U.S. Joint Fire Science Program


Date of this Version


Document Type



Project Active ID: 03-1-1-22


U.S. Government Work


• We identified annual area burned for 37 National Forests in Washington, Oregon, and California between 1929 and 2004 from annual fire reports. Variation in area burned each year on National Forest lands is strongly related (r2 = 0.87) to total area burned in California. This indicates area burned in National Forests is a good indicator of total area burned in a state. • Four large groups of National Forests were identified that had similar temporal patterns of fire extent. Three of these groups were spatially coherent and included mainly: 1) Washington and northern Oregon; 2) northern California and California Coast Ranges; 3) southern Oregon. The fourth group consists of National Forests in each state adjacent to areas with relatively high populations, heavy travel corridors, and dry summers. • The areas of western Washington, southwestern Oregon, and northern California are each primarily represented by a single group of National Forests. These areas receive more precipitation and heavier fuels associated with conifer forests than National Forests to the east and south so fire years are more synchronized by dry years in these areas than in the drier climates were fuels are dry enough to burn most years. • Average summer Palmer Drought Severity Index (PDSI) was lower (drier) in years with large fires in each National Forest Group. The difference in summer PDSI between large and small fire years was strongest for Washington and northern Oregon and weakest in northern California where dry summer conditions prevail in most years and extensive fires are associated with outbreaks of dry lightning. • Average annual PDSI and average winter and spring PDSI preceding the summer fire season were lower in years with large fires. Thus, dry winter and spring conditions, as reflected by PDSI, are an indicator of dry summers associated with large fire years.