Modeling Stand Density Index for Sustainable Silviculture of Kayu Bawang (Azadirachta excelsa (Jack) Jacobs) on Community Forest in Bengkulu Province, Indonesia

Kayu Bawang (Azadirachta excelsa (Jack) Jacobs) is a fast-growing and economically valuable tree species widely cultivated in community-managed forests in Bengkulu Province, Indonesia. Despite its importance, stand density management tools tailored to this species are scarce, leading to suboptimal productivity and unsystematic harvesting practices. This study aims to develop species-specific silvicultural models to regulate stand density effectively, including growth models, stand density–diameter relationships, and a Stand Density Index for sustainable management of Azadirachta excelsa plantations. Data was collected from 45 permanent sample plots established across five districts in Bengkulu Province, covering a range of elevations and silvicultural systems. Tree diameter at breast height and total height were measured periodically over several years. Growth models for quadratic mean diameter, stand height, and stand volume were developed using nonlinear regression analysis. Logarithmic regression was applied to describe the relationship between stand density and quadratic mean diameter. The stand density index was calculated using a reference diameter of 25 centimeters. The results showed the growth equation to predict of quadratic mean diameter (QMD), stand height (SH) and stand volume (SV), were QMD = 78.76 * exp (-exp (0.864-0.062 * A)), SH = 35.40 * exp (-exp (0.555-0.108 * A)), and SV = 7.93 * exp (-exp (1,725-0,060 * A)), respectively. The quadratic mean diameter (equation at the maximum stand density was ln N = 11.72 - 1.68 ln QMD (R² = 98.3%). Based on this formula, the maximum stand density index of A. exelsa was 551, which showed the maximum number of trees per hectare with the diameter reference was 25 cm. The actual density index (SDI) assessment of A. exelsa can be estimated from the equation SDI = N (25/QMD)^-1.68.