Compression wood in conifers (Compression Wood)

Summary

Examining the formation of compression wood in conifers and its effect on timber quality

Compression wood forms on the underside of leaning and twisted stems and trunks. Its structure and chemical composition differs from normal wood, producing low quality timber with poor mechanical properties. Forest Research led this project and worked with 10 partners from across Europe in field, and laboratory studies to evaluate the impact of compression wood on softwood timber and to develop solutions to minimise its detrimental effects.

Compression Wood – Literature review – Study of compression wood

Topics

• Study of compression wood
  • Introduction
  • Colour as a characteristic feature
• Classification of compression wood
• Detection of compression wood
  • Detection of compression wood in standing trees
  • Detection of compression wood in wood logs
  • Detection of compression wood in discs
  • Detection of compression wood in sawn wood
• References

Review performed by:

Philipp Duncker
Albert-Ludwigs-University Freiburg
Institut für Waldwachstum
Bertoldstr. 17
79085 Freiburg
Germany

Mats Warensjoe
Swedish University of Agricultural Sciences
Department of Forest Management and Products
901 83 Umeå
Sweden

Compression Wood – Literature review – Study of compression wood 2

Topics

• Study of compression wood
  • Introduction
  • Colour as a characteristic feature
• Classification of compression wood
• Detection of compression wood
  • Detection of compression wood in standing trees
  • Detection of compression wood in wood logs
  • Detection of compression wood in discs
  • Detection of compression wood in sawn wood
• References

Review performed by:

Philipp Duncker
Albert-Ludwigs-University Freiburg
Institut für Waldwachstum
Bertoldstr. 17
79085 Freiburg
Germany

Mats Warensjoe
Swedish University of Agricultural Sciences
Department of Forest Management and Products
901 83 Umeå
Sweden

Compression Wood – Review of literature – Study of compression wood

Introduction

The study of compression wood has attracted the attention of numerous scientists during the last 150 years. Much of what is known about compression wood today was first discovered more than 100 years ago.

Most of the work that has been reported is summarised by Timell (1986) in a comprehensive three-volume publication. According to this work, Karl Gustav Sanio was the first to observe and describe the anatomical structure of compression wood. His findings were published in 1860 and most of them have been confirmed by more recent researchers. As a matter of fact the majority of the anatomical features that Yumoto et al. use in their classification system, published in1983, were described by Sanio in 1860.

Even earlier than Sanio, one of the first to discuss compression wood was Carl von Linne, during the 18^th century. During his travel to the most northern county of Sweden Linne described crooked pines that on their lower side had hard and dark wood that he described as box wood.

It is the colour and the hardness of compression wood that have given this type of tissue a variety of names, as shown in the table below. Today the term compression wood and the direct translation to other languages, referring to the compressive forces in the tissue that strive to reorient the tree into a more stable position (vertical), is the valid term.

Colour as a characteristic feature

As can be seen from the table it is clearly apparent that compression wood is often recognised on a macroscopic level by eye from its colour. Compression wood appears dark because it absorbs more light (due to a high lignin content) and scatters less light (due to thick tracheid walls). Different authors have described this characteristic feature of compression wood, and discussed how to maintain the best contrast when seeking to make a visual identification of compression wood.

Timell (1986 Vol.I) states that mild compression wood is less deeply coloured than moderate compression wood which in its turn is paler than the severe form of compression wood. He further states that pronounced or severe compression wood is distinctly different from normal wood especially when freshly cut.

The colour of compression wood seems to differ between species. According to Timell (1986 Vol.I) and Trendelenburg (1932) compression wood is darker in Abies and Picea species than in Pinus and Pseudotsuga.

To enhance the contrast between normal and compression wood several techniques have been suggested:

• Wetting
• Polishing, e.g. sanding
• Coating with vaseline
• Staining.

When wetted compression wood appears more clearly. However this difference in colour tends to fade as the wood dries (Mer 1887 and later Hartig 1901). Wetting of compression wood restores the original “green state” colour (Timell 1986 Vol.I). When the wood is polished the green state colour is also restored. According to Timell (1986 Vol.I) wetting is only necessary to bring out the full contrast in pines. Furthermore, Timell (1986 Vol.I) states that polishing is preferable to wetting since:

• Cracks and fissures develop easily when discs are rewet
• The structure becomes clearer
• Wetting blurs structural details.

