Estimating ecosystem-scale non-structural carbon dynamics in a shortleaf pine forest

Plants allocate photosynthetically fixed carbon (C) between metabolic needs, biomass production (BP), and non-structural carbon (NSC), a buffer between carbon supply and demand. While conceptual representation of alternative regulatory mechanisms of NSC has advanced significantly in recent decades, quantitative assessments of NSC's contribution to the ecosystem C budget have not been possible. We estimated C flux to or from the NSC pool (F NSC ) as the difference between gross primary productivity (GPP), plant respiration (R p ), and BP in a shortleaf pine forest. Annual GPP remained relatively stable, but F NSC varied during the 3-year study period: 77-350 g C m⁻² yr⁻¹ in dry years (2022-2023) and 41 g C m⁻² yr⁻¹ in the wet year (2024), representing 2-18% of annual GPP. In contrast, C allocation to BP was ∼43% higher in the wet year. Flux-based F NSC (37 ± 84 g C m⁻² yr⁻¹) closely matched F NSC estimates from monthly tissue NSC content (65 ± 77 g C m⁻² yr⁻¹) during 2024 (Feb-Dec). F NSC was inversely related to BP, Biomass Production Efficiency, and plant water status. These findings are consistent with the surplus carbon hypothesis, which posits sink strength-driven C allocation, and can serve as the conceptual foundation for next-generation carbon allocation algorithms.