To identify a cutting method that reduces damage from upper-story thinning to lower-story trees, we studied a multi-storied stand of karamatsu (Larix leptolepis) and hinoki (Chamaecyparis obtusa). First, we investigated the effect of felling direction on the rate of damage to lower-story trees. While felling upper-story trees downhill showed a damage rate of 23.2%, felling them uphill more than halved the damage rate to 10.4%. Second, we investigated the relationship between the damage rate of lower-story trees and pruning upper-story trees prior to thinning. For each felling direction, we removed 40% of branches from upper-story crowns in one area and compared it to a control area, but no clear difference was found. We also investigated the damage rate of lower-story trees within felled crowns of upper-story trees. A significant difference was found between uphill felling and downhill felling, but not between pruning and not pruning for both felling directions. The damage rate of lower-story trees within upper-story crowns is related to the rotation angle of the upper-story trees before they land. In the case of a 30-degree slope, 49.0% of lower-story trees within felled crowns were damaged when the trees were felled downhill, yet the damage was reduced to 8.8% by felling them uphill. In summary, felling upper-story trees uphill rather than downhill is an effective cutting method to reduce damage to lower-story trees. However, removing 40 % of branches from upper-story crowns does not effectively reduce the damage.

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To evaluate an experimental felling tree method scientifically, the influence of back-cut placement on internal stress around hinges was examined. Observations of the felling process revealed that the corners of back-cut began to crack at the initial stage. Then, the relationship between back-cut placement and the direction of cracking was investigated, and the internal stress of trees until cracking occurred was analyzed by finite element method (FEM). The tree model was regarded as isotropic material, and the loading condition was set so that the amount of horizontal displacement at 1m in height would reach 3cm. High stress was induced at one of the two corners placed on the end of a back-cut which seemed to be the starting point of cracking caused by large displacement. The direction of the first cracking was related to the placement of the back-cut, namely, it moved upward when the back-cut was lower than the notched corner, and moved downward when the back-cut was higher. Cracking that moves upward is not desirable because it not only reduces the commercial value of logs but also poses risks for loggers. A more precise analysis in which the simulated form of the notched corner more closely resembled an actual felling method showed the possibility that cracking would move upward even when the back-cut was placed at the same height as the notched corner. This suggests that the back-cut should be made a little higher than the notched corner to prevent cracks from moving upward. The results of FEM analysis and observations of cracking directions confirmed that methods used for teaching about felling in Japan are practical.

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