Further suggestions for maintaining the maximum contrast between compression wood and normal wood that have been reported include:

• In panchromatic light a blue filter increases the contrast (Westing 1965)
• When using black and white film, a moderately sensitive film, slightly overexposed, and printed on hard paper improves the contrast (Timell 1986 Vol.I)
• Compression wood is sharply contrasted with normal wood in transmitted light, but there is a lack of detail that is best brought out when the wood is polished (Pillow 1941)
• Compression wood becomes darker when it is coated with Vaseline or wetted, probably because the cavities and lumen are filled in which reduces the scattering intensity (Hartig 1901).

As a general rule the intensity of the colour of stem compression wood increases with increasing severity. However, exceptions have been reported. Yumoto and Ishida (1982) found no difference in the colour of the compression wood in their trees displaced 45° and 90° from the vertical.

What’s of interest

Literature review performed by:
Philipp Duncker
Albert-Ludwigs-University Freiburg
Institut für Waldwachstum
Bertoldstr. 17
79085 Freiburg
Germany

Mats Warensjoe
Swedish University of Agricultural Sciences
Department of Forest Management and Products
901 83 Umeå
Sweden

Compression Wood – Review of literature – Classification of compression wood

From the consideration of colour differences and microscopic studies it is obvious that different degrees of severity of compression wood development exist Yumoto et al. (1983) describe the variability of compression wood tracheids, “Compression wood tracheids are known to have rounded outline in cross-section with intercellular spaces and thicker walls in which spiral grooves run obliquely and lignin is distributed in a characteristic pattern, and to lack in the S3 layer (…). However, it is only a so-called typical compression wood tracheid that shows all these structural features and indeed, there are intermediates between the typical compression wood tracheid and the normal wood tracheid, in other words, compression wood tracheids show various degrees of development” (Yumoto et al. 1983).

This knowledge about variability raises the question of a grading system to classify different degrees of compression wood severity. The table gives an overview of grading systems applied by different authors.

The grading system developed by Harris (1977) uses colour differences to classify compression wood visually in four grades.

Burdon (1975) classified Pinus radiata discs in transmitted light into six different grades:

• (0) Normal wood
• (1) Latewood partly opaque
• (2) Latewood generally opaque
• (3) Latewood and earlywood partly opaque
• (4) Latewood opaque and earlywood generally opaque
• (5) Latewood and earlywood highly opaque.

The boundaries and grades of compression wood were traced onto translucent squared paper. The zones of different compression wood classes were excised, and the areas were measured by weighing:

Compression wood rating was defined as A*G/BA where:

• A = Area
• G = Grade
• BA = Total area of disc

The grading system presented by Yumoto et al. (1983), which is probably one of the most detailed, is based on various anatomical features. The grading system holds for compression wood tracheids in the middle of a growth ring. According to the authors, helical cavities, UV-absorption and cell-wall thickness are considered as primary properties of compression wood tracheids while other properties such as the outline of the boundary between S1 andS2(L) layers, the outline of the bordered pits and the presence or absence of intercellular spaces and an S3 layer are of “less importance”. The latter are probably properties that vary more than the former. According to the authors the grading criteria hold true for compression wood tracheids in the middle of a growth ring but are not applicable for those situated near the growth ring boundary.

What’s of interest

Literature review performed by:
Philipp Duncker
Albert-Ludwigs-University Freiburg
Institut für Waldwachstum
Bertoldstr. 17
79085 Freiburg
Germany

Mats Warensjoe
Swedish University of Agricultural Sciences
Department of Forest Management and Products
901 83 Umeå
Sweden

Compression Wood – Review of literature – Detection of compression wood

In the literature various destructive and non-destructive methods for detecting compression wood are described. Some of these methods are summarised below:

Detection of compression wood in standing trees

The presence of compression wood can be suspected when stems are leaning or have a pronounced curvature. However even straight, vertical trees with circular boles can contain large amounts of compression wood.

