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The growth stresses of woody stems

Reference Number: AA-02439 Views: 852 Created: 28-02-2014 10:00 Last Updated: 14-03-2014 11:09 0 Rating/ Voters

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Title:The growth stresses of woody stems
Author/s:Maxwell Ralph Jacobs
Abstract:(Recorded in Vol. 8, p. 60 of Doctoral dissertation) The growth stresses discussed in this paper are those which develop in the woody tissues of growing trees. They are distinct from the tissue tensions of succulent stems. Erect hardwood stems were found to have a longitudinal strain gradient from the periphery inwards along any radius. The outer layers invariably contracted longitudinally on being removed from the stem, and the innermost layers expanded. The conclusion reached is that diameter growth in an erect hardwood stem is accompanied by a progressive shortening of the distance between any two points on the longitudinal axis. It is thought that the cause of this phenomenon is a tension stress which develops in each new sheath of wood. Erect coniferous stems were found to have an irregular longitudinal strain gradient. Larger stems behaved like hardwoods; smaller stems behaved irregularly, a tendency to the development of a compressive stress in the outer layers being evident up to a stem age of some years. It was concluded that erect coniferous stems tend to increase in length between any two points on the longitudinal axis during the first few years of life. The woody tissues developed in coniferous stems after that time behave like hardwood tissues. Leaning stems were found to possess modifications of the strain gradients found in erect steins. These modifications cause the leaning stem to tend to attain or regain an erect habit. In leaning hardwood stems eccentric diameter growth takes place on the upper side of the lean, sometimes accompanied by the development of a special tissue known as tension wood. In either case the resulting longitudinal compression of the stem is then greater on the top side of the lean, leading to a recovery of the erect habit. In leaning softwood stems the special tissue compression wood develops on the under side of the leaning stem. A definite compressive stress may develop in this tissue and tend to push the stem back to an erect position, or the tissue may act as a bracket and maintain its length while a tension stress in normal tissues on the top side tends to pull the tree erect. As regards lateral stresses, all stems examined were found to be in tangential compression near the periphery and in radial tension near the pith. Of several possible contributing factors, present evidence as to the origin of the forces suggests that tensions which develop in the sap stream during the growing season are responsible for most of the primary strain. These growth stresses influence the behaviour of green timber during conversion, and appear to contribute to the cause of certain defects. They may also be of considerable significance in the growth processes of the tree. [This abstract is from Bulletin No. 28, Commonwealth Forestry Bureau. The Growth Stresses of Woody Stems, by M.R. Jacobs. Canberra, 1945. Commonwealth Government Printer. The greater part of the original manuscript of this paper was submitted as a dissertation in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Yale University.'] "
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