Objective  To establish a scientific site quality evaluation system, guide forestry production practice.

Method Based on the number of major coniferous forest stand plots across the country, 16 coniferous tree groups were classified. The Richards, Logistic, and Korf models were applied to fit the guiding curves and establish a site form index model, which was then validated through a scatter plot test. Data from four consecutive forest inventory periods from 1999 to 2018 were used to analyze dynamic changes in site form levels.

Result The mean coefficients of determination for guiding curves fitted with the Richards, Logistic, and Korf models were all above 0.95. The scatter plot validation of the established site form index model showed test values exceeding 90.00%, with an average value of 96.59%, indicating its feasibility for practical use. Over 20 years, site quality for coniferous stands improved, with a combined proportion increase of 7.60 percent in grades I and II, a 3.50 percent decrease in grade III, and a combined reduction of 4.10 percent in grades IV and V.

Conclusion  Developing a national unified site quality assessment model based on DBH-height relationships is feasible and reasonable. This approach reduces the impact deviations of climate-induced and age-based growth rate on site quality assessment, allowing for comparable site quality evaluations across regions. The model demonstrates good applicability on a large scale, though it is essential to consider potential uncertainties due to management practices and limited sample data.