The mechanics of Norway spruce [Picea abies (L.) Karst]:: mechanical properties of standing trees from different thinning regimes

In the first part, the authors discuss the mechanical stability of dominant and suppressed Norway spruce trees (Picea abies) grown under differing thinning regimes with a focus on the stem shape and the mechanical properties of the standing tree. In the second part, they concentrate on the variation of the modulus of elasticity of green wood (MOEfresh) and the density within the stem to analyse the effect of the underlying patterns in variation of these wood properties on the mechanics of the entire stem. At the standing-tree level, the investigations comprised st morphological description and static bending tests on the trees to measure flexural stiffness and structural Young's modulus of the stem. Discs sampled from different stem heights were examined to distinguish between different tissues (cortex, wood, pith) and wood types (adult, adolescent, juvenile), and to measure quantitatively their contribution to the axial second moment of area of the stem. An analysis of the variation of density and MOEfresh within the stem was carried out on small wood samples cut out of stem sections taken from the same height as the stem discs. The flexural stiffness of the trees is mainly influenced by stem radius, resulting in a higher flexural stiffness for thicker trees. Independently from the stem dimensions, the structural Young's modulus was slightly higher for the suppressed trees within each site. In an individual tree the structural Young's modulus decreases with increasing stem height. The cross-sectional analyses of wood type proportions and their characterisation by density and MOEfresh showed that the distribution of adult, adolescent and juvenile wood within the stem affects the structural Young's modulus and the flexural stiffness of the stem. Finally, a method is presented that allows a re-calculation of the mechanical bending properties of the entire standing trees using MOE and axial second moments of area of the different wood types. Our results suggest that further modelling of mechanical behaviour of trees might be possible taking into account the composite nature of conifer stems. (C) 2000 Elsevier Science B.V. All rights reserved.