Forest structure and carbon dynamics of an intact lowland mixed dipterocarp forest in Brunei Darussalam

Tropical forests play a critical role in mitigating climate change because they account for large amount of terrestrial carbon storage and productivity. However, there are many uncertainties associated with the estimation of carbon dynamics. We estimated forest structure and carbon dynamics along a slope (17.3°–42.8°) and to assess the relations between forest structures, carbon dynamics, and slopes in an intact lowland mixed dipterocarp forest, in Kuala Belalong, Brunei Darussalam. Living biomass, basal area, stand density, crown properties, and tree family composition were measured for forest structure. Growth rate, litter production, and litter decomposition rates were also measured for carbon dynamics. The crown form index and the crown position index were used to assess crown properties, which we categorized into five stages, from very poor to perfect. The living biomass, basal area and stand density were 261.5–940.7 Mg ha⁻¹, 43.6–63.6 m² ha⁻¹ and 6,675–8400 tree ha⁻¹, respectively. The average crown form and position index were 4, which means that the crown are mostly symmetrical and sufficiently exposed for photosynthesis. The mean biomass growth rate, litter production, litter decomposition rate were estimated as 11.9, 11.6 Mg ha⁻¹ a⁻¹, and 7.2 g a⁻¹, respectively. Biomass growth rate was significantly correlated with living biomass, basal area, and crown form. Crown form appeared to strongly influence living biomass, basal area and biomass growth rate in terms of light acquisition. However, basal area, stand density, crown properties, and biomass growth rate did not vary by slope or tree family composition. The results indicate that carbon accumulation by tree growth in an intact lowland mixed dipterocarp forest depends on crown properties. Absence of any effect of tree family composition on carbon accumulation suggests that the main driver of biomass accumulation in old-growth forests of Borneo is not species-specific characteristics of tree species.