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UCSB Biogeography Lab Publications Abstracts
Controls on disturbance regime dynamics: Fire in Los Padres National Forest (California)
Moritz, Max Alan 1999
The goal of this work has been to identify the dominant controls on the fire regime of chaparral shrublands and to understand the impact that fire suppression has had on these ecosystems. The mapped fire history of Los Padres National Forest (LPNF) is an excellent resource for this type of work, containing both spatial and temporal data over the period 1911-1995. There has been much debate about whether fire suppression has brought about large wildfires in shrublands of California by homogenizing the age-patch mosaic on the landscape. I argue that we can answer this question by assessing the relative importance of primary controls on fire regime dynamics. Fire suppression will have varying impacts on fire regimes, depending on the relative strengths of controls. To make this case, I have developed a conceptual model of the primary controls (i.e., vegetation, climate, and ignitions), and I have applied it to chaparral ecosystems of LPNF. Assuming there have always been ignition sources during extreme weather conditions that produce large fires, the issue further reduces to determining the relative importance of vegetation-versus climate-related controls. The peak occurrence of LPNF lightning fires in September supports this assumption, as does evidence from paleo-charcoal studies by other researchers. Because the constraining effect of a fine-grained age-patch mosaic on fire spread is really a matter of fire not being able to burn through young vegetation, the issue largely becomes one of quantifying age dependence. In summary, I have found fire suppression in LPNF not to have altered the likelihood of large fire occurrence, based on pre- and post-1950 comparisons. There appears to be a threshold at a fire size of 4000 ha, below which fire suppression has had a consistent impact; however, this impact is relatively minor, given the role small fires play in the fire regime. Fires above this size threshold are occurring during more extreme fire weather conditions, driving a fire dynamic that is affected little by fuel characteristics or suppression efforts. By quantifying the degree of age dependency within chaparral-dominated portions of LPNF, I have shown that this fire regime exhibits a dynamic that is largely independent of fuel age. Spatial differences in ignition regime and vegetation do not impact the degree of age dependence in different regions. However, spatial differences in fire weather patterns do relate to differences in age dependence, indicating that fire weather is the dominant control on fire dynamics in chaparral of LPNF. Using published data concerning age dependence in fire regimes outside LPNF, I conclude that chaparral fire regimes in general appear to be driven by weather extremes. Therefore, most chaparral ecosystems can and should be considered crown fire ecosystems, as extreme fire weather is the dominant control on fire regime dynamics. This has implications for both the ecology and management of chaparral-dominated areas, and it requires a shift in the way we view these ecosystems.