1 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 Reaction Wood (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 – macroscopic appearance Compression wood (Hoadley)

4 Light microscope Scanning electron microscope Transmission electron microscope Compression wood – microscopic appearance

5 Ultrastructure of longitudinal tracheids (Josza)

6 Normal wood Compression wood Compression wood – microscopic appearance

7 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 *

8 Tension wood – macroscopic appearance Tension wood (Hoadley)

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

10 Microfibril orientation Tension wood – appearance of G layer

11 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) *

Location of reaction wood formation 12 (Haygreen & Bowyer) Compression wood formation in horizontally oriented stem. Compression wood is found on underside of stem – not on side of stem under compression.

Location of reaction wood formation 13 (Haygreen & Bowyer) 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