Jean-Michel Leban

Linking growth modelling to timber quality assessment for Norway spruce

Abstract The aim of this paper is to propose a consistent framework for analyzing the influence of silviculture, site quality and, to some extent, genetics on the wood production of Norway spruce from both a quantitative and a qualitative point of view. Tree and stand volume, stem taper, wood basic density, proportion of juvenile wood as well as knottiness are considered as the result of growth processes. Two complementary applications are presented. (1) An average-tree growth model which is built of several interrelated processes: site quality has an effect on height growth and hence on all other tree and stand characteristics; crown development is driven by height growth and controlled by stand density; stand basal area increment is predicted from empirical rules; tree basal area increment is then distributed along the stem. (2) A model that aims at assessing timber quality of a standing tree from usual inventory measurements such as tree age, height and diameter at breast height: growth equations are used to reconstruct the past growth of a tree and to predict its current internal structure, namely ring distribution. Both models are linked to allometric equations that estimate the characteristics of branchiness, to densitometric models that predict wood basic density from ring distribution and to a software that simulates the grading of any board located in a stem whose morphology is known in detail. The aim of these models is not to make precise quantitative predictions but to show how different pieces of knowledge of silviculturists, forest biometricians and wood scientists may be brought together in simulation software in order to help forest managers and wood industrialists to make decisions. This framework could be extended to other fast-growing coniferous species.

Determination of Young's modulus for spruce, fir and isotropic materials by the resonance flexure method with comparisons to static flexure and other dynamic methods

SummaryDynamic methods provide rapid and accurate means to determine Youngs modulus, i.e. the modulus of elasticity, of wood. For dry, clear specimens of épicéa commun (Norway spruce, picea excelsa) and sapin pictiné (silver fir, abies amabilis) we present a comparison of results from tests by a resonance flexure method with results obtained from four-point static flexure tests. For a wide range of specimen size the resonance flexure method provides a simpler, more rapidly performed alternative to the classical static flexure method, giving Youngs modulus values which are for the spruce and fir specimens of this study, nearly identical to those calculated from the static flexure tests. Results are also presented which show that a resonance longitudinal method yields higher values of Youngs modulus and an ultrasonic method yields still higher values. We provide also a comparison of the four test methods applied to isotropic materials.

Wood dowel bonding by high-speed rotation welding

High-speed rotation-induced wood dowel welding, without any adhesive, is shown here to rapidly yield wood joints of considerable strength. The mechanism of mechanically-induced highspeed rotation wood welding is shown here to be due, as already observed in vibration welding, to the temperature-induced softening and flowing of some amorphous, cells-interconnecting polymer material in the structure of wood, mainly lignin, but also of hemicelluloses and consequent high densification of the bonded interface. Wood species, relative diameter differences between the dowel and the receiving hole, and pressing time were shown to be parameters yielding significant strength differences; while relative orientation of the fibre grain of the dowel in relation to the fibre grain of the substrate, relative rate of rotation within a limited range and the use of rough or smooth dowels did not have any significant influence. X-ray microdensitometry and scanning electron microscopy were used to determine the limits of wood dowel welding by high-speed rotation. The type of parameters that had an influence on strength indicated that the strength values obtained, although often rather high, were often due to welding of only a limited part of the dowel to the substrate. This is due to the forcing of the dowel into a truncated conical shape by the pressure of insertion and the consequent disruption of bonding in some areas. Notwithstanding this effect, the welded contact area is sufficient to yield strength results comparable to or even slightly higher than that obtained by PVAc adhesive bonding. The use of dry dowels inserted hot in the substrate after preheating them at high temperature (100°C) yielded consistently better results than that obtained with PVAc gluing.

