Fine root traits are critical to the plant's capacity and efficiency to uptake water and nutrients. Although plant diversity is decreasing, our understanding of its effects on fine root traits remains elusive. By synthesizing 103 studies, we found that the effects of plant mixtures were highly dependent on species richness in mixtures, stand age, and soil depth. The positive mixture effects on root biomass increased with species richness, soil depth, and mean annual temperature. Plant mixture effects on root length density shifted from negative to positive, from young to older stands, topsoil to deep soils, and warm to cold climates. The mixture effects on specific root length shifted from positive to negative, from two to higher number species mixtures and topsoil to deep soils, and then negative to positive with increasing stand age. Our results demonstrate the profound plasticity of root traits in response to productivity dynamics in plant mixtures.