Growth stress distribution in leaning trunks of Cryptomeria japonica

The distribution of growth stresses in leaning trunks of Cryptomeria japonica (L.f.) D. Don was determined by measuring the stresses released by the kerf method with strain gauges glued at specified positions along the trunks. Effects of both tree height and peripheral positions on the surface of leaning trunks on surface growth stress were determined. The inner residual growth strains in leaning trunks were also measured. We found high compression stresses in the lower side of leaning trunks that differed greatly from the tensile stresses in normal erect trunks. However, transverse compression stress was found around the tree trunk in both normal and compression wood. In leaning trees, the distribution of internal stresses in the bent trunk portion differed from that in the erect trunk portion, being compressive on the outside and tensile on the inside. The resistant moment introduced by compression stress generated in compression wood is released by the bending of the leaning trunk. The bending stresses are then superimposed on the original internal growth stress. We demonstrated that Poisson's effect of longitudinal stresses should be considered when evaluating transverse surface growth stresses. The existence and intensity of compression wood development can be assessed by growth stress measurements. We conclude that the compressing force of compression wood functions physiologically to give an upward righting response in a leaning trunk.     

Relation between growth stress and lignin concentration in the cell wall: Ultraviolet microscopic spectral analysis

Lignin content in the cell wall was investigated to examine its relation with growth stress, using an ultraviolet microscopic spectrum analyzer. Although a weak correlation existed between the growth stress and lignin concentration in the compound middle lamella, it was believed that the compound middle lamella did not contribute to compressive growth stress generation as there was no correlation between growth stress and lignin concentration in the cell corner part of the intercellular layer. In the secondary wall, larger compressive growth stress was associated with higher lignin concentration especially in the outer part. This finding confirms that lignin contributes positively to the generation of compressive longitudinal growth stresses in the compression wood and more substantially in the outer part of the secondary wall. This experimental result strongly supports our hypothesis of growth stress generation given by the model.This paper was presented at the International Academy of Wood Science Meeting at Vancouver, Canada, July 1997     

Tensile growth stress and lignin distribution in the cell walls of black locust (Robinia pseudoacacia)

Seven specimens that contained a continuous gradient of wood from normal to tension wood were collected from an inclined black locust (Robinia pseudoacacia), and the released strain of growth stress was quantified. Lignin distribution in the cell wall was investigated using ultraviolet (UV) microspectrophotometry to examine its relation to the intensity of growth stress. The UV absorption at cell corner middle lamella and in the compound middle lamella remained virtually constant, irrespective of the contractive released strain (i.e., tensile growth stress). The gelatinous (G)-layer began to differentiate, and the UV absorption decreased there in accordance with increases in the contractive released strain. The absorption maximum (max) remained virtually constant at the cell corner middle lamella and in the compound middle lamella at 277–280nm, irrespective of the released strain. The max for the secondary wall of normal wood was 272nm and shifted to 268nm in the G-layer of tension wood as the contractive released strain increased. The percentage of the cross-sectional area, consisting of the G-layer, with respect to the whole cross-sectional area increased with the contractive released strain.     

Tree growth stress and related problems

Tree growth stress, resulted from the combined effects of dead weight increase and cell wall maturation in the growing trees, fulfills biomechanical functions by enhancing the strength of growing stems and by controlling their growth orientation. Its value after new wood formation, named maturation stress, can be determined by measuring the instantaneously released strain at stem periphery. Exceptional levels of longitudinal stress are reached in reaction wood, in the form of compression in gymnosperms or higher-than-usual tension in angiosperms, inspiring theories to explain the generation process of the maturation stress at the level of wood fiber: the synergistic action of compressive stress generated in the amorphous lignin–hemicellulose matrix and tensile stress due to the shortening of the crystalline cellulosic framework is a possible driving force. Besides the elastic component, growth stress bears viscoelastic components that are locked in the matured cell wall. Delayed recovery of locked-in components is triggered by increasing temperature under high moisture content: the rheological analysis of this hygrothermal recovery offers the possibility to gain information on the mechanical conditions during wood formation. After tree felling, the presence of residual stress often causes processing defects during logging and lumbering, thus reducing the final yield of harvested resources. In the near future, we expect to develop plantation forests and utilize more wood as industrial resources; in that case, we need to respond to their large growth stress. Thermal treatment is one of the possible countermeasures: green wood heating involves the hygrothermal recovery of viscoelastic locked-in growth strains and tends to counteract the effect of subsequent drying. Methods such as smoke drying of logs are proposed to increase the processing yield at a reasonable cost.     

