Contrasting nitrogen-use strategies and photosynthetic carbon-water trade-offs in Pinus tabulaeformis plantations under different forest management modes.

INTRODUCTION: Forest management practices can alter canopy light environments and competitive intensities, thereby reshaping leaf functional traits and photosynthetic physiology. However, how different management modes coordinate leaf morphology, carbon gain, water use, and nitrogen utilization in coniferous plantations remains unclear.

METHODS: A field experiment was conducted in a Pinus tabulaeformis plantation under four management modes: structure-based forest management (SM), randomized forest management (RM), close-to-nature forest management (NM), and an unmanaged control (CK). Leaf morphological traits, gas exchange parameters, carbon and nitrogen biogeochemical traits, and nitrogen use efficiency were analyzed and compared.

RESULTS: All modes significantly decreased leaf water content (LWC), with the largest reduction (8.44%) under SM. Only SM significantly reduced specific leaf area (SLA) by 27.3%. Net photosynthetic rate (Pn), stomatal conductance (Gs), and transpiration rate (Tr) were significantly increased by management, particularly under SM and RM. RM achieved the highest Pn (+59% vs. CK), whereas SM increased Gs and Tr the most (2.38 and 2.07 times CK). All treatments decreased instantaneous water use efficiency (WUE) and stomatal limitation (Ls). SM and RM significantly increased total nitrogen (TN) but decreased C/N. Contrasting nitrogen-use patterns emerged: SM increased leaf nitrogen content per unit area (Na) to 1.55 times CK and elevated δ¹5N by 16.08%, suggesting nitrogen accumulation; RM increased photosynthetic nitrogen use efficiency (PNUE) by 28% without markedly increasing Na, suggesting more efficient nitrogen utilization. Correlation analysis further indicated that Pn was significantly positively correlated with TN and negatively with C/N, while Na was positively correlated with Pn, Gs, Tr, and δ¹³C, but negatively with LWC and SLA.

DISCUSSION: Different forest management modes exhibited contrasting nitrogen-use patterns and altered the carbon-water trade-off. RM achieved the highest photosynthetic rate and PNUE while avoiding excessive nitrogen accumulation, thereby appearing to strike a better balance between carbon gain and nitrogen economy. Therefore, RM appears promising for enhancing carbon fixation and nitrogen use efficiency in P. tabulaeformis plantations, though multi-year, multi-site validation is needed. This study provides early-stage mechanistic insights into trait-based forest management and highlights the importance of integrating carbon-water-nitrogen synergies, while acknowledging the need for further biochemical validation and broader spatial-temporal replication.