Tree growth stress and related problems

J Wood Sci (2017) 63:411–432 DOI 10.1007/s10086-017-1639-y REVIEW ARTICLE Tree growth stress and related problems Joseph Gril1 • Delphine Jullien1 • Sandrine Bardet1 • Hiroyuki Yamamoto2 Received: 23 May 2016 / Accepted: 15 May 2017 / Published online: 29 June 2017 Ó The Author(s) 2017. This article is an open access publication Abstract Tree growth stress, resulted from the combined effects of dead weight increase and cell wall maturation in the growing trees, fulfills biomechanical functions by enhancing the strength of growing stems and by controlling their growth orientation. Its value after new wood formation, named maturation stress, can be determined by measuring the instantaneously released strain at stem periphery. Exceptional levels of longitudinal stress are reached in reaction wood, in the form of compression in gymnosperms or higher-than-usual tension in angiosperms, inspiring theories to explain the generation process of the maturation stress at the level of wood fiber: the synergistic action of compressive stress generated in the amorphous lignin– hemicellulose matrix and tensile stress due to the shortening of the crystalline cellulosic framework is a possible driving force. Besides the elastic component, growth stress bears viscoelastic components that are locked in the matured cell wall. Delayed recovery of locked-in components is triggered by increasing temperature under high moisture content: the rheological analysis of this hygrothermal recovery offers the possibility to gain information on the mechanical conditions during wood formation. After tree felling, the presence of residual stress often causes processing defects during logging and lumbering, thus reducing the final yield of harvested resources. In the & Joseph Gril & Hiroyuki Yamamoto 1 LMGC, University of Montpellier, CNRS, Montpellier, France 2 Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan near future, we expect to develop plantation forests and utilize more wood as industrial resources; in that case, we need to respond to their large growth stress. Thermal treatment is one of the possible countermeasures: green wood heating involves the hygrothermal recovery of viscoelastic locked-in growth strains and tends to counteract the effect of subsequent drying. Methods such as smoke drying of logs are proposed to increase the processing yield at a reasonable cost. Keywords Mechanical stress  Tension wood  Compression wood  Biomechanics  Hygrothermal recovery Introduction Tree growth stress refers to the mechanical stress permanently supported by wood in a living tree during tree growth. It results from the combined action of two mechanisms, i.e., cell wall maturation and the increase of dead weight [1]. The following scenario is commonly admitted to explain the contribution of maturation—here, the term ‘‘maturation’’ refers to the latest stage of cell wall formation, from the completion of polysaccharides deposition until cell death, and includes lignification. During secondary-wall maturation, the newly differentiated xylem fiber tends to deform in its axial and transverse directions. These dimensional changes are restricted by the alreadyformed xylem. The restraint induces a mechanical stress, or the so-called ‘‘maturation stress’’, at the outermost surface of the secondary xylem, located beneath the layer of differentiating xylem. It provokes, in the older xylem, during each growth increment, a counteractive stress distribution which is superimposed on the pre-existing stress. In 123 412 addition, the increasing tree weight is supported by the older part of the stem (this term is used in this paper in the botanical sense, referring either to trunk or branch). As a result, each stage of growth produces an additional stress distribution balancing the effect of gravity. A complex distribution of mechanical stress, called ‘‘growth stress’’, is thus installed in the living stem. Growth stress does not include, in its definition, the effect of non-permanent loads, such as wind or snow, that impose temporary stress modification only. Growth stress measured at the outermost xylem surface, the ‘‘surface growth stress’’, is more or less the same as the maturation stress. Due to progressive application of stress on the structure, the growth stress cannot be simply released by the removal of all external mechanical actions. Where gravity suddenly eliminated, the stress distribution would be somewhat modified but not return to zero: most would remain as selfbalanced ‘‘residual stress’’. This is almost the case in a cut log, where the effect of gravity is much reduced as soon as the log is laid horizontally on the ground. Growth stress performs essential functions for the tree; it maintains its huge body for a long period against the gravitational force [1]. Reaction wood formation participates in this function, especially when a drastic response is needed. Growth stress becomes extremely high in reaction wood, whereas it is reduced in the opposite side, which causes an upward or downward bending moment in the stem. Thanks to the capacity to control stem orientation, growth stress in reaction wood allows the newly formed xylem to perform a function analogous to that of muscles in animals. Whereas in animals, the muscles would not be effective without bones, in trees, the wood itself fulfills the supporting function of a skeleton. It will be shown in next section that in addition to the ‘‘muscular’’ function, the stress distribution by itself contributes to the ‘‘skeletal’’ function through enhanced bending strength [2, 3]. The growth stress is instantaneously released by cutting operations that isolate a small wood portion from the surrounding part of the tree. The resulting strain recovery, or released strain, combined with measurement of material rigidity, permits to evaluate the pre-existing stress. Wood being a viscoelastic material, a delayed recovery is caused. This time-dependent recovery is a temperature-activated process. As will be discussed later, it can be used to gain information on the mechanical conditions of wood at an arbitrary position in the stem during cell wall maturation and subsequent deposition of new wood layers, that is, secondary growth of the stem [4]. The presence of growth stress in tree stems often causes problems when using logs as raw material for timber products. Examples are radial cracks at the edge of cut logs, crooked sawn lumber, and so forth. When the harvested logs contain reaction wood, processing defects 123 J Wood Sci (2017) 63:411–432 become unpredictably serious and diminish, to a notable degree, the final yield. The resulting economic loss amounts to untold millions of dollars across lumber industry. Wood scientists and engineers are required to find practical solutions to solve those problems. Thus, the topic of tree growth stress, including that of reaction wood [5], is interesting no (...truncated)