History and Uses of Wood
– Discovery of wood in Canadian province of New Brunswick, dating back 395-400 million years ago
– Wood has been used for thousands of years for various purposes
– Wood used for fuel, construction, tools, weapons, furniture, packaging, artworks, and paper
– Recent use of wood enhanced by addition of steel and bronze in construction
– Dominant uses of wood in 2008 were for furniture and building construction
– Woody materials are of intense interest as a source of renewable energy
– As of 2020, the growing stock of forests worldwide was about 557 billion cubic meters
Physical Properties of Wood
– Wood is yielded by trees through secondary growth
– Woody layers envelop the entire stem, living branches, and roots
– Growth rings can be seen on the end of a log and indicate annual or seasonal growth patterns
– Earlywood or springwood is lighter and formed early in the growing season
– Latewood or summerwood is darker and formed later in the season
Growth Rings and Knots
– Growth rings provide clues to prevailing climate at the time a tree was cut
– Differences in growth ring width and isotopic abundances indicate year-to-year variation
– Growth rings can be indistinct or absent in regions with little seasonal difference
– Bark removal can deform growth rings as the plant overgrows the scar
– Width of growth rings decreases as a tree gets larger in diameter
– Knots are formed by lower branches that die and become enclosed by subsequent layers of trunk wood
– Knots affect the technical properties of wood and can reduce tension strength
– Knots may be exploited for visual effect in longitudinally sawn planks
– Knots can weaken timber and lower its value for structural purposes
– Position, size, number, and condition of knots determine their impact on the strength of a beam
Heartwood, Sapwood, and Color
– Heartwood is wood that has become more resistant to decay due to a chemical transformation
– Heartwood formation is a genetically programmed process
– Some uncertainty exists as to whether the wood dies during heartwood formation
– A tree can thrive with its heart completely decayed
– Some species begin to form heartwood very early in life, while in others the change comes slowly
– Thin sapwood is characteristic of certain species, while thick sapwood is characteristic of others
– Heartwood is visually distinct from the living sapwood
– Heartwood is usually darker than sapwood
– Color variation does not imply a significant difference in mechanical properties
Water Content, Wood Structure, and Discoloration
– Water occurs in living wood in three locations: cell walls, protoplasmic contents, and cell cavities
– Heartwood retains water only in the cell walls
– Air-dried wood retains 8-16% water in the cell walls
– Oven-dried wood is considered dry for most purposes
– Wood is a heterogeneous, hygroscopic, cellular, and anisotropic material
– It consists of cells with cell walls composed of cellulose, hemicellulose, and lignin
– Coniferous wood cells are mostly tracheids, resulting in a more uniform structure
– Hardwoods have a more complex structure
– Abnormal discoloration of wood can indicate a diseased condition
– Rot-producing fungi can cause characteristic colors in wood, indicating weakness
– Sap-staining is due to fungal growth but does not necessarily weaken the woodSources: https://en.wikipedia.org/wiki/Wood
