Native shrubbery that died during a severe 7-year-long drought helped spread the 2017 Thomas and 2018 Woolsey Fires, according to research published in the journal, Ecosphere. These wildfires were ignited by electric power line failures and spread through vast areas of dead vegetation, directly impacting communities in Santa Barbara, Ventura and Los Angeles counties. 

Evergreen, densely packed shrubs cover much of Southern California. These shrubs, called chaparral, are adapted to California’s wet winters and dry summers, but when one of the driest and longest lasting droughts hit the state from the end of 2011 to spring of 2017, landscapes normally filled with living green plants were left with grey-colored drought-killed remains. 

Although many people think of forest fires when they think of large fires, forests only cover 10% of natural landscapes in Southern California. “Chaparral dominates the same landscapes that people dominate,” said Jon Keeley, a USGS research ecologist who led the study. “Our new research helps us understand how drought-stricken chaparral can help spread big fires that are close to where people live.” 

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To better understand how the drought affected chaparral, researchers studied Landsat satellite images of the burned areas. The stark difference in chlorophyll levels between 2010 and 2016 meant that a lot of shrubs that were alive in 2010 had died by 2016, their leaves dehydrated and unable to produce the green pigment. 

“It’s reasonable to assume that the dieback in vegetation would increase fire severity because that dead vegetation would provide fuel for a bigger fire,” Keeley said, but instead, he found that plots with dead chaparral had significantly lower fire severity than plots without dieback. 

The issue? The difference between fire severity and fire intensity. The former measures the loss of biomass due to a fire while the latter measures the fire’s emitted heat. If a plot of land already lost a lot of vegetation due to drought, it had less to lose during an actual fire. 

However, the drought-stricken chaparral did help spread and intensify the Thomas and Woolsey fires. Embers, which travel further in dead stems, were blown ahead of the fire by the Santa Ana winds and landed on dead vegetation, which ignited and contributed to the rapid spread of the fire. 

“Current fire danger indices used to predict the upcoming fire season typically do not incorporate drought-caused dieback,” Keeley said. Instead, they use estimates of dead fuels based on vegetation age. “This study highlights the potential importance of drought-induced dieback in chaparral wildfires and suggests that we can improve existing fire danger indices by incorporating current dieback estimates.” 

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