Root system of the giant Cryptomeria japonica tree collapsed at Shinmei shrine in Okute town, Gifu, in July 2020

On the night of July 11, 2020, the giant Cryptomeria japonica tree collapsed during the heavy rainfall event in Okute Town, Gifu. The purpose of this study is to clarify the characteristics of the root system of the fallen tree together with the weather conditions such as heavy rainfall at the time. In July 2020, there were few sunshine hours which could induce higher water contents of the tree and soils. A large area of decay was observed in the root system close to the center of the stem and roots remaining in the soil showed signs of stripping. Using a laser scanner, the three-dimensional structure of the tree was digitally reproduced, and the volume of root system was estimated to be 43.2 m3. The diameter of the root-soil plate and the volume ratio to the aboveground part suggested that the root system of the tree was relatively small compared with those of other C. japonica trees. Based on the evaluation of the root system and weather conditions, the soil reinforcement by roots could be reduced due to decay during the long-term growth periods, and the balance between the above- and below-ground parts of the tree might be disrupted by the increase in the water contents of the tree and soils caused by prolonged rainfall and lack of sunlight.