Katri Luostarinen

Birch (Betula pendula) Wood Discolouration during Drying. Effect of Environmental Factors and Wood Location in the Trunk

Summary Birch wood discolours during artificial drying, which is a considerable problem for the use of birch as raw material in the mechanical wood industry. In this investigation sample boards of Betula pendula were sawn to the dimensions used as raw material for parquet billets. The variables studied were those that may affect wood discolouration: growing site, felling date, length of the storage period for logs and location of the wood in the trunk. Discoloration of birch wood was studied in conventional and vacuum drying processes. The wood layer of about 2–5 mm under the yellow surface of conventionally dried boards remained light-coloured, while the inner wood of the boards became discoloured. During vacuum drying, however, the inner wood of sample boards clearly became discoloured. The radial location of the wood in the trunk affected the colour of dried wood more than the longitudinal location did. To keep the colour of dry wood as light as possible, it may be best to fell birch in winter and store the wood for several weeks before conventional drying. On the other hand, the lightest coloured wood obtained during vacuum drying was from autumnfelled five weeks stored birches, which may indicate differences in the process of discoloration in these two drying methods.

Effects of coppicing on the root and stump carbohydrate dynamics in birches

Whole birch stems were cut off in order to determine how coppicing affects root and stump starch, glucose, fructose and sucrose concentrations and their correlation with shoot regeneration capacity. The Betula pubescens Ehrh. and B. pendula Roth studied included intact trees, trees that had been coppiced 8 years earlier, trees coppiced at the beginning of the experimental season, and birches that had been coppiced twice, 8 years earlier and at the beginning of the experimental season. Carbohydrate accumulation differed between 8 years earlier coppiced and intact trees. Recent coppicing clearly decreased the starch and sugar concentrations of the roots, which were often highest in the thin roots. The concentrations of these compounds in the stumps were always low, although the carbohydrate concentrations of stumps, in particular, correlated with shoot regeneration capacity. Starch was the most labile of the carbohydrates measured and most clearly reacted to coppicing. Differences in starch- and sugar-reserve dynamics indicate a difference between these birch species in the use and replenishing of root and stump reserves. This information may also be of help when the effects of other stresses, for example, severe animal damage or burning, on the regrowth of young birch stands are estimated.

Discolouration and deformations of birch parquet boards during conventional drying

Abstract As a light coloured and relatively hard species of wood, birch is much in demand for wooden floors in Scandinavia. The problem with birch wood is that it easily becomes discoloured during conventional drying. Visual sorting by colour is impossible before the boards are processed into parquet blocks because the discolouration, except for yellowing of the surface, occurs inside the board. In addition to discolouration, harmful deformations occur in birch boards during drying. Colour changes and deformations were compared in birch parquet boards dried according to three different schedules.

Effects of log storage and drying on birch (Betula pendula) wood proanthocyanidin concentration and discoloration

In conventional drying, sawn birch (Betula sp.) timber darkens and reddens from the inside while the layer a few millimetres under the yellowish surface remains light in color. Lack of information concerning the chemical basis of the discoloration hinders the development of a reliable solution for this problem. In this study, the role of soluble proanthocyanidins in discoloration of birch wood was investigated because the polymerization and oxidation of these compounds are known to yield insoluble reddish compounds. Different periods of log storage affected the synthesis of soluble proanthocyanidins during conventional drying. Concentration of proanthocyanidins also correlated with changes in the color of birch wood. Discoloration appeared differently in conventionally dried and vacuum-dried wood, which indicates that the discoloration mechanism in these drying methods may differ chemically, and/or the compounds that take part in discoloration may be different at different drying temperatures.

