Effect of thinning on the development of compression wood in stems of Corsican pine

This study considered the effects of thinning on the development of compression wood in stems of 35-year-old stand of Corsican pine (Pinus nigra L.). Part of the stand had been thinned at 5-yearly intervals and part left unthinned. Twenty trees each from the thinned and unthinned stands were randomly selected and felled. Measurements were made on tree height, stem diameter, stem slenderness and canopy depth. Wood samples were removed from the central part of the main log and cross-sectional measurements made on ring width, basic density and compression wood content. Cross-sectional area of compression wood was found to be three time higher in stems from the unthinned trees in comparison with those from the thinned trees. No significant differences in mean radial ring width or basic density were found between treatments. Correlations indicated that, with increasing in stem diameter, compression wood content increased in the unthinned trees, while a decline in compression was observed in the thinned trees. Tree height was also positively correlated with compression wood content in unthinned trees, while no equivalent relationship was observed in thinned trees. Observations from this study, while not conclusive, suggest that phototropic stimulus may be producing stem inclinations in the unthinned stand as trees compete for space in the canopy, whereas crown competition has been largely eliminated in the thinned stand; and that this is responsible for compression wood levels recorded in this study.     

Measuring compression wood severity in spruce

As the severity of compression wood influences the mechanical and chemical properties of wood it is desirable to be able to measure compression wood severity. However, so far no satisfactory method has been reported in the literature. Here we describe how scanning FTIR micro-spectroscopy can be employed to achieve CW severity measurements on increment cores of Norway spruce (Picea abies (L. Karst.) and Sitka spruce (Picea sitchensis (Bong.) Carrière). Radial wood strips were converted into sawdust by a process that maintained their spatial orientation. Samples prepared in this way were scanned with an FTIR-microscope in reflective mode and from the spectra obtained a CW-indicator was calculated representing aromatic and carboxyl signals. This FTIR CW-indicator correlated well with alternative CW identification techniques (namely microfibril angle, transmitted light and immunolabelling of beta 1–4 galactan), which have been used to validate the method. Repeatability of the measurements was good and no systematic difference between spruce species was found. The achievable resolution of the measurements was of sub-mm order. The CW indicator described offers the opportunity to correlate CW severity with mechanical wood properties in spruce.     

Studies on pre-treatment by compression for wood impregnation I: effects of compression ratio,compression direction,compression speed and compression-unloading place on the liquid impregnation of wood

To improve the impregnation of wood, the pre-treatment by compression was systematically studied in terms of effects of compression ratio, compression direction, compression speed and compression-unloading place on the liquid impregnation in poplar and Chinese fir. The results showed: the impregnation increased 0.0065 or 0.0074 g/cm³ for every 1% increase of compression ratio when the compression ratio was lower or equal to 50 and 40% for poplar and Chinese fir, respectively; it continued to increase afterwards while the variation was quite big. There existed a significant difference of the impregnation of wood compressed at different directions in Chinese fir, but not in poplar. There existed a significant difference of the impregnation of wood compressed at different speed in both species. The impregnation of wood is likely to be in favor of radial compression in terms of the amount of impregnation. 5 and 10 mm/min were recommended as a compromise of impregnation and pre-treatment efficiency. The impregnation of wood that the compression unloaded in water was about 18.2 (poplar) and 9.2% (Chinese fir) higher in amount and was much quicker in speed than that the compression unloaded in air, and the difference between them was significant, suggesting that compression unloaded in water is significant to improve the impregnation.     

