Wood

• 1. Introduction

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  Wood grew on Earth long before any human could shape it. A 2011 discovery in the Canadian province of New Brunswick yielded the earliest known plants to grow wood, roughly 395 to 400 million years ago. That ancient invention still surrounds us. As of 2020, the growing stock of forests worldwide measured about 557 billion cubic meters, and in 2023 almost 4 billion cubic meters were harvested. Yet for a material so familiar, wood hides a strange double life. It is the structural tissue called xylem, found in the stems and roots of trees and other woody plants. Inside a living tree it does two jobs at once. It holds the plant upright and it carries water and nutrients between leaves, growing tissues, and roots. How does a bundle of dead cells become a beam strong enough to raise a house? Why is one plank weak where another is tough? And why do we call bamboo, a grass, by the same name as oak? The answers begin inside the cell wall.

• 2. A Composite Of Fibers

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  Cellulose makes up 40 to 50 percent of a wood cell wall, a crystalline polymer derived from glucose and strong in tension. These cellulosic fibers sit inside a matrix of lignin, which resists compression, alongside hemicellulose. Lignin runs around 27 percent in coniferous wood against 23 percent in deciduous trees, and its aromatic rings give wood its water-repelling, hydrophobic character. The three components are interwoven, with direct covalent linkages tying the lignin to the hemicellulose. By dry weight, wood is roughly 50 percent carbon, 42 percent oxygen, 6 percent hydrogen, 1 percent nitrogen, and 1 percent other elements such as calcium, potassium, sodium, magnesium, iron, and manganese. This architecture makes wood heterogeneous, hygroscopic, cellular, and anisotropic, meaning its properties change with direction. The split between hardwood and softwood shows up even in chemistry. Hardwood lignin is primarily derived from sinapyl alcohol and coniferyl alcohol, while softwood lignin comes mainly from coniferyl alcohol alone. The paper industry exists largely to pull these threads apart, separating lignin from the cellulose that becomes paper.

• 3. How A Tree Lays Down Wood

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  Between the existing wood and the inner bark, a tree builds new woody layers that wrap the entire stem, living branches, and roots. This is secondary growth, driven by cell division in the vascular cambium, a lateral meristem, followed by the expansion of new cells and the thickening of their secondary walls. Where seasons differ sharply, as in New Zealand, growth happens in a discrete annual or seasonal pattern that produces growth rings, most clearly seen on the cut end of a log. In equatorial regions such as Singapore, where the distinction is annual, these are called annual rings; where seasonal difference is slight, rings may be faint or absent. Earlywood, also called springwood, forms early when growth is rapid, with wider elements and a lighter color. Latewood, or summerwood, forms later and runs darker. A tree grown in the open makes its widest rings in youth, then narrows as it ages and its crown opens. Some trees, such as southern oaks, hold the same ring width for hundreds of years.

• 4. Knots And Their Burden

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  A dead lower branch, overgrown by later layers of trunk wood, becomes a knot, an imperfection where the grain direction can run up to 90 degrees off the surrounding wood. In a knot the wood parts and rejoins around a roughly circular, usually darker piece. Knots carry real consequences for strength. They weaken timber most when forces run perpendicular to the grain or place it under tension, far less under compression along the grain. A knot on a beam's upper side is compressed, one on the lower side stretched, and a season check inside a tensioned knot offers little resistance. Position matters as much as size. Knots near the ends of a beam do not weaken it, and sound knots within the central portion, one-fourth of the beam's height from either edge, are not serious defects. Small knots along the neutral plane can even add strength by preventing longitudinal shearing. Sound knots do not weaken wood under compression parallel to the grain. The trouble outlives the sawmill. On painted surfaces like skirting boards and door frames, resin can keep bleeding through a knot for months or years, staining it yellow or brown, a problem knotting primer reduces but rarely cures.

