Since the start of the 21st century, large wildfires have become increasingly more frequent in California. Two recent publications by Dr. Jon Keeley (USGS Western Ecological Research Center) and collaborator Dr. Alex Syphard (Conservation Biology Institute) provide insight into the drivers of these fires and the factors that lead to their destructiveness.

The first publication examines selected recent California wildfires, assessing the locations, vegetation types, ignition causes, and other drivers of fire activity along with area burned, structures destroyed, and lives lost. Keeley and Syphard write that many California wildfires can be categorized into one of two types: fuel-dominated vs. wind-dominated wildfires.  

Lightning-ignited fires that burn in montane forests in the summertime are typically fuel-dominated. The behavior of fuel-dominated fires is driven by the amount of vegetation in the forest (bottom-up control). Montane forests in California are often dense with fuel due to fire suppression that has reduced the natural fire frequency.  In contrast, western California shrubland fires are overwhelmingly wind-driven (top-down control) by autumn or early winter foehn wind events coupled with human ignitions in densely populated areas.

Using this framework, the authors classify selected high-profile 21st century fires, including the 2003 Cedar Fire (wind), the 2013 Rim Fire (fuel), the 2017 Tubbs and Thomas Fires (both wind), and the 2018 Camp Fire (wind). High fuel loads in dense forests are often thought to cause the most destructive fires, yet the most catastrophic fires in terms of total lives and structures lost fit the profile of wind-dominated fires. Understanding these two types of fires can help managers better assess the causes of fires in their region and choose the most appropriate strategies, such as fuels management techniques or focus on prevention and protecting homes.

"There is often little recognition of the differences between fuel-driven forest fires versus wind-driven shrubland fires in the news," says Dr. Keeley, "but this distinction is critical to understanding appropriate management responses."

In a second publication, Keeley and Syphard dive deeper into the aftermath of these 21^st century California fuel and wind-driven wildfires. This study used thousands of public records of wildfire structure losses to examine if home characteristics, defensible space distance, or defensive action by firefighters or civilians increased a home’s likelihood of surviving fire. The scientists analyzed the effectiveness of Home Ignition Zone (HIZ) characteristics for about 40,000 California buildings exposed to wildfire between 2013 and 2018 to determine which were the most important features in preventing structure loss. After sorting the buildings into “survived” (about 10%) and “destroyed” (about 90%), statistical comparisons of the two groups showed that “hardened home” details were most strongly associated with surviving wildfire across California during the period of study. Closed eave structure was the most important feature for surviving fire, followed by multiple pane windows.

Defensible space distance, as defined within this study (sorted into coarse, 30 meter distance categories) was not found to be a significant factor in preventing structure loss relative to structure characteristics. This doesn’t mean that defensible space more broadly can’t be an effective tool for limiting fire damage and protecting firefighters, but the results indicate that features of a home itself may matter more than just extending the space around it.

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“Home survivorship is less a function of the extent of defensible space as it is the specific structural characteristics that harden homes against ember entry inside,” explains Dr. Keeley.

These results underscore the idea that many of the most destructive wildfires in California are wind-driven, with wind carrying embers long distances to combustible fuels in or on houses. Under these conditions, approaches that prevent embers from entering and igniting the home will be most effective.

The drivers of wildfire behavior in California and across the nation can be complex, and vary considerably depending on location, vegetation type, climate, and management history. USGS wildfire scientists and their collaborators are working to better understand how these pieces fit together so that land managers, first responders, and homeowners can better protect lives, property, and natural resources.

 

Learn more:

Keeley, J.E., and A.D. Syphard. 2019. Twenty-first century California, USA, wildfires: fuel-dominated vs. wind-dominated fires. Fire Ecology 15:24 doi.org/10.1186/s42408-019-0041-0.

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Syphard, A.D., and J.E. Keeley. 2019. Factors associated with structure loss in the 2013-2018 California wildfires. Fire 2(49): 15pp. doi:10.3390/fire2030049

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