BIOMEChANICAL pERfORMANCES Of TREES IN ThE phASE Of ACTIVE REORIENTATION

DLOUhá, J., ALMÉRAS, T., CLAIR, B., GRIL, J., hORáčEK, p.: Biomechani-cal performances of trees in the phase of active reorientation. Acta univ. agric. et silvic. Mendel. Brun., 2008, LVI, No. 5, pp. 00–00 The purpose of the present paper was to investigate the accumulation of growth stresses in a cross section of a tree in active reorientation process and its biomechanical performances i.e. up-righting efficiency and stem flexibility. Effect of two factors was analysed in details: occurrence of juvenile wood and viscoelasticity of wood tissues. In a phase of active re-orientation, wood tissues close to the pith are submitted to significant levels of compressive stresses. production of juvenile wood in earlier stage of a tree life seems to increase the stem flexibility during active reorientation for both softwoods as well as hardwoods. Concerning the viscoelasticity of wood tissues, only minor effect has been observed in softwoods while an important positive impact has been pointed out in hardwoods. Set of simulations with increasing level of maturation strains in reaction tissues indicate possible trade-off between the stem flexibility and the up-righting efficiency. tree reorientation, stem flexibility, up-righting efficiency, juvenile wood, viscoelasticity During its life, a tree is submitted to an incremental diameter growth together with an increase of external forces, mainly due to the weight of its crown. To improve the stiffness of newly formed cells, wood tissues are pre-stressed during their maturation. Due to the lignin deposition, the newly formed wood cells have a tendency to shrink along the fibres howe ver this deformation is restricted by older (already stiff) wood layers thus inducing considerable prestresses in the new layer. Asymmetric distribution of pre-stresses around its circumference allows the tree to control the verticality of its stem and the shape of branches (Archer, 1986). It is well known that the distribution and magnitude of growth stresses depends on the relative speed of loading and of secondary wood formation. however, the viscoelastic nature of wood introduces an additional time dependency through the diff eren tial relaxation of various tissues. This aspect is only poorly documented and probably introduces a significant bias in the biomechanical models used for tree assessment or growth description. Mechanical analysis of progressively loaded growing structure submitted to maturation strains was done by fournier (fournier et al., 1991a; fournier et al., 1991b). Alméras (Alméras et al., 2005) investigated the impact of maturation strain and Young modulus differential between normal and reaction wood and of the growth eccentricity on the efficiency of reorientation process. finite element method was also applied to analyse the shape regulation in trees (fourcaud & Lac, 2003). however only few modelling approaches take into account viscoelastic properties of wood (Gril & fournier, 1993) and no references were found about the influence of juvenile wood transition. In the present study, we have investigated the accumulation of growth stresses in a tree stem based on simple cross-section model allowing general distribution of wood properties. The aim was to investigate the impact of viscoelasticity of wood tissues and juvenile wood occurrence on the reorientation process. further, the impact of the growth ec cen tri ci ty was investigated. presented simulations are based on estimated values of wood properties that will be replaced, when available, by experimental values. MAteRiAls And Methods A simple model based on beam theory has been developed to simulate the dynamics of the stem reorientation and the evolution of its fle xi biha l-0 07 97 01 4, v er si on 1 5 M ar 2 01 3 Author manuscript, published in "Acta Universitatis Agriculturae Et Silviculturae Mendelianae Brunensis LVI, 5 (2008) 1-6"