Reaction Wood

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 macroscopic appearance
Compression wood
(Hoadley)
4
Compression wood microscopic appearance
Light microscope
Transmission electronmicroscope
Scanning electronmicroscope
5
Ultrastructure of longitudinal tracheids
(Josza)
6
Compression wood microscopic appearance
Compression wood
Normal wood
Compression wood
7
Compression wood characteristics and properties

• Anatomy
• Wider growth rings
• More latewood
• Shorter longitudinal tracheids
• Rounded cells with intercellular spaces
• Helical striations (following S2?)
• Ultrastructure
• Larger S2?
• 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
Tensionwood
(Hoadley)
9
Tension wood microscopic appearance
(Hoadley)
G-layer
Aspen normal wood
Aspen tension wood
10
Tension wood appearance of G layer
Microfibril orientation
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 ? in G-layer
• Higher S1?
• Chemistry
• More cellulose
• Less lignin
• Hemicelluloses differ

• Properties
• Higher wood density
• Compression strength ?
• Seasoning defects
• Higher pulp yields
• Poor workability (fuzzy grain)

12
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)
13
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)