Negative gravitropism and growth stress in GA3-treated branches ofPrunus spachiana Kitamura f.spachiana cv.Plenarosea

One of the roles of growth stress in branch shape formation was investigated using a weeping-type Japanese cherry,Prunus spachiana. Negative released strains, caused by longitudinal tensile growth stresses, were detected in the upper side of gibberellin A₃-treated (GA₃-treated) and control branches. The mean value of the released strain in the upper side of the treated branches was –0.104%, which was larger than the value (–0.067%) observed in the control branches. Both branches formed tension wood in the upper side of the xylem, and the treated branches formed tension wood near the pith as well. This suggested that the treated branches generated larger tensile growth stress from the early growth stages. The successive generation of growth stress from the early growth stages was considered to generate forces large enough to bend the branch upward.     

Growth strain in the trunk and branches of Chamaecyparis formosensis and its influence on tree form

Distributions of growth strains in branches, straight trunks and basal sweeping trunks of Chamaecyparis formosensis Matsum. trees were measured with strain gauges. Microfibril angles (MFAs) of the S2 layer of the cell wall were measured by the iodine deposition method and their relationships with growth strain examined. The magnitude of the compressive stress on the lower side of trunks with a basal sweep was greater than that of the tensile stress at the surface of straight trunks. However, transverse compressive stress was similar around the trunk regardless of whether normal wood or compression wood was present. The released surface growth strains varied with MFA. At MFAs of 20-25 degrees , growth stress changed from tension to compression, and compressive stress increased dramatically in the compression wood region. Branches suffer bending stress due to self-loading. This stress is superimposed on the growth stress. Growth strains on the upper or lower sides of branches were larger than those in the trunks, suggesting that generation of growth stress on the lower sides of branches with extensive compression wood is affected by the gravitational bending stress due to self-loading. We conclude that branch form is affected by the interaction between the bending moment due to self-loading and that due to the asymmetric distribution of growth stress. Growth strain distribution in a branch differed depending on whether the branch was horizontal, upward bending or downward bending.     

On the origin of growth stresses in trees

Summary A mechanism for growth stress generation is studied which involves a contractive strain in the microfibril direction and swelling strain in the transverse direction in the developing wall of wood cells. A cylindrically anisotropic elastic model is used to calculate the accumulation of residual stresses in the S₂ wall as it is formed. An explicit relation between the shrinkage/swelling strains in the growth increment of the cell wall and the resulting axial and circumferential stresses induced in the cell is derived. For gymnosperm cells the transition from tensile stress in normal wood cells to compressive stress in compression wood cells is found with increasing microfibril angle.     

The generation of longitudinal maturation stress in wood is not dependent on diurnal changes in diameter of trunk

A hypothetical mechanism for the generation of maturation stress in wood was tested experimentally. The hypothesis was that the maturation stress could partly originate in a physical mechanism related to daily changes in water pressure and associated diurnal strains. The matrix of lignin and hemicellulose, deposited in the cell wall during the night, would be put in compression by the effect of water tension during the next day. The cellulose framework, crystallizing during the day, would be put in tension by the decrease in tension at night and subsequent cell-wall swelling. This was tested on young saplings of sugi and beech. Half of the saplings were submitted to continuous lighting, which canceled diurnal strains. Saplings were tilted 40 degrees, and their uprighting movement was measured. The uprighting movement is directly due to the production of reaction wood and the concomitant development of large longitudinal maturation stress. It occurred in the continuously lighted plants at least as much as in control plants. We conclude that the generation of longitudinal maturation stress in tension or compression wood is not directly related to variations in water pressure and diurnal strains.     

