Wood - WikiHQ

Wood is a structural tissue/material found as xylem in the stems and roots of trees and other woody plants. Being a natural material, it is characterized as an organic materiala natural composite of cellulosic fibers that are strong in tension and embedded in a matrix of lignin and hemicelluloses that resists compression.

Wood is sometimes defined as only the secondary xylem in the stems of trees, or more broadly to include the same type of tissue elsewhere, such as in the roots of trees or shrubs. In a living tree, it performs a mechanical-support function, enabling woody plants to grow large or to stand up by themselves. It also conveys water and nutrients among the leaves, other growing tissues, and the roots. Wood may also refer to other plant materials with comparable properties, and to material engineered from wood, woodchips, or fibers.

Wood has been used for thousands of years for fuel, as a construction material, for making tools and weapons, furniture and paper. More recently it emerged as a feedstock for the production of purified cellulose and its derivatives, such as cellophane and cellulose acetate.

As of 2020, the growing stock of forests worldwide was about 557 billion cubic meters. As an abundant, carbon-neutral renewable resource, woody materials have been of intense interest as a source of renewable energy. In 2023, almost 4 billion cubic meters of wood were harvested. Dominant uses were for furniture and building construction.

Wood is scientifically studied and researched through the discipline of wood science, which was initiated since the beginning of the 20th century.

History

A 2011 discovery in the Canadian province of New Brunswick yielded the earliest known plants to have grown wood, approximately 395 to 400 million years ago. Wood can be dated by carbon dating and in some species by dendrochronology to determine when a wooden object was created.

People have used wood for thousands of years for many purposes, including as a fuel or as a construction material for making houses, tools, weapons, furniture, packaging, artworks, and paper. Known constructions using wood date back ten thousand years. Buildings like the longhouses in Neolithic Europe were made primarily of wood.

Recent use of wood has been enhanced by the addition of steel and bronze into construction. The year-to-year variation in tree-ring widths and isotopic abundances gives clues to the prevailing climate at the time a tree was cut.

Early humans progressively invented tools and techniques for trapping animals. The earliest spears were crafted from wood, with tips toughened by burning. By 15,000 BC, hunters employed wooden and bone spear-launchers to enhance force and distance. These devices were frequently adorned with carvings of creatures.

Physical properties

thumb|Diagram of [[secondary growth in a tree showing idealized vertical and horizontal sections. A new layer of wood is added in each growing season, thickening the stem, existing branches and roots, to form a growth ring.]]

Growth rings

Wood, in the strict sense, is yielded by trees, which increase in diameter by the formation, between the existing wood and the inner bark, of new woody layers which envelop the entire stem, living branches, and roots. This process is known as secondary growth; it is the result of cell division in the vascular cambium, a lateral meristem, and subsequent expansion of the new cells. These cells then go on to form thickened secondary cell walls, composed mainly of cellulose, hemicellulose and lignin.

Where the differences between the seasons are distinct, e.g. New Zealand, growth can occur in a discrete annual or seasonal pattern, leading to growth rings; these can usually be most clearly seen on the end of a log, but are also visible on the other surfaces. If the distinctiveness between seasons is annual (as is the case in equatorial regions, e.g. Singapore), these growth rings are referred to as annual rings. Where there is little seasonal difference growth rings are likely to be indistinct or absent. If the bark of the tree has been removed in a particular area, the rings will likely be deformed as the plant overgrows the scar.

If there are differences within a growth ring, then the part of a growth ring nearest the center of the tree, and formed early in the growing season when growth is rapid, is usually composed of wider elements. It is usually lighter in color than that near the outer portion of the ring, and is known as earlywood or springwood. The outer portion formed later in the season is then known as the latewood or summerwood. There are major differences, depending on the kind of wood. If a tree grows all its life in the open and the conditions of soil and site remain unchanged, it will make its most rapid growth in youth, and gradually decline. The annual rings of growth are for many years quite wide, but later they become narrower and narrower. Since each succeeding ring is laid down on the outside of the wood previously formed, it follows that unless a tree materially increases its production of wood from year to year, the rings must necessarily become thinner as the trunk gets wider. As a tree reaches maturity its crown becomes more open and the annual wood production is lessened, thereby reducing still more the width of the growth rings. In the case of forest-grown trees so much depends upon the competition of the trees in their struggle for light and nourishment that periods of rapid and slow growth may alternate. Some trees, such as southern oaks, maintain the same width of ring for hundreds of years. On the whole, as a tree gets larger in diameter the width of the growth rings decreases.

Knots

thumb|left|A knot on a tree trunk

As a tree grows, lower branches often die, and their bases may become overgrown and enclosed by subsequent layers of trunk wood, forming a type of imperfection known as a knot. The dead branch may not be attached to the trunk wood except at its base and can drop out after the tree has been sawn into boards. Knots affect the technical properties of the wood, usually reducing tension strength, but may be exploited for visual effect. In a longitudinally sawn plank, a knot will appear as a roughly circular "solid" (usually darker) piece of wood around which the grain of the rest of the wood "flows" (parts and rejoins). Within a knot, the direction of the wood (grain direction) is up to 90 degrees different from the grain direction of the regular wood.

