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.