Very fine roots respond to soil depth: biomass allocation, morphology, and physiology in a broad-leaved temperate forest

Very fine roots (< 0.5 mm in diameter) of forest trees may serve as better indicators of root function than the traditional category of < 2 mm, but how these roots will exhibit the plasticity of species-specific traits in response to heterogeneous soil nutrients is unknown. Here, we examined the vertical distribution of biomass and morphological and physiological traits of fine roots across three narrow diameter classes (< 0.5, 0.5-1.0, and 1.0-2.0 mm) of Quercus serrata and Ilex pedunculosa at five soil depths down to 50 cm in a broad-leaved temperate forest. In both species, biomass and the allocation of very fine roots were higher in the surface soil but lower below 10-cm soil depth compared to values for larger roots (0.5-2.0 mm). When we applied these diameter classes, only very fine roots of Q. serrata exhibited significant changes in specific root length (SRL; m g(-1)) and root nitrogen (N) concentrations with soil depth, whereas the N concentrations only changed significantly in I. pedunculosa. The SRL and root N concentrations of larger roots in the two species did not significantly differ among soil depths. Thus, very fine roots may exhibit species-specific traits and change their potential for nutrient and water uptake in response to soil depth by plasticity in root biomass, the length, and the N in response to available resources.