Form, physiology, and fire response explain key dimensions of litter flammability in fire-adapted longleaf pine savanna.

With fire frequency predicted to increase globally, a more refined understanding of flammability, including the traits that drive its variation, will be critical to better predict postfire vegetation responses. Pausas, Keeley, & Schwilk recently proposed a multidimensional framework comprising three plant strategies for responding to fire: nonflammable, fast-flammable, and hot-flammable species. However, empirical data are still needed to validate this framework. We measured six flammability-related traits on over 2000 leaf samples from the fire-prone longleaf pine-wiregrass ecosystem. We assessed whether traits group into 'dimensions' of flammability and attempt to explain variation in these dimensions using species' growth form, leaf physiology, and population response to fire. We also investigated whether these dimensions were phylogenetically conserved. Individual samples and species were distributed continuously across three key axes (heat release, flame spread rate, and ignitability) and clustered into functional groups. Further, the axis of flammability related to flame spread rate exhibited a phylogenetic signal. A different combination of growth form, percent leaf carbon, and population response to fire best explained each axis. Here, we identify three dimensions of flammability in our system, providing support for the Pausas, Keeley, and Schwilk framework.