Variations in physical and mechanical properties between tension and opposite wood from three tropical rainforest species

Growth strains were measured in situ in nine trees of three species from a French Guiana tropical rainforest in a clearly active verticality restoration process. The aim was to detect tension wood within the samples. Wood specimens were cut in the vicinity of the growth strain measurements in order to determine the microfibril angle and some mechanical and physical properties. As suspected, tensile growth strain was much higher in tension wood zones, as shown by the slightly higher longitudinal modulus of elasticity. Conversely, tension wood showed reduced compression strength. Longitudinal shrinkage was much higher in tension wood than in opposite wood. Clear relationships between the microfibril angle and longitudinal properties were noted in comparison (i) with those observed in gymnosperm compression wood and (ii) with expected relationships from the organization of wood fibres cell wall structure.     

Distribution of lignin in normal and compression wood of Pinus taeda L.

Summary The distribution of lignin in normal and compression wood of loblolly pine (Pinus taeda L.) has been studied by the technique of lignin skeletonizing. Hydrolysis of the wood carbohydrates with hydrofluoric acid left normal wood tracheids with a uniform distribution of lignin in the S1 and S2 cell wall layers. However, the S3 region of both earlywood and latewood tracheids consistently retained a dense network of unhydrolyzable material throughout, perhaps lignin.Lignin content in compression wood averaged about 7% more than in normal wood and appears to be concentrated in the outer zone of the S2 layer. The inner S2 region, despite helical checking, is also heavily lignified. The S1 layer, although thicker than normal in compression wood tracheids, contains relatively little lignin.Ray cells, at least in normal wood, appear to be lignified to the same extent, if not more so in certain cases, than the longitudinal tracheids. Other locations where lignin may be concentrated include initial pit border regions and the membranes of bordered pits.This report is a detailed excerpt from the Ph. D. dissertation of R. A. P. Financial support provided by the College of Forestry at Syracuse University and the National Defense Education Act is hereby gratefully acknowledged.     

Effects of the lateral growth rate on wood quality parameters of Eucalyptus grandis from different latitudes in Brazil and Argentina

Climate change resulting from increased atmospheric carbon dioxide (CO₂) and shortages of fossil fuels such as petroleum are major problems worldwide. Under these conditions, demand for woody biomass resources is increasing. We investigated the feasibility of using fast-growing Eucalyptus grandis for material production. Samples of E. grandis were collected from four plantations in different latitude divisions, including tropical and subtropical Brazil and subtropical Argentina. Various xylem qualities were measured and related to the lateral growth rate. Lateral growth rate did not significantly affect the longitudinal released strain of the surface growth stresses or the xylem density at any of the sampling sites. Higher lateral growth rate, higher values of xylem density, and lower absolute values of the released strain were observed in plantations closer to the equator. Higher growth rates in tropical climate promote longer fiber length. In subtropical plantations, smaller diameter trees will produce tension wood with smaller microfibril angles. Planting E. grandis closer to the equator thus produces higher quality wood than in plantations at lower latitudes.     

Patterns of longitudinal and tangential maturation stresses in Eucalyptus nitens plantation trees

Context

Tree orientation is controlled by asymmetric mechanical stresses set during wood maturation. The magnitude of maturation stress differs between longitudinal and tangential directions, and between normal and tension woods.

Aims

We aimed at evaluating patterns of maturation stress on eucalypt plantation trees and their relation with growth, with a focus on tangential stress evaluation.

Methods

Released maturation strains along longitudinal and tangential directions were measured around the circumference of 29 Eucalyptus nitens trees, including both straight and leaning trees.

Results

Most trees produced asymmetric patterns of longitudinal maturation strain, but more than half of the maturation strain variability occurred between trees. Many trees produced high longitudinal tensile stress all around their circumference. High longitudinal tensile stress was not systematically associated with the presence of gelatinous layer. The average magnitude of released longitudinal maturation strain was found negatively correlated to the growth rate. A methodology is proposed to ensure reliable evaluation of released maturation strain in both longitudinal and tangential directions. Tangential strain evaluated with this method was lower than previously reported.

Conclusion

The stress was always tensile along the longitudinal direction and compressive along the tangential direction, and their respective magnitude was positively correlated. This correlation does not result from a Poisson effect but may be related to the mechanism of maturation stress generation.     

