Disturbance Gradients Reshape Biodiversity, Stand Structure, and Regeneration Dynamics in Sal (Shorea robusta) Forests of Dharmapur National Park, Bangladesh

Disturbance gradients strongly influence biodiversity, stand structure, and regeneration dynamics in Shorea robusta (Sal) forests. This study provides the first empirical assessment of such impacts in Dharmapur National Park, Bangladesh, one of the last remaining Sal Forest remnants in the region. Despite its ecological importance, the disturbance ecology of Dharmapur has remained unexamined. To quantify disturbance effects, 105 plots (20 × 20 m) were established across low (LD), moderate (MD), and high disturbance (HD) categories, classified using a composite Disturbance Index. Within each plot, trees (DBH ≥ 5 cm), saplings, and seedlings were inventoried, and diversity, structure, regeneration, and community composition were analyzed using diversity indices, non metric multidimensional scaling (NMDS), and PERMANOVA. A total of 39 tree species (29 genera, 20 families) were recorded. Species richness declined from 35 species in LD plots to 17 in HD plots, while Shannon diversity decreased from 2.36 to 2.12 (p < 0.001). Basal area dropped from 25.7 to 13.4 m² ha⁻¹ and tree density from 802 to 397 individuals ha⁻¹ between LD and HD plots. Regeneration of S. robusta remained relatively strong, but late successional and shade tolerant associates declined sharply. NMDS ordination (stress = 0.14) revealed significant compositional turnover, with HD plots dominated by S. robusta and disturbance tolerant taxa. Overall, disturbance gradients in Dharmapur’s Sal forests reduce diversity, simplify structure, and constrain regeneration of non Sal species, emphasizing the urgent need for disturbance sensitive management and restoration strategies.