Parameter interactions in two-block welding and the wood nail concept in wood dowel welding

The interactions between parameters found to be determinant in wood dowel welding by high speed rotation have been evaluated. Of these, the interactions that proved to be the most significant, in descending order, were rotation rate/dowel moisture content, followed by rotation rate/ethylene glycol, and finally, at a lower level of significance, the interactions rotation rate/dowel temperature, wood grain direction/wood species and dowel temperature/wood species. Of the individual factors, once the most determinant factor already optimized in previous studies, namely the dowel/hole diameter difference, was fixed, the most significant were wood grain direction, dowel moisture content (dryness) and wood species. The optimized process yielded excellent strength results. The regression equations developed were able to predict the strength obtainable. The torque for insertion of the dowel in the substrate hole has been measured for several cases and the results are presented. In no cases the value of the torque needed for insertion was excessive and insertion was, therefore, easy. Wood joints composed of two pieces of timber held together by a dowel welded to both of them were assembled for the first time. Two further new concepts have also been advanced and tested: (i) the conical dowel, to maximize welding area and (ii) the concept of the wood nail in which a slightly conical fast-rotating hardwood dowel is inserted rapidly into a softwood substrate into which no hole has been pre-drilled. X-ray densitometry of the samples prepared with the latter approach showed some interesting mechanical interlocking features that might contribute to dowel bonding in softwoods.

Wood joints by through-dowel rotation welding: microstructure, 13C-NMR and water resistance

Two-block wood joints were obtained by insertion and welding without adhesives of dowels by high-speed rotation. Their strengths were better than that obtained by poly(vinyl acetate) (PVAc) gluing. X-ray-microdensitometry analysis showed that a complete welding of the dowel to the substrate occurred and that a perfectly tight joint was formed. Isolation of the flow material allowed CP-MAS 13C-NMR analysis of its composition with possibly low interference from the constituents from the substrate. The flow material appeared to be composed of hemicelluloses, apparently xylans, and lignin. Scanning electron microscopy coupled with the NMR analysis results showed that microfibrils of cellulose, in both amorphous and crystalline states, torn from the wood surface during welding, as well as very small proportions possibly of recrystallized xylans and furanic compounds formed by heat transformation of the carbohydrates, were present. The geometry of the dowel joint allowed the joint to maintain up to 88% of its initial tensile strength after 24-h immersion in cold water.

Picea abies sapwood width: Variations within and between trees

Abstract This study focused on the amount of sapwood and its variation by means of computed tomographic (CT) imaging. Twenty-four trees were selected from four Norway spruce [Picea abies (L.) Karst.] stands in north-eastern France, varying in age, density and fertility. In each stand, sampled trees represented the dominant, co-dominant and suppressed strata. The heartwood/sapwood boundary was detected from the CT images, and the heartwood and sapwood amount and their variations were then evaluated. At the within-tree level sapwood width was relatively constant along the tree stem above the butt swelling and below the living crown. The between-tree sapwood width variations were partially explained by the total cross-sectional area of living branches. This result opens up the possibility of investigating within-tree allometric relationships. Sapwood width was found to be highly correlated with tree slendemess (tree height/breast height diameter) and with the relative height of the crown. This suggests that sapwood width could be readily predicted from conventional forest inventory measurements. The number of sapwood rings within the stem was largely dependent on cambial age, and could be determined dynamically using the concept of mean lifetime of sapwood rings.

X-Ray Microtomography Studies of Tannin-Derived Organic and Carbon Foams

Tannin-based rigid foams of different bulk densities and their carbonized counterparts were investigated for the first time by X-ray microtomography. This method allowed acquisition of three-dimensional pictures of such highly porous materials. Through mathematical treatment of the images, extremely useful physical characteristics such as porosity, fraction of open cells, connectivity, tortuosity, and pore-size distribution were determined as a function of the foams density. The obtained information was compared with independent data derived from pycnometry measurements and scanning electron microscope image analysis. The agreement was shown to be acceptable in the limit of the accuracy of the laboratory microtomograph (4 microm). Moreover, recalculating properties like permeability were shown to be quite possible based on the results of standard microtomography data.

