Abstract: Chinese fir (Cunninghamia lanceolata) is a key resource in China’s timber industry. However, there are no site-specific cultivation guidelines for producing large-diameter trees and enhancing forest productivity and timber quality while addressing the complexities of Chinese tropical terrains. This study identifies key site factors using the Boruta algorithm and establishes diameter growth curves through K-means clustering and mixed-effects modeling to estimate how many years are required to reach target diameters. It is based on sample plot data from Chinese fir plantations across seven provinces in subtropical China. The results show that: (1) soil type and depth, and elevation have a strong influence on growth; (2) of the six basic growth models compared, the Korf model performed best, with the coefficient of determination (R²) of 0.6154. The predictive accuracy was significantly improved by introducing a mixed-effects model, which increased the R² value to 0.7535; (3) at a designated harvesting age of 26 years, diameters at breast height (DBH) were approximately 30, 27, and 26 cm under STG6, STG4, and STG8, respectively, all exceeding the large-size timber threshold (DBH ≥ 25 cm), demonstrating strong cultivation potential; and, (4) to attain a 25 cm DBH, STG6, STG4, and STG8 required 18, 21, and 24 years, respectively. Growth rate slowed significantly after 30 years, suggesting that the rotation period should be limited to within 30 years to optimize management efficiency. In this study, we developed a robust growth prediction model suitable for diverse site conditions and propose a site-specific management strategy that provides the basis for the precision cultivation and sustainable management of large-size Chinese fir plantations.

Key words: Site conditions, Chinese fir, Large-size tree, Mixed-effect model, Target diameter