Delphine Jullien

Maturation Stress Generation in Poplar Tension Wood Studied by Synchrotron Radiation Microdiffraction

Tension wood is widespread in the organs of woody plants. During its formation, it generates a large tensile mechanical stress called maturation stress. Maturation stress performs essential biomechanical functions such as optimizing the mechanical resistance of the stem, performing adaptive movements, and ensuring the long-term stability of growing plants. Although various hypotheses have recently been proposed, the mechanism generating maturation stress is not yet fully understood. In order to discriminate between these hypotheses, we investigated structural changes in cellulose microfibrils along sequences of xylem cell differentiation in tension and normal wood of poplar (Populus deltoides × Populus trichocarpa ‘I45-51’). Synchrotron radiation microdiffraction was used to measure the evolution of the angle and lattice spacing of crystalline cellulose associated with the deposition of successive cell wall layers. Profiles of normal and tension wood were very similar in early development stages corresponding to the formation of the S1 layer and the outer part of the S2 layer. Subsequent layers were found with a lower microfibril angle (MFA), corresponding to the inner part of the S2 layer of normal wood (MFA approximately 10°) and the G layer of tension wood (MFA approximately 0°). In tension wood only, this steep decrease in MFA occurred together with an increase in cellulose lattice spacing. The relative increase in lattice spacing was found close to the usual value of maturation strains. Analysis showed that this increase in lattice spacing is at least partly due to mechanical stress induced in cellulose microfibrils soon after their deposition, suggesting that the G layer directly generates and supports the tensile maturation stress in poplar tension wood.

Modelling Log-End Cracks Due to Growth Stresses: Calculation of the Elastic Energy Release Rate

Summary The occurrence of log-end cracks, due to the release of growth stress pre-existing in the standing tree, causes severe damage at the early stage of wood transformation. A mechanical model based on Griffiths theory for elastic-fragile materials has been developed to explain the observed patterns: a crack can only progress when the elastic energy release rate (G) exceeds the toughness (Gc) of the material for the given fracture mode and orientation. At each stage of the crack propagation, G was calculated using the finite-element method. The influence of various parameters related to the rigidity components, the initial growth stress field or the crack geometry has been investigated, based on a set of experimental data gathered on a population of Eucalyptus. In all cases the high G values just after crack initiation are followed by a marked decrease until the periphery has been reached. Their order of magnitude for a typical log is similar to Gc values measured independently on similar material, thus supporting the validity of the approach.

Tree growth stress and related problems

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 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.

Modelling, Evaluation and Biomechanical Consequences of Growth Stress Profiles Inside Tree Stems

The diameter growth of trees occurs by the progressive deposition of new wood layers at the stem periphery. These wood layers are submitted to at least two kinds of mechanical loads: maturation stress induced in wood during its formation, and the effect of the increasing self-weight. Interaction between growth and these loads causes mechanical stress with a particular distribution within the stem, called growth stresses. Growth stresses have technical consequences, such as cracks and deformations of lumber occurring during sawing, and biological consequences through their effect on stem strength. The first model for computing the field of stress inside a growing stem was set long ago by Kubler. Here, we extend these analytical formulations to cases with heterogeneous wood properties, eccentricity and bending stresses. Simulated profiles show reasonable agreement with measured profiles of released strains in logs. The particular shape of these profiles has consequences on stem bending strength. During bending in response to transient loads such as wind, most of the load is supported by outer parts of a stem cross section. The tensile maturation stress at this level increases the bending strength of the stem by delaying compression failure. Compressive stress in reaction to this tension does not reduce the bending strength because it is located near the centre of the stem and thus not loaded during bending, except if growth is strongly eccentric. Permanent bending stresses are concentrated at the mid-radius of the section, so that they do not cumulate with above-mentioned sources of stress. This smart distribution of stresses makes it possible that the stem is stronger than the wood it is made of, and that a growing stem can bend considerably more than its non-growing beam equivalent without breaking.