Analyzing Extreme Disturbance Events: Fire in Los Padres National Forest

Abstract

Extreme disturbance events may strongly influence the structure and functioning of many ecosystems, particularly those in which large, infrequent events are the defining forces within the region. This paper introduces the extremal fire regime (i.e., the time series of the largest fire per year) and the assumptions implicit in its analysis. I describe the statistics of extremes and demonstrate their application to the fire regime of Los Padres National Forest, California, to compare two regions (i.e., Main and Monterey divisions), to test for a shift in fire regime due to fire suppression, and to examine climatic events as a forcing mechanism for large fires. Despite their similarity and proximity, the Main Division exhibited a much higher frequency of large fires (and shorter return time) compared to the Monterey Division. Comparison of time periods 1911–1950 and 1951–1991 indicated that fire suppression had no effect on the distribution of very large fires in the Main Division, although the frequency of fires smaller than ∼4000 ha declined. Comparing distributions of an index for severity of Santa Ana conditions (i.e., characterized by hot, dry winds) and extreme fire events in the Main Division indicated a convergence of distributions with increasing event size. The distribution of fire events larger than ∼4000 ha appears to be coupled with that of severe Santa Ana conditions, suggesting a strong climatic forcing for extreme fires and a threshold for the transition from small‐ to large‐fire dynamics. Results indicate the usefulness of extremal fire regime analysis for comparisons over space and time and for examining a potential forcing mechanism. This approach can be applied to any disturbance regime in which large events play an important role, providing ecologists and land managers with a useful tool for understanding and predicting dynamics of extreme disturbance events.

Projection of wildfire activity in southern California in the mid-twenty-first century

“…However, unusually strong Santa Ana winds in late September 1970 resulted in an extremely large (1.1×10 5 ha) wildfire near San Diego (http://www.wildfirelessons.net/documents/Laguna_Fire_Analysis_1970.pdf). Figure 9 indicates that though Santa Ana events increase the likelihood of large wildfires, some large fires occur in the absence of Santa Anas, a finding also reported by Moritz (1997).…”

“…Most of the extremely large wildfires in southern California are associated with Santa Ana winds, strong offshore winds characterized by low humidity (Schroeder et al 1964; Moritz 1997; Keeley et al 2009). These winds develop as a consequence of a steep pressure gradient between the Great Basin and the Los Angeles area (Raphael 2003) and are strengthened as they channel through the complex local topography (Jones et al 2010).…”

Wildfire Selectivity for Land Cover Type: Does Size Matter?

“…If, in fact, fuels (age, load and continuity) are determinant for the occurrence of severe fire seasons, then fuel management may be efficient in reducing fire hazard [15]. However, if area burned tends to coincide with episodes of severe weather, poorly constrained by fuel landscape properties, then investments in large scale fuel management programs are unlikely to succeed [16], [17]. Furthermore, future climate change is likely to exert stronger impact in fire regimes that are primarily climate-driven.…”

Dynamic Resource Allocation in Disaster Response: Tradeoffs in Wildfire Suppression

“…In the case of California wildfires, a dominant factor is wind. The largest, most destructive wildfires are typically associated with extreme, high wind, low humidity Santa Ana conditions [54], [55]. Fluctuations in wind speeds during these events dominate both the dynamics of fire spread and the effectiveness of response.…”