Abstract: Cunninghamia lanceolata is the primary timber species in artificial forests in southern China. Increasing the large-diameter timber yield of this species is crucial for achieving high-quality development of artificial forests. We focused on close-to-nature stands of C. lanceolata-Phoebe bournei mixed forests with varying mixing ratios in Linwu County, Hunan Province, and constructed a structural equation model to analyze the mechanisms by which stand growth, spatial structure, and understory vegetation diversity influencing the yield of large-diameter timber. The results showed that the model exhibited a good fit (R²=0.729). Diameter at breast height of C. lanceolata was the key direct factor affecting the yield of large-diameter timber (path coefficient=0.82, P<0.001), and crown width (path coefficient=0.36, P<0.05) and moderate competition intensity (represented by Hegyi competition index, path coefficient=0.33) had positive effects. In contrast, retained density of C. lanceolata (path coefficient=-1.19, P<0.001), tree height structure heterogeneity (represented by Gini coefficient, path coefficient=-0.45), and shrub layer diversity (Simpson index, path coefficient=-0.34, P<0.05) indirectly reduced the large-diameter timber yield by limiting the increases of diameter at breast height. The diversity and biomass of the herbaceous layer (path coefficient=0.16) exhibited positive indirect effects. In conclusion, regulating retained density and spatial structure of C. lanceolata (e.g., reducing density and size ratio), promoting radial growth of retained trees, and maintaining a balanced understory composition (e.g., promoting herb layer development and controlling shrub layer) are key strategies for enhancing the yield of large-diameter C. lanceolata timber in close-to-nature stands of C. lanceolata-P. bournei mixed plantations.

Key words: Cunninghamia lanceolata, large-diameter timber, structural equation model, spatial structure, understory vegetation, close-to-nature silviculture