Some authors have tried to classify trees according to their outer shape (Low 1964, Dyson 1969). Different authors have classified specific types of curvature differently, i.e. crook and sweep, see Timell (1986, Vol. II, s 755).

Several researchers have also tried to establish correlations between the deviation from the vertical and the extent of CW-formation (Low 1964, Shelbourne 1966).

According to Low there is no close correlation between outer shape and compression wood formation. Shelbourne found that severe compression wood content was correlated to stem straightness, but slight and moderate compression wood was not correlated.

According to Timell an increment borer could be used for detection of compression wood in standing trees, by making a visual assessment of the cores extracted. If torque drilling is measured this “torsiometer” can be used to evaluate the wood-specific-gravity and hence obtain an estimate of compression wood content.

Detection of compression wood in logs

The presence of compression wood can be suspected when logs have a pronounced curvature, are oval or have an eccentric pith position in the log ends (Koch et al. 1990, Timell 1986). However, compression wood can also occur in perfectly concentric stems.

According to Lundgren (2000) the external shape of a trunk is a good indicator of the internal quality of the log and is therefore a cornerstone for quality assessment of logs. With new log scanners it is possible to obtain a detailed assessment of the true shape of the log. Variables such as unevenness, taper, ovality and straightness can be used in models for classification of logs into different quality grades (Lundgren 2000, Grace 1993a, Grace 1993b).

Gjerdrum and Warensjo (2001) developed shape parameters from 3D-data that could be used for automatic detection of logs with specific curvature (different crook types). The aim with the study was to characterise the geometric shape of these logs and see how they could be separated. According to the authors logs with sharp curvatures, that are prone to contain compression wood, could be detected by using these parameters in a model.

Detection of compression wood in discs

Use of transmitted light

The method that was developed by Pillow (1941) is considered an accurate and convenient method especially for detection of mild forms of compression wood. The procedure is based on the observation that compression wood is opaque to transmitted light while normal wood is translucent, (helical cavities scatter the light). The method has been found to be especially useful for ascertaining the presence of compression wood within large areas of normal wood. The method has been widely applied since it was introduced, as can be seen from the authors cited below.

There are however some limitations for the method

• Sanded surfaces cannot be used – the dust “destroys” the fine structure and makes the normal wood more opaque
• In some species with a high resin content such as Larix spp. the compression wood appears translucent which makes it difficult to detect
• Stained wood (decay) is opaque
• Branches are opaque – Of course the branches contain large amounts of CW. The different orientation of cells is probably also an explanation
• The method is less suitable for highly coloured species such as Juniperus spp and Larix spp.
• The method does not allow recognition of fine details
• Thickness of wood specimens is crucial. Samples should be of even thickness in the range 2.5 mm – 3.5 mm (1/8 inch – 3/16 inch)
• The surface of the wood specimen should be smooth (a circular saw gives the best result).

The best contrast is observed in species with white sapwood such as Picea spp. or Abies balsamea.

In species with highly coloured heartwood detection of compression wood is difficult. Fungal infection and other stains can also have the same effect.

Low and Shelbourne (1966) examined discs in transmitted light, delineated areas of compression wood, and measured them with a dot grid.

Burdon (1975) classified Pinus radiata discs in transmitted light into 6 different grades as described before.

According to the Tappi Standard (1955, 1972) for measurement of compression wood-content, 3-6 mm discs are cut and viewed in transmitted light. The compression wood areas are outlined with a pencil and then measured with a planimeter.

Another less time consuming method was developed by Andersson and Walter (1995).

The method uses image analysis techniques to divide an image of a disc viewed in transmitted light into different compression wood grades.

• The computer software was developed by Fredrik Walter SLU, Centre for Image analysis
• The method is based on the findings by Pillow and is conducted on thin wood discs that are placed on a light scattering table and viewed in transmitted light
• Mild compression wood appears light orange to red, and severe compression wood appears dark brown to black
• Images are registered with a digital camera
• The operator marks typical areas of the different wood types in the image
• The computer software uses supervised multivariate classification for dividing the disc into normal wood, mild and severe compression wood
• The computer software also automatically extracts shape parameters that can be used for calculation of pith eccentricity, ovality and different diameters.

