The interrelation between microfibril angle (MFA) and hygrothermal recovery (HTR) in compression wood and normal wood of Sugi and Agathis

Abstract

Abstract Tree growth stress (GS) consists of an elastic component and a viscoelastic locked-in component. The elastic component is released instantaneously by cutting wood, whereas the locked-in component remains. The latter can be released by hot water treatment, which is known as hygrothermal recovery (HTR). In this paper the mechanism behind HTR is described and interpreted in terms of the microfibril angle (MFA) in the cell wall as follows: during cell-wall maturation, axial tensile stress is generated in the cellulose microfibrils (CMF), whereas isotropic compressive stress is generated in the matrix of lignin-hemicellulose (MT). Some amount of microscopic stresses remains following the removal of the wood from the living stem. Hygrothermal (HT) treatment induces recovery of remaining compressive stress in the MT, which causes its expansion. Axial tensile stress in the CMF are released by HT softening of the MT. This causes the CMF to contract along its length and to expand laterally. The combined effect of the expansions of the MT and contraction of the CMF causes the wood to deform anisotropically. This is the HTR of wood. The degree of anisotropy is determined by the MFA on the basis of reinforced-matrix theory.