Simulation of effects of wood microstructure on water transport

A tracheid-level model was used to quantify the effects of differences in wood microstructure between coastal and interior Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii and var. glauca) wood on larger scale properties like hydraulic conductivity. The model showed that tracheid length, the ease of flow through a bordered pit and effective tracheid diameter can all limit maximum hydraulic conductivity. Among the model parameters tested, increasing bordered pit conductivity and tracheid length resulted in the greatest increase in maximum conductivity in both the inland and coastal ecotypes. A sensitivity analysis of the uncertainty between parameters governing flow through the bordered pit and air-seeding potential showed that, although decreased pit flow resistance increased maximum hydraulic conductivity, increased cavitation led to lower conductivity over time. The benefits of increasing the number of bordered pits depended on the intensity of the meteorological driving function: in drier environmental conditions, wood with fewer pits was more conductive over time than wood with more pits. Switching the bordered pit characteristics between coastal and interior wood indicated that the conductivity time course of coastal and interior wood was primarily governed by differences in the number of bordered pits and not differences in tracheid dimensions. The rate at which tracheids refilled had little effect on the conductivity time course of either coastal or interior wood during the first two summers when the wood was highly saturated, but had a marked influence in subsequent years once the cavitation profile stabilized. Our work highlights the need for more empirical work on bordered pits to determine whether variation in their number and properties is related to changing environmental conditions. In addition, a detailed simulation model of a bordered pit is needed to understand how variation in pit properties affects the relationship between ease of flow through a bordered pit and its potential for facilitating air-seeding.     

Influence of region of provenance and climate factors on wood anatomical traits of <Emphasis Type="Italic">Pinus nigra</Emphasis> Arn. subsp. <Emphasis Type="Italic">salzmannii</Emphasis>

Region of provenance is defined as an area with uniform ecological conditions where stands with similar phenotypic or genetic features are found. This study assesses the effect of differing climate conditions of eight Spanish regions of provenance of Pinus nigra Arn. subsp. salzmannii on earlywood anatomical traits measured in samples from basal discs from mature trees. Results showed that variation in wood biometry between provenances was high and more pronounced than intrapopulation variation. When comparing P. nigra with other Mediterranean pines, high intertracheid wall strength values are associated with better adaptation of pines to arid conditions. However, the intraspecific variations of this parameter in P. nigra did not follow the same pattern, due to the influence of mechanical support requirements. Trees subject to greater aridity were characterised by short tracheids, apparently resulting from their poorer growth, and high frequency of rays and ray parenchyma cells, which would allow trees to store greater amounts of starch, which is the source of metabolites invested in minimising the limitations imposed by water stress. Severe winter cold spells were strongly associated with high axial resin canal frequency and large radial resin canals, creating a powerful, preformed defence system. Increased tracheid lumen involved an increase in the size of bordered pits, favouring sap flow between tracheids, in addition to an increase in the maximum diameter of cross-field pits, favouring the flow of water and metabolites between the axial and radial systems. The high influence of region of provenance on structural variation in P. nigra shows the importance of provenance in the selection of seed origin for reforestation.     

On the fine structure of bamboo fibres

Summary The thick-walled bamboo fibres exhibit a polylamellate structure with alternating broad and narrow lamellae. Characteristically the cellulose fibrils in the broad lamellae are oriented almost parallel to the longitudinal axis of the fibre (2–20°), whereby there is a gradual but only slight increase in the angle from middle lamella to lumen. The narrow lamellae consist of fibrils oriented almost perpendicular to the cell axis (85–90°); this angle remains constant in all the successive narrow lamellae. The concentration of lignin is higher in the narrow lamellae than in the broad ones. Xylan seems to occur in a higher concentration in the narrow lamellae. The pits between the fibres are bordered. The results are discussed in relation to earlier data on wall structure and development. A model for the thick-walled bamboo fibre is presented with a new terminology for the various lamellae.We are thankful to Mrs. R. Schultze and Miss B. Schröder for their technical assistance. Grateful acknowledgement is also due to Dr. D. Keyser and Mrs. K. Hoffmann for the SEM facilities and to Prof. Dr. J. Bauch and Mrs. R. Endeward for the UV-microspectrophotometry. Prof. T. E. Timell (Syracuse) and Dr. O. Faix were helpful with the preparation of hydrofluoric acid concentrations.     

