the positive biodiversity and ecosystem functions relationship (bef) has been well established, especially in controlled experiments. however, a coherent study to examine the multiple relationships in bef in natural ecosystems is lacking. using the observational data of canadian forests, i aim to examine the species diversity-productivity relationships in natural forest ecosystems across forest strata, under the influences of the climate factors and local site conditions. i found the positive effects of energy on tree species diversity while accounting for potentially confounding evolutionary effects and show evidence for the effects of local site conditions and secondary succession on diversity. the positive species diversity effects on productivity were common across forest strata but to different extent while the canopy tree diversity had negative effects on understorey plant biomass. furthermore, tree size inequality may be a central process for the positive diversity effects on productivity, and potentially as the shared mechanism in regulating productivity and species diversity simultaneously via interactions among individuals in natural forests. this study sheds lights to deepen our understanding of the key features of natural ecosystems. i highlight that relationships between biodiversity and ecosystem functions and services are multifaceted, subject to several covariates including climate, local site conditions, time since stand-replacing disturbances, and the reciprocal interactions between diversity and ecosystem functions and services. however, our findings call for the complementary experiments to evaluate those potential mechanisms and drivers.