Abstract
Forest damages by natural disasters such as typhoons and heavy rain have recently increased and adequate forest management is required to enhance the resistance against the disasters. Thinning is one of the important forest management to develop sound forests. However, the effects of thinning on the tree resistance against wind-fall and mud flood remained unclear. In this study, we investigated the effects of the thinning on the critical turning moment by tree-pulling test in a Cryptomeria japonica stand where 17 years passed after with or without thinning and analyzed the factors concerning the critical turning moment. We measured the parameters of above-ground (DBH, tree height, and crown width) and below-ground (hard-pan depth, radius of root-plate, and depth of center point of rotation) that are related to the critical turning moment. As a result, we found the regression lines between the critical turning moment and H × DBH2 were significantly different between the treatments of the thinning. Namely, the critical turning moment of trees after the thinning increased significantly compared with the same-sized trees without the thinning. For the contribution of belowground parameters on the critical moment, the radius of root plate after the thinning increased significantly compared to those without the thinning. Our results suggested that the growth of horizontal roots has been promoted after the thinning, leading to increasing the critical turning moment.
