Contrasting behavior and economic impact of wildfires in California’s Mediterranean ecosystems

Background/Question/Methods
The shrub-dominated ecosystems in Southern California are vulnerable to two distinct fire regimes: episodic and large fall fires driven by strong offshore Santa Ana (SA) winds, and frequent but relatively smaller fires in hot and dry Mediterranean summer. A dramatic increase in large and costly fires during the past decade caused billions of dollars in economic loss, and affected air and water quality and ecosystem services. A systematic analysis of the fire behaviors and the socio-economic losses, and an improved understanding of the associated drivers for these two contrasting fire regimes are critical for better fire management. Past studies haven’t distinguished these fires, and a single strategy for fire management is often assumed to be appropriate for all fires. We recently developed an approach to classify fires using fire history and high resolution climate data. We here used the MODIS satellite since 2002 to quantify the fire behaviors, including fire spread, intensity, and duration. The structure losses and fire-fighting costs were quantified for 1990-2009 fires using the fire incident reports and USFS database. We also used the stepwise regression model to analyze the environmental and human drivers in controlling the size and economic impacts of these two fire types.  

Results/Conclusions

SA fires typically spread to west, while non-SA fires progressed much slower toward east. SA fires had much higher fire intensity than non-SA fires, especially during night time (115.9±8.4 vs. 93.6±7.1 W m^-2), consistent and considerable across all age groups. We also found that fire probability was not significantly correlated with stand age during SA periods, but increased with stand age during non-SA conditions. Fire weather, intensified by terrain, was the predominant control for SA fire size, while the accessibility constrained by topography was critical for non-SA fire size. SA fires caused more risks and damages to structures due to the fast spread and proximity to the urban areas, about sevenfold larger than those from non-SA fires. Non-SA fires had much higher suppression costs as a result of rugged terrain and longer duration, about 1.3 billion dollars during 1990-2009 and twice of the corresponding property losses. Our results supported that SA fires were typically weather driven and non-SA fires depend more on fuels and human activities. This study provides additional information to reconcile the debate on what drives the Southern California’s fire regimes, and underscores the importance of adopting separate fire mitigation strategies for summer versus SA fire prone areas.