Can a fire-vegetation-microclimate feedback maintain community boundaries under simulated global change?
In pyrogenic communities, interactions between fire, vegetation, and microclimate (FVM) result in positive, fire-promoting feedbacks. Conversely, adjacent pyrophobic communities maintain their own positive, fire-hindering FVM feedbacks. The transition in flammability (i.e.flammable to inflammable) between these communities is controlled by the FVM, resulting in distinct boundaries and ecosystems. Therefore, as fire can only exert direct control on vegetation it actually burns, determining where fires stop between these communities is important. Vegetation structure and microclimate have previously been identified as determinants of fire spread between longleaf pine savanna (pyrogenic) and adjacent wetland (pyrophobic) in North Carolina, USA. While these community distributions appear to be relatively stable under recent conditions, the ability of FVM feedbacks to maintain boundaries under threats from global change have not been previously investigated. We developed a cellular automaton to simulate fire spread along longleaf pine – wetland ecotonal gradients, using data from 114 gradients comprised of 548 plots and three fire seasons. Future conditions were simulated by manipulating fire return intervals (FRI). We tracked the location of fire failure along the gradient to estimate boundary movement. We also analyzed the simulated vegetation structure and microclimate to quantify the contribution of these FVM components to fire spread sensitivity.
Once the alteration of a FRI reached a threshold value, it resulted in the movement of the fire failure location. In general, shorter FRIs resulted in fires that spread further into the wetland and longer FRIs resulted in fires that stopped nearer the savanna. Additionally, fire spread was more sensitive to vegetation structure than microclimate in this simulation. While the FVM feedback was able to withstand some measure of simulated global change, its ability to maintain community boundaries should not be considered absolute. Flammability transitions located within the ecotone are fundamentally what keep these communities distinct. Conservation and management of these systems are focused on fire, and the ability to predict the extent of fire spread under change should prove to be valuable.