Tree growth forces and wood properties

Abstract

Living wood in the tree performs a muscular action by generating forces at the sapwood periphery and residual strains in dead sapwood fibres. Dissymmetric force generation around the tree trunk is the motor system allowing movement, posture control and tree reshaping after accidents. Rather young trees are able to restore the verticality of their trunk after accidental rotation of the soil-root system due to wind or landslide, leading to typically curved stems shape. The very high dissymmetry of forces for the motor action is associated with the occurrence of reaction wood on one side of the inclined stem during many successive years. A method to reconstitute this biomechanical history from observations after tree felling, on either green or dry wood, is discussed. A selection of 17 trees from 15 different species (13 different families), tropical and temperate, hardwoods and softwoods, were selected and peripheral residual strains were measured in situ before felling, on 8 positions for each stem. Matched wooden rods were sawn and measured for their mechanical and physical properties in the green and dry states, allowing the estimation of tree growth stress, i.e., the force created by the living wood. It was possible to build easy-to-use conversion coefficients between the growth stress indicator (GSI), measured in situ by the single hole method, and growth strain and growth stress with the knowledge of basic density and green longitudinal elastic modulus. Maturation strain, specific modulus (as a proxy of micro-fibril angle) and longitudinal shrinkage are properties independent from basic density, whose variation among species was very large. For the whole range of compression wood, normal wood and tension wood, strong relationships between these 3 properties were observed, but together no single model, based on cell-wall microfibril angle only, could be defined. Growth forces are the product of 4 parameters: ring width, basic density, basic specific modulus and maturation strain, all of them being the result of wood formation. Thanks to the wide range of wood types and species, simple and highly significant formulas were obtained for the relationship between basic and dry density, green and dry longitudinal modulus of elasticity, basic and dry specific modulus. To estimate ring width in the green state from values in dry state, radial shrinkage needs to be measured afterwards on dry specimens. Maturation strains is less accurately linked to late measurements on dry wood, but longitudinal shrinkage offers a rather good solution for an estimation provided that the wood type (softwood, hardwood with-G layer, hardwood without G-Layer) is known.

Structural Characteristics of Reaction Tissue in Plants

“…It is common for young trees to accidentally tilt due to the rotation of their root systems, which may be caused by strong winds or landslides. While they may be restored to a vertical position in the following years, such growth changes often result in unique eccentric growth in their bases [5]. The Monterey cypress, which often has an asymmetric appearance due to the influence of one-sided sea winds, is a typical example of this [6].…”

Oak beams in medieval frameworks: Constraints and advantages for restoration

“…There is always a very strong proportional relationship between green and air dry properties with coefficient of determination R² around 80% [29]. We can use data on the mechanical properties for oak species in the US oaks at green and dry state [24] in order to have a good estimate of the changes in mechanical properties from green to dry (12% moisture content) wood (Table 5).…”

Modelling the growth stress in tree branches: eccentric growth vs. reaction wood

“…The stress values in the normal wood were fixed according to the average maturation strains advised by (Thibaut and Gril, 2021). Similarly, the green wood MOE were given by the correlation between dry and green MOE identified by (Thibaut and Gril, 2021): Eg = 0.89 * Ed. Dry MOE were provided by the tropix database of CIRAD (Gérard et al, 2011).…”

“…In most cases, a tensile maturation stress is produced in the newly formed 'normal wood'. But observations on inclined trunks (Alméras et al, 2005;Coutand et al, 2007;Thibaut and Gril, 2021), seedlings (Hung et al, 2016) and branches (Fisher and Stevenson, 1981;Huang et al, 2010;Hung et al, 2017;Tsai et al, 2012) have evidenced a clear difference between hardwood and softwood. Hardwoods produce 'tension wood', inducing a much higher tensile stress on one side, while for softwood, a compressive stress is induced in 'compression wood'.…”