1. The diversity–productivity debate has so far been focused above-ground, despite that below-ground production can account for approximately half of total annual net primary production, mostly from fine roots.
2. Here, we investigate the fine root productivity of mature, fire-origin stands of Populus tremuloides–Picea spp.–Abies balsamea (mixed-species stands) and relatively pure P. tremuloides (single-species stands) in two regions of North American boreal forest to better understand the link between plant diversity and below-ground productivity in forest ecosystems. We hypothesized that: (i) mixed-species stands have higher fine root productivity compared with single-species stands and (ii) this difference may be the result of greater soil space filling by the fine roots due to the contrasting rooting traits of the component species in the mixed-species stands.
3. We found that fine root productivity, measured by annual production and total biomass, was higher in mixed- than single-species stands. We also found that mixed-species stands had lower and higher horizontal and vertical fine root biomass heterogeneity, respectively, indicating that soil space is more fully occupied by fine roots in the mixed- than single-species stands.
4. In all, our study supports that below-ground niche differentiation may be a key driver of higher fine root productivity in mixed stands of species with contrasting rooting traits than single-species stands by facilitating greater soil space filling of fine roots and soil resource exploitation.