Abstract Species mixtures have been widely reported to increase aboveground productivity; however, how tree species mixtures affect root systems in natural forests remains unclear. We hypothesize that mixtures have a greater fine root length compared to single species‐dominated stands to support their greater productivity. Here, we collected monthly root images from the minirhizotrons installed in 18 stands either dominated by Populus tremuloides, Pinus banksiana, and their mixtures for three years (2015–2017) in post‐fire boreal forests of two stand ages (8 and 34 years old) to test our hypotheses. We found that the fine root length was higher in mixtures than in single species‐dominated stands, and the magnitude of mixture effects was greater in the 34‐ than in the 8‐year‐old stands in the third year. The mixture effects on fine root length revealed a positive relationship with forest net primary productivity. Root length production, which is the growth of new roots within a year, was not affected by tree species mixtures except for the 8‐year‐old stand in 2015. Tree species mixtures did not affect root length turnover. Root length and root biomass were not significantly correlated at stand level. Synthesis. Our results show that tree species mixtures positively affect fine root length, with that positive effect increasing with stand development in the studied natural forests. Our results suggest that the greater root length in species mixtures supported the greater forest productivity in species mixtures as the greater root length benefits plant uptake of nutrients and water. Therefore, conserving tree species diversity has implications for improving forest productivity and carbon sequestration in forest ecosystems.