to the increase in wildfire area from 1940 to 2000. To be effective across the diverse forest types and conditions in the USA, fire policy should better recognize and respond to the diversity of US forests and how they have burned in the past. This analysis determined that there is high geographical diversity on wildfire occurrence and causes. Local input is therefore important in designing diverse, ground-based solutions to address fire management challenges in the United States.
THE UNITED STATES FOREST SERVICE
