Françoise Huber

Measurement of free shrinkage at the tissue level using an optical microscope with an immersion objective: results obtained for Douglas fir (Pseudotsuga menziesii) and spruce (Picea abies)

Shrinkage at the tissue level has been evaluated satisfactorily using relatively simple equipment, comprising an optical microscope equipped with reflected light, a standard objective, a water immersion objective of same magnification and a digital camera connected to a computer. Shrinkage is calculated from pairs of images taken at the same magnification, one collected during immersion in water and the other in air-dry state. A novel software program has been developed to determine shrinkage based on a closed chain of reference points chosen from the anatomical markers at the external part of the zone of interest. Measurements were performed on earlywood, latewood and compression wood zones from two softwood species (Douglas fir and spruce), isolated from the rest of the annual ring with the aid of a diamond wire saw. As main results, reference should be made to the low degree of shrinkage and high anisotropy factor of earlywood, the marked and practically isotropic shrinkage in latewood and the low shrinkage (with respect to cell wall thickness) and inverse anisotropy ratio in compression wood.RésuméLe retrait a été évalué de façon satisfaisante à l’échelle tissulaire en utilisant un équipement relativement simple : un microscope optique, un objectif standard, un objectif à immersion de même grossissement et une caméra digitale reliée à un ordinateur personnel. Le retrait est déterminé par comparaison de deux images : l’une obtenue en immersion dans l’eau et l’autre à l’état sec à l’air. Un logiciel nouveau a été développé pour extraire le retrait à partir d’une chaîne fermée de points situés à la périphérie de la zone d’intérêt. Les mesures ont été effectuées sur des zones de bois initial, de bois de compression et de bois final de deux espèces (Douglas et sapin). Ces zones sont isolées du reste de l’accroissement initial à l’aide d’une scie à fil diamanté. Les principaux résultats montrent le faible niveau de retrait et la forte anisotropie du bois initial, le fort retrait, presque isotrope, du bois final et la faible valeur, en rapport à sa densité, avec une anisotropie inversée, du bois de compression.

Comparison of radial increment and volume growth in stems and roots of Quercus petraea

Dendrochronological studies dealing with roots, stems and branches are very rare or often take the form of short notes. The difficulties of detecting rings and of quantifying the radial growth in roots have already been described for various species. In oak the anatomical root structure differs from stemwood. The roots are radial-porous or diffuse-porous, and there is often no clear distinction between individual rings. In our study visual and radiographic techniques were used to examine radial increment in roots of sessile oak (Quercus petraea L.) which was compared with radial growth in branches and along the stem. Coarse roots were cut from four 30- to 34-year-old trees that had been uprooted mechanically and disks were taken at different distances from the stem-root base. Ring widths were measured in the stem at height of 0.3 m, at breast height (1.3 m), beneath the crown, in branches of the crown, and in roots every 20 cm. The ring widths were cross-dated, and the heterogeneity of growth within a root and within the root system were analysed. Asymmetric growth frequently occurred in roots so that ovals, I-beam and T-beam shapes were developed. With the method used in our study the annual growth layers close to the central cylinder could be distinguished as well as beneath the bark. Pointer years were detected in all sections of the tree and permitted correction of ring widths in roots. Root system, stem and branch showed a basic similarity in their radial sequence of ring width. The annual biomass increment was weaker and more variable with several consecutive changes in the roots than in the stems. The root/shoot ratio reached a minimum rather early, beginning at the cambial age of 20 years.

Contribution of gums to natural durability of Prosopis africana heartwood

Abstract The exceptional natural durability of Prosopis africana heartwood was investigated to find potential new biocides for wood preservation. Extractions carried out with different solvents indicated high levels of extractives which explained wood durability towards fungal and insects attacks. However, the extractives were not enough to explain the durability. The hydrophobic character of the wood also likely had a significant effect. Contact angle measurements before and after extraction, indicated that extractives have only a minor effect on wood hydrophobicity. Microscopic analysis reveals the presence of high levels of gums filling the wood cell lumens, limiting the penetration of water.

Free shrinkage of wood determined at the cellular level using an environmental scanning electron microscopE

The shrinkage at the cellular level was determined in the transverse plane of wood using an environmental scanning electron microscope (ESEM). The whole procedure is based on pairs of images grabbed with the same imaging conditions (magnification, working distance, voltage) without removing the sample from the chamber: one image collected at saturated conditions (1067 Pa, 1°C) and the other at air-dry state (533 Pa, 16°C). A closed chain of the same reference points chosen from the anatomical markers was defined on both images at the external part of the zone of interest. A custom software program, called MeshPore, allowed the shrinkage coefficients to be extracted from the slight difference of shape between these two chains. Measurements of transverse shrinkage were performed on earlywood and latewood zones from Norway spruce (Picea abies), only normal wood was studied. The interested zones were isolated from the rest of the annual ring with the aid of a microtome blade, insuring the observations to be done under free shrinkage. As additional features, the changes of the cellular morphology and the cell wall thickness were also evaluated thanks to the resolution provided by the ESEM.

Variabilité de la constitution anatomique d'accroissements annuels de chênes indigènes

Summary There are 8 species of oaks in France. This article shows the anatomical structure of annual increments, in transverse section, of only 5 french oaks species. Differences in wood feature are regarded more specially for the two species (pedunculate and sessile oaks) which hold a high reputation in the industry. To improve the quality of the oak wood, there have been many studies on the anatomy of oak wood. Inside the tree, vessel distribution and diameter depend on the position of the sample in the tree (juvenile and adult wood, duramen and sapwood). For a same growth ring width, in the adult wood, the percent of the different tissues varies between the trees and between the different oaks species. In the pedunculate oak, there are many vessels, but few fibers, counter to wood of the sessile oak. To conclude, we show a transverse section of a “chene paradoxal”.