Spatial and temporal variation of tree biomass in a tropical rainforest in Southwest Sri Lanka

The long-term patterns and spatial distribution of aAbove Ground Biomass (AGB) in Mixed Dipterocarp Forests (MDF) in tropical Asia remain understudied. This research investigates changes in AGB over a 25-year period within the Sinharaja Forest Dynamic Plot (FDP), a 25-hectare area in southwest Sri Lanka. The study uses 25 years of Diameter at Breast Height (DBH) measurements from the 20 most dominant species in the FDP, along with their wood-specific gravity data, to estimate AGB. The AGB estimates yielded an average of 300 Mg ha^-1 (±17 Mgha^-1, 95% confidence interval). The total AGB exhibited wide spatial variation (3.4 to 1198 Mgha^-1 per 0.04 ha, n= 625), with significant spatial variation (p < 0.05) over 25 years. AGB showed a consistent decline from 1994 (324 Mgha^-1) to 2012 (289 Mgha^-1), with a slight increase in 2019 (299 Mgha^-1), showing a decreasing trend in AGB over the 25-years. Repeated ANOVA tests revealed that the temporal variation of AGB among the censuses was not statistically significant. However, at the diameter class level, statistical tests indicated significant (p < 0.05) AGB variation. The 1-3 cm diameter class had the highest stem percentage (66%) but <1% of the total AGB. The 30-50 cm diameter class contributes the most to total AGB (25%). Large trees (>70 cm diameter) have low stem percentage (0.14%), but carry 24% of the AGB. Findings of this study are important for estimating AGB and carbon sequestration in Mixed Dipterocarp Forests (MDF) in Asia, especially in Sri Lanka. The study area shows a noticeable decline in AGB, mainly due to the loss of large canopy tree species such as Mesua ferrea. This shift is transforming the area from a carbon sink into a carbon source.