Spatiotemporal Analysis of Controls on Shrubland Fire Regimes: Age Dependency and Fire Hazard

Large fires in chaparral-dominated shrublands of southern and central California are widely attributed to decades of fire suppression. Prehistoric shrubland landscapes are hypothesized to have exhibited fine-grained age-patch mosaics in which fire spread was limited by the age and spatial pattern of fuels. In contrast, I hypothesize that fires during extreme weather conditions have been capable of burning through all age classes of the vegetation mosaic. Using the mapped fire history (1911–1995) of Los Padres National Forest, I analyzed burning patterns for hundreds of fires using a geographic information system (GIS). To estimate the degree of age dependency exhibited by the fire regime at different spatial scales, I employed methods of fire frequency analysis (i.e., fitting a generalized Weibull function to fire interval distributions). Statistics were also calculated using a temporal breakpoint of 1950 to assess possible effects of suppression. Results indicated that shrubland fires have frequently burned through young age classes of vegetation, exhibiting a minimal degree of age dependency. Findings were not scale dependent and were consistent for all but one region of the study area. The anomalous region exhibited a more rapid increase in the hazard of burning with fuel age, reflecting a moderately age-dependent fire regime; this difference probably resulted from the fact that the region is somewhat sheltered from extreme fire weather that commonly affects other shrublands. Exposure to extreme fire weather therefore appears to override the sensitivity of a fire regime to fuels characteristics at the landscape scale. Fire suppression has affected characteristics of smaller fires much more than those of larger fires. Since 1950, there has been a decrease in size and an increase in the number of smaller fires. Findings support the claim that fire suppression could offset ecological risks posed by increasingly frequent human-caused fires in specific areas, but with a net decrease in annual burning rate of ∼14% across the landscape. Findings contradict the assertion that, in the absence of fire suppression, large fires would be constrained by more complex age-patch mosaics on the landscape. Corresponding Editor: W. J. Platt.