Effects of density and microfibril orientation on the vertical variation of low-stiffness wood in radiata pine butt logs

The roles of density and microfibril angle in causing low stiffness in radiata pine butt logs were studied in detail on a 17-yearold tree. Distributions of these variables were compared with stiffness variations in the vertical direction. Results supported the hypothesis that cell ultrastructure is responsible for the vertical variation in stiffness. The microfibril orientation in tangential wall is considered to be an important factor contributing to wood stiffness because of the smaller microfibril angles compared with radial microfibril angles, and also because of the larger decrease of the microfibril angles with the rapid increase of wood stiffness in vertical direction especially in corewood zone. The microfibrils in the S3 layer fall from over 80 to angles of 54degrees and 51degrees for radial and tangential cell walls at the top of the butt log. Further study is needed for fully understanding the characteristics of S3 layers.