When properly managed, the longleaf pine (LLP) savanna ecosystem is among the most biologically diverse habitats in the world due to the high density of understory plant species. The ecosystem has played a vital role in building the economy and culture of the U.S. southeast over centuries, but anthropogenic influences, such as fire suppression, have decimated this ecosystem and its biodiversity, making restoration a priority. Here we determine key factors controlling the variability in biodiversity and understory dynamics in North Carolina (NC) LLP and address the potential for dominant grass species to structure these communities and drive biodiversity. Once the factors influencing the understory are recognized, the management and health of these lands can improve. To address these objectives, 15 study sites were established in NC spanning from the Sandhills to the Outer Coastal Plain. At each site, abiotic factors (e.g. resource availability, elevation, etc.) potentially affecting the understory were documented. Each site was further divided into subplots where all species rooted within 1m2 quadrats were identified and their percent cover estimated, and all aboveground herbaceous biomass was clipped within nested quadrats to provide estimates of aboveground net primary productivity.
Results/Conclusions
Considerable variation exists across the 15 study plots in all biodiversity metrics, including mean richness which ranged from 2 to 11.8 per m2. The relative cover of the dominant grass species, Aristida stricta (wiregrass), also varied greatly within (0%-100%) and across plots (15%-85%), with a median of ~30% relative cover per subplot. Wiregrass was a significant driver of biodiversity at multiple spatial scales. The mere presence of this dominant grass has a positive effect on biodiversity, but with increasing abundance, there is a decrease in every biodiversity metric, with the ideal relative cover of wiregrass being 16% for enhancing and maintaining biodiversity at small scales. Likewise, as wiregrass abundance is a primary driver of ANPP, we found a unimodal richness-ANPP relationship, with peak richness occurring at ~275g/m2. If LLP savanna land owners and managers can implement this ideal relative cover of wiregrass in their restoration strategies, the health and function can be restored to this ecosystem, as well as the biodiversity. Our sampling suggests that the drivers of biodiversity are interactive and complex in LLP savannas, and more work needs to focus on the mechanisms behind species coexistence in this unique system.