Monitoring bending stress of trees during a snowy period using strain gauges

The load created by snow greatly affects forest trees; however, almost no studies have reported on the mechanical stress in tree trunks due to snow under field conditions. In this study, we monitored trunk bending stresses using strain gauges during the snowfall season at two sites, Tadami and Kaneyama, both in a cool-temperate forest in Japan. Measurements were made of the various sizes and trunk shapes of beech trees at Tadami, and of beech trees and Japanese cedars at Kaneyama. Young’s modulus and bending strength were also measured. At Tadami, beech trees with substantially curved trunks often bent down to the ground surface soon after snowfall started. The strain no longer increased once a tree had lodged; however, individuals with larger diameters had stresses exceeding the proportional limit stress. By contrast, among beech trees with erect trunks, those with larger diameters had smaller maximum strain and estimated stress values. These results imply that in areas greatly affected by snow, beech trees struggle both to remain erect and to grow to diameters of 10 cm or more. At Kaneyama, large strain values exceeding 1 % were observed in trees with a diameter at breast height ≤7 cm, all of which were cedars. Among trees of the same diameter, strain values were lower in beech trees than in cedars, and Young’s modulus was three times larger for beech trees. These results indicate that it is more difficult to grow erect cedars compared to erect beech trees in regions with deep snow.