In the tree a knot is either the base of a side branch or a dormant bud. A knot (when the base of a side branch) is conical in shape (hence the roughly circular cross-section) with the inner tip at the point in stem diameter at which the plant's vascular cambium was located when the branch formed as a bud.

In grading lumber and structural timber, knots are classified according to their form, size, soundness, and the firmness with which they are held in place. This firmness is affected by, among other factors, the length of time for which the branch was dead while the attaching stem continued to grow.

thumb|upright|Wood knot in vertical section

Knots do not necessarily influence the stiffness of structural timber; this will depend on the size and location. Stiffness and elastic strength are more dependent upon the sound wood than upon localized defects. The breaking strength is very susceptible to defects. Sound knots do not weaken wood when subject to compression parallel to the grain.

In some decorative applications, wood with knots may be desirable to add visual interest. In applications where wood is painted, such as skirting boards, fascia boards, door frames and furniture, resins present in the timber may continue to 'bleed' through to the surface of a knot for months or even years after manufacture and show as a yellow or brownish stain. A knot primer paint or solution (knotting), correctly applied during preparation, may do much to reduce this problem but it is difficult to control completely, especially when using mass-produced kiln-dried timber stocks.

===Heartwood and sapwood===

thumb|right|A section of a [[Taxus|yew branch showing 27 annual growth rings, pale sapwood, dark heartwood, and pith (center dark spot). The dark radial lines are small knots.]]

Heartwood (or duramen) is wood that as a result of a naturally occurring chemical transformation has become more resistant to decay. Heartwood formation is a genetically programmed process that occurs spontaneously. Some uncertainty exists as to whether the wood dies during heartwood formation, as it can still chemically react to decay organisms, but only once.

The term heartwood derives solely from its position and not from any vital importance to the tree. This is evidenced by the fact that a tree can thrive with its heart completely decayed. Some species begin to form heartwood very early in life, so having only a thin layer of live sapwood, while in others the change comes slowly. Thin sapwood is characteristic of such species as chestnut, black locust, mulberry, osage-orange, and sassafras, while in maple, ash, hickory, hackberry, beech, and pine, thick sapwood is the rule. Some others never form heartwood.

Heartwood is often visually distinct from the living sapwood and can be distinguished in a cross-section where the boundary will tend to follow the growth rings. For example, it is sometimes much darker. Other processes such as decay or insect invasion can also discolor wood, even in woody plants that do not form heartwood, which may lead to confusion.

Sapwood (or alburnum) is the younger, outermost wood; in the growing tree it is living wood, and its principal functions are to conduct water from the roots to the leaves and to store up and give back according to the season the reserves prepared in the leaves. By the time they become competent to conduct water, all xylem tracheids and vessels have lost their cytoplasm and the cells are therefore functionally dead. All wood in a tree is first formed as sapwood. The more leaves a tree bears and the more vigorous its growth, the larger the volume of sapwood required. Hence trees making rapid growth in the open have thicker sapwood for their size than trees of the same species growing in dense forests. Sometimes trees (of species that do form heartwood) grown in the open may become of considerable size, or more in diameter, before any heartwood begins to form, for example, in second growth hickory, or open-grown pines.

thumb|Cross-section of an oak log showing growth rings

No definite relation exists between the annual rings of growth and the amount of sapwood. Within the same species the cross-sectional area of the sapwood is very roughly proportional to the size of the crown of the tree. If the rings are narrow, more of them are required than where they are wide. As the tree gets larger, the sapwood must necessarily become thinner or increase materially in volume. Sapwood is relatively thicker in the upper portion of the trunk of a tree than near the base, because the age and the diameter of the upper sections are less.

When a tree is very young it is covered with limbs almost, if not entirely, to the ground, but as it grows older some or all of them will eventually die and are either broken off or fall off. Subsequent growth of wood may completely conceal the stubs which will remain as knots. No matter how smooth and clear a log is on the outside, it is more or less knotty near the middle. Consequently, the sapwood of an old tree, and particularly of a forest-grown tree, will be freer from knots than the inner heartwood. Since in most uses of wood, knots are defects that weaken the timber and interfere with its ease of working and other properties, it follows that a given piece of sapwood, because of its position in the tree, may well be stronger than a piece of heartwood from the same tree.

Different pieces of wood cut from a large tree may differ decidedly, particularly if the tree is big and mature. In some trees, the wood laid on late in the life of a tree is softer, lighter, weaker, and more even textured than that produced earlier, but in other trees, the reverse applies. This may or may not correspond to heartwood and sapwood. In a large log the sapwood, because of the time in the life of the tree when it was grown, may be inferior in hardness, strength, and toughness to equally sound heartwood from the same log. In a smaller tree, the reverse may be true.

Color

thumb|The wood of [[Sequoia sempervirens|coast redwood is distinctively red.]]

