Evaluation of the section modulus for tree-stem cross sections of irregular shape

The effect of section modulus, as estimated for tree stems of irregular cross section with hollow trunks, on windthrow resistance is discussed. The sample trees were 12 aged poplar trees growing along the roadside in Sapporo. Binarized bitmap images of photographs of the crosscut surface of the sample tree stems were used to calculate numerical solutions for section moduli. The error for image resolution was simulated to less than 1% under the condition in which the image was divided into more than 400 pixels. The coefficient of variation in section modulus concerning six neutral axis directions in a horizontal plane was 11%–14%. The reduction in section modulus caused by decay or hollow trunk was 36%–56% of the full cross section. The effect of section modulus on critical wind velocity was found to be considerable. The critical wind velocity calculated for some of the sample trees was less than 40m/s, and these trees were considered to be in danger of bending failure of the stems.