Understanding flammability and bark thickness in the genus Pinus using a phylogenetic approach.

Pinus species dominate fire-prone ecosystems throughout the northern hemisphere. Their litter drive fires that control plant community flammability and multiple ecological processes. To better understand the patterns and mechanisms of pine flammability, we measured leaf characteristics (needle length and thickness) and conducted combustion experiments on litter from 31 species. We paired flammability results with bark accumulation data and used phylogenetic generalized least squares regression to examine relationships between physical traits and flammability. Pine flammability varied widely among pines: flame heights and fuel consumption varied three-fold, and flaming and smoldering durations varied three- to six-fold. Subgenus Pinus species were the most flammable and subgenus Strobus species had the lowest flammability. Needle length was the best predictor of flammability with a significant interaction with subgenus, suggesting that flammability of pines in subgenus Strobus was more affected by physical traits than pines in subgenus Pinus. Species in the subgenus Pinus that accumulated outer bark rapidly also had high flammability, while the relationship was not significant in subgenus Strobus. These results highlight the diverse patterns of flammability in North American pines and the complexity in the mechanisms causing differential flammability.