ABNORMAL WOOD
Juvenile Wood
Makes up the first 5 to 20 growth rings in a cross section.
Is thought to be created in the cambium region while it was still influenced by the apical meristem.
As a tree grows taller, the influence of the apical meristem on that region becomes less and less.
Therefore, the wood produced in a given year may be juvenile at the top but mature at the butt (see Figure 6.1. on Page 110 in Textbook).
The percentage of the wood that is juvenile seems to vary between species and between trees.
It seems to be increasing in amounts in plantation grown trees and becoming a growing concern for softwood users.
Table 6.1. on Page 112 in Textbook compares some properties of juvenile and mature wood. It is obvious that the properties of juvenile wood are inferior to mature wood. The trouble seems to stem from the fact that the microfibrils in the S2 layer of juvenile wood makes a greater angle to the longitudinal axis of the cell.
As you remember, the S2 layer is the thickest layer in the cell wall; Therefore, it exerts more influence on the behavior of the cell.
See Figures 6.2 and 6.3 on Pages 116 in the Textbook
Because of the gradual change in wood properties, it is difficult to tell where juvenile wood ends and mature wood begins.
In the transition region within a growth ring, some properties may be that of juvenile wood and some may be that of mature wood.
REACTION WOOD
Compression Wood -- found on the lower side of leaning softwood trees.
Properties:
A. tracheids are shorter
B. Less cellulose -- more lignin and hemicellulose -- resulting in lower pulp yield.
C. longitudinal shrinkage 1-2% and sometimes up to 6-7% versus 0.1 - 0.2% for normal wood -- causes severe warpage in drying.
D. low strength to weight ratio -- it has the same strength but greater weight.
Appearance:
1. wide growth rings on lower side and narrow growth rings on upper side.
2. more latewood cells but the transition is more gradual.
3. tracheids are more round and have bent and folded ends.
4. only S1 and S2 layers with the fibril angle in S2 layer at 45 degrees.
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Tension Wood -- found on the upper side of leaning hardwood trees.
Properties:
A. higher cellulose content
B. 5 to 10% increase in density
C. requires special care in pulping
D. produces a fuzzy surface in solid products -- causes saws to overheat and make finishing difficult.
Figure 6.8 on page 123 in textbook.
E. tendency to collapse while drying
F. longitudinal shrinkages -- 1% or less
G. lower strength to weight ratio
Appearance:
1. wider growth rings in reaction wood and narrower rings on the other side resulting in an elliptical shaped cross section
2. fuzzy surface
3. maybe darker in color
4. fewer and smaller vessels
5. loosely attached secondary walls
6. Gelatinous layer -- almost pure cellulose of highly crystalline structure.
Branch Wood
1. higher proportion of bark
2. hardwoods have more vessels and rays than stem wood
3. softwood have more ray tissue than stem wood
4. narrower growth rings
5. smaller longitudinal cells
6. higher specific gravity
7. particleboard made of logging slash -- lower strength
8. lower yields and lower strength in pulping
Root Wood
1. dirty
2. in softwoods
a. lower ray volume
b. larger diameter cells
c. thinner cell walls
3. in hardwoods
a. vessels and parenchyma in greater volume -- low fiber volume
b. large diameter and long fibers
c. vessels abnormally larger diameters to small
d. specific gravity -- low
4. lower Kraft pulp yield