WOOD STRUCTURE, CHEMICAL-COMPOSITION AND GROWTH STRAINS IN EUCALYPTUS CLONES

Contrary to a lot of hardwoods, the tension wood of Eucalyptus species is not characterised by G-fibres. It can however be characterised by its growth strains. The measurement of residual growth strains at the stem surface, by relaxing stresses at the periphery of trunks, allowed a continuous and quantitative classification of wooden samples coming from clones of hybrids Eucalyptus. This kind of measurement allows a mechanical identification of tension wood. The connections between structure and mechanical properties of these different woods were studied by ultrastructural and chemical analysis of samples, especially by a quantitative investigation of the monomeric compound of lignins by thioacidolysis. The results presented in this study show: i) a large variation of the residual growth strains with high values, in spite of the weak eccentricity, the good verticality and the absence in G-fibre in our trees; ii) a negative correlation between the level of the residual growth strains, the lignin content and the microfibril angle of cellulose (S2 layer of fibre); and iii) a positive correlation between the level of the residual growth strains and the ratio of the syringyl units to the guaiacyl units in lignin monomeric units. The significance of these correlations is discussed (direct mechanical causality or more intricate correlations between structural variables).