Application of percolation modelling on end-grain water absorption in aspen (Populus tremula L.)

Abstract

The aim of this study was to improve our understanding of the capillary uptake of water in aspen sapwood and heartwood, which may help to explain the variable performance of aspen used outdoors. The study uses a percolation model developed for softwoods to examine capillary liquid absorption in aspen and compares predicted results with ones obtained experimentally using computed tomographic scanning. The study shows that there is an equilibrium saturation level where new flow paths are being found at the same rate as old ones are being blocked. This is seen as a plateau where the water content maintains a relatively constant height within the material. This makes it possible to model the capillary behaviour of aspen sapwood. In heartwood, however, the uptake of moisture seems to be mainly restricted to bound water, except for a short region in the sample wet end. This absorption is thus governed mainly by bound water diffusion. This improved understanding of the capillary behaviour of aspen may contribute to a future market expansion where aspen may be utilized to a greater extent outdoors and above ground.     

Predicting anisotropic shringkage of softwood Part 1: Theories

 The phenomenon of wood shrinkage by losing moisture can be analysed at four levels: molecular, ultrastructural, microstructural and macrostructural levels. To predict the shrinkage of wood cells, the model of Barber and Meylan is modified in the current work to reflect combined effects of shrinkage of the cell wall, changes of the lumen shape and effects of rays and bordered pits. Where a piece of wood contains a multi-layer of earlywood and latewood or multi-layer of normal and defect wood with variable properties, a model is proposed to relate the total, measurable shrinkage to the shrinkage of each layer. The model can be applied to a specimen with asymmetric properties through the thickness. In such a board, bow (or crook), cup and twist are often observed. The modified and proposed models involve several mechanical properties of the cell wall which are difficult to measure. These properties vary with wood types, such as earlywood, late wood, compression wood, or wood with spiral grain. However, an alternative method may be used to obtain these properties from experimentally measured shrinkage data, and this method will be presented in a subsequent paper. Received 25 January 1999     

Moisture content gradient in a softwood board during drying: simulation from a 2-D model and measurement

Summary A 2-D mathematical model was developed to simulate moisture movement and heat transfer in width and thickness directions within a softwood board during drying. The model is based on wood physiological features and behaviour observed during drying. In sapwood, liquid water movement is assumed to be a consequence of capillary action between liquid and gas phases inside the cell lumens. However, liquid flow does not occur in wood close to the exposed surfaces because at timber surfaces the wood cells are damaged during the sawing process and consequently the liquid column is broken. In heartwood, liquid flow is negligible since the pits are normally aspirated during the formation of heartwood in the growing tree. Water vapour moves under a partial vapour pressure gradient while bound water diffuses within the wood material due to differences in chemical potential. The model was solved numerically to predict moisture-content profiles. Experiments were undertaken to measure the moisturecontent gradient. Samples were removed from a tunnel dryer at intervals throughout drying, frozen overnight and then cut into slices for moisture-content determination. The experimental results were used to verify the model.This work is supported by the New Zealand Foundation of Research, Science and Technology under contract CO4415     

Study of the transverse liquid flow paths in pine and spruce using scanning electron microscopy