Longitudinal shrinkage behaviour of compression wood in radiata pine

The behaviour of longitudinal shrinkage was investigated in the corewood of a swept, 17-year-old New Zealand radiata pine stem. Wood categories in terms of normal wood, mild compression wood and severe compression wood were identified microscopically using autofluorescence of lignin. Average longitudinal shrinkage was collated according to corewood location and wood category within corewood in the leaning and the vertical parts of the stem, and then maximum radial difference of longitudinal shrinkage within growth ring was examined. The results show that the average longitudinal shrinkage is significant (2.4%) in the corewood of the leaning part of the stem. Among wood categories, severer compression wood displays the highest (2.9%) average longitudinal shrinkage. In the context of this study, growth rings may consist of one of three types of wood: (1) only normal wood; (2) a single compression wood type; and (3) mixed-type wood. Where multiple compression woods co-existed with normal wood, the maximum radial difference of longitudinal shrinkage within the growth ring was found to be 4.0%. A strong correlation (R ² = 0.90) between average MFA and average longitudinal shrinkage suggests a significant influence of the average MFA on average longitudinal shrinkage across the three growth ring types.     

A numerical study of the shape stability of sawn timber subjected to moisture variation Part 1: Theory

A three-dimensional theory for the numerical simulation of deformations and stresses in wood during moisture variation is described. The constitutive model employed, assumes the total strain rate to be the sum of the elastic strain rate, the moisture-induced strain rate and the mechano-sorption strain rate. Wood is assumed to be an orthotropic material with large differences between the longitudinal, radial and tangential directions in the properties found. The influence of the growth rings, the spiral grain and the conical shape of the log on the orthotropic directions in the wood is taken account of in the model. A finite element formulation is used to describe the deformation process and the stress development during drying.The research presented in this paper is a part of the national research programme in Sweden concerning wood physics and drying. It was financially supported by the Research Foundation of Swedish Sawmills and the Swedish Council for Forestry and Agricultural Research.     

杨木应拉木微区结构可视化及化学成分分析

木材微区结构与木材宏观性质密切相关,杨木应拉木与对应木宏观性质存在较大差别,探究杨木应拉木和对应木微区结构和化学成分,可为了解杨木应力木的宏观性质提供理论根据。借助光学显微镜、荧光显微镜、显微拉曼成像光谱仪、透射电镜对杨木应拉木微区结构进行可视化研究,并借助X射线衍射技术和美国可再生能源实验室方法,分析杨木应拉木的微晶尺寸、结晶度以及化学成分。结果表明:杨木应拉木中应拉区和对应区纤维细胞微区结构差异显著。光学显微镜下显示应拉区木纤维中胶质层清晰可见,荧光显微镜和拉曼显微镜下显示胶质层的木质素浓度比对应区低。透射电镜下显示应拉区木纤维细胞壁结构由初生壁、次生壁和胶质层组成,未见次生壁外层,各层的平均厚度分别为0.61,1.22和2.53μm。对应区木纤维为典型的初生壁和次生壁结构,次生壁各层平均厚度分别为0.33,2.28和0.14μm。杨木应拉区纤维素含量(58.91%)比对应区(41.53%)高,木质素含量和半纤维素含量均比对应区的低,应拉区木质素和半纤维素含量分别为21.99%和12.01%,对应区分别为28.10%和17.08%。杨木应拉区结晶度(48.06%)比对应区(41.01%)高,应拉区晶区宽度为2.66 nm,长度为8.84 nm;对应区晶区宽度为2.65 nm,长度为9.87 nm。     

Moisture diffusion coefficient of reaction woods: compression wood of Picea abies L. and tension wood of Fagus sylvatica L.

The moisture diffusion coefficient of compression wood in spruce (P. abies) and tension wood in beech (F. sylvatica) was examined. The results indicated that the diffusion coefficient measured under steady-state condition (cup method) could well characterize the drying kinetics of the reaction woods. The compression wood offered more resistance to the moisture diffusivity when compared with the corresponding normal wood. The thick cell wall rich in lignin explains the small mass diffusivity in compression wood. In contrast, the mass diffusivity in beech is almost always higher in tension wood than in normal wood, in spite of similar density values. The high moisture diffusion in tension wood can be explained by the ease of bound water diffusion in the gelatinous layers (G-layers).     

