Abstract: To provide theoretical support for the selection and promotion of Chinese fir (Cunninghamia lanceolata) families adapted to changing climate, we investigated how elevated atmospheric CO₂ concentration influenced photosynthetic carbon allocation in one-year-old seedlings of two Chinese fir families (NO.020 and NO.061). Using a ¹³C isotope pulse-labeling approach, we established three CO₂ concentration treatments, i.e. ambient (400±50 μmol·mol^-1), elevated (800±50 μmol·mol^-1), and super-elevated (1000±50 μmol·mol^-1), to trace the fixation and distribution of carbon across different plant tissues. We analyzed the differences of total carbon uptake, non-structural carbohydrate (NSC) dynamics (including soluble sugars and starch), and biomass allocation across treatments and families. Results showed that both families increased total ¹³C accumulation and NSC content with rising CO₂ levels, with the highest values being observed under the 1000 μmol·mol^-1 treatment (C1000). Family NO.020 exhibited rapid ¹³C assimilation within the first 5 days after labeling, particularly under C800 and C1000, followed by a decline over time, suggesting an initial burst of metabolic activity. In contrast, NO.061 displayed a more gradual but sustained increase in ¹³C accumulation, indicating a conservative carbon-use strategy. After 30 days, the aboveground soluble sugar content in NO.020 under C800 and C1000 decreased by 40% and 26.1%, respectively, whereas the aboveground starch content increased by 32.8% and 85.3% compared to ambient CO₂ (C400). In NO.020, root NSC levels consistently followed the order C1000>C800>C400. Under C1000, soluble sugar content in the aboveground tissues of NO.061 increased by 54.5% at 15 days post-treatment (P<0.05), while the starch content increased 10% compared to C400 at 30 days. Elevated CO₂ stimulated growth, as it increased biomass and plant height in both families. However, the two families had distinct adaptive strategies to elevated CO₂. NO.020 demonstrated a faster rate of carbon uptake and metabolism, accompanied by a greater translocation of carbon from aboveground tissues to roots, which promoted root development. In contrast, NO.061 maintained higher NSC reserves in aboveground parts and prioritized shoot growth, likely optimizing resource capture under enhanced carbon availability. These findings reveal distinct carbon allocation strategies among Chinese fir families, highlighting the significance of family-specific responses in adapting to elevated CO₂ and supporting climate-resilient reforestation.

Key words: elevated CO₂,  non-structural carbohydrate, carbon allocation, photosynthetic carbon, Cunninghamia lanceolata, isotope labeling