Samples of pine (Pinus sylvestris) and spruce (Picea abies) were impregnated with a low-viscous epoxy resin using a vacuum process. The epoxy was cured in situ and the specimens sectioned. Deposits of the cured epoxy was then observed in the wood cavities using a scanning electron microscope. The investigation concentrated on tracing the transverse movements of a viscous liquid in the wood, and special attention was therefore given to the cross-field area between ray cells and longitudinal tracheids. A damage hypothesis is proposed based on the results obtained in the present investigation in combination with those from earlier studies on linseed oil-impregnated pine: In addition to the morphology of the bordered pits, viscous liquid flow in wood is dependent on damage that occurs during the impregnation procedure. For pine sapwood, liquid flow is enabled through disrupted window pit membranes, which divide the longitudinal tracheids and the ray parenchyma cells. A mechanism accounting for the reduced permeability of pine heartwood is believed to be deposits of higher-molecular-weight substances (extractives) in the ray parenchyma cells and on the cell walls. In spruce the thicker ray cells in combination with the smaller pits, which are connected to the longitudinal tracheids, reduce permeability considerably.     

The effects of cell wall swelling on the permeability of grand fir wood

Summary The longitudinal permeability of Abies grandis wood swollen to varying degree by water vapour sorption has been measured using a non-swelling liquid. The results obtained show that cell wall swelling causes a reversible decrease in permeability in both air dried and solvent exchange dried specimens.Direct microscopic measurements have shown that change in lumen diameter cannot explain the observed effect.Mathematical analysis of the data suggests that the permeability decrease may be due to increase in thickness of bordered pit membrane fibrils resulting from sorption of water.     

Anatomical differences in the structural elements of fluid passage of Scots pine sapwood with contrasting treatability

Treatability of wood is a function of anatomical properties developed under certain growing conditions. While Scots pine sapwood material normally is considered as easy to impregnate, great variations in treatability can be observed. In order to study anatomical differences in the structural elements of transverse fluid passage, wood material with contrasting treatability has been compared. Ray composition and resin canal network, membrane areas of fenestriform pits in the cross-field as well as dimension and properties of bordered pits were investigated. The results showed large anatomical differences between the two contrasting treatability groups. Refractory Scots pine sapwood samples developed more rays per mm² tangential section, while they were on average lower in cell numbers than rays found in easily treatable material. Easily treatable material had more parenchyma cells in rays than refractory material. At the same time, a larger membrane area in fenestriform pits in the cross-field was observed in the easily treatable sample fraction. Differences in the composition of resin canal network were not observed. Refractory samples developed on average smaller bordered pit features, with relatively small formed pit apertures compared to the easily treatable samples. In refractory Scots pine sapwood material, the structural elements of fluid passage such as bordered pit dimensions, fenestriform pits in the cross-field and parenchyma cells were altogether developed in smaller dimensions or number. Wood samples from better growing conditions and sufficient water supply showed a better treatability in this study.     

Unexpected experimental results for capillary suction in wood: Analysis on the fibre level

Abstract

A series of simple experiments on capillary water uptake in rectangular sticks of Scots Pine (Pinus sylvestris L.) gave some unexpected results. Sticks of sapwood and heartwood with lengths of 100, 200 and 300 mm, with the four long sides sealed, were used in the experiments. The samples were allowed to suck water, one group for 1000 h and one group for 1900 h. After exposure, as anticipated, the heartwood samples had a moisture content (MC) distribution with high values at the bottom and steeply decreasing values towards the top. The sapwood samples, in contrast, had moisture distributions with high MC at the bottom and the top, and low MC in the middle. Additional experiments to study flow paths and timescales of the process were performed using dyed water. A Fickian approach is discussed in some detail. It is clear that the process represented by the experimental results cannot be explained within a Fickian framework. The analysis has to be done on the fibre level, where an explanation is proposed. The influence from bordered pits is of significant importance and their resistance dominates the timescales. A key factor in the proposed explanation is the high lateral capillary conductivity within the saw-damaged top surface. However, some questions remain unanswered.     

