<p>Forest biomass serves as a cornerstone for understanding ecological dynamics and global carbon cycling. However, the relative contributions of stand structural diversity and tree competition to aboveground biomass in species-poor forests remain unclear. In this study, we calculated three facets of structural diversity (i.e., horizontal diversity, vertical diversity, and three-dimensional diversity) by the Shannon–Wiener biodiversity index, and neighbourhood competition by the Voronoi-based competition index, from five pure birch forests (PB), two pure larch forests (PL), and seven mixed birch-larch forests (BL) in the mountainous forests of northwest Hebei, China. Our results showed that stand biomass, horizontal diversity, and vertical diversity of PB were significantly higher than PL and BL, but there was no significant difference in total tree competition and three-dimensional diversity among the three forest types. Moreover, we found that intraspecific competition and horizontal diversity linearly increased stand biomass in pure forests. Similarly, interspecific competition, horizontal diversity, and three-dimensional diversity were positively related to stand biomass in mixed forests, while total and intraspecific competition had convex relationships with stand biomass. Across pure and mixed forests, there was an opposite relationship between structural diversity and tree competition. At last, the most important factor affecting stand biomass was identified as horizontal diversity in pure forests, while it was intraspecific competition in mixed forests. Our findings suggested that stand structure and tree competition in pure and mixed forests exhibit different regulatory mechanisms on forest aboveground biomass, providing a scientific reference for further research on forest functions at larger scales.</p>