The Stand Structure, Species Diversity, and Carbon of Mendawak Protected Forest, West Kalimantan-Indonesia

• Mendawak Protected Forest is a key part of Indonesia's peat swamp ecosystems in West Kalimantan. • Few species such Calophyllum sp., Dactylocladus stenostachys, Xerospermum noronhianum , and Shorea pachyphylla retain as threaten in peatland forests. • Diverse communities with priority species highlight large trees' role in biomass /carbon and sustainability in Mendawak Protected Forest • Large trees drive 122.7 Mg ha -1 control biomass/carbon stocks, vital data for climate mitigation • Minimal pole contribution (<10 Mg ha -1 ) underscores urgency for pole regeneration needs focus Peat swamp forests are an important ecosystem located in the Mendawak Protected Forest in West Kalimantan. All scientific names of the observed types were standardized using the World Flora Online database through the WorldFlora R package, and the dominant types in the sampling plots used the Importance Value Index (IVI). The results of the study showed that the species were dominant throughout the growth stage, starting from early-stage and sapling species to important species, namely Calophyllum sp., Xerospermum noronhianum , and Lithocarpus bennettii at the pole and tree levels. IVI analysis showed that a small number of species especially Calophyllum sp., Dactylocladus stenostachys, X. noronhianum , and Shorea pachyphylla played a dominant role in shaping forest structures through high base planes and trunk dominance in the DBH class of 20 cm ≥. A diverse tree community that includes species with high conservation value, this reflects that the presence of large-diameter trees is important for maintaining stand biomass, forest structure, and forest ecological sustainability. Also, above-ground biomass and its carbon stock are dominated by tree layers (≥ DBH 20 cm) in all plots, where large-diameter trees contribute more than 70-90% to the total AGB, while pole level vegetation contributes only a small biomass (<10 Mg ha⁻¹). The presence of biomass and carbon storage shows a clear positive relationship, highlighting the important role of forest structure and maturity in regulating carbon sequestration, with the average AGB across sites being around 122.7 Mg ha⁻¹. These findings provide concrete evidence for management practices in the future sustainability of Indonesia’s peat swamp forests.