Temporal variation in fine-root biomass, production and mortality in a cool temperate forest covered with dense understory vegetation in northern Japan

To understand the temporal pattern of fine-root dynamics and the factors that affect it, we investigated the seasonal and interannual variation in fine-root production (FRP) and fine-root mortality (FRM) rates, as well as fine-root biomass (FRB) and necromass in a cool temperate forest in northern Japan that was covered with dense understory vegetation of Sasa senanensis. We measured the root length density (RLD) and the rate of root production and mortality over 3 yr using minirhizotrons, and compared these rates with temperature, precipitation, soil moisture, and plant area indices (PAI). We also measured the FRB and the necromass of fine roots four times per year for 2 yr using soil cores and calculated dry weight-based FRP and FRM. FRB in the uppermost 15 cm of the surface-soil layer accounted for 41-61% of the biomass up to 60 cm soil depth, and decreased with increasing soil depth. The biomass of fine roots with root diameters <0.5 mm was almost equivalent to that of roots measuring 0.5-2 mm in diameter. Sasa roots accounted for 59-88% of the total FRB. FRB did not fluctuate seasonally, whereas RLD did. The FRP rate was high in mid- to late summer and correlated significantly with air and soil temperatures, indicating that temperature affects FRP. However, the relationship between FRP and soil moisture was weak. FRP was significantly correlated with the PAI of oak trees and the increment in the PAI of Sasa, suggesting that endogenous factors also affect FRP. Depending on the method used to calculate turnover, mean FRP for the 3-year study period was 589 or 726 g m−2 yr−1, accounting for 36% or 41% of forest net primary production, respectively. The results of this study illustrate the substantial seasonal and interannual fluctuations in FRP, and indicate that a significant proportion of assimilated carbon was allocated to below-ground root systems in an oak-Sasa stand.