Résumé
The long -term (as opposed to short-term intra-ring) radial variation of wood properties in European beech (Fagus sylvatica L.) from pith to bark are largest in the young ages of the tree (internal core). This so-called juvenility reflects both cambium ageing (ontogenetic juvenility) and adaptation to the changing mechanical constraints during secondary growth (adaptive juvenility). Ring width (RW), specific gravity (SG) and specific modulus (SM) are important parameters for each new wood layer, needed for the study of mechanical stability of a standing tree. They should be sensitive to the mechanical adaptation of growth. They were measured on diametrical boards (North/South direction) issued from 86 trees from several high forest stands in European countries. Analysis of variance showed very significant influence of position within the tree (core/external), of trees within a plot and of plots, but not for North/South orientation. The share of variance was similar for SG and SM (importance of tree effect) but different for RW (importance of plot effect). The occurrence of red heartwood in the core on some trees had a significant influence, mostly on SM, but the differences between white and red wood was very small. Globally the variability was high for RW, rather small for SM and very small for SG. Accordingly, the variations of the modulus of elasticity (product of SG and SM) were much more influenced by SM than by SG for beech. The radial variations of each parameter were fitted by both a linear (2 coefficients: zero value and mean slope) and a parabolic curve (3 coefficients: zero value, initial slope and curvature). They were used to classify types or radial profile in terms of flat, up & down and straight, convex & concave. Median values of coefficients per plot (or total) were used to draw median profiles for each parameter per plot and at the global level. The median global profiles differed from the typical radial pattern (TRP) of juvenility for plantation softwoods for SG (down concave instead of up concave) and SM (convex like TRP but with a clear decrease in the mature wood). The main result was the very large variability of profiles between trees or even between plots. Even if there is a part of ontogenetic influence in the juvenile patterns for RW, SG and SM, the results suggest that the influence of mechanical constraints on tree growth (adaptive juvenility) dominates largely.