MEASUREMENTS OF RESIDUAL GROWTH STRAINS A T THE STEM SURFACE OBSERVATIONS ON DIFFERENT SPECIES

The wood in standing trees undergoes internal stress during the entire life of the tree. This stress, commonly named growth stress, originat es in maturation strains and is impeded by the mass of the entire trun k. Released strain at the stem surface (ie strain on a small piece of wood isolated from the stem by cutting grooves, drilling holes etc) me asures maturation strain. Our research program on 'Tree architecture, anatomy and mechanics' aims at i) understanding the biological control of maturation strain with regard to tree morphogenesis (branching pat terns, crown form, stem and shoot positions in relation to vertical di rection); ii) qualifying correlations between maturation strain and th e anatomical features of the wood and other wood characteristics (stif fness, shrinkage, hygrothermal recovery); iii) modelizing and qualifyi ng the strew in the entire tree that results from the cumulative matur ation of successive layers during cambial growth, in order to understa nd cracks and strains when the wood is processed. This paper focuses o n results concerning longitudinal maturation strain at the stem periph ery measured on different species. Two methods have been used. i) the single hole method with a special sensor designed in the Centre Techni que Forestier Tropical; and ii) measurements of strain due to 2 groove s sawn above and below a classical electric sensor. Comparisons betwee n the 2 methods show quite good, but not perfect, agreements in beech and eucalyptus but not in chesnut. These results are discussed from th e sensor dimensions, the principles of the method and the anatomical a nd mechanical properties of wood. Measurements on one tree show import ant variations with height and angular position, which are correlated to tree morphogenesis (proximity of branches and righting movements of stems). High strain values are never homogeneous in the tree but conc entrated in small angular sectors. This angular asymmetry of maturatio n strain is obviously related to stem bending movement as one side of the stem 'pulls' or 'pushes' the other. Furthermore, histograms of val ues measured on Pinus pinaster, clones of Eucalyptus (PF1 1.45, UAIC-C TF7, Congo) and poplar (Populus euramericana cv 1214), Castanea saliva , Fagus sylvatica and Eperua falcata show that the distribution of str ains is not Gaussian. The long tail of tensile (in hardwoods) or compr essive (in softwood) values correlates with the formation of reaction wood (compression or tension). The main difference between populations is not the mean value (out of the tail, in normal standard wood) but the width and maximum of the tail. Hence, to study variability of grow th stress in a population of trees, we must study the frequency and th e magnitude of peaks of high maturation strains' within trees, rather than mean strains. Therefore, growth stress should be analyzed in corr elation with the regulation of the form of the tree and particularly i n correlation with the kinetics of stem movement (changes of curvature and lean). Finally, uncommon patterns of release strain with 2 opposi te angular peaks have been observed in some tropical trees as Dichoste mma sp, Saccoglotis gabonensis, Eperua falcata and Castanea sativa. Th ese patterns can be related to the tree architecture. In sympodial tre es (ie trees in which the trunk is built by stacks of branches as stem s formed from axillary buds take over from the former leader), peaks o f high maturation strains seem to be induced by the different axes (th e present leader and the former) and thus, in a cross-section, 2 flows of highly strained wood can be observed. A functional explanation of such patterns is not obvious.