Comparison of mechanical properties of tension and opposite wood in <Emphasis Type="Italic">Populus</Emphasis>

In this paper we focused on the differences of mechanical properties of tension and normal wood of 1-year-old poplar trees, artificially tilted. Elastic and fracture properties have been measured and linked to the anatomy. Tension wood is well known because it prevents good surface finishing and leads to difficulties with sawing. We studied three main mechanical properties: young modulus, energy of cutting and longitudinal residual strain of maturation (with strain gauges) because of their importance in wood technology. Moreover, this work takes place in a larger project of study, the phenomena of axes re-orientation in trees (allowing by the production of reaction wood), where these data are required for biomechanical modelling. The results show that tension wood has a higher young modulus, needs a higher energy to be cut and exhibited a higher level of longitudinal residual strain of maturation than those of normal wood. The results suggest that these differences require deeper analysis of the wood than anatomy: measurement of microfibril orientation in the S2 layer and also the lignin composition in monomeric units.     

Growth strain assessment at the periphery of small-diameter trees using the two-grooves method: influence of operating parameters estimated by numerical simulations

The two-grooves method used for estimating the surface locked-in strains on standing stems was studied using a finite-element model simulating the gauge measurement resulting from the groove cutting, having in mind the particular of small diameter trees. The assumed growth stress distribution was described by simple polynomial expressions of the relative radius with the possible existence of a tension wood sector characterized by higher residual stresses in the longitudinal direction. The end effect of the crosscutting vanished for a height/diameter ratio higher than 3 and the simulated gauge measurement reasonably approached the local growth strain average. For very small trees with a diameter of about 2 cm, the distance between gauge end and groove should not exceed 3 mm, a value of up to 5 mm is allowed in case of larger stems, 5–20 cm. For any combination of stem diameter, gauge length, groove distance and depth, and assumptions on the internal stress distributions, the underestimation of the surface growth strain by the gauge can be evaluated using the developed numerical tool.     

Origin of the characteristic hygro-mechanical properties of the gelatinous layer in tension wood from Kunugi oak (Quercus acutissima)

The mechanism responsible for unusual hygro-mechanical properties of tension wood containing the gelatinous layer (G-layer) was investigated. Tension and normal wood specimens were sampled from the leaning stems of a 75- and a 40-year-old Kunugi oak (Quercus acutissima) tree, and the moisture dependencies of the longitudinal Young’s modulus and longitudinal dimensions were measured. The results, which were analyzed in relation to the anatomical properties of the specimens, revealed that the ratio of increase in the longitudinal Young’s modulus with drying was higher in the G-layer than in the lignified layer (L-layer); the longitudinal drying shrinkage displayed a similar pattern. It was found that the lattice distance of the [200] plane in the cellulose crystallite increased with drying, moreover, the half-width of the [200] diffraction peak increased with drying, which was remarkable in the tension wood. Those results suggest that in the green state, the polysaccharide matrix in the G-layer behaves like a water-swollen gel; however, it is transformed into a condensed and hard-packed structure by strong surface tension during moisture desorption, which is a form of xero-gelation. However, in the L-layer, condensation and subsequent xero-gelation of the polysaccharide matrix was prevented by the hydrophobic lignin that mechanically reinforces the matrix.     

Reaction wood in Pseudowintera colorata — A vessel-less dicotyledon

Summary Inclined branches of Pseudowintera colorata exhibit pronounced growth promotion to the lower (abaxial) side similar to that found in gymnosperms. The only other significant difference between the anatomy of the upper and lower regions is that the tracheids on the lower side have a larger microfibril angle. Other microscopic features normally associated with compression wood or tension wood are completely absent. The longitudinal shrinkage of samples from the upper and lower regions is shown to be related to the mean microfibril angle in a highly non-linear way, and a relatively small change in microfibril angle is associated with a large change in longitudinal shrinkage. This result is in agreement with the hypothesis that compression wood force generation arises during the lignification phase of secondary wall deposition and is critically dependent on mean microfibril angle.The author is indebted to Mr R. R. Exley of this laboratory who prepared the samples and made all the measurements in this project