Objective To explore the adaptive changes in tree structure, leaf photosynthesis, and biomass allocation patterns of Phoebe bournei underplanted in the stands of different densities of Chinese fir plantations, and to provide scientific basis for precise transformation and improvement of the quality of near-mature Chinese fir.

Methods Based on 21-year-old Chinese fir plantations with four thinning treatments with residual densities of 375 trees·hm⁻², 570 trees·hm⁻², 630 trees·hm⁻², and 810 trees·hm⁻² 900 trees·hm⁻² of P. bournei were underplanted in the Chinese fir stands to form a multilayer-mixed forests. The differences in growth indices, such as crown shape, side branch length, branching angle, leaf morphology, chlorophyll fluorescence parameters, SPAD value, and root, stem, and leaf biomass of various organs, were analyzed to explore the effects of residual stand density of Chinese fir on the growth patterns of P. bournei.

Results In the stands of Chinese fir plantation with densities 375 trees·hm⁻² and 570 tree·hm⁻² , the lengths of secondary side branches, the densities with first and secondary side branches of P. bournei were significantly larger than stands with 810 trees·hm⁻² density. Leaf area and specific leaf area of P. bournei. leaves showed an increase with increasing density of Chinese fir. In the stand with density 375 trees·hm⁻² of Chinese fir plantation, leaf area, specific leaf area of P. bournei were significantly smaller than 810 trees·hm⁻² density treatments, with a difference of 80.97 cm² and 10.43 cm²·g⁻¹. In the stand with density 810 trees·hm⁻² of Chinese fir plantation, Chlorophyll fluorescence parameters and SPAD values of P. bournei were higher than 375 trees·hm⁻² density treatments. Low stand density(375 trees·hm⁻² and 570 trees·hm⁻²), lower canopy foliage total leaf biomass, and lateral and stem biomass of P. bournei were significantly higher than those of stand with 810 tree·hm⁻².In the stand with density 375 trees·hm⁻², lateral branch biomass was increased by increasing tree height, while in the stand with density 810 tree·hm⁻² increased leaf biomass accumulation by decreasing the investment in plant height.

Conclusion In the stand with density 375 tree·hm⁻² and 570 tree·hm⁻² of Chinese fir plantation, sufficient understory light is conducive to the biomass accumulation and lateral branch growth of P. bournei, and the growth of P. bournei is better In the stand with densities 630 tree·hm⁻² and 810 tree·hm⁻², P. bournei can enhance its light capture ability by promoting the length of leaves, increasing leaf area, increasing leaf SPAD value, and improving photosynthetic electron transfer rate and light energy capture efficiency, thereby adapting to the living environment with insufficient light resources.