Recurrent fires are strong filters in terrestrial plant communities. Predictable fire regimes filter for plant species with life histories that enhance persistence and filter out species that are unable to successfully survive or recruit under those regimes. In pine savannas of the North American Coastal Plain, communities might have historically been filtered by fire regimes with a predictable seasonality that followed climatic cycles. More recently, increasing human influence has changed fire regime seasonality, which is of concern in this highly biodiverse ecosystem. We tested the hypothesis that fires occurring consistently during different seasons result in divergent pine savanna community composition. We expected that pine savanna communities burned in different seasons over time would reflect the cumulative effect of the fire filter on survival and recruitment. We examined the interactive effects of season and sampling year in wet-mesic (flatwoods) and dry (sandhills) pine savannas in north Florida after eight years and again after an additional fifteen years of biennial prescribed fires in different seasons. We sampled species frequencies at the 24 m2 scale, and classified fire seasonality as: (1) phenological (occurring in the dormant or growing season), and (2) wet/dry (occurring during the early dry, mid-dry, late dry, or wet season).
Results/Conclusions
We found little evidence that fire seasonality was a significant filter on species composition. Based on species frequencies, neither sandhills nor flatwoods species composition differed significantly between seasons but did differ between sampling years. There was a significant interactive effect of wet/dry season and sampling year on species composition in both environments, but a significant interactive effect of phenological season and sampling year in sandhills only. There were no significant differences in post-hoc comparisons between seasons. Subtle changes likely drove the interactive effect of dry/wet fire season and sampling year in both environments, with communities burned in some seasons changing slightly more over time than others. When seasons were defined more generally (i.e., phenological), we found some evidence that sandhill communities tended towards compositional convergence. Many pine savanna plant species are long-lived perennials; if recruitment from seed is rare, changes are likely very gradual or linked to relatively infrequent events that open space and reduce competition in the groundcover. Fire regimes with diverse seasonality should maintain diversity over time as well as fires during a single, consistent season. On long time scales, large-scale interactions between climate and fire are likely stronger drivers of community change.