Plants & Human Affairs (BIOL106)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe

Wood and Wood Products – A Quick Introduction

 I. What is wood?
    The simple answer - xylem cells. Recall that food and water are transported throughout a plant by the vascular system that is comprised of two major tissues, xylem and phloem. The phloem transports organic materials (i.e., food) from the sites of production (usually the leaves) to the sites of need (usually the roots). In contrast, xylem transports water (and dissolved minerals) from the roots to other parts of the plant. Wood is comprised of the cells of the xylem.

II. Xylem – the water transport tissue revisited
    There are four major types of cells in the xylem:

1. Tracheids – cigar-shaped, tapered ends, lots of pits (thin circular areas);
2. Vessel elements, - shorter, wider, ends attached;
3. Fibers - long and skinny with thick secondary wall, mostly for support; and
4. Parenchyma - alive, thin, store starch and other materials, lateral transport. The primary water transport cells are tracheids and vessels.

A few take-home-lessons:

1. Gymnosperms (conifers like pines) only have tracheids for water transport whereas angiosperms (flowering plants like oaks, maples) have both and primarily rely on vessels for water transport.
2. In the final stage of their development, vessels, tracheids, and fiber cells die. The cell contents are reabsorbed and a hollow, rigid cell wall remains.
3. Because xylem cells have thick, rigid cell walls, an additional function of the xylem is support. That’s why redwood trees can get so big.
4. The cell wall is very porous and allows water to freely pass through it.

III. Wood Structure
    In class we will look at a cross section of a woody stem and identify the following structures:

• Vascular cambium – meristematic tissue that produces the xylem and phloem. Occurs between the bark and wood, lateral meristem (vs. apical meristem), results in growth in width of trees
• Wood – xylem
• Inner bark – phloem
• Outer bark (or periderm) – comprised of three layers: cork (phellum), a cork cambium (phellogen) and phelloderm (inner layer)
• Sapwood – lighter color, functional xylem
• Heartwood – darker colored wood near center of stem, non-functional xylem, clogged with tannins, resins and other materials
• Rays – for lateral (inside/outside) movement of water and other materials
• Spring (or early) wood – larger xylem cells resulting from better growing conditions early in the growing season when moisture is generally plentiful and temperatures are ideal
• Summer (or late) wood – smaller xylem cells produced latter in the growing season
• Annual rings – caused by the transition from the summer wood of one growing season to the spring wood of the next.
• Ring-porous - pores in early wood larger than in late wood and appear in a ring.  Common in oak, elm and ash
• Diffuse-porous - spring wood pores the same size as those produced laters.  Common in buckeye, basswood, maple, cherry

IV. Hardwood vs. softwood
    These terms refer to the species of tree from which the wood is obtained. Hardwoods are angiosperm (flowering) trees like the oaks, birches, maples, and basswood. Softwoods refer to conifers (gymnosperms) like pines, firs, and spruces. These terms are also roughly equivalent to the degree of "hardness" of the wood. In general, the hardwoods have harder wood than softwoods. However, there are hardwoods with "soft" wood such as basswood and the poplars, and similarly there are softwoods with relatively "hard" wood like southern yellow pine. The following table compares the two:

V. Wood Appearance

A. Cuts – the "Jelly Roll" or Onion Model

• Transverse (Cross) Section – across the stem, perpendicular to the long axis; annual growth rings appear as concentric circles
• Radial Section – parallel to the long axis, through the center; grain pattern a series of parallel lines
• Tangential Section – parallel to the long axis, anywhere but through the center; grain pattern wavy and variable, not all parallel

B. Boards

• Quarter-sawn – radial cuts – note grain pattern (linear with perpendicular rays)
• Plain-sawn – tangential cuts – not grain pattern ("wavy")

C.  Grain

• due to annual rings and cell structure
• runs in the direction of the tree
• coarse grained wood usually with conspicuous annual rings, large pores

VI. Lessons from Wood

A. Forensics
    Wood structure has helped to solve several crimes including the conviction of Bruno Hauptmann for the abduction and murder of Charles Lindbergh’s infant son.  Lindberg's son was abducted on March 1, 1932 from his bedroom in their Hopewell (NJ) home.  By carefully studying the anatomical structure of the wood in the ladder left at the scene of the crime, technologist Arthur Koehler, was able to determine that:

