How well do fire-related plant traits predict species pattern and fire behavior along a hydrologic gradient?
In fire-adapted systems, fire behavior can be strongly controlled by plant species composition because of the variability in flammability across species. Conversely, fire behavior can also drive species composition. To better understand the feedbacks between local fire behavior and community composition at a small spatial scale, our research investigates the distribution of fire-related plant traits as a response to fire intensity along soil moisture gradients. In other words, is the flammability of species a response to where they occur and the temperature of fires? Our research was performed along 32 moisture gradients in the Sandhills of Fort Bragg, North Carolina where there are abrupt transitions from upland longleaf pine (Pinus palustris) savanna to streamhead pocosin that is separated by a distinct shrub-dominated ecotone. We conducted vegetative surveys and collected live leaf samples to perform single species litter burn-trials for the most abundant species at known positions along the gradient. Using Principle Component Analysis, a series of fire-related plant traits (e.g. sustainability and combustibility) from the burn trials were chosen to explore interactions between species distribution, community-weighted fire traits, and fire temperature along the moisture gradient.
We found significant relationships between fire-related plant traits and the average position of 25 species along the moisture gradient. In particular, we found positive relationships between the average soil moisture of plots in which species were found and maximum temperature. Furthermore, we found negative relationships between the soil moisture of a plot and its community-weighted means of burn time and smolder time. In addition, community-weighted burn time showed a strong positive relationship with fire temperature measured from on-site prescribed burns. Generally speaking, these results demonstrate that individual species sort along the hydrologic gradient based on their fire traits, although these patterns weaken at the community scale and that community-level traits can predict fire behavior. Our results exemplify the need for further investigation of additional fire-related plant traits (e.g. twig dry matter content, branching and height) as predictors of fire behavior in this fire-adapted system.