Limitations:

• The method is dependent on the operator
• Discrimination between mild and severe compression wood is sometimes difficult for the operator
• It is important that operators are well trained and use the same colour display throughout the whole investigation
• The thickness of wood discs is crucial.

The repeatability was tested by letting two people classify the compression wood content in 10 Norway spruce discs. The classification was repeated four times during one week. In total each disc was classified eight times. The mean and standard deviation as a percentage of the mean were calculated for mild-, severe- and total compression wood content. The result from the comparison indicated a good repeatability for the total compression wood content, a rather good repeatability for the mild compression wood content and a poor repeatability for the severe compression wood content. According to the authors this indicates that the operators had difficulty in discriminating between mild and severe compression wood. Therefore it is very important that operators are well trained and use the same colour display throughout the whole investigation.

A t-test showed that there were no significant differences between the two operators.

The repeatability of the angle from the pith to the gravity of the severe and mild compression wood was also tested. The result showed that the algorithm computes the angles well. However, a t-test showed a significant difference between each operator’s classification of the angle for severe compression wood. However, this could be explained by the large standard deviations in three of the discs that contained small amounts of compression wood.

According to the authors the method by using transmitted light increased the differences between the different wood types. The image analysis method is both faster and more objective compared to methods using a planimeter or counting points in a dot grid.

Detection of compression wood in discs – use of reflected light

As already described in section 1 the reddish-brown colour is used by different authors as a means of detecting and classifying compression wood. The discussion about the colour fading while drying indicates a certain degree of uncertainty about the reliability of this feature when viewed in reflected light. According to Low (1964) the use of a green filter increases the contrast between normal wood and compression wood.

Westing recommends reflected light of wavelength of 480 nm for viewing compression wood.

Timell (1986) states:

“In doubtful cases examination has to be performed with a light microscope or a good stereomicroscope”

Another approach to improve the contrast between compression and normal wood is to use different staining methods. They are widely applied in conjunction with microscopic studies.

Different staining solutions that could be used include:

• Malachite green → stains the compression wood dark green
• Malachite green in conjunction with Methylene blue → compression wood becomes darker
• Malachite green + Iodine → stains the lignin
• Acridine orange → improves the optical resolution
• Phloroglucinol-hydrochloric acid, Safranin and Rhodamin → gives bright red colour.

Detection of compression wood in discs – use of a shrinkage method

This method was developed by Trendelenburg and Meyer-Wegelin in 1955. Discs are cut in the green state and then dried. The greater longitudinal shrinkage of compression wood results in compression wood appearing more depressed on the surface of the discs than the adjacent normal tracheid.

Detection of compression wood in discs – use of density measurements

Most methods for determining the density and proportion of latewood could be used for estimation of compression wood-content.

X-ray techniques:

• Instruments for radiation densitometry developed by Phillips B-ray procedure
• According to Timell (1986) a more widely used technique using x-rays was introduced by Polge (1966). The method is especially useful in dendrochronological growth ring analyses
• The x-ray technique is superior with respect to both accuracy and speed and it is also more widely applicable than the B-ray procedure
• Examples of modern applications are Silviscan and Windendro that use high-speed x-ray scanning diffractometry.

A scanning microphotometer uses visible light to measure ring width and percentage of latewood. The advantage of this method are convenience and low cost (Windendro).

Detection of compression wood in sawn wood

Warped lumber often contains compression wood. However the main cause of distortion may be that the lumber contains juvenile wood or spiral grain.

Mild and moderate forms of compression wood are often difficult to detect in sawn timber.

Prediction of compression wood using microwaves

Studies have been conducted at Luleå University of Technology with the aim of predicting moisture content and density in sawn wood of Scots pine using microwaves (Johansson 2001). The primary result from his work is a well-functioning apparatus for microwave scanning of wood in a laboratory environment. On of the major problems seems to be calibration, due to the richness in the microwave signals from the inhomogeneous wood material.