Relationship of selected cell characteristics and colour of silver birch wood after two different drying processes

Abstract Change in the colour of silver birch wood is a serious problem in the mechanical wood industry. Here, colour was correlated with microscopic characteristics of wood, such as cell types and dimensions, by drying processes. In conventional drying, with lower temperature than in vacuum drying used here, the most important factor causing darkened wood was wide latewood. In vacuum drying, thickness of the vessel walls affected wood darkening, as did broad rays and large amount of axial parenchyma. Axial and terminal parenchyma cells contained very small amounts of phenolics, but after drying at elevated temperature, a thin dark layer could be observed on the innerside of their walls. Phenolics were abundant in ray parenchyma; these compounds darkened at elevated temperatures, less in conventional drying than in vacuum drying. Phenolics were observed only inside cells, mainly in the parenchyma, but in vacuum-dried wood also in fibres and vessels. Anatomical characteristics are known to be affected by both environmental and genetic factors. Thus it might be possible to influence the colour reaction of birch wood during the drying process by choosing wood according to growing-site conditions, or by choosing the seed source for birch plantations according to given anatomical characteristics.

Effect of growing site, sampling date, wood location in trunk and drying method on concentration of soluble proanthocyanidins in Betula pendula wood, with special reference to wood colour

Proanthocyanidins (PAs), which are common phenolic compounds in plants, are considered to be mainly defensive compounds against different kinds of stress, e.g. herbivory. In addition to increasing after stress, they may condense and/or oxidize to form coloured compounds when cells die, which may affect both the resistance of xylem to rot and its colour. This is important in the mechanical wood industry. The effects of some biological factors (growing site, sampling date, wood location in the trunk) on the concentration of soluble PAs (sPAs) were studied in fresh and dried birch wood. Those factors that affected sPA concentration most were sampling date and radial location. Drying method also had a great effect on the sPA concentration, which in turn correlated with the colour of the wood.

Relation of arabinogalactans to density, growth rate and shear strength in wood of cultivated Siberian larch

Present in high concentrations in the heartwood of Larix species, water-soluble arabinogalactans affect the density, moisture content and processing properties of larch wood. The aim of this study was to chart arabinogalactan concentration (AC) and to define the relations between arabinogalactans, and density and shear strength at different axial and radial locations in Larix sibirica Ledeb. trunks. In addition, growth rate (ring width, fibre dimensions) was compared with AC to determine the link between them. Although quite low AC levels were observed, values differed significantly between sapwood and heartwood, with a slight increasing trend from pith to mature heartwood. Variation in growth rate affected AC levels in juvenile wood only. In conclusion, AC levels were clearly lower in the studied cultivated, fast-grown Siberian larches than those measured earlier in trees of natural origin. AC had a weak correlation with growth rate and shear strength.ZusammenfassungWasserlösliche Arabinogalactane, die in hohen Konzentrationen im Kernholz von Lärchen vorhanden sind, haben einen Einfluss auf die Dichte, die Holzfeuchte und die Verarbeitungseigenschaften dieses Holzes. Ziel dieser Studie war es, die Arabinogalactankonzentration (AC) zu ermitteln und die Beziehung zwischen Arabinogalactanen und der Dichte sowie der Scherfestigkeit in verschiedenen axialen und radialen Lagen im Stamm von Larix sibirica Ledeb. zu bestimmen. Darüber hinaus wurde die Wuchsleistung (Jahrringbreite, Faserabmessungen) mit der Arabinogalactankonzentration verglichen, um einen möglichen Zusammenhang aufzuzeigen. Trotz relativ niedriger Arabinogalactankonzentrationen unterschieden sich die Werte von Splint- und Kernholz signifikant, wobei eine leichte Zunahme vom Mark hin zum adulten Kernholz festzustellen war. Unterschiedliche Wachstumsgeschwindigkeiten hatten nur einen Einfluss auf die Arabinogalactankonzentration in juvenilem Holz. Zusammenfassend ist zu sagen, dass die Arabinogalactankonzentration der hier untersuchten, angebauten, schnellwachsenden sibirischen Lärchen eindeutig niedriger war als die der früher untersuchten Lärchen aus natürlichen Beständen. Die Arabinogalactankonzentration korrelierte schwach mit der Wuchsleistung und der Scherfestigkeit.

