Forest Structure and Potential of Carbon Storage at Khao Nam Sab, Kasetsart University, Sri Racha Campus, Chonburi Province

Abstract

Background and Objectives: Tropical Forest ecosystems are globally significant for their roles in biodiversity conservation, climate regulation, and carbon sequestration. In Thailand, two predominant forest types, Dry evergreen forest (DEF) and mixed deciduous forest (MDF), are of high ecological value but differ in species composition, canopy structure, and successional status. DEF is characterized by multi-layered canopies and high species richness, while MDF often supports fast-growing, early-successional species and exhibits more open canopy structures. Khao Nam Sap Forest, located within the Kasetsart University Sriracha Campus in Chonburi Province, is a secondary forest landscape that has undergone natural regeneration following historical logging and land conversion. This study aimed to assess forest structure, species diversity, aboveground biomass, and carbon sequestration potential in both DEF and MDF areas. In addition, the study investigates how environmental variables such as elevation, soil pH, and organic matter influence tree community distribution. These findings contribute to understanding forest dynamics in secondary tropical forests and support Thailand’s national carbon neutrality policies.

Methodology: The study was conducted using a systematic line plot survey across Khao Nam Sap Forest, which spans approximately 54.4 hectares. A total of 51 vegetation plots, each measuring 30 × 30 meters, were established along transects distributed throughout the DEF and MDF zones. In each plot, all trees with a diameter at breast height (DBH) ≥ 4.5 cm were identified, measured, and geo-referenced. Soil samples were collected at 0–15 cm depth at five points per plot and analyzed for pH, texture, and organic matter. Biomass was estimated using species-specific allometric equations, and carbon stocks were calculated by applying a standard conversion factor (47% of biomass). Carbon dioxide sequestration was estimated based on the carbon content multiplied by the CO₂ conversion ratio (44/12). Cluster analysis and Canonical Correspondence Analysis (CCA) were used to classify vegetation communities and examine the influence of environmental gradients on species distribution.

Main Results: A total of 120 species belonging to 90 genera, 39 families were identified across the study area. The overall tree density was 1,264.49 individuals ha^-1, with a mean basal area of 14.64 m²·ha^-1. The Shannon-Wiener Diversity Index was calculated at 3.69, reflecting high species diversity. Dominant species identified based on the Importance Value Index (IVI) included: Terminalia nigrovenulosa (Combretaceae): a late-successional species dominating DEF with tall straight trunks and strong wood. Dialium cochinchinense (Fabaceae): a canopy species contributing significantly to biomass in both forest types. Diospyros coaetanea (Ebenaceae): a shade-tolerant species common in DEF, known for its dense wood. Vitex limonifolia (Lamiaceae), Atalantia monophylla (Rutaceae), and Hydnocarpus ilicifolia (Achariaceae): each with distinct ecological niches in both DEF and MDF zones. Cluster analysis based on species composition and forest structure separated the vegetation into 8 distinct forest communities at approximately 50% similarity. These were classified as: (1) Leucaena leucocephala – Atalantia monophylla community, (2) Diospyros castanea – Leucaena leucocephala community, (3) Vitex limonifolia – Leucaena leucocephala community, (4) Cratoxylum formosum – Vitex limonifolia community, (5) Lannea coromandelica – Vitex limonifolia community, (6) Walsura trichostemon – Briedelia ovata community, (7) Walsura trichostemon – Bixa orellana community, and (8) Hydnocarpus ilicifolia – Walsura trichostemon community respectively. These forest communities varied along gradients of elevation, canopy closure, and soil conditions. Canonical Correspondence Analysis (CCA) identified soil pH, elevation, and organic matter as the most significant environmental factors influencing species distribution. DEF communities were associated with moderately acidic soils (pH 5.7–7.2), height above mean sea level (above 150 m a.s.l.), and finer-textured soils. In contrast, MDF communities were linked to more alkaline, sandy soils with higher organic content, found in foothill areas and valleys. These findings support the hypothesis that species composition and forest structural attributes are strongly shaped by edaphic and topographic variability. The average total aboveground biomass was 612.70 t· ha^-1, comprising stem biomass (477.94 t·ha^-1), branch biomass (122.04 t·ha^-1), and leaf biomass (12.72 t·ha^-1). This corresponds to an average carbon stock of 287.97 tC ha^-1 and an estimated tCO₂-equivalent sequestration of 1,055.88 tCO₂·ha^-1. When analyzed by forest type, Dry Evergreen Forests exhibited significantly higher carbon storage and biomass than Mixed Deciduous Forests. Specifically: DEF plots had an average biomass of 445.06 t ha^-1 and stored 209.18 tC ha^-1 of carbon. MDF plots had significantly lower values, with 167.63 t ha^-1 of biomass and 78.79 tC ha^-1of carbon. A notable ecological concern is the widespread presence of Leucaena leucocephala, an invasive alien species dominating the forest edge, particularly in MDF zones. This fast-growing legume forms dense thickets that suppress native regeneration and may alter successional dynamics, reduce biodiversity, and diminish long-term carbon sequestration potential.

Conclusion: Khao Nam Sap Forest demonstrates considerable ecological potential, particularly within Dry Evergreen Forest areas, where structurally complex tree communities, high diversity, and substantial biomass contribute to robust carbon storage. The data highlight the crucial role of late-successional forests in maintaining ecosystem stability and resilience. Mixed Deciduous Forest areas, while less carbon-dense, offer diverse species assemblages and are valuable for understanding regeneration pathways in disturbed tropical landscapes. Integrating species data, biomass estimates, and environmental factors provides a comprehensive framework for forest restoration planning and management. These insights are vital for Thailand’s strategy toward achieving carbon neutrality, promoting forest-based climate solutions, and conserving native biodiversity under changing environmental conditions.