By using microwaves it is possible to measure electromagnetic properties of wood. Dry wood is a rather good electric insulator. With increased moisture content the ability to conduct electricity increase. The dielectric parameters for wood mostly depend on the density and the moisture content, but also on temperature, field frequency and the field orientation in relation to the grain of the angle. The polarisation ability in wood is very important. Dipole and interfacial polarisation play the main role at frequencies in the range from 10⁵ to 10¹⁰ Hz.

The dipole polarisation is created by the orientation of the dipole molecules in the direction of an electric field. Interfacial polarisation arises from charges that are built up in interfaces between components like cell walls, interfibrillar channels and water. The degree of polarisation is influenced by the grain angle.

The scanner system that was developed consisted of three parts, the sensor system, the PC-system and the conveyer system. The sensor system consisted of a transmitting antenna, a receiving unit, a RF receiver, a multiplexer unit and a data processing unit.

The principle of measurement is electromagnetic transmission by a quasi plane through the wood.

To verify the results from the density measurements a medical CT-scanner was used.

The result show that microwaves are a good tool for predicting density and moisture content both as an average as well as a distribution prediction.

The very high accuracy of the density prediction indicates that it may be possible to predict strength and stiffness of wood using microwaves based on density predictions since density is one of the most important variables when predicting strength and stiffness of wood. Since compression wood tends to have a higher density than normal wood this method could probably also be used for prediction of CW-content in sawn boards.

Detection of compression wood in sawn wood – image based methods for non-destructive detection of compression wood in sawn timber

In a Licentiate thesis from 1999 Nyström developed and tested four different image based methods for detection of compression wood.

The methods were:

• Spectral imaging
  Uses many narrow wavelength bands to reveal much smaller differences in colour than what is possible for a normal colour camera or even for the human eye.
  
  Uses a camera sensitive for red green and blue (RGB) light. According to the author compression wood can be classified with this technique in green conditions using non-linear models of RGB colour data. However, the same method did not work as well on dry surfaces.
  
  Results in an image of the x-ray absorption through the object. This absorption is closely related to density. Since the average density of compression wood is relatively high this method could be used for detection The method is also useful for volume estimations of the compression wood since X-ray scanning sees through the wood. This is something that the “surface scanning techniques” cannot achieve. In a comparison of x-ray images from the same board in dry and green conditions it became evident that the method did not work in green conditions. The reason was that the large moisture variation in the board masks the higher density of compression wood.
  
  Measures the ability of the wood fibres to scatter light. Normal wood fibres transmit light much better along the grain than in the transverse direction. Infected or stained wood fibres lose much of their ability to transmit light. Compression wood, with a steeper microfibril angle and helical cavities, also tends to scatter light less than normal wood (it has a lower longitudinal transmission). This feature can therefore be used for detection of compression wood. The tracheids act as optical conductors and transmit light in the grain direction better than across the fibres.
• Colour imaging
• X-ray scanning
• Tracheid effect

Spectral imaging is a relatively simple method which has shown good results.

The use of RGB-colour scanning also shows promising results but with some variations depending on the moisture content as in sap and heartwood. Tracheid effect scanning shows very promising results on dry wood surfaces and X-ray imaging can be used to show compression wood of higher concentration inside dry timber (see table above).

Detection of compression wood in sawn wood – visual assessment

According to Nordic Timber (Anon 1997), the Nordic grading rules for sawn timber, compression wood should be considered during grading if the darker part covers more than 1/3 of the width of the year ring (annual increment) and has affected the shape of the board resulting in distortion.

The compression wood volume in the board is estimated by multiplying the compression wood area on the faces and thus constructing an imaginary box that encloses the compression wood within the board.

Detection of compression wood in sawn wood – measurement of the green shape of boards

In a Doctoral thesis from 2001 Ohman states:

“If the exact position of compression wood within the sawn product could be determined before sawing all problems related to compression wood could be avoided. Examples of such techniques are X-rays and gamma-radiation, nuclear magnetic resonance and microwaves. X-ray techniques or gamma-radiation are both capable of depicting the internal density variation well, but the unknown amount and distribution of water makes it impossible to identify areas of compression wood with the level of accuracy needed.

