Plant fuels play a central role in Longleaf Pine Savannas due to their importance in shaping fire characteristics related to fire severity. Since fire characteristics are largely a function of plant fuel load, processes that modify plant fuels may affect future fires. One such process is microbial decomposition, which directly controls the amount of plant fuels available. Despite strong potential to alter future fire characteristics, microbial mortality increases during severe fires, which could slow post-fire decomposition if surviving microbes are poor decomposers or their ability to breakdown new fuels is reduced. We hypothesized that increasingly severe fires would slow microbial decomposition of plant fuels and indirectly increase the build-up of new plant fuels. In an old-growth Longleaf Pine Savanna, we established six fuel manipulation treatments that mimicked increasing levels of fire severity, and measured microbial decomposition the year following following fire. We also quantified soil nutrient availability, fire characteristics, and proximity to Longleaf pines (P. palustris) to assess their roles in these processes. We used linear mixed effect regressions (LMERs) and structural equation modeling (SEM) to assess the effect of and mechanisms through which severity associated fire characteristics modified microbial decomposition.
Results/Conclusions
Increasingly severe fires were negatively associated with microbial decomposition, however it did not appear that this was due to differential effects of fire on different microbial groups (i.e. bacteria and fungi). Furthermore, our final SEM model supported our hypothesis, with clear relationships between microbial decomposition and severity associated fire characteristics. Specifically, more severe fires were negatively associated with microbial decomposition and higher nutrient availability was linked to increased decomposition. Surprisingly however, there was no relationship between fire severity and nutrient availability. Furthermore, sites near Longleaf pines, where pine need density was highest, had higher decomposition early after fires and slower decomposition at later points when compared with adjacent grassland savanna areas. This suggests that increasingly severe fires and recalcitrant plant fuels slow microbial decomposition following fires, and may lead to an accumulation of new plant fuels. The increased plant fuel load in turn could favor higher severity future fires, and establish a positive feedback mechanism that favors the recurrent fires necessary to maintain Longleaf Pine Savannas and potentially other pyrophilic ecosystems.