Dry Live Fuels Increase the Likelihood of Lightning‐Caused Fires.

Live fuel moisture content (LFMC) is a key determinant of landscape ignition potential, but quantitative estimates of its effects on wildfire are lacking. We present a causal inference framework to isolate the effect of LFMC from other drivers like fuel type, fuel amount, and meteorology. We show that in California when LFMC is below a critical flammability threshold, the likelihood of fires is 1.8 times as high statewide (2.25% vs. 1.27%) and 2.5 times as high in shrubs, compared to when LFMC is greater than the threshold. This risk ratio is >2 times when LFMC is 10% less than the threshold. Between 2016 and 2021, the risk ratio was highest in 2020 (2.3 times), potentially contributing to the record‐breaking wildfire activity in 2020. Our estimates can inform several wildfire prediction and management applications, including wildfire suppression, prescribed burn planning, and public safety power shutoff implementation. Plain Language Summary: Wildfires are complex phenomena and can occur under a range of conditions, making it difficult to determine how different drivers affect wildfire occurrence. For example, dense contiguous vegetation, dry litter, dry vegetation, high winds, and warm air all contribute to high fire likelihood. Under such co‐occurring conditions, how can we determine each driver's effect on fire likelihood? Here, we present a framework to attribute an individual driver's effect on fire occurrence, while controlling for other correlated factors that might provide a false signal. We consider lightning‐based ignitions, assuming they are randomly distributed, and compare the fire outcomes for different vegetation dryness levels, all else held constant. We test our framework in California and find that when vegetation is dryer than the critical flammability threshold, wildfire likelihood increases by almost a factor of two. These results help clarify the uncertainty in the effect of vegetation dryness on fire occurrence, and may also help improve wildfire preparedness, management, and response strategies. Key Points: We provide a causal inference framework to understand the contribution of natural drivers to wildfire occurrenceWhen live fuels are extremely flammable, lightning strikes are about twice as likely to cause wildfiresThe increase in fire likelihood is highest in shrubs and lowest in grass [ABSTRACT FROM AUTHOR]