Flammability of chaparral species along a water deficit gradient in southern California

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Indra Boving, ERI, UC Santa Barbara, Santa Barbara, CA, Joe Celebrezze, UC Santa Barbara, Santa Barbara, CA and Max A. Moritz, Bren School of Environmental Science and Management, University of California, Santa Barbara, Santa Barbara, CA

Background/Question/Methods
Fire risk assessments incorporate measurements of live fuel moisture (LFM) to characterize fire risk on the landscape scale, with thresholds in LFM corresponding to increased fire likelihood and fire size. These thresholds trigger regional fire danger warnings, which inform management decisions related to recreation and restoration, as well as act as inputs for future fire severity models. In chaparral-dominated areas in southern California, LFM is most widely measured and monitored in the chaparral species Adenostoma fasciculatum due to its abundance on the landscape; live fuel moisture in Ceanothus megacarpus, another widespread chaparral species, is less widely monitored and may also be a good indicator of fire risk. While the threshold relationship between LFM and fire risk has been documented at the landscape scale, few studies have done so at the tissue level under controlled laboratory environments. In this study, we measured the flammability of A. fasciculatum and C. megacarpus during a laboratory dry-down by burning samples and recording time to ignition and flame height, two key components of flammability, across a LFM gradient.
Results/Conclusions
Our laboratory studies of flammability showed an increase in flammability at lower moisture contents in both species. Unlike landscape-level studies, our results did not indicate a threshold in flammability, but instead exhibited a gradual increase in flame height and a decrease in time to ignition as moisture levels decreased. This relationship was true for both species, but with C. megacarpus exhibiting lower flammability (lower flame heights and longer time to ignition) than A. fasciculatum across moisture levels. These results confirm the use of LFM as an indicator of fire risk and tissue-level flammability, as well as the use of A. fasciculatum as a primary indicator species. However, in areas where C. megacarpus is more dominant, LFM monitoring initiatives could benefit from increased LFM measurements of this species.