1. ”Wood Defects” Knots, Spiral Grain, Juvenile Wood, and Reaction Wood
FW1035
Lecture 6
Bowyer et al,
Chapter 2, pp , Chapter 6, pp
”Wood Defects” Knots, Spiral Grain, Juvenile Wood, and Reaction Wood

2. Grain Orientation in Wood
Grain direction - direction of the long axis of longitudinal wood cells
Spiral grain caused by anticlinal division with new primary cell wall formation in one direction only
Interlocked grain
genetically controlled
common in elms
may cause warping in lumber upon drying
may exhibit nice figure in veneers. “ribbon figure”

3. Ribbon Figure
Quarter-sawn lumber from tree stem with interlocked grain.

4. Knots are Branch Stubs
Tight knots - incorporation of living branches into the stem
knots are integral part of the surrounding wood
= “intergrown knots”
Loose knots – stem growth encases a dead branch
may fall out upon lumber drying
= “encased knots”
“Grain” of wood deviates around knots – weak point

5. Juvenile Wood
Short fibered xylem with high microfibril angles and low specific gravity.
Wood produced during the first 5-15 years of growth
As tree grows, the SG increases and the fibers lengthen.
Gradual transition from juvenile wood to mature wood
Caused by effects of hormones from apical meristems on cambium
As cambium in stem becomes farther from and less influenced by the apical meristem, transition to mature wood

6. Physical Characteristics of Juvenile Wood that Affect its Use
Cells are shorter than mature wood
Thin cell walls and less latewood
Leads to lower density and strength
More spiral grain
For Softwoods in Particular:
Density
10-15% lower than mature wood
Strength
15-50% lower than mature wood

8. Transition from Juvenile Wood to Mature Wood is Gradual!
Not the same as transition from sapwood to heartwood, which is abrupt.

9. Juvenile Wood in Solid Wood Products
Greater tendency for spiral grain
Shrinkage
often shows up to 10 times the longitudinal shrinkage of mature wood due to the greater S2 microfibril angle
Trees with high juvenile wood content may yield only 20-50% as much high grade dimension lumber as older trees
Sawmill loss may be reduced when planning specifically for cutting juvenile wood

10. Reaction Wood

12. Compression Wood and Tension Wood
Reaction wood is called compression or tension wood in response to where it forms in a stem
Softwoods form compression wood
Hardwoods form tension wood to correct growth irregularity in a stem
Reaction wood forms in branches of most trees
Tension
Compression

13. Reaction Wood in Softwoods and Hardwoods
Compression Wood
Softwoods
Underside of branches or leaning stem
Commonly in juvenile wood
Appearance is similar in most species
Tension Wood
Hardwoods
Top of branches or leaning stem
Common in juvenile wood also
Appearance and microanatomy is less consistent than for compression wood

14. General Appearance of Compression Wood
Eccentric growth rings that appear to contain an abnormally large proportion of latewood in the widest portions
Non-centrally located pith in stem
Often darker color (red/brown)

15. Microanatomical Characteristics of Compression Wood
Latewood longitudinal tracheids are most affected
Rounded cross-section, rather than prismatic
Intercellular spaces are present
Greater cell wall thickness
No S3 layer
Larger S2 microfibril angle, ~45°
Spiral cavities in S2 layer
Longitudinal tracheids are 10-40% shorter
Tips of tracheids are distorted

18. Effects of Compression Wood on Utilization
Large longitudinal shrinkage (1-2%) causes warping and bending of boards upon drying
Higher lignin content (average of 38% versus 29% in normal wood) gives lower pulp yields
Lower strength properties than density would lead you to predict

19. General Appearance of Tension Wood
Cut surfaces have “wooly” or fibrous appearance
causes overheating and dulling of saws
difficult to sand and finish

20. Appearance of Tension Wood in Stem

21. Microanatomy of Tension Wood
Cell modifications are usually in the earlywood
Most commonly affects fiber cells
more numerous fibers, fewer rays, vessels, etc.
secondary cell wall is significantly different

22. Microanatomical Differences
Changes in the secondary cell wall
almost entirely made of cellulose
forms a floppy layer that is loosely attached to the primary wall
gelatinous layer called the “G” layer

23. Appearance of the Gelatinous Layer

24. Utilization of Tension Wood
Can produce good paper properties if pulping conditions are modified
Good for “dissolving pulps”
cellulose source for making cellophane, rayon, and nitrocellulose
Solid wood products have lower quality
tendency to ‘collapse’ upon drying
higher longitudinal shrinkage 1-5x normal wood) may lead to warp and bending
lower strength properties