Effects of site conditions on forest stand attributes and above-ground biomass in a seasonal tropical forest of Laos

Forest above-ground biomass (AGB) is influenced by a combination of environmental factors and their effects on forest structure and composition, yet the primary drivers of AGB variation in tropical regions remain poorly understood. This study analysed data from 32 0.25-ha plots in the tropical forests of Phou Khao Khouay National Park, located in central Laos, to examine how elevation and soil conditions shape forest community structure, species diversity and AGB. Linear mixed effects and piecewise structural equation modelling were employed to assess these relationships. The results revealed that (i) soil nutrient availability exhibited significant spatial variability across the study area; (ii) elevation was positively correlated with species evenness but negatively correlated with AGB; and (iii) AGB, as a stand attribute, was directly influenced by stand density and species evenness (with opposing effects), while elevation indirectly affected AGB through its relationship with species evenness. These findings elucidate that site conditions (elevation and soil) govern stand structure and species diversity, which collectively determine AGB in tropical forests. Furthermore, while biodiversity conservation is fundamental, maintaining a sustainable stand structure (shaped by underlying environmental conditions) is critical for sustaining high AGB levels. This study underscores the importance of forest management strategies that prioritise site-specific ecological drivers to enhance carbon sequestration and ecosystem resilience.