杉木冷冻干燥材和气干材液体浸注性的比较研究

A comparative study was conducted on liquid penetration of the freeze-drying and air-drying sapwood and heartwood lumber of plantation Chinese fir （Cunninghamia lanceolata）. The maximum amount of dyeing solution uptake by the capillary rise method was used to evaluate the liquid penetration properties of the treated wood. The pit aspiration ratio was determined by semithin section method. Changes in wood microstructure were investigated using scanning electron microscopy. The results showed that compared with air drying, the freeze drying had a significant effect on liquid penetration of sapwood and heartwood of Chinese fir. The liquid penetration of sapwood is significantly higher than that of the heartwood for both drying treatments. Low pit aspiration ratio and cracks of pits membrane of some bordered pits are the main reasons for increasing liquid penetration after freeze drying treatment.     

Hydraulic and mechanical properties of young Norway spruce clones related to growth and wood structure

Stem segments of eight five-year-old Norway spruce (Picea abies (L.) Karst.) clones differing in growth characteristics were tested for maximum specific hydraulic conductivity (k(s100)), vulnerability to cavitation and behavior under mechanical stress. The vulnerability of the clones to cavitation was assessed by measuring the applied air pressure required to cause 12 and 50% loss of conductivity (Psi(12), Psi(50)) and the percent loss of conductivity at 4 MPa applied air pressure (PLC(4MPa)). The bending strength and stiffness and the axial compression strength and stiffness of the same stem segments were measured to characterize wood mechanical properties. Growth ring width, wood density, latewood percentage, lumen diameter, cell wall thickness, tracheid length and pit dimensions of earlywood cells, spiral grain and microfibril angles were examined to identify structure-function relationships. High k(s100) was strongly and positively related to spiral grain angle, which corresponded positively to tracheid length and pit dimensions. Spiral grain may reduce flow resistance of the bordered pits of the first earlywood tracheids, which are characterized by rounded tips and an equal distribution of pits along the entire length. Wood density was unrelated to hydraulic vulnerability parameters. Traits associated with higher hydraulic vulnerability were long tracheids, high latewood percentage and thick earlywood cell walls. The positive relationship between earlywood cell wall thickness and vulnerability to cavitation suggest that air seeding through the margo of bordered pits may occur in earlywood. There was a positive phenotypic and genotypic relationship between k(s100) and PLC(4MPa), and both parameters were positively related to tree growth rate. Variability in mechanical properties depended mostly on wood density, but also on the amount of compression wood. Accordingly, hydraulic conductivity and mechanical strength or stiffness showed no tradeoff.     

对防腐处理前后遭细菌侵蚀的木材超微构造研究

用透射电镜观察了隧道式细菌在未经防腐剂处理的素材和处理过的木材[欧洲白桦(Betula pendula),欧洲赤松(Pinus silvestris)和挪威云杉(Picea abies)]中形成的腐朽现象,研究了细菌入侵木材细胞壁的过程。侵蚀是从木材细胞腔内开始,通过S_3层后,细菌在细胞壁的S_1层、S_2层及胞间层内移动并且分裂,引起木材细胞壁的降解。还观察了隧道式细菌、隧道的形态和超微结构。本研究表明具缘纹孔和单纹孔的纹孔膜比纹孔缘对细菌活动更有抗性。细菌可以侵入低浓度防腐剂(0.5％Tanalith NCA)处理过的木材,表明细菌具一定解毒能力。     

Tracheary elements that resemble secondary xylem in calli derived from the conifers, Torreya nucifera and Cryptomeria japonica

Differentiated cells were recognized in calli derived from needles of Torreya nucifera and in calli derived from immature zygotic embryos of Cryptomeria japonica. Some differentiated cells resembled tracheary elements of primary xylem with spiral or reticulate thickening of cell walls. Other cells resembled tracheary elements with thick cell walls and bordered pits, which are features of secondary xylem. These tracheary elements were formed in cell clusters. Tracheary elements in calli of T. nucifera formed more highly developed structures, such as bordered pits and spiral thickening, than those of C. japonica. Cultured cells derived from conifers might provide a good model for studies of the differentiation of secondary xylem in vitro.     