Effect of radial origin on final moisture content and gradient, casehardening, cracking and deformations of dried Siberian larch (Larix sibirica) timber

Siberian larch (Larix sibirica Ledeb.) timber is a good raw material, particularly for the carpentry industry. However, in the radial direction varying wood properties, like density, may have an effect on the drying quality of sawn larch timber. Thus, final moisture content (MC), MC gradient, casehardening, cracking and deformations were studied according to radial origins, taking into account the orientation of annual rings. The effect of radial origin on MC properties was mainly caused by the differences in the basic density between different radial locations. Differing ring orientation between planks of different radial origin most probably affected the MC gradient. As casehardening and deformations had clear trends in radial direction, differences in them may be largely caused by MC differences. Thus including larch timber of similar density and ring orientation in the same drying lot would help achieve larch timber of uniform drying quality.ZusammenfassungSibirische Lärche (Larix sibirica Ledeb.) ist ein sehr guter Rohstoff insbesondere für den Holzbau. Allerdings können die in radialer Richtung variierenden Holzeigenschaften, wie zum Beispiel die Dichte, einen Einfluss auf die Trocknungsqualität von Lärchenschnittholz haben. Deshalb wurden der Einfluss der Lage im Stammquerschnitt und der Jahrringverlauf auf die Endfeuchte (MC), den Feuchtegradienten, die Verschalung, die Rissbildung und die Verformungen untersucht. Der Einfluss der Lage auf die Feuchteeigenschaften ist hauptsächlich auf Unterschiede in der Rohdichte in radialer Richtung zurückzuführen. Unterschiedlicher Jahrringverlauf im Schnittholz hatte höchstwahrscheinlich einen Einfluss auf den Feuchtegradienten. Da sich bei der Verschalung und den Verformungen ein klarer Trend in radialer Richtung zeigte, dürften die Unterschiede zu einem großen Teil auf die Holzfeuchteunterschiede zurückzuführen sein. Daraus folgt, dass Lärchenschnittholz mit einheitlicher Trocknungsqualität erzeugt werden könnte, wenn das Holz einer Trocknungscharge ähnliche Rohdichte und ähnlichen Jahrringverlauf aufweisen würde.

Dependence of shear strength on wood properties in cultivated Larix sibirica

Abstract Mechanical properties are very important in structural uses of wood. In the case of larch wood, shear strength (SS) may be a limiting property, because the wood is brittle and it splinters easily. In this study, SS of Siberian larch (Larix sibirica) wood was studied in relation to its axial and radial location and to some other wood properties in the part of the trunk that produces logs of good carpentry quality. SS increased by 21–23% from the pith to the outer heartwood at each studied height (butt, 4.5 and 9 m), and it was at its most approximately 7% higher at the butt than at the other studied heights. The main reason for the radial increase in SS was the increase in density caused by wood maturation, but other factors also affected SS, depending on the grouping mode (combined material, radial/axial location). An exception was sapwood, for which no factors affecting SS could be identified. Thus, sapwood may behave differently from heartwood regarding use, even unexpectedly. The results suggest that mature heartwood, but not juvenile wood and sapwood, of Larix sibirica should primarily be used in products that require high SS.

Effects of basic density, annual growth and drying schedule on selected quality factors of dried Siberian larch (Larix sibirica) timber

Abstract Siberian larch (Larix sibirica Ledeb.) timber is suitable for many uses in the mechanical wood industry. Drying it without any decrease in value, however, is difficult and slow. The purpose of this research was to compare the drying quality of Siberian larch timber dried with three different conventional schedules taking into account the density and annual growth of wood. Five drying tests were performed. The final moisture content (MC), MC gradient, cracks, deformations (bow, crook, twist and cup) and case hardening were measured from the dried timber. The basic density particularly affected the MC, as shown in the differences regarding dried wood, with the denser wood having higher final MC and MC gradient. It was also found that large annual growth increased some deformations. Most of the measured factors were best after drying at the highest temperatures used; however, a slightly different trend was observed for bow, twist and cup. MC factors and twisting were the most problematic properties in drying according to this study. Sorting Siberian larch timber, particularly according to density, would improve the MC properties of dried timber by ensuring sufficient drying time, as economically as possible, for each timber piece.