Vegetation Structure and Environmental Influences on Carbon Stocks in Lower Montane Evergreen Forest in Community Forests, Mae Hong Son Province

Background and Objectives: Lower montane forest (LMF) is indispensable in mountain ecosystems; it was important for carbon sequestration in forest ecosystems and mostly found that in northern Thailand. The areas where this forest type is located in the national forest reserve, are often found that inhabited by hill tribe communities, making the lower montane forest vulnerable to encroachment and destruction. Therefore, to prevent this problem, the community forests have been established to enable local communities to help protect these forest types. Despite several studies on the vegetation structure and carbon sequestration in lower montane forest but mostly focused on the conservation area. This leads to a lack of data in national forest reserves, especially community forests. As well as the study of the limitation’s factors of carbon storage in such a plant community is also limited. Therefore, this study aims to investigate the vegetation structure characteristics, carbon sequestration, and environmental factors that affect carbon sequestration in lower montane forest areas that are managed as community forests. Methodology: This study was conducted in the lower montane forest located in community forests of Ban La-Oob, Ban Dong Kao, Ban Dong Mai, and Ban Tun, at Huai Hom Subdistrict, Mae Lan Noi District, Mae Hong Son Province. Twelve - 0.16 ha (40 x 40 meters) of purposive sampling plots were established to cover the community forest area of four villages. The diameter at breast height (DBH) and total height of all trees within the DBH ≥ 4.5 cm in the sample plots were measured. Cluster analysis was performed to classified sub-communities. Plant community characteristics were analyzed for each sub-community, including diversity index (H′), density (D), basal area (BA), and importance value index (IVI). We also analyzed biomass, carbon sequestration, and carbon dioxide sequestration of each sub-community. Furthermore, the relationship between carbon sequestration of lower montane forest with the factors of topographical as elevation above mean sea level, aspect, and slope, moreover the plant community characteristics as BA, D, H′ and number of species were analyzed using a Generalized Linear Mixed Model (GLMM) with the “lme4 package” and “MuMin package”, respectively, in R version 4.2.2. Main Results: We found that a total of 138 tree species, 99 genera, and 54 families were founded including 2,548 trees. The tree density and basal area were 1,327 trees ha-1 and 29.93 m2 ha-1, respectively. The Shannon-Wiener index (H′) was high (H′=4.18). Plant communities can be divided into four sub-communities, three of which are secondary forest which dominated by Alstonia glaucescens (LMF-ALGL), Eurya nitida (LMF-EUNI) and Ostodes paniculate (LMF-OSPA), and one sub-community of primary forest of lower montane forest dominated by Castanopsis diversifolia (LMF-CADI). The forest structure and carbon sequestration varied among sub-community types. First, the sub-community of lower montane forest dominated by Alstonia glaucescens (LMF-ALGL) showed that 76 species 65 genera and 40 families were found. The Shannon-Wiener index (H′) was 3.64 with tree density and basal area were 2,014.58 trees ha-1 and 45 m2 ha-1, respectively. The biomass, carbon sequestration, and carbon dioxide sequestration values were 238.48±34.45. t ha-1, 112.08±16.19 tC eq ha-1 and 410.98±59.36 tCo2eq ha-1, respectively. Second, the sub-community of lower montane forest with Eurya nitida (LMF-EUNI) showed that 65 species 54 genera and 33 families were found. The Shannon-Wiener index (H′) was 3.46 with tree density and basal area were 1,472.91 trees ha-1 and 25.91 m2 ha-1, respectively. The biomass, carbon sequestration, and carbon dioxide sequestration values were 175.57±41.30 t ha-1, 82.52±19.41 tC eq ha-1 and 302.57±71.18 tCo2eq ha-1, respectively. Third, the sub-community of lower montane forest with Ostodes paniculata (LMF-OSPA) showed that 73 species 62 genera and 37 families were found. The Shannon-Wiener index (H′) was 3.81 with tree density and basal area were 887.50 trees ha-1 and 28.41 m2 ha-1, respectively. The biomass, carbon sequestration, and carbon dioxide sequestration values were 266.99±28.50 t ha-1, 125.48±13.39 tC eq ha-1 and 460.12±49.11 tCo2eq ha-1, respectively. Fourth, the sub-community of lower montane forest with Castanopsis diversifolia (LMF-CADI) showed that 59 species 46 genera and 30 families were found. The Shannon-Wiener index (H′) was 3.36 with tree density and basal area showed that 1,229.16 trees ha-1 and 29.85 m2 ha-1, respectively. The biomass, carbon sequestration, and carbon dioxide sequestration are shown 245.63±16.02 t ha-1, 115.44±7.53 tC eq ha-1 and 423.30±27.61 tCo2eq ha-1, respectively. Furthermore, the carbon sequestration of the lower montane forest in the study areas had negatively significantly with the elevation above mean sea level (p < 0.001), but positively significantly with the slope (p<0.05) and basal area (p<0.001). Indicating the carbon sequestration of LMF is positively associated with high elevations, steep slopes, and large basal areas. Conclusion: This study indicates that community forests of Huai Hom subdistrict, Mae Hong Son Province, consist of secondary lower montane forest. These forests maintain high biodiversity and possess significant carbon sequestration capacity. However, alongside tree growth, topographic factors such as elevation and slope remained crucial determinants of carbon sequestration. Therefore, topography must be taken into consideration when managing lower montane community forests to enhance their carbon sequestration efficiency.