Evidence of Nitrogen and Phosphorus Limitation in Longleaf Pine Savanna Understories

Due to anthropogenic pressures, only 3% of the historic extent of the biodiverse longleaf pine ecosystem remains, much of which is degraded. Fire is necessary for maintaining longleaf pine savanna structure, function, and biodiversity; however, it also creates resource constraints, as nutrients are volatilized, especially in the already nutrient-depleted soils of many longleaf pine savannas. Nutrient limitation and subsequent competition between plants can lead to changes in species diversity and productivity. Using a multiyear, chronic nutrient addition experiment, we explore how resource limitation influences restoration outcomes in longleaf pine savannas by affecting (1) productivity, (2) biodiversity metrics, and (3) community composition. In the field, we established a factorial N and P nutrient addition (10 g m^-1 year^-1) experiment. Nutrient additions were administered for 4 years, and plant composition and biomass were collected yearly. We measured biomass each year by functional group and calculated diversity metrics and community composition changes. Understory productivity typically increased with N and P additions, with N × P together having no additive effect. There were several significant interacting effects of nutrient addition treatments with year on our biodiversity metrics; however, the main nutrient addition effects were not significant for any biodiversity metric. Finally, community composition was significantly different in nutrient addition plots compared to control. Our results show that xeric Sandhill longleaf pine savannas exhibit distinct responses to fertilization, as fertilization led to increased productivity of the groundcover without reducing biodiversity. Low-level, chronic nutrient inputs could influence understory structure in ways that help meet numerous management objectives, such as promoting fire spread or increasing forage availability. However, this may also present challenges, such as encouraging woody encroachment or threatening rare species. Our findings highlight the need for context-specific approaches to longleaf pine savanna management and a careful evaluation of how nutrient dynamics interact with long-term conservation and restoration goals.