1. Reaction Wood
Reaction wood is formed as a response by the tree to a triggering event such as tipping from the vertical.
It is also known to regulate the orientation or angle of branches relative to the main stem.
(Haygreen & Bowyer)

2. The terminology used to describe reaction wood formed in softwoods and hardwoods comes from the stresses normally present in those locations.
However, those stresses themselves are NOT responsible for the formation of reaction wood.
(Haygreen & Bowyer)

3. Compression Wood
Wind
(Haygreen & Bowyer)
(Hoadley)

4. Compression wood – characteristics and properties
Anatomy
Wider growth rings
More latewood
Shorter longitudinal tracheids
Rounded cells with intercellular spaces
Helical striations (following S2Ө)
Ultrastructure
Larger S2Ө
S3 absent
New S1L layer
Chemistry
More lignin
Less cellulose
Hemicelluloses differ
Properties
Higher wood density
Compression strength ↑
All other strengths ↓
Brittle failure
Greater longitudinal shrinkage
Lower pulp yields

5. Compression wood – macroscopic appearance
(Hoadley)

6. Compression wood – microscopic appearance
Light microscope
Scanning electron microscope
Transmission electron microscope

7. Ultrastructure of longitudinal tracheids
(Josza)

8. Compression wood – microscopic appearance
Normal wood
Compression wood
Compression wood

9. Tension Wood
Wind

10. Tension wood – macroscopic appearance
(Hoadley)

11. Tension wood – microscopic appearance
(Hoadley)
G-layer
Aspen tension wood
Aspen normal wood

12. Tension wood – appearance of G layer
Microfibril orientation

13. Tension wood – characteristics and properties*
Tension wood – characteristics and properties
Anatomy
Fibers affected not vessel elements
Gelatinous fibers (G-layer)
Ultrastructure
SG - after S3 - replaces S3 - replaces S2 + S3 - replaces some of S1 + S2 + S3
Microfibrils less closely packed
Low Ө in G-layer
Higher S1Ө
Chemistry
More cellulose
Less lignin
Hemicelluloses differ
Properties
Higher wood density
Compression strength ↓
Seasoning defects
Higher pulp yields
Poor workability (“fuzzy” grain)

14. Location of reaction wood formation
Compression wood formation in horizontally oriented stem.
Compression wood is found on underside of stem – not on side of stem under compression.
(Haygreen & Bowyer)

15. Location of reaction wood formation
Reaction wood formation in growing looped stem.
Compression wood is found consistently on underside of stem and tension wood is found consistently on upper side of stem (regardless of the nature of the stresses experienced in those locations).
a. Softwood
b. Hardwood
(Haygreen & Bowyer)