Over the past 30 years, the restoration of presumed historical fire regimes on over 3 million ha in the southeastern USA has yielded a landscape dotted with fire-maintained savanna fragments. Within-site studies have shown that fire-maintained savannas contain higher plant diversity than fire-suppressed shrublands, but little is known about their functioning, or whether they will be resilient in the face of future change. Species richness within functional groups is thought to be a primary contributor to ecosystem resilience, because functional redundancy may allow ecological functions to continue being performed even as some species are lost. The purpose of this study was to explore drivers of functional group composition on a broad ecoregional sample of fire-maintained savanna fragments. Our focus was on sites with little to no history of anthropogenic disturbance other than fire suppression (e.g. old growth savannas). We used nested quadrats to sample the understories of 30 frequently-burned savannas in Florida and Georgia, on preserves ranging in size from 50 to 230,000 ha. The understory data are being analyzed in conjunction with spatial and site data to identify patterns and relationships among species-level (i.e. functional type), site-level (i.e. fire history), and landscape-level (i.e. preserve size) variables.
Results/Conclusions
Species richness, both overall and within functional groups, was highly variable between sites. Overall species richness ranged from 36 to 119 species/site. Species richness of N-fixing legumes ranged from 0 to 13 species/site, while C4 grass species richness ranged from 3 to 17 species/site, and species richness of mast-producing shrubs/vines ranged from 6 to 28 species/site. We used three different nestedness metrics (Matrix Temperature, Brualdi-Sanderson Discrepancy, and NODF) to explore species distribution patterns across sites, and found that N-fixing legume, C4 grass, and mast-bearing shrub/vine communities were strongly nested according to all metrics, both within soil types and across all soil types. Functional group compositions on lower-diversity sites therefore represent nested subsets of those on higher-diversity sites, a pattern that may be due either to underlying ecological drivers or mass effects. Ongoing analyses include additional functional groupings (such as seed dispersal method), and analysis techniques (non-metric multidimensional scaling, stepwise multiple linear regression, species-area curves, and hierarchical clustering) to identify underlying relationships between site/landscape variables and species distribution patterns. Our preliminary results suggest that frequently-burned old growth savanna fragments may vary greatly in their resilience, despite having similar management regimes.