微生物对长白鱼鳞云杉木材渗透性的影响

本文利用水池贮存处理木材研究了细菌对改善长白鱼鳞云杉木材渗透性的有效性。渗透性的测定采用水上升置换气流法。试验结果表明,水池贮存处理长白鱼鳞云杉气干材10周,不仅可以非常显著地增加其边材的渗透性(渗透性平均增加29倍),而且其心材渗透性亦有明显改善(平均渗透性增加1.52倍)。但心材试样渗透性增加不均匀,仅有50％的心材试样在水池贮存处理后渗透性得到了增加。渗透性增加的原因主要是由于边材和部分心材试样受到短芽孢杆菌(Bacillus brevis)等细菌的侵蚀,据电镜观察,边材大多数具缘纹孔的纹孔塞被细菌降解后形成空洞,心材的管胞内壁及具缘纹孔的纹孔缘上有细菌存在。     

Effects of steam explosion on wood appearance and structure of sub-alpine fir

The technology of steam explosion was applied to pre-treat sub-alpine fir (Abies lasiocarpa (Hook.) Nutt.) lumber and to improve its drying characteristics. Effects of steam explosion on the appearance and structure of the lumber are discussed in this paper. The structure of the wood was examined using light microscope (LM) and scanning electron microscope (SEM). The following results were obtained. With increasing temperature, pressure, and explosion cycles the color of the lumber darkened gradually. No significant structural difference between treated and untreated samples was observed using LM when the treatments were carried out at temperatures below 130°C with ten explosion cycles (group A or B). Some fractures were observed in bordered pit pairs between tracheids after 20 explosion cycles at 130°C (group C). More fractures occurred in bordered pit pairs between earlywood tracheids at a temperature of 160°C (group D). More or less fractures in pits between ray parenchyma cells and earlywood tracheids were observed using SEM in all four cases of treatments. Although no change in bordered pits in the tracheid walls between group A and the control group was discovered, groups B, C, and D showed different extents of ruptures in bordered pits, which may lead to break aspirated pits and improve permeability. In these groups, wrinkles and separations in the inner tracheid walls and detachments in middle lamella also occurred and became more serious as temperature or cycles of the treatment increased.     

Mechanistic study of microstructural deformation and stress in steam-exploded softwood

Steam explosion pretreatment results in the formation of microcracks in the cell walls of wood. In the present study, steam explosion experiments were performed and structural changes in Norway spruce were identified using scanning electron microscopy. The cellular structure of the softwood spruce was simulated using the finite element method, and the effects of pressure generated during the steam explosion pretreatment on the deformation of the cells were investigated. The simulated model included earlywood, latewood, and ray cells. The effects of bordered and cross-field pits on the stresses in the cell wall were studied as well. Many similarities were observed between the microcracks in the steam-exploded wood and the high-stress regions predicted by the model. The experimental and simulation results showed that the radial cell walls in the earlywood cells experienced major deformation. The presence of the pits created stress localization and facilitated the formation of microcracks in the cell walls.     

Microfibril angle non-uniformities within normal and compression wood tracheids

The pattern and extent of variation of microfibril angle (MFA) in normal and compression tracheids of softwood were investigated by using confocal laser scanning microscopy technique. All measurements support the idea that the orientation of microfibrils in single wood tracheids is not uniform. MFA of the radial wall of earlywood tracheids was highly non-uniform and had an approximately circular form of arrangement around the bordered pits (inside the border). Between the bordered pits the measured MFAs were less than the other parts of the tracheid. In the latewood tracheids MFA was less variable. The average orientation of simple pits in the crossfield region was consistent with the mean MFA of the tracheids; however some of the measurements showed a highly variable arrangement in the areas between the simple pits. In many cases the local measured MFAs of compression wood tracheids agreed with the orientation of natural helical cavities of compression wood. Comparing the measured results in different growth rings showed that MFAs in juvenile wood are generally larger than in perfect wood.