FW1035
Lecture 6
Bowyer et al, Chapter 6
‘Wood Defects’
Knots, Spiral Grain, Juvenile
Wood and Reaction Wood
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Grain Orientation in Wood
• Grain direction - direction of
the long axis of 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”
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Ribbon Figure
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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
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Juvenile Wood
• Xylem produced by the
cambium when it is affected
by growth hormones from an
apical meristem
• pith + wood, for the first 5-
25 years
• As tree grows, cambium in
stem becomes farther from
and less influenced by the
apical meristem
• gradual transition from
juvenile wood to mature
wood
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Physical Characteristics of Juvenile
Wood that Affect its Use
• Cells are shorter than For Softwoods in Particular
mature wood • Density
• Thin cell walls and less • 10-15% lower than mature
latewood wood
• Leads to lower density • Strength
and strength • Typically, 15-30% lower
• More spiral grain (up to 50% for some
species)
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Transition from
Juvenile Wood to
Mature Wood is
Gradual
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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
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Reaction Wood
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Compression Wood and Tension Wood
• spp.) Reaction wood often
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
Tension irregularity in a stem
• Reaction wood forms in
branches of most trees
(except drooping
Compression branches like in Picea
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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 is less consistent than compression wood
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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)
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Microanatomical Characteristics of
Compression Wood
• latewood longitudinal • no S3 layer
tracheids are most • larger S2 microfibril
affected angle, ~45°
• rounded cross-section, • spiral cavities in S2
rather than prismatic layer
• intercellular spaces are • longitudinal tracheids
present are 10-40% shorter
• greater cell wall • tips of tracheids are
thickness distorted
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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
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General Appearance of Tension Wood
• cut surfaces have
“wooly” or fibrous
appearance
• causes overheating and
dulling of saws
• difficult to sand and finish
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Appearance of Tension Wood in Stem
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Microanatomy of Tension Wood
• Cell modifications are
usually in the earlywood
• most commonly affects
fiber cells
• more numerous fibers,
fewer rays, vessels, etc.
• longer and thinner cell
walls
• secondary cell wall is
significantly different
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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 (very low
lignin content)
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Appearance of the G-Layer
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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
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