Tangential Shrinkage of Serial Sections Within Annual Rings of Douglas- Fir and Western Red Cedar

An attempt was made to discover why tangential shrinkage of gross wood structure is greater than radial shrinkage, and also why the ratio of tangential to radial shrinkage for springwood in a conifer is considerably greater than that for summerwood. Using data of other researchers, the average specific gravity, green-volume basis, of Douglas-fir used was 0.28 for springwood, and 0.67 for summerwood; the ratio of specific gravities was 2.4. To measure the variation in tangential shrinkage, the magnified image of a thin tangential section was projected onto a large, smooth surface and the length was measured while the section was wet and after it had dried completely. In nearly all cases the least shrinkage occurred in the cell layers in the first part of the springwood. An appreciable increase in tangential shrinkage of springwood occurs before the visible transition to summerwood. Tangential shrinkage within the summerwood was likewise not uniform, increasing through the transition zone. In some rings it reaches a maximum near the middle of the summerwood. The tangential shrinkage of the first half of the springwood was 4.3 percent. measured on thin sections, and for summerwood the average maximum was 9.3 percent. The results are shown for five trees of second-growth Douglas-fir from two sites and three degrees of closure of crown canopy. In western redcedar, the average tangential shrinkage of the springwood was 3.9 percent, and for summerwood 7.4. percent. The lower shrinkage as compared with Douglas-fir indicates that the overall low shrinkage of the cedar is due not only to the low percentage of summerwood, but also to the lower shrinkage in each part of the ring. Photographs of the radial surfaces of cross sections of a 2- by 2-inch stick show the nature of the adjustment between springwood and summerwood during drying.

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