The importance of fine roots in forest ecosystem processes is well known. However, the contribution of understory vegetation to underground ecosystem processes is not well understood. We tested the hypothesis that fine-root biomass (FRB) and performance of the overstory and understory independently decrease with increasing soil N availability in cool-temperate deciduous broad-leaved natural forests and larch plantations in Japan. The mean contribution of understory FRB to total FRB (tree + understory) ranged from 4% to 78% (mean 37%). Tree FRB was negatively correlated with understory FRB, and understory FRB was dominant to tree FRB in infertile soil. Understory and total FRB were negatively correlated with soil net N mineralization rate, whereas tree FRB showed a quadratic relationship with soil N mineralization rate with the peak observed at mineralization of 58.4 kg N ha-1 y-1. The low tree FRB at infertile sites may be due to a belowground competitive effect of understory fine roots on tree FRB. Understory fine-root nitrogen concentration (FRN) and leaf to fine-root (L/FR) ratio were positively correlated with N mineralization rate. However, tree L/FR was not significantly correlated, whereas tree FRN was positively correlated, with soil N mineralization rate, suggesting that the leaf production efficiency of trees might not increase even on infertile soil. We suggest that belowground processes of overstory trees might change depending on understory vegetation, and that understory vegetation might affect the fine roots of overstory trees, which did not increase mass allocation but increased N use efficiency under low FRN.
2015 Japanese Society for Root Research