OOS 36-3 - Linking leaf-scale flammability traits to landscape-scale patterns of wildfires: Findings from southern California chaparral shrublands

It is clear that the amount of moisture in living plant foliage is central to ecophysiological functioning under drought stress and also flammability characteristics. While there are increasing leaf-scale studies of pyro-ecophysiology, it is often challenging to link findings to patterns observed at the landscape-scale. Through analyses of hundreds of wildfires over several decades in chaparral shrublands, we have identified multiple thresholds in live fuel moisture (LFM) that appear to drive fire dynamics (e.g., in terms of numbers, sizes, and area burned). These thresholds break the fire season into three distinct domains, roughly corresponding to management ratings of low, moderate, and high fire danger. There is a relatively broad range of LFM values associated with the transition of low-to-moderate (i.e., averaging ~85%), and this is interpreted as an ignition-related threshold. A sharper LFM threshold (i.e., consistently ~60%) is observed at the moderate-to-high transition, which appears to relate to the point at which fires are able to spread much more aggressively. In conjunction with landscape-scale analyses, we have performed a suite of experimental tests on the foliage of 2 common chaparral species, to examine how well LFM explains different flammability characteristics. Findings vary by species, water status, and the type of flammability trait examined. However, water content is almost always an important driver, and thresholds identified in these leaf-scale tests translate roughly to the LFM transitions described above for the three domains of the fire season. Admittedly, these linkages ignore factors like the local weather, topography, and fuel amount/structure that influence individual fires. Even so, our early findings show that experimental flammability tests at fine scales may be helpful in understanding wildfire patterns at relatively broad scales, allowing for mechanistic explanations of how fire regimes operate.