Tension Wood in Cottonwood: Its Effect on Density, Toughness, and Compression

Tangentially matched specimens were taken from the upper and lower sides of five leaning eastern cottonwood trees. After cutting, humidifying at 65 percent R.H., and final sizing, the specimens were tested for growth eccentricity, moisture content, specific gravity, and several physical properties. The presence of tension wood was apparent in all specimens from the upper side of the tree. This was indicated by the near impossibility of sawing due to relieved stresses and subsequent warping, bundles of torn fibers protruding from the fuzzy surfaces of all tension wood sticks, concave bowing on the bark side, and abnormal dulling of the saw. Despite the presence of tension wood, no growth eccentricity was noted, and under similar conditions the tension wood tended to increase in moisture content slower than normal wood. Average specific gravity was higher in the tension wood. Toughness showed significantly higher values in the tension wood. Tension wood had higher modulus of elasticity, probably related to higher specific gravity. Fiber stress at proportional limit was the same for both tension and normal wood, and maximum crushing strength was lower in tension wood. In the compression-parallel-to-grain tests, tension wood specimens often exhibited bending and crushing type failures, but seldom the shear failures common to normal wood.

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