• The ladder was constructed from scraps of ponderosa pine, Douglas fir and birch
• The wood was low grade lumber, used for joists, construction
• Nail holes had no rust and the wood was not weathered - wood had been stored inside
• Planer marks on the rails showed that the planer was belt-driven and had 8 knives.  The mill that produced the board was located in South Carolina and sold to a lumber yard in the Bronx.
• The edge of one of the rails had been smoothed by a plane found in Hauptmann’s toolbox
• art of the ladder had been constructed from wood removed from a joist in Hauptmann’s attic - the nail holes lined up perfectly, were produced by the same square nails

B. Past History – Dendrochronology
    Trees rings provide a window on the past. Tree rings can be used to:

• Date archaeological artifacts by comparing tree rings in wood samples taken from the site with rings of known age.
• Monitor climate trends can be monitored since the ring width in many species is sensitive to growth conditions (i.e., temperature, moisture, carbon dioxide concentration).
• Determine past geological events like volcanic activity, sunspots, fire
• Determine past forest types (i.e., alder wood wheels on ancient carts were much larger than those of trees today, canoe from bog 50 foot log – none that big today)

C. "Spirit of the Trees" (not on exam)
    This video features information about dendrochonology and the bristlecone pine, the oldest tree (organism) in the world (ca. 4000 years old). They are able to get so old because they have resinous wood resistant to decay, grow in arid region less prone to decay, and don’t drop their needles like regular pines.

VI. MN Forest Types  (not on exam)
    Roughly speaking, MN divided up into three main regions: prairie, deciduous forest, coniferous forest.

VII. Wood Products

A. Veneer – thin piece of board, glued onto a less expensive board; various methods of cutting, most commonly, the log is rotated after steaming and a knife essentially peels a thin layer off.

B. Plywood – glue odd numbers of veneers together

C. Particle/fiber board – small fragments glued together

D. Species/uses

• Red oak – flooring, furniture
• White oak – barrels
• Basswood – creates, boxes, general millwork
• Ash – strong, stiff, good shock resilience, bats, mallets, tool handles
• White pine – construction
• Walnut - furniture, gun stocks
• Cherry – carving, furniture, musical instruments
• Birch – variety (furniture, plywood, toys)
• Maple � furniture, bowling pins, floors
• Click here for a key to wood types

E. Cork � bark of cork oak tree. Mediterranean. Subject to fire. Thick bark to protect. Many uses for the cork including stoppers (pressed directly from sheets of cork. The sheets are prepared by glue the ground up cork), insulation, shoes.

F. Bamboo – product from a plant in the grass family (not on exam)

G. Rayon/Cellophane/Acetates - derived from wood. Chips treated with sodium hydroxide. Then spun. (not on exam)

V. Paper Production (not on exam)

A. General
    International Paper, a local mill, makes coated papers like that used for Time, Newsweek, TV Guide and advertising supplements.

B. Process
    Debarked → chipped (takes about 8 seconds) → screened → TMP plant → refiners (plates with knives turn ground mix to pulp fibers about 1 mm long) → bleached to yellowish white (normally brown) → spread on machines → rollers → heat to remove water → reels → "super calendar"

C. Wood

• Use a mix of about 50% aspen (also called popple, a "soft" hardwood) and 50% balsam fir (a softwood)
• Wood cut in about 8 foot lengths
• They use 350 – 400 cords per day

D. Cool Trivia

• Use about 10 million gal water per day. Gets treated and returned to the river
• Electric bill about $1.0 million per month
• Hydropower accounts for about 8% of the energy, steam for 15%
• Eucalyptus trees now a major provider of pulp. Grow extremely rapidly in suitable climates, like Brazil. Native to Australia. Pulp competitor for our native trees
• 40% of the paper is recycled

E. Other Paper making
    International Paper uses a mechanical process. Other papers are produced by chemical means – sulfite (acids) and sulfates. These are used to separate the fibers to prepare pulp.

Exercises:   click here for exercises and study questions

References:

• Perlin, J. 1991. A Forest Journey: The Role of Wood in the Development of Civilization. Harvard University Press, Cambridge, MA.

Last updated:  11/19/2008 / � Copyright  by SG Saupe /