There is a rising interest in the relationship between diversity and belowground productivity due to the critical contribution of belowground systems to overall terrestrial productivity. However, temporal change of diversity effects on belowground productivity, and their underlying mechanisms, remains unclear. We hypothesized that: (i) the magnitude of diversity effects on fine root production increases with stand development, and (ii) the diversity effects on productivity is positively associated with increased soil volume filling horizontally and foraging towards higher nutrient availability vertically. We investigated the effects of tree species diversity on fine root productivity through stand development in post-fire natural boreal forests of two stand ages, i.e., 8 and 34 years. Overstory tree species diversity varied from: pure broadleaf Populus tremuloides Michx., to pure conifer Pinus banksiana Lamb., to their various levels of mixtures. We also explored how diversity effects on fine root productivity related to soil volume filling and nutrient concentration.
Significant overyielding of annual fine root production occurred in both stand ages with higher magnitude in mature stands. Transgressive overyielding only occurred in mature stands which indicated complementary effects. Mixtures had higher horizontal soil volume filling than single-species-dominated stands and the difference was more pronounced in mature stands. Fine root productivity was driven by soil nutrient concentration regardless of stand types. Higher magnitudes of overyielding was found in deep soil layers for both stand ages. Our results provide some of the first evidence for temporal changes of belowground diversity effects in heterogeneous natural forests. Similar to findings in aboveground systems, we demonstrate that the magnitudes of diversity effects on fine root productivity increase with stand development mainly by increasing soil volume filling/exploiting and growing more roots in deep soil layers.