Ultrastructure of compression wood in Ginkgo biloba

Summary Compression wood in the ancient Ginkgo biloba differs from that in most of the younger gymnosperms in the more angular outline of its tracheids, their thinner walls, and their lack of helical cavities. Both normal and compression woods of Ginkgo contain two types of tracheids, one wide, with a thin wall, and another, narrow, with a thicker wall. In all other respects the compression wood tracheids in Ginkgo are ultrastructurally similar to those in other gymnosperms. Helical cavities probably developed relatively late in the evolution of compression wood, since they are missing not only in Ginkgo but also in the Taxales and the Araucariaceae. The occurrence of compression wood in Ginkgo biloba indicates that this tissue probably has existed since the Devonean period. Very likely, the arborescent habit of the gymnosperms has always been dependent on their ability to form compression wood.This investigation was carried out under the McIntire-Stennis Program, Cooperative State Research Service. I am indebted to Mr. A. C. Day of this College and to Mr. A. Rezanowich of the Pulp and Paper Research Institute of Canada for kindly providing the scanning electron micrographs.     

Significance of laricinan in compression wood tracheids

Summary After a -D-1,3-linked glucan had been isolated from compression wood, identified, and named laricinan, other researchers suggested that it occupied the helical cavities in the S₂ layer of those tracheids. They postulated that the glucan was responsible for the capacity of compression wood to generate the large forces associated with reorientation of displaced stems and branches, and also caused its severe shrinkage with drying.Analyses herein indicate that it is improbable that such a glucan could be the primary factor responsible for those characteristics of compression wood. An alternative significance is proposed, namely that its presence strengthens the already well-supported deduction that the helical cavities have a schizogenous origin.     

Mechanical behaviour of saturated wood under compression Part 2: Behaviour of wood at low rates of strain some effects of compression on wood structure

Summary The mechanical behaviour of three species of hardwoods, soaked in different liquids, had been investigated at high rates of strain using a Hopkinson pressure bar system. In order to determine the influence of the rate of strain, samples from the same species were subjected to compression-tests at low rates of strain. It was noticed that, at low rates of strain, the saturated samples were always less stiff than the dry ones, which is in agreement with the literature, but differs from the behaviour at high rates of strain. This difference is attributed to the behaviour of the liquid present in the large cavities of the material, which must depend on the rate of strain. It was also noticed that the samples could support higher stresses at high rates of strain. Although permanent sets were measured after the tests, the samples were not always visibly damaged, but some typical failures were detected by means of microscopy. The damaged zones presented similar aspects, whatever the rate of strain.     

Mechano-sorptive creep mechanism of wood in compression and bending

Summary A model is introduced which links the mechano-sorptive behaviour of wood subjected to moderate and high compression or bending stresses parallel to grain to the formation of slip planes in the cell wall. Slip plane formation is dependent on the breaking of hydrogen bonds, which process is directly related to the amount of moisture change. The dramatic change of microfibril orientation in slip plane zones cause an increase of the longitudinal shrinkage/swelling and a decrease of the modulus of elasticity. These features of slip plane formation account for both the magnitude and the oscillation of the excessive mechano-sorptive creep associated with compression and bending parallel to grain. A summary is given of the characteristics of the mechano-sorptive effects, and the model is discussed in the light of these effects.The paper is one of the results of a project on the influence of changing moisture content on the mechanical behavior of wood, currently underway in a co-operation between College of Environmental Science and Forestry, State University of New York, and the Technical University of Denmark. Support for this project is provided by the Danish Technical Research Council and by the USDA C--operative Research Program (proj. 85-FSTY-9-0112). The support is gratefully acknowledged     

