Abstract:

Objective: Taking natural mixed conifer-broadleaved forests in Changbai Mountains as the research object, this study proposes four estimation methods of stand dominant height and the corresponding calculation methods of site form (SF), compares the differences among various dominant height estimation methods, and analyzes the relationship between dominant height and stand factors in order to determine a reasonable estimation method for stand dominant height in mixed forests and to evaluate the applicability of site form for assessing site quality. Method: Totally 127 sample plots of natural mixed conifer-broadleaved forests were investigated and analyzed at Bajiazi Forestry Bureau of Jilin Province. The height, diameter at breast height (DBH) and age of six dominant trees (three conifers and three broadleaves) were measured in each sample plot. Dominant heights were estimated using four different methods based on weighted averages, and these included the unweighted arithmetic mean height of all dominant trees (H_T1), the basal area-weighted mean height considering both conifer and broadleaf trees (H_T2), the basal area-weighted mean height considering only dominant tree species (H_T3), and the basal area-weighted mean height considering all species within a plot (H_T4). The corresponding mean DBH of dominant trees (D_T1, D_T2, D_T3, D_T4) and the mean age of dominant trees (A_T1, A_T2, A_T3, A_T4) for a plot were also calculated using four different methods. Through correlation analysis and paired t-tests, differences among the four methods of estimating dominant height were compared. The study further analyzed the relationships between these dominant heights (H_T1, H_T2, H_T3, H_T4) and stand density, productivity, mean DBH of dominant trees, and mean age of dominant trees. Power function, Hossfeld II, and Richards’ equations were selected as candidate models describing the mean height-DBH relationship of stand dominant trees. The final model was determined according to model evaluation indices. The reference diameter was the rounded value with most frequency of diameter distribution of dominant trees, and the site form was derivated using the height-diameter model and the reference diameter. By comparing the relationships between the site forms corresponding to different dominant heights and stand factors, the optimal methods for estimating dominant height and calculating site form in natural mixed conifer-broadleaved forests were determined. Result: Significant correlations were found among the four methods of dominant height estimation. H_T1, H_T2, and H_T3 showed high correlation coefficients above 0.96, with no significant differences. However, H_T4 demonstrated the lowest correlation with the other three methods and there was a significant difference between H_T4 and the other three methods. The basal area-weighted mean height considering all tree species within a plot H_T4 showed no significant correlation with three stand density indicators (stand basal area, stem number, additive stand density index). Furthermore, H_T4 and its corresponding diameter (D_T4) and age (A_T4) showed the highest correlation coefficients of 0.815 and 0.657, respectively. The correlation coefficients between H_T3 and its corresponding D_T3 and A_T3 were 0.420 and 0.227, respectively. The models for H_T3-D_T3 and H_T4-D_T4 were established using Power and Richards equations, respectively, with adjusted determination coefficients (R_a²) of 0.16 and 0.68. Based on the most frequent DBH of dominant trees in the sample plots, the reference DBH was set as 30 cm after rounding. The corresponding site forms, S_F,T3 and S_F,T4, were then determined using guide curves. Among the two site forms, only S_F,T4 showed no correlations with additive stand density index, while both S_F,T3 and S_F,T4 were significantly related to stand productivity, with correlation coefficients of 0.224 and 0.264, respectively. Conclusion: The basal area-weighted mean height considering all tree species within a plot (H_T4) performed the best, and the site form (S_F,T4) calculated from the dominant height showed significant correlations with site productivity, making it suitable for assessing site quality in natural mixed conifer-broadleaved forests.

Key words: stand dominant height, site form, natural mixed conifer-broadleaved forest, site productivity, site quality assessment