Effect of Tension Wood in a Leaning Eastern Cottonwood

An eastern cottonwood tree (Populus deltoides Bartr.) with a lean of 13 percent from the vertical was selected. The upper side of the tree was marked. After felling, the tree was bucked into four logs, each 10 feet in length except one log, which was 8 feet. The logs were sawn into 1-1/8 inch boards; four boards were obtained from each of the lateral sides and five or six boards from the upper and lower side of the lean. A section 1 foot in length was cut from the butt end of every board and the top end of all boards from the 8-foot log. After sections were cut and coded they were immersed in water. From each 1-foot section, five pieces 1-inch in width were cut to obtain straight grain samples. The samples were immersed in water and taken to laboratory for tests. A total of 380 samples were used. The specific gravity for each sample was computed on the green volume, oven-dried weight basis, and the green length was measured to 1/1000 inch. After measuring, the samples were put into a conditioning chamber and seasoned from the saturated state to that of 22, 12, and 8 percent moisture content. At each of these stages, the samples were remeasured and weighed. The four outside boards on the upper and lower side of the leaning tree underwent a microscopic analysis. Transverse sections approximately 14 microns in thickness were cut on a sliding microtome and stained with zinc chloriodide. To estimate the comparative number of gelatinous fibers in each transverse section, a frequency distribution system was used. Each microtome section was examined microscopically at magnifications of 430x and 630x for diagnostic characteristics of the gelatinous fibers. Results showed that the specific gravity increased from the pith outward and from the base of the tree to the top. Longitudinal shrinkage was greater on the upper side of the lean than on the lower side. The largest longitudinal shrinkage for the upper side occurred at the base of the tree and decreased with increasing height. The staining reagent swelled the gelatinous layer and turned it to a deep purple or black color, while the remainder of the fiber wall and the walls of normal wood fibers were not swelled and turned yellow in color. Frequently, determinations of gelatinous fibers showed that the upper side contained the greatest number of gelatinous fibers. The test data also showed that the greater the concentration of gelatinous fibers the higher the longitudinal shrinkage; for every increase of 1 in the comparative number of gelatinous fibers there was a corresponding increase of 0.003 inch in longitudinal shrinkage.

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