Bernard Thibaut

On the detachment of the gelatinous layer in tension wood fiber

The detachment of the gelatinous layer (G-layer), often observed on microtome cross sections, has led some authors to believe that the G-layer cannot act as the driving force of longitudinal shrinkage in tension wood. The aim of this study was to observe the detachment of the G-layer along fibers. Green wood blocks were cut transversely into two samples. One sample was kept in water and the other was oven-dried. With one face being common to both samples, the detachment of the G-layer was studied on the same fibers. Observations were performed after blocking deformation by embedding. This revealed that the detachment of the G-layer is an effect produced by the act of cutting the transverse face of the wood block to be embedded. At distances greater than 100 µm from this primary surface of the sample, no detachment was observed. Drying shrinkage shows little or no effect on this detachment. The result seems to explain well why the detachment of the G-layer occurs during sectioning using conventional sliding microtomy. These observations prove the adhesion of the G-layer in massive wood and confirm the active role of the G-layer in tension wood properties.

Mechanics of wood and trees: some new highlights for an old story

The main aspects of wood mechanics are approached through the role played by wood as the building material of the tree. The main concepts used by nature: progressive setting up of tree weight and architecture, multifunctional role of wood and adaptability to hazard and long-term evolution, are clarified together with their consequence on the multilayered, anisotropic nature of wood. The technological choices of the plant world as the cellular structure, the use of composite with oriented fibres and hydro-carbonated polymers, as well as the systematic prestressing of every new layer, bear obvious consequences on the type of mechanical behaviour of wood. The local control of the level of prestressing through minor modifications of the cell-wall biosynthesis allows the tree to adapt to conditions of light exploration that will evolve according to time and risk occurrence. This analysis of wood genesis process permits in return to improve our understanding of the mechanical behaviour of the material in relation with the parameters of the microstructure.

The decreasing radial wood stiffness pattern of some tropical trees growing in the primary forest is reversed and increases when they are grown in a plantation

BackgroundThis study examines the radial trend in wood stiffness of tropical rainforest trees. The objective was to determine if the type of growing environment (exposed plantation or dense primary forest) would have an effect on this radial trend.MethodsThe axial elastic modulus of wood samples, representing a pith to bark cross-section, of six trees from several French Guianese species (two of Eperua falcata, one of Eperua grandiflora, two of Carapa procera and one of Symphonia gloubulifera) was measured using a dynamic “forced vibration” method.ResultsPrimary forest trees were observed to have a decrease in wood stiffness from pith to bark, whereas plantation trees, from the same genus or species, displayed a corresponding increase in wood stiffness. Juvenile wood stiffness appears to vary depending on the environment in which the tree had grown.ConclusionWe suggest that the growth strategy of primary forest trees is to produce wood resistant to self-buckling so that the height of the canopy may be obtained with the maximum of efficiency. In contrast, the growth strategy of the trees growing in an exposed plantation is to produce low-stiffness wood, important to provide flexibility in wind. Further experiments to study the behaviour of more species, with more individuals per species, growing across a range of physical environments, are required.

Shrinkage of cane (Arundo donax) I. Irregular shrinkage of green cane due to the collapse of parenchyma cells

Shrinkage of green cane (Arundo donax L.) was measured during air-drying at room temperature. The cane began to shrink at 150% moisture content due to a remarkable collapse of parenchyma cells. The collapse recovered after boiling in water, but more serious collapse (recollapse) was induced by the following drying. On the other hand, the collapse recovered almost completely after steaming with saturated water vapor at 92°–96°C without recollapse. By comparing the thickness of cane specimens before and after steaming, the degree of cell collapse remaining in dry cane was evaluated. When the green cane was frozen prior to drying, the degree of collapse was reduced whereas the drying rate remained unchanged. The effect of prefreezing was interpreted as the generation of air bubbles in the cell lumen which hinder the effective loading of liquid tension on the cell wall. Even when the cane was carefully dried using a conventional method used by reed manufacturers, the degree of collapse was very large and it increased with elevating internode position.

Physical and mechanical properties of reaction wood

Reaction wood produces very peculiar maturation stresses at the tree periphery, i.e. compressive stress or very high tensile stress, for compression and tension wood, respectively, as compared to moderately high tensile stress for normal wood. This means that both its mechanical state and its mechanical and physical properties differ from normal wood.

Commercial Implications of Reaction Wood and the Influence of Forest Management

In general reaction wood creates problems in timber processing and service whether for solid timber, panel products or pulp and paper production. For example, timber containing reaction wood is more difficult to saw and takes a poorer surface. Compression wood also creates difficulties for papermaking because of its high lignin content although pulp is easier to produce from tension wood because of its lower lignin content. Part of the problems in performance of products incorporating reaction wood arises from the generally inferior mechanical properties of reaction wood compared to normal wood and part of the difficulty arises from the increased variability in wood properties introduced by the presence of reaction wood. Avoiding reaction wood formation in forest trees to reduce the problems in processing requires careful attention to site, species choice and management at all stages of the life of a tree. Although there can be sometimes conflicting evidence for the benefits or otherwise of a particular management option, in general any action that leads to unstable root systems, stem sweep or lean, unbalanced root to shoot biomass allocation, eccentric crowns or increased wind or snow loading will have a tendency to produce reaction wood. The key is for stand management to avoid, whenever possible, large scale changes that result in the requirement for major adaptation of the trees to new growing conditions.

The molecular mechanisms of reaction wood induction

Reaction wood originates from cambial activity which adjust cell division activity, proportion of fibres, cell wall structure and properties, so that the resulting growth event will be the appropriate response to endogenous and environmental stimuli.