Effects of species and tree size diversity on above-ground biomass in diverse heterogeneous tropical evergreen forests, Quangbinh Province, north central Vietnam

The relationships between species diversity and size structure complexity on above ground biomass (AGB) have theoretical and practical applications for biodiversity conservation and sustainable forest management. The relationships become more complex in the high-species diversity of tropical forests and heterogeneous environmental conditions. To disentangle the complicated relationships, structural equation models were applied to examine the relative effects of species and tree size diversity on above ground biomass under different topographical conditions of two tropical evergreen forest stands. Our results showed that: (i) species diversity contributes greater to AGB in the forest stand under homogeneous topography. (ii) Structural diversity positively affected AGB in the heterogeneous topographical stand. (iii) Slope revealed a negative effect on species diversity but positive effects on structural diversity and AGB in both studied plots. We concluded that maintaining high-stand structural diversity enhances above ground biomass and local topographical conditions constrain the relationship between species and size diversity in the study area.