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Spiral grain in second growth Douglas fir and western hemlock Elliott, Geoffrey Kenyon

Abstract

Spiral grain in timber may cause severe twisting of lumber and plywood. In the primary forest products industries the presence of spiral grain in the tree results in cross-grained products. Cross grain affects the strength properties of lumber to a marked degree. Thus a grain deviation of 1 In 25 (2°18') results in decreased tensile strength whereas a slope of 1 in 10 (5°43') will reduce compression strength. Spiral grain is a condition well known to the wood technologist and the silviculturalist. Until recently it was considered the exception rather than the rule. From recently published papers, however, and from unpublished data available to the author, strong evidence has been produced to show that spiral grain is the normal growth pattern in trees. This thesis is designed to investigate the spiral pattern of second-growth Douglas fir and western hemlock. Accordingly three sites were chosen: a good, a medium and a low site in a typical British Columbia coastal forest of second growth. Two crown classes for each species were sampled from each site and three trees in each crown class for each species were felled and their spiral patterns investigated. A general trend of spirality was established, the twist being initially left (at first), decreasing to the left and becoming right with increasing age. This pattern holds good for both species. The effect of site on spiral development was established as highly significant with both species. On high quality sites the chief factor influencing spiral development was found to be distance from the pith. On sites of lower quality, age from the pith was found to have the most significant influence on spiral development.

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