AbstractReforestation is a feasible option for the eventual restoration of biodiversity and ecosystem services on deforested oil‐sands mining sites. The associations between plant diversity and productivity spanning the different strata of restoration forests, in conjunction with specific factors that may impact these relationships, remain uncertain. We sampled 94 sites that encompassed conifer, mixed‐wood, and broadleaved overstory types as three exemplar substrates following the reforestation of oil‐sands mining sites in Alberta, Canada. We employed structural equation modeling to investigate the correlations between species diversity and aboveground biomass production spanning forest vegetation strata, while concurrently accounting for the effects of overstory composition, functional diversity/identity, soil fertility, and restoration age. We found that the relationships between species diversity and biomass were negative, or inconsequential across restoration forest strata. Overstory biomass was linked to the coniferous tree proportion and community‐weighted mean of leaf nitrogen, rather than the species and functional diversity of overstory trees. Further, overstory composition, diversity, and biomass were the key features involved in the diversity and biomass of understory strata. Our results demonstrated the importance of overstory tree species composition and selection effects toward driving overstory and understory productivity in restoration forests at post‐oil sands mining sites. Coniferous trees were observed to have lower leaf nutrient levels and negative impacts on the productivity and diversity of overstory and shrub layers. Therefore, it was recommended that a greater proportion of broadleaved trees should be incorporated to promote productivity and species diversity in restoration forests through the enhancement and utilization of available resources.