1. fine roots (< 2 mm) play a key role in terrestrial ecosystem processes. species diversity loss has been recognized as one of the primary global change drivers that can have profound negative effects on ecosystem functionality and services to humanity. however, our understanding of the effects of plant diversity on fine root production remains limited. 2. we investigated the effects of species diversity on fine root production in a boreal forest that had grown naturally for eight years, following stand-replacing fire, by comparing the generation of fine-roots in single-species stands (populus tremuloides michx. (populus) and pinus banksiana lamb. (pinus)], and their mixtures (populus+pinus). 3. we hypothesized that: (i) fine root production is higher in mixed stands, (ii) across a given growing season, the effects of diversity on fine root production would be the greatest in august in a boreal forest, and (iii) fine root production in a mixed stand is strongly influenced by the dominant species. 4. we found no evidence of positive diversity effects on fine root production in a young natural boreal forest. moreover, fine root production was not altered during sampling dates between may to october; however, the effects of tree species diversity on fine root production was positive in august. rather, fine root production differed significantly with the composition of overstory tree species, with populus stands having the highest root production. 5. our results suggested that mixtures of two shade-tolerant tree species at an early stage of development did not benefit fine root production, except during the mid-growth season. further, our results supported the mass ratio hypothesis for species compositional effects on the belowground processes that we investigated.