• 5. Heartwood And Sapwood

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  Heartwood, also called duramen, is wood that has undergone a naturally occurring chemical change to resist decay. The name comes only from its position. A tree can thrive with its heart completely decayed, proof that heartwood holds no vital importance to the living plant. Sapwood, or alburnum, is the younger outer wood that conducts water from roots to leaves and stores the reserves prepared there. All wood begins as sapwood, and by the time its tracheids and vessels can conduct water, the cells have lost their cytoplasm and are functionally dead. Species behave differently. Chestnut, black locust, mulberry, osage-orange, and sassafras carry thin sapwood, while maple, ash, hickory, hackberry, beech, and pine carry thick sapwood, and some never form heartwood at all. Vigorous, fast-growing trees in the open need more sapwood, sometimes reaching 30 centimeters or more in diameter before any heartwood forms. There is a hidden reward in this geometry. Because the outer wood of an old forest-grown tree is freer of knots than its inner heartwood, a piece of sapwood may actually be stronger than heartwood cut from the same log.

• 6. Reading Color And Water

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  In longleaf pine, resin-saturated heartwood called fat lighter grows almost impervious to rot and termites, and very flammable. Tree stumps of old longleaf pines are dug, split, and sold as kindling, and a dug stump may remain a century or more after being cut. Spruce impregnated with crude resin and dried gains strength the same way. Color can hint at strength because latewood runs darker than earlywood, a useful cue in coniferous woods, though otherwise color is no indication of strength. Abnormal discoloration tells a different story. The black check in western hemlock comes from insect attacks, and the reddish-brown streaks common in hickory mostly come from injury by birds, marks of damage rather than weakness. Water lives in wood in three places: in the cell walls, in the protoplasmic contents of the cells, and as free water in the cell cavities. Thoroughly air-dried wood holds 8 to 16 percent of its water in the cell walls and almost none elsewhere. Drying transforms strength. A completely dry spruce block 5 centimeters in section sustains a permanent load four times as great as a green block of the same size.

• 7. Density And The Grading Of Strength

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  Specific gravity is the single most revealing property of wood quality, since both pulp yield and lumber strength follow from it. Wood density answers to age, diameter, height, radial growth, geographical location, site, silvicultural treatment, and seed source, so variation is expected. The USDA Forest Service measured a coefficient of variation of 10 percent, meaning about 68 percent of samples fall within plus or minus 10 percent of a species average. Within a single tree, variation can be as great as the difference between trees. In ring-porous woods such as oak, ash, and hickory, faster growth tends to mean heavier, harder, stronger, stiffer wood, because the strength-giving latewood fibers grow more abundant. U.S. Forest Service tests on hickory found shock-resisting ability greatest in wide-ringed wood with 5 to 14 rings per inch, and concluded that first-class stock shows 5 to 20 rings per inch. Diffuse-porous woods like maple and birch behave more like conifers, where medium growth gives the strongest material. The hardwood and softwood labels mislead. Balsa, a hardwood, is softer than any commercial softwood, while yew, a softwood, is harder than many hardwoods. One of the densest woods of all is black ironwood.

• 8. The Many Lives Of Timber

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  Bamboo, botanically a member of the grass family, is called wood and carries real economic weight, its larger culms used in building and in engineered flooring, panels, and veneer. Palms produce a similar material, as do lesser plants like Pandanus, Dracaena, and Cordyline, though their structure differs from ordinary wood. People have built with wood for ten thousand years, from the longhouses of Neolithic Europe to today. The earliest spears were crafted from wood with tips toughened by burning, and by 15,000 BC hunters used wooden spear-launchers, often adorned with carvings of creatures. Global roundwood production rose from 3.5 billion cubic meters in 2000 to 4 billion in 2021, when wood fuel held the largest share at 49 percent. Tradition still clings to specific species. Elm resisted decay while kept wet, serving for boats and water pipe; medieval Europe favored oak for beams, walls, doors, and floors. Cricket bats use white willow, baseball bats ash, hickory, or maple, and golf woods once carried heads of persimmon. The story keeps reaching forward, into recyclable food packaging, anti-bacterial medical agents, and high-strength composites drawn from a material that first grew nearly 400 million years ago.