A quantitative evaluation of soil mass held by tree roots

Key message Tree roots hold soil that is dramatically heavier than the tree biomass, wet or dried. This soil might compensate for the imbalance between above- and belowground mass. Abstract Root–soil plates are recognized to play an important role in root anchorage of plate-like root system, however, actual measurements of their mass have rarely been reported. Even though the root–soil plate mass is often estimated using aboveground allometric indices, no research confirms the validity. Seven root–soil plates of Cryptomeria japonica fallen by Typhoon Jebi were divided into roots and soil, and their weights were directly measured. Mass of the seven plates ranged from 251 to 3070 kg on a dry basis. Roots accounted for 8% of total plate mass and soil for 92%. The mass of the soil held in the plates was 2.8 times greater than tree biomass. The root-to-shoot biomass ratio was 0.26, whereas the ratio of root–soil plate mass to shoot biomass was 3.9, meaning that the root–soil plate mass was much greater than aboveground biomass. These results suggest that the soil mass held in the plate is the main component of whole-tree mass including the plate. The root system holds soil weighing as much as 13 times the root system’s mass. The soil might balance the aboveground weight of the tree by adding mass. Aboveground allometric indices are good indicators of root–soil plate mass and allow the belowground mass to be estimated to understand tree anchorage without soil disturbance.