A relative density index for natural mixed forests dominated by Masson pine in southern China

Natural forests account for 63.55% of China’s total forest area and 80.14% of its total stock volume. Due to their complex species composition and structure, these forests are often managed using qualitative approaches, which might not optimally achieve diverse management objectives. Stand density significantly influences tree growth, mortality, and recruitment, thereby shaping forest structure and species composition. Therefore, stand density indices have been extensively developed to guide density management for pure or structurally simple forests. Unfortunately, however, these existing stand density indices often exhibit poor performance when applied to complex natural forests; furthermore, studies focusing on density assessment and optimization for such complex natural forests remain scarce in the Chinese mainland. Relative density indices (RDs) that quantify the degree of crowding within a stand relative to its maximum carrying capacity, have been widely produced for complex natural forests to provide objective baselines for density control, determining the exact timing and intensity of thinning operations. In this present study, we, therefore, developed a novel RD based on the negative relationship between maximum stand density and wood specific gravity using 512 sample plots from the Chinese National Forest Inventory for natural uneven-aged mixed-species forests dominated by Masson pine in southern China. The index is additive and robust, making it suitable for diverse species compositions and diameter distributions and enabling the precise stand density management of natural forests. The proposed RD is the first quantitative density metric for natural uneven-aged mixed-species forests in the Chinese mainland.