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 outlinein 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 thenormal 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.