Fire and vegetation have a complex feedback in fire-frequented ecosystems. Fire intensity filters for plant species that can survive and recruit after fire. Those species that are able to persist provide fuel for subsequent fires, thereby altering local fire intensity. In fire-frequented southeastern U.S.A. pine savannas, local variation in fuel loading has been shown to alter plant communities. We tested the relative importance of fuel loading in addition to other interacting environmental factors in filtering the composition of understory plant species in pine-dominated habitats that varied in fire history: long-unburned (>20 years) and infrequently- (3-5-year fire intervals) or frequently-burned (1-2 year fire intervals). To determine fire-vegetation relationships, we counted aboveground stems and estimated percent cover of understory vegetation 1- and 2-years post-fire and measured environmental variables, including fuel loading, fire intensity (i.e., temperature and resident time), soil characteristics, and canopy openness during and after fires in both dry and mesic pine savannas. Within the three fire frequencies and two savanna types, we haphazardly selected trees and arranged plots to test whether fire intensity and vegetation differed under tree driplines (i.e., where pine needles increase fuel loads) as compared to inside driplines where duff often accumulates, especially in long unburned sites.
Results/Conclusions
Plant community composition and species richness differed between and within the mesic and dry pine savannas. Mesic savannas had higher species richness overall with the frequently- and infrequently-burned sites dominated by diverse flowering perennials and Serenoa repens, respectively. Dry savannas had sparser vegetation (i.e., fewer stems per plot), often with patches of bare sand and were dominated by bunchgrasses and shrubby oaks (mainly Quercus laevis). In both pine savanna types, we found an interaction between fire frequency and tree dripline location 1-year post-fire on plant species richness. Frequently burned sites had significantly more species than long unburned sites, but only under driplines. A single fire in long-unburned sites after many years of fire suppression had no effect on plant species richness. In contrast, frequently burned sites gained species 1-year post-fire, especially under tree driplines, but those differences decreased 2-years post-fire. Both soil heating and canopy openness differed significantly under and inside tree driplines, which could be among the mechanisms driving differences in plant communities. Variation in fire intensity and canopy openness resulting from the spatial distribution of pine trees, in addition to other environmental variables, likely contribute to maintaining high biodiversity in pine savanna ecosystems.