Recent progress in the chemistry and topochemistry of compression wood

Summary A review of the chemistry and topochemistry of compression wood with 200 references. Compression wood contains on the average 30% cellulose, 35–40% lignin, 10% galactan, 9% galactoglucomannan, 8% xylan, and 2% of a 1,3-glucan (laricinan). The cellulose is less crystalline, and the xylan has fewer arabinose side chains than in normal wood. The lignin is composed of guaiacylpropane and p-hydroxyphenylpropane units. It is more condensed, has a higher proportion of carbon-carbon bonds, and contains fewer arylglycerol--aryl ether structures than a normal conifer lignin. The ray cells and the primary wall of the tracheids have the same chemical composition in normal and compression woods. The galactan is largely located in the outer region of the secondary wall. Only 5–10% of the lignin in compression wood tracheids is extracellular. The middle lamella is less lignified than in normal wood, while the S₁ and inner S₂ layers have a lignin concentration of 30–40% which is twice as high as in normal wood. The lignin content of the S₂ (L) layer is equal to or higher than that of the intercellular region along the wall. The review is concluded with a brief reference to areas where present information is incomplete or lacking.A portion of an Academy Lecture of the International Academy of Wood Science, presented at the International Symposium on Wood and Pulping Chemistry (Ekmandagarna 1981), held in Stockholm, Sweden, June 9–12, 1981. Reprints of the unabridged review, published under the title Recent Progress in the Chemistry, Ultrastructure, and Formation of Compression Wood in the preprints of the symposium (SPCI Report 38, Vol. 1, p. 99–147) are available from the author. I wish to express my gratitude to my colleague Professor Robert A. Zabel for generous travel assistance     

Mechanical behaviour of saturated wood under compression

Summary The mechanical behaviour of three species of hardwoods soaked in different swelling liquids, compressed at high rates of strain, was investigated using a split Hopkinson pressure bar system. Variations in elastic moduli, proportional limit and maximum stress with respect to the treatments were studied. It was found that the saturated specimens could be as stiff as the dry ones. This result was explained by the behaviour of the liquid present in the large cavities of the wood, i.e. the lumens of the cells, which must be different from that observed at low rates of strain. At large rates of strain, this liquid cannot flow out of the pores and must behave like a solid; therefore the structure of the material is reinforced and, as a consequence, the softening effect of the soaking agent can be masked.     

Finite element techniques and models for wood fracture mechanics

Numerical models for wood fracture and failure are commonly based on the finite element method. Most of these models originate from general theoretical considerations for other materials. This limits their usefulness because no amount of complexity in a model can substitute for lack of an appropriate representation of the physical mechanisms involved. As for other materials, wood fracture and failure models always require some degree of experimental calibration, which can introduce ambiguity into numerical predictions because at present there is a high degree of inconsistency in test methods. This paper explores avenues toward achieving models for wood fracture that are both appropriate and robust.     

Helical thickenings and helical cavities in normal and compression woods of Taxus baccata

Summary The longitudinal tracheids in compression wood of Taxus baccata contain helical thickenings but no helical cavities. The thickenings are as frequent and well developed and have the same ropelike appearance as in normal wood of this species. They are an integral part of the S₃ in normal and of the S₂ in compression wood and have the same orientation as the innermost microfibrils in these layers. Except for the absence of cavities and presence of thickenings, compression wood tracheids of Taxus baccata possess all the anatomical features typical of such cells, including a rounded outline, intercellular spaces, a thick S₁ layer, a highly lignified S₂ (L) layer, and no S₃ layer. Pronounced compression wood of Pseudotsuga menziesii contains helical cavities but no helical thickenings. Thickenings and cavities seem to be mutually exclusive in Pseudotsuga and Taxus.This investigation was carried out under the McIntire-Stennis Program, Cooperative State Research Service. I am indebted to Mr. A. Rezanowich of the Pulp and Paper Research Institute of Canada for kindly providing the scanning electron micrographs.     

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.     

Basal sweep and compression wood in young Scots pine trees

This study demonstrates the correlation between stem form and compression wood content in 36 sampled trees from a 6-yr-old container grown Scots pine ( Pinus sylvestris L.) plantation in central Sweden. Root analyses were carried out to examine causes for the high incidences of basal sweep. On the sampled trees basal sweep, stem sweep and maximum bow-height were measured. Discs were cut at different heights and the compression wood content was analysed using digital image analysis. Pith eccentricity and out-of-roundness were calculated for each disc as well as per stem. Compression wood and pith eccentricity was most pronounced near the stem base, where sweep tends to be high. In general, pith eccentricity increased with degree of basal sweep. As a conclusion, expression of basal sweep can be used to predict compression wood content in young Scots pine trees. This study also shows the importance of anchoring of a tree to prevent basal sweep formation due to instability.