Fuel architecture drives shoot flammability

Abstract Background Understanding the relationship between plant traits and flammability is important for accurately predicting fire behavior and assessing fire hazard. However, plant flammability is multidimensional and multi-scale in nature, and it is not well understood how plant traits measured on leaf or partial crown scales influence crown fire behavior. In this study, we burned entire branches of 16 shrub species native to Texas and measured four shoot-level traits (total dry mass per 70 cm branch, shoot bulk density, leaf stem mass ratio, and shoot moisture content), and three leaf traits (leaf mass per area, leaf area, and leaf length). We aimed to answer two questions: (1) Are shoot-level traits or leaf traits better at predicting the key aspects of shoot flammability? (2) How well do shoot-level flammability measurements correspond to the flammability category rankings developed by the fire hazard categories listed in the National Fire Protection Association’s Firewise program? Results We found that shoot-level traits were more important than leaf-level traits in shoot flammability, with linear density (dry mass per 70 cm of branch length) and leaf stem mass ratio predicting flame duration, and linear density was the most important predictor of time to ignition. We also found that shoot flammability rankings based on flame duration and Firewise hazard categories aligned reasonably well, but not for time to ignition—species that burned longer tended to take longer to ignite. Conclusions Contrary to some studies, our study shows that shoot-level traits, not leaf traits, primarily influence shoot flammability. Therefore, conducting rigorous tests to determine whether lab-based shoot flammability can scale to stand-scale fire behavior is an important future step needed to build process-based fire behavior models for ecosystems prone to crown fires. Our study also emphasizes the importance of testing flammability in experiments because we found that current species-specific fire hazard designations offered to homeowners and managers are not consistent across Firewise groups, and species within some taxa are assigned similar hazard categories, despite exhibiting notable variation in shoot flammability.