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Tree resistance to uprooting can be estimated as the critical turning moment in tree-pulling experiments. The depth at the center point of rotation (Dcp) in tree-pulling experiments is measured as an indicator of below-ground traits and is related to this critical turning moment. However, few researchers have investigated the relationship between the Dcp and maximum root depth. Our objective in this study was to clarify whether the Dcp in tree-pulling experiments can be estimated as the maximum root depth of Pinus thunbergii Parl. in sandy soils. We also estimated which position of displacement of the center of rotation (Cp) can be applied as the Dcp. We conducted tree-pulling experiments, and compared the Dcp obtained from images with the measured maximum root depth. We found significant positive correlations between the Dcp and maximum root depth. The Cp displacement concentrated immediately below the stem when the maximum critical turning moment was reached. This position should be measured as the Dcp, which is related to the maximum root depth. We found that the Dcp can serve as a parameter, preventing the need for uprooting, when tree-pulling experiments are performed to obtain an important below-ground trait for understanding the critical turning moment.

Masako and HIRANO, Yasuhiro: Effect of thinning on the critical turning moment of Sugi (Cryptomeria japonica (L. f.) D. Don) 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 DBH 2 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.

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