Nuclear magnetic resonance is a method which measures the water content above the fibre saturation point and is therefore not capable of detecting compression wood.

Using microwaves it is possible to separate moisture content and wood density. In samples of the size of a log the damping of the signals is large and the ability to detect any density variations is small.”

In his studies Ohman found that by using the secondary features related to compression wood, such as magnitude of log sweep, ovality and amount of visible compression wood in log ends, he could achieve a rough indication of the amount of compression wood.

The relationship between the amount of compression wood in the sawn products and different log features were generally weak. Strongest correlation to the total amount of compression wood were the magnitude of log sweep and the amount of visible compression wood-content in the butt-end cut of the log. But in neither of the papers did the correlation coefficient exceed 0,60. Other features such as ovality in top and butt ends displacement of the pith and amount of juvenile wood demonstrated an even poorer correlation to the amount of compression wood.

The location of compression wood within a single plank can be predicted by the shape of the green plank. A convex shaped plank is a very strong indicator of the presence of compression wood and the larger the observed convex bow, the more compression wood can be expected.

According to one of the papers in his thesis “Modelling compression wood in sawn timber of Scots pine and Norway spruce” it is possible to separate logs with large amounts of compression wood from those with small amounts. The best prediction models (PLS – Partial Least Squares) of compression wood content in boards had an R² value of 0.66 for Scots pine and 0.64 for Norway spruce. The variable that contributed the most was the size of the span that occurred between the centreboards directly after sawing (green-span). Compression wood-content in the butt end also contributed to the model.

The conclusion from the study was that these PLS-models had better predictability than the compression wood-log variable that is in use in Sweden today. However, this variable green-span is impossible to detect during log grading since it appears after sawing. Therefore this method can not be considered as a better method for log grading.

No correlation between compression wood content and twist was found.

According to his conclusions, compression wood content in the butt end of the log is a rather poor indicator of the quality of the sawn products if only warp is considered.

What’s of interest

Literature review performed by:
Philipp Duncker
Albert-Ludwigs-University Freiburg
Institut für Waldwachstum
Bertoldstr. 17
79085 Freiburg
Germany

Mats Warensjoe
Swedish University of Agricultural Sciences
Department of Forest Management and Products
901 83 Umeå
Sweden

Compression Wood – Programme objectives

Overall objective

The overall objective of the project was to obtain along-term improvement in the quality of softwood timber products in terms of mechanical properties (strength and stiffness) and geometrical stability (warp, distortion) related to the occurrence of compression wood.

The general effects of compression wood on the performance of sawn timber are reduction in the strength, stiffness and dimensional stability, resulting in a decrease in yield of high quality end products and consequent financial losses.

Species selected

Work was undertaken on Sitka spruce, Norway spruce, Scots pine and European larch. These are commercially important conifer species throughout Europe and will be used to develop methodologies appropriate to other conifer species. The growth patterns of spruce, pine and larch are very different, as a result of which the formation and presentation of compression wood varies among these species.

By working on different species, common characteristics in compression wood formation and behaviour can be identified and knowledge obtained that can be readily applied to other conifers.

Specific objectives

Specifically the objectives of the project were:

• To develop a better understanding of the formation of compression wood during growth, by considering possible cause-effect relationships between environmental constraints, silvicultural practices and individual tree growth characteristics.
• Based on 1 to develop predictive (short-term) methodology and preventive (long-term) silvicultural strategies aimed at minimising the occurrence and the severity of compression wood in high value softwood stands.
• To assess quantitatively the impact of compression wood on micro-structural, physical and mechanical properties.
• To minimise the negative impact of compression wood during the various steps of processing by developing adequate processing and treatment methodologies at laboratory and industrial scale.
• To develop new methods for identifying the presence of compression wood in saw logs and sawn timber.
• Based on the above, to compile an overall evaluation of the impact of compression wood on the utilisation of softwood timber and suggest solutions to minimise this impact both at the silvicultural and the processing levels.
• To incorporate the effect of compression wood into existing tree growth, yield and wood-quality models.
• To communicate the results and conclusions to the relevant forestry and industrial sectors and seek their feedback and comments.