Objective Estimating the potential productivity of stand basal area and volume using the potential mean annual increment method, and provide a scientific basis for the site quality evaluation of Larix olgensis plantations.

Method The dominant height growth model of Larix olgensis plantations was developed using the generalized algebraic difference method, based on the permanent sample plot data from the national forest inventory and the forest management unit-level inventory in Jilin Province. Site grades were ranked according to the site index. The growth models of basal area and volume, incorporating site grades as dummy variables, were constructed, and the optimal stand density index was searched using the golden section method. The corresponding potential productivity was calculated and verified by the self-thinning line. Realized productivity was calculated from the actual stand age and stand density index. The productivity improvement gap was comprehensively evaluated by combining the difference between realized and potential productivity.

Result (1) The models of dominant height, basal area and volume growth of Larix olgensis plantations showed good performance. The adjusted coefficient of determination ( R_\mathrma^2) values were 0.877, 0.985 and 0.966. The root mean square error (RMSE) values were 1.037 m, 0.886 m²/(ha·year) and 9.756 m³/(ha·year), while the relative RMSE (rRMSE) values were 7.353%, 6.588% and 11.890%. The average site index values for the five site grades were 19.8 m, 17.8 m, 15.7 m, 13.5 m and 11.6 m, respectively. (2) For the same stand age, the optimal stand density index differed for basal area and volume potential productivity. At the base age of 30 years, the potential productivity for stand basal area ranged from 0.861 to 0.941 m²/(ha·year), while the potential productivity for volume ranged from 5.319 to 8.775 m³/(ha·year). The productivity gap between realized and potential productivity was larger in young and middle-aged stands compared to near-mature, mature and overmature stands, indicating a greater potential for productivity improvement. (3) During stand growth, the optimal stand density corresponding to potential productivity was always less than or equal to the maximum density determined by self-thinning process.

Conclusion The potential productivity calculated after constraining the initial stand density was more realistic, further validating the feasibility of the site quality evaluation method based on potential mean annual increment. During the young and middle-aged stages, the realized productivity of Larix olgensis differed significantly from its potential productivity, with the difference increasing as site quality declined. The results provided the optimal stand density corresponding to potential productivity, offering a basis for the tending operations and management of Larix olgensis plantations.