Measuring wood density by means of X-ray computer tomography

Abstract• Wood density is a characteristic of major interest. Usually, it is used as an indicator of wood quality; however, in the context of global change, it is increasingly used for biomass and carbon storage estimations. X-ray computer tomography is a method which enables quick estimates of wood density after applying a calibration procedure.• A review of the literature is presented in this article. Most of the previous studies have been performed in the 80’s or at the beginning of the 90’s.• In this study, the relationship between wood density and Hounsfield numbers was investigated using a recent medical scanner. A linear relationship was fitted using a calibration data set which consisted in tropical wood samples representing a large range of densities ranging between 133 and 1319 kg m−3, and then validated using an independent data set (mainly temperate tree species). The fitted relationships were very strong (R2 > 0.999), whichever the tested scanner settings, with slight but significant effects of the current voltage and reconstruction filters. The RMSE values computed from the validation data set ranged between 5.4 and 7.7 kg m−3 for densities ranging between 364 and 821 kg m−3.• In conclusion, this method of calibration enables the use of a medical scanner to obtain maps of wood density, in a fast and non destructive way, and with a very good accuracy. Very interesting perspectives are opened regarding biomass distribution within trees.Résumé• La densité du bois est une caractéristique d’intérêt majeur. Généralement, elle est utilisée comme un indicateur de la qualité du bois ; cependant, dans le contexte du changement global, elle est de plus en plus utilisée pour des estimations de biomasse et de stockage de carbone. La tomographie à rayons X est une méthode permettant des estimations rapides de la densité du bois moyennant une procédure de calibration.• Une revue de la littérature sur le sujet est présentée dans cet article. La plupart des études précédentes ont été réalisées dans les années 80 ou au début des années 90.• Dans cette étude, la relation entre la densité du bois et les nombres Hounsfield a été étudiée en utilisant un scanner médical récent. Une relation linéaire a été ajustée sur un jeu de données de calibration constitué d’échantillons de bois tropicaux représentant une large gamme de densités allant de 133 à 1319 kg m−3, et validée sur un jeu de données indépendant (principalement des essences tempérées). Les relations ajustées étaient très fortes (R2 > 0.999) quels que soient les réglages utilisés pour le scanner, avec des effets légers mais significatifs de la tension d’accélération et des filtres de reconstruction. Les valeurs des RMSE calculées à partir de l’échantillon de validation sont comprises entre 5.4 et 7.7 kg m−3 pour des densités allant de 364 à 821 kg m−3.• En conclusion, la méthode de calibration proposée permet l’utilisation d’un scanner médical pour obtenir de façon rapide et non destructive des cartes de densité du bois avec une très bonne précision. Des perspectives très intéressantes sont ouvertes concernant la répartition de la biomasse dans les arbres.

Nitrogen footprint in a long-term observation of forest growth over the twentieth century

Environmental drivers of forest productivity increases have been much debated. Evidence for the suggested role of increasing nitrogen supply is lacking over long-term time scales. Tracking the footprint of environmental factors by using long-term growth records may thus prove decisive. We analysed growth chronologies of common beech in two areas of contrasting nutritional status in France. Dominant height growth was used as a proxy for productivity. Growth was compared between old and young paired stands sampled at the same sites to factor out effects of ageing and site. Growth chronologies were estimated with a statistical modelling procedure. The environmental causality of growth changes was addressed by combining (1) a comparison of growth changes between regions, (2) a regional comparison of growth chronologies with chronologies of environmental factors and (3) growth–environment relationships established from climate/soil data. Historical growth increases followed very similar courses in the two areas. Remarkably, the magnitude of change was 50% lower in the area that had reduced nutritional status and nitrogen deposition. Historical variations in environmental factors and growth were congruent with the roles of nitrogen availability and deposition, and of atmospheric CO2 increase. Low-frequency variations in climate and growth were not coincident. However, our analysis demonstrated the role of climatic anomalies in short-term growth variations. Growth–environment relationships further indicated a nitrogen constraint. These observations corroborate the enhancing role of increased nitrogen availability on forest biomass accumulation previously reported in ecosystem experiments and process-based modelling explorations.

Improved water resistance of UF adhesives for plywood by small pMDI additions

Addition of small amounts of isocyanate (pMDI), between 10% and 15% by weight, to UF resins for plywood considerably improve the UF glue line water resistance. Analysis of the plywood by X-ray microdensitometry indicate that on both sides of the glue lines there is a local density increase which reflects either some adhesive diffusion in the wood layers immediately in contact with the glue line, or some extent of wood densification as a result of the gluing process, or both.