Response of growth, photosynthetic characteristics and carbon metabolism of Pinus tabuliformis seedlings to long-term moderate warming.

Global climate warming profoundly affects the physiological metabolism and adaptive capacity of forest trees. However, the responses of photosynthetic and carbon metabolic traits in coniferous species to long-term experimental warming simulating future climate scenarios remain poorly understood. In the present study, 3-year-old Pinus tabuliformis seedlings were subjected to ambient air temperature (CK) and elevated temperature (ET, ambient +2 °C) in open-top chambers over two consecutive growing seasons. The growth parameters, photosynthetic gas exchange, photosynthetic pigment contents, photosynthetic enzyme activities, photosynthetic product accumulation, and carbon metabolism-related enzyme activities of P. tabuliformis seedlings were determined to explore the growth and photosynthetic physiological adaptive strategies under warming. The results showed that warming significantly increased needle length and needle relative water content (RWC), whereas plant height, basal diameter, and seedling biomass exhibited no significant treatment differences. Additionally, warming enhanced maximum carboxylation rate (Vcmax), maximum electron transport rate (Jmax), respiration rate (Rd), net photosynthetic rate (Pn), water use efficiency (WUE), and photosynthetic pigment contents. Sucrose, soluble sugar, and non-structural carbohydrate (NSC) concentrations were consistently enhanced under warming, accompanied by altered activities of sucrose synthase (SUS), acid invertase (AI), and neutral invertase (NI). Principal component and correlation analyses revealed close interrelationships among growth, gas exchange, photosynthetic pigments, and carbon metabolism under warming. These findings indicate that such physiological adjustments collectively strengthen seedling thermal adaptability under moderate warming, driven by coordinated changes in needle morphology, carbon assimilation, carbohydrate allocation, and related metabolic processes. This study improves our understanding of the physiological adaptation mechanisms of coniferous species to climate warming and provides a scientific reference for forest management under future climate scenarios.