Objective This study aims to elucidate the effects of woody plant diversity (in terms of α-diversity and β-diversity) on aboveground biomass in tropical natural forests of Hainan, and to investigate the relative importance and the scale dependence, in order to enrich the theoretical framework of biodiversity and ecosystem function relationships.

Method This study, based on 30 1.0-hectare (ha) forest dynamic plots established in the tropical natural forests of Hainan, utilized data from vegetation surveys, tree spatial distribution, and environmental factor measurements. By employing a spatial multi-scale design and multiple linear regression models, we explored the relationships between woody plant diversity, environmental heterogeneity, and aboveground biomass in tropical natural forests, as well as their variation across different spatial scales.

Result (1) The mean values of α-diversity and β-diversity in the 30 1.0-ha plots were (10.85 ± 3.63) and (0.30 ± 0.16), respectively. α-diversity did not show significant variation across different spatial scales. While β-diversity exhibited an increasing trend with spatial scales. (2) The effects of α-diversity and β-diversity on aboveground biomass were all significantly positive, and the strength of the diversity effects showed a consistent trend with spatial scale. (3) The impact of woody plant diversity on aboveground biomass increased with spatial scale initially (from smaller scales of 400 m² to 3 600 m²), then plateaued (at intermediate scales of 3 600 m² to 6 400 m²), and finally decreased (at larger scales of 6 400 m² to 10 000 m²). (4) The positive effects of α-diversity on aboveground biomass were significantly stronger than that of β-diversity. (5) Environmental factors had positive effects on aboveground biomass, and the strength of these effects increases with the spatial scale.

Conclusion This study confirms a significant scale dependency in the relationship between biodiversity and ecosystem function, indicating that the main mechanisms influencing the BEF relationship may vary across different spatial scales. Research limited to a single spatial scale or at a local small scale is insufficient for a comprehensive understanding of the mechanisms affecting the BEF relationship. Although β-diversity has a lower explanatory power for variations in aboveground biomass compared to α-diversity, this empirical study highlights the important role of β-diversity in promoting ecosystem functions. This provides new insights and analytical perspectives for further exploration of the relationships between biodiversity and ecosystem function across spatial scales. Future research should thoroughly consider the relationships between β-diversity and multiple ecosystem functions (i.e., ecosystem multifunctionality), as well as the mechanisms by which β-diversity affects ecosystem multifunctionality at different spatial scales (such as local, regional, and landscape scales).