Flammability and fire feedbacks across North American woodlands and forests
Fire and fuels interact in fire-prone landscapes. Fires affect community composition and structure via differential mortality. Fuels (senesced and living plants) affect fire behavior as a function of interspecific differences in flammability. The dampening or accentuation of fire behavior and effects via interspecific variation in flammability has been linked to feedback models of vegetation dynamics and community stability. With frequent fire communities are maintained by negative feedbacks caused by the persistence of more flammable species. In the absence of fire, communities become destabilized through the positive feedback mechanisms caused by the presence of less flammable species. Isolated cases of this phenomenon have been reported, but no synthesis or cross-ecosystem analysis has been done over species-change gradients. We used laboratory flammability studies across 12 communities and 45 dominant species across fire-prone landscapes of the US to measure the direction (positive or negative) and magnitude of changes in community flammability.
Species differ widely in their relative flammability. Within ecosystems, we found a consistent trend in decreased community flammability across successional gradients. Evidence for dampened flammability and positive feedbacks were found in Quercus garryana and Q. kelloggii to Pseudotsuga transitions, western Pinus to other conifer (Abies, Thuja, Picea) transitions, eastern Pinus-Quercus to mixed hardwood and Acer rubrum transitions, Sequoia sempervirens to Pseudotsuga or Tsuga heterophylla transitions. The magnitude of these changes in community flammability ranged from subtle to orders of magnitude. In one case, caused by a shift in composition due to a pathogen, a negative feedback was found from Torreya taxifolia and Taxus floridana to Liquidambar, Pinus, and Fagus grandifolia transitions. Apparent mechanisms for these changes are related to leaf traits that affect drying rates, ignition, and fire intensity. Just how widespread these phenomena are is not known, but shifts in community composition in changing landscapes will surely reveal other examples. The ability to overcome these state transitions will be a challenge for the restoration and management of these communities over time.