Tree Diversity, Carbon Storage and Carbon Dioxide Sequestration Assessment in Dry Evergreen Forest and Mixed Deciduous Forest of Selected Botanical Gardens and Arboreta in Thailand

Abstract

Background and Objectives: The increase in greenhouse gases concentrations is a major driver of climate change, which in turn affects to forest ecosystems. Forest plays a role in the release and storage of carbon, thereby influencing global warming. However, forest ecosystems also mitigate climate change by absorbing carbon dioxide through various processes and serve as reservoirs of biodiversity for both flora and fauna. In Thailand, the forests mostly are tropical dry forests and forests can be categorized into two types; 1) deciduous forests (such as deciduous dipterocarp forest and mixed deciduous forest) and 2) evergreen forests (such as dry evergreen forest, montane forest and moist evergreen forest). These two types of forest are distributed throughout the country, with most found in protected areas. Botanical gardens and arboreta are classified as urban forest that there are people using regularly and serve as important sites in protected areas. It comprised many rare or economically valuable plant species and also serves as a biodiversity conservation area and a research site but less documents were found in species diversity and the potential of carbon stock and carbon dioxide (CO₂) sequestration. This study aimed to clarify species diversity in dry evergreen forest (DEF) and mixed deciduous forest (MDF) within selected botanical gardens and arboreta and study the potentials of botanical gardens and arboreta as carbon storage.

Methodology: A total of 241 concentric sample plots were established in 2024 with inter-plot spacing of either 250 m or 2.5 km, depending on the size of the botanical gardens and arboreta and are divided into 115 sample plots within dry evergreen forest, covering an area of 11.5 ha, and 126 sample plots within mixed deciduous forest, covering an area of 12.6 ha. Concentric sample plots with radius about 17.84 m were established. All trees with diameter breast height (DBH) >4.5 cm and height greater than 1.3 m were tagged, identified and mapped azimuth. The data was used to calculate aboveground biomass using allometry equations, and the resulting aboveground biomass were used to quantify belowground biomass, carbon storage and carbon dioxide (CO₂) sequestration.

Main Results: The results revealed that 19, 514 individuals were found. In DEF contained 10,139 individuals across 481 species with 268 genera and 78 family and the dominant species based on importance value index (IVI) was Dipterocarpus alatus with 15.54 and followed with Streblus asper, Afzelia xylocarpa, Eucalyptus camaldulensis and Delonix regia, with species diversity and species richness index was 4.96 and 52.80 respectively. While, MDF contained 9,375 individual of trees across 406 species with 243 genera and 77 family and the dominant species based on IVI was Tectona grandis with 19.84 and  followed with Pterocarpus macrocarpus, Xylia xylocarpa, Combretum quadrangulare and Streblus asper, with species diversity index and species richness index was 4.99 and 44.28 respectively. Carbon storage and carbon dioxide (CO₂) sequestration assessment were estimated using allometry equation. Biomass in DEF and MDF were 190.18 t/ha and 142.26 t/ha, respectively, Carbon storage in DEF and MDF were 89.38 tC/ha and 66.86 tC/ha, respectively and Carbon dioxide (CO₂) in DEF and MDF were 328.04 tCO₂/ha and 245.38 tCO₂/ha, respectively. Diameter class distribution in both communities exhibited a negative exponential pattern, indicating stable population structure. The high proportion of small-sized trees suggests strong regeneration potential, with many individuals likely to develop into mature trees in the future. Furthermore, in terms of carbon storage potential, larger trees (DBH > 30 cm) had the larger of carbon storage, even though there are less number of trees in the group. However, small to medium sized trees can still grow into mature trees in the future, thereby enhancing future carbon storage potential. A t-test was used to compare the mean of biomass, carbon storage and carbon dioxide (CO₂) sequestration indicated that two forest types is non-significant (p-value = 0.22) in potentials of carbon storage and carbon dioxide (CO₂) sequestration.

Conclusion: This study highlights the potential of botanical gardens and arboreta to function as large carbon storage area. This study indicates that many tree species within natural forests had high potentials of carbon storage and carbon dioxide (CO₂) sequestration. The results can serve as a valuable database for assessing carbon storage in botanical gardens and arboreta. Furthermore, this study can inform conservation planning within these areas. Effective management should prioritize not only the preservation of large trees but also keep smaller trees with the potential to develop into larger trees in the future, thereby enhancing overall carbon storage potential. In addition, future monitoring of the dynamics within the sample plots may provide greater clarity changes in tree populations and their impact of potential of carbon storage in botanical gardens and arboreta. Overall, our findings can support the sustainable management of botanical gardens and arboreta, contributing to their role towards a low-carbon society.