Influencing factors of large-diameter timber yield of <i>Cunninghamia lanceolata</i> under the close-to-nature silviculture stands mixed with <i>Phoebe bournei</i>.

<i>Cunninghamia lanceolata</i> 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 <i>C</i>. <i>lanceolata</i>-<i>Phoebe bournei</i> 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 (<i>R</i>²=0.729). Diameter at breast height of <i>C</i>. <i>lanceolata</i> was the key direct factor affecting the yield of large-diameter timber (path coefficient=0.82, <i>P</i><0.001), and crown width (path coefficient=0.36, <i>P</i><0.05) and moderate competition intensity (represented by Hegyi competition index, path coefficient=0.33) had positive effects. In contrast, retained density of <i>C</i>. <i>lanceolata</i> (path coefficient=-1.19, <i>P</i><0.001), tree height structure heterogeneity (represented by Gini coefficient, path coefficient=-0.45), and shrub layer diversity (Simpson index, path coefficient=-0.34, <i>P</i><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 <i>C</i>. <i>lanceolata</i> (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 <i>C</i>. <i>lanceolata</i> timber in close-to-nature stands of <i>C</i>. <i>lanceolata</i>-<i>P</i>. <i>bournei</i> mixed plantations.