The determinants of large-scale patterns of alpha and beta diversity are still poorly understood, especially as they relate to changes in diversity caused by historical and contemporary anthropogenic disturbance. Using the highly diverse understory plant communities found in longleaf pine savannas, we evaluate the contribution of historical and current land management practices in explaining alpha and beta diversity. Our sampling regime was conducted at 279 sites spread among three locations: Fort Bragg, NC, Fort Stewart, GA, and the Savanna River Site, SC. Locations spanned over 300km in the southeastern U.S. This unique combination of large-scale spatial extent and intensive within-location sampling provides a robust way to evaluate patterns of alpha and beta diversity. For each location, we stratified our sampling efforts across environmental and degradation gradients based on land use history (historical agricultural fields vs. remnant forests) and the frequency of prescribed fires (low vs. high fire return intervals). In addition to these categorical factors, we examined the potential role of several environmental covariates in affecting diversity (canopy cover and basal area, and soil moisture holding capacity, organic matter, and total N availability).
Results/Conclusions
Our work reveals a consistent, large-scale effect of historical agricultural land use: for all three locations, fewer species occurred in historical agricultural fields than in remnant forests (>10% reduction in richness). In contrast, the effect of fire management on species richness varied among locations. Beta diversity was shaped by the interaction of both historical land use and the frequency of prescribed fires. For example, at Fort Bragg, understory plant communities in remnant forests were most similar in species composition regardless of fire management practices. At Fort Stewart and the Savanna River Site, plant communities situated on historical agricultural fields were most similar and fire management had a greater influence on diversity patterns. Furthermore, the environmental factor that best predicted diversity patterns differed among locations: Fort Bragg (soil moisture holding capacity), Fort Stewart (soil organic matter), and the Savanna River Site (basal area). The complex interactions among geographic location, land use history, and environmental factors suggest that restoration strategies should vary according to the nature and degree of degradation.
