Researchers are actively working to improve our understanding of the critical ecological thresholds that, when crossed, result in drought impacts and/or outbreaks of wildfire. Many of these studies focus on aspects of plant water status that control plant hydraulic function and flammability. However, the methods and approaches for assessing drought and wildfire risk are often done independently of one another, with each focusing on slightly different but related traits and indices. An opportunity exists to better integrate these different approaches that would allow for greater translation of findings across disciplines. Therefore, our research aims to determine the relationship between critical drought and flammability thresholds at the leaf level. We used a laboratory approach to determine critical thresholds in LFM for chamise (Adenostoma fasciculatum). In the laboratory we followed a benchtop desiccation procedure and measured both LFM and flammability characteristics repeatedly, as the plants dried down. At each time point, LFM measurements were compared with several flammability characteristics to determine which ones are most impacted by LFM and where potential thresholds lie. The flammability setup included an epiradiator for ignition, thermocouples to measure flame and ignition temperatures, and video cameras to record flame height, flame duration, and time to ignition.
Results/Conclusions
Not surprisingly, we found that each of the flammability characteristics we measured increase with decreasing LFM. Lower LFM samples led to higher maximum temperatures, taller flame heights, shorter time to ignition, and longer flame duration. A regression analysis showed a significant relationship (P<0.05) between both LFM and flame height, and LFM and time to ignition. Maximum temperature and flame duration were more variable metrics of flammability, but still held trends with LFM. There were no apparent thresholds in LFM where any of the flammability traits drastically increased. Because time to ignition was strongly associated with LFM, it is suggested that drought may cause increased ignitions which lead to more frequent fires. Extrapolating our results from leaf to landscape level requires some caution. Further studies using a wider variety of chaparral species may help in defining these relationships better and determining if a critical threshold exists.