Lengthwise Shrinkage in Ponderosa Pine

Second growth ponderosa pine in the West Sierra area of California grows very rapidly. Lumber from these trees exhibits abnormally high longitudinal shrinkage in the 2 to 3 inches of wood surrounding the pith. Drying behavior is similar to that of cross-grained or compression wood in that considerable warpage may occur. Samples taken from 18 trees at three heights (butt, mid-height, and top) were measured in the green condition. At equilibrium moisture contents of 14, 6.7 and 0 percent respectively, sticks in the first 3 inches from the pith averaged 0.13, 0.25, and 0.34 percent shrinkage while sticks 4 inches and farther from the pith averaged only 0.03, 0.08, and 0.19 percent shrinkage. The difference in total shrinkage was most pronounced in the butt section. No significant differences were found between dominant, co-dominant, and intermediate crowned trees. Microscopic examinaton at several heights revealed a relatively large fibril angle (from the longitudinal direction) in the cells near the pith.

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