Wood Macroanatomy

1
Wood Macroanatomy
FW1035 Lecture 2 Bowyer et al - Chapter 2, 26-41

• Planes of view in wood
• Growth rings
• Sapwood and heartwood

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Planes-of-View in Wood Samples

• Wood differs in appearance and characteristics on
  the three primary anatomical planes.
• R Radial Plane
• T Tangential Plane
• X Transverse Plane

Cross Section or X
R
T
3
Appearance of the Different Planes in Wood
Transverse Plane (cross section)
Radial Plane
Tangential Plane
4
Two Basic Types of Boards
Tangential Plane
A - Quartersawn Radial plane on broad face B -
Flatsawn Tangential plane on broad face
A
B
Radial Plane
5
Flatsawn
Quartersawn
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Growth Rings

• Mark annual growth boundaries in trees grown in
  temperate climates
• Often composed of 2 distinct segments
• earlywood (springwood)
• latewood (summerwood)
• Earlywood and latewood cells have different
  characteristics

7
Cell Differences Within Growth Rings

• Latewood
• smaller diameter cells
• thicker cell walls

• Earlywood
• large diameter cells
• lower density than latewood

Generally true for both hardwoods and softwoods
8
Effect of Growth Rate on Appearance Ring Porous
Hardwoods
Average
Fast
Slow
Growth rate can have a large affect on gross
appearance. All pictures are of red oak (Quercus
rubra) samples.
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Irregularities in Annual Ring Formation

• False rings
• - growth interrupted by environment (e.g.
  defoliation)
• - slow growth may cause formation of latewood
  type cells
• Discontinuous rings
• - cambium was dormant in one region
• - one-sided crowns, suppressed, or overmature
  trees
• Trees grown in tropical environments
• - almost continual growth can limit occurrence of
  rings
• - In some climates, stopping and restarting of
  growth can give more than one growth increment in
  a year

10
Discontinuous growth rings in redwood (Sequoia
sempervirens)
11
Discontinuous growth ring in redwood (Sequoia
sempervirens)
False growth ring in bald cypress (Taxodium
distichum)
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Sapwood and Heartwood

• In mature trees, the xylem has both living and
  dead cells.
• - Sapwood contains the only living cells in the
  xylem (not all sapwood cells are alive either)
  and has a conductive function.
• - 1 of the cells in a mature conifer are alive
• - Heartwood is composed of dead cells and lends
  mechanical support only.
• Size of sapwood region is related to size of the
  tree crown
• - Generally, larger crown leads to more sapwood
• - varies widely with species and individual trees
• - sapwood requirements are lessened upon maturity
  and layer width may shrink with age

13
Sapwood

• contains all of the live cells in the xylem
• rays provide water and nutrient transport in from
  phloem
• nutrients stored in specialized cells called
  parenchyma cells
• Also provides strength to the tree stem

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Heartwood

• Formation from reduced water and oxygen
  availability leads to death of parenchyma cells
• formation of extractives in the cells
• often (but not always) results in a coloring of
  the wood
• Function is strength/support to tree stem

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Live Cells in the Sapwood
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Distribution of Extractives Across Stem Diameter
Heartwood
Sapwood
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Heart Rot
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What do extractives do?

• Extractives can impart valuable properties to the
  heartwood
• Color - black walnut and black cherry (for
  decorative use)
• Decay resistance - cedars, Douglas-fir, redwood,
  cypress
• Low Water Permeability Douglas-fir (may cause
  difficulty in drying, though)
• Odor - cedars (cedar chests, closet linings)