In species which show a distinct difference between heartwood and sapwood the natural color of heartwood is usually darker than that of the sapwood, and very frequently the contrast is conspicuous (see section of yew log above). This is produced by deposits in the heartwood of chemical substances, so that a dramatic color variation does not imply a significant difference in the mechanical properties of heartwood and sapwood, although there may be a marked biochemical difference between the two.

Some experiments on very resinous longleaf pine specimens indicate an increase in strength, due to the resin which increases the strength when dry. Such resin-saturated heartwood is called "fat lighter". Structures built of fat lighter are almost impervious to rot and termites, and very flammable. Tree stumps of old longleaf pines are often dug, split into small pieces and sold as kindling for fires. Stumps thus dug may actually remain a century or more since being cut. Spruce impregnated with crude resin and dried is also greatly increased in strength thereby.

Since the latewood of a growth ring is usually darker in color than the earlywood, this fact may be used in visually judging the density, and therefore the hardness and strength of the material. This is particularly the case with coniferous woods. In ring-porous woods the vessels of the early wood often appear on a finished surface as darker than the denser latewood, though on cross sections of heartwood the reverse is commonly true. Otherwise the color of wood is no indication of strength.

Abnormal discoloration of wood often denotes a diseased condition, indicating unsoundness. The black check in western hemlock is the result of insect attacks. The reddish-brown streaks so common in hickory and certain other woods are mostly the result of injury by birds. The discoloration is merely an indication of an injury, and in all probability does not of itself affect the properties of the wood. Certain rot-producing fungi impart to wood characteristic colors which thus become symptomatic of weakness. Ordinary sap-staining is due to fungal growth, but does not necessarily produce a weakening effect.

Water content

Water occurs in living wood in three locations, namely:

in the cell walls
in the protoplasmic contents of the cells
as free water in the cell cavities and spaces, especially of the xylem

thumb|Equilibrium moisture content in wood.

In heartwood it occurs only in the first and last forms. Wood that is thoroughly air-dried (in equilibrium with the moisture content of the air) retains 8–16% of the water in the cell walls, and none, or practically none, in the other forms. Even oven-dried wood retains a small percentage of moisture, but for all except chemical purposes, may be considered absolutely dry.

The general effect of the water content upon the wood substance is to render it softer and more pliable. A similar effect occurs in the softening action of water on rawhide, paper, or cloth. Within certain limits, the greater the water content, the greater its softening effect. The moisture in wood can be measured by several different moisture meters.

Drying produces a decided increase in the strength of wood, particularly in small specimens. An extreme example is the case of a completely dry spruce block 5 cm in section, which will sustain a permanent load four times as great as a green (undried) block of the same size will.

The greatest strength increase due to drying is in the ultimate crushing strength, and strength at elastic limit in endwise compression; these are followed by the modulus of rupture, and stress at elastic limit in cross-bending, while the modulus of elasticity is least affected.

In coniferous or softwood species the wood cells are mostly of one kind, tracheids, and as a result the material is much more uniform in structure than that of most hardwoods. There are no vessels ("pores") in coniferous wood such as one sees so prominently in oak and ash, for example.

The structure of hardwoods is more complex. The water conducting capability is mostly taken care of by vessels: in some cases (oak, chestnut, ash) these are quite large and distinct, in others (buckeye, poplar, willow) too small to be seen without a hand lens. In discussing such woods it is customary to divide them into two large classes, ring-porous and diffuse-porous.

In ring-porous species, such as ash, black locust, catalpa, chestnut, elm, hickory, mulberry, and oak,

In diffuse-porous woods the pores are evenly sized so that the water conducting capability is scattered throughout the growth ring instead of being collected in a band or row. Examples of this kind of wood are alder, birch,

The effect of rate of growth on the qualities of chestnut wood is summarized by the same authority as follows:

:"When the rings are wide, the transition from spring wood to summer wood is gradual, while in the narrow rings the spring wood passes into summer wood abruptly. The width of the spring wood changes but little with the width of the annual ring, so that the narrowing or broadening of the annual ring is always at the expense of the summer wood. The narrow vessels of the summer wood make it richer in wood substance than the spring wood composed of wide vessels. Therefore, rapid-growing specimens with wide rings have more wood substance than slow-growing trees with narrow rings. Since the more the wood substance the greater the weight, and the greater the weight the stronger the wood, chestnuts with wide rings must have stronger wood than chestnuts with narrow rings. This agrees with the accepted view that sprouts (which always have wide rings) yield better and stronger wood than seedling chestnuts, which grow more slowly in diameter." as both pulp yield and lumber strength are determined by it. Specific gravity is the ratio of the mass of a substance to the mass of an equal volume of water; density is the ratio of a mass of a quantity of a substance to the volume of that quantity and is expressed in mass per unit substance, e.g., grams per milliliter (g/cm3 or g/ml). The terms are essentially equivalent as long as the metric system is used. Upon drying, wood shrinks and its density increases. Minimum values are associated with green (water-saturated) wood and are referred to as basic specific gravity (Timell 1986).

{| class="wikitable"

!Symbol

!Mass basis

!Volume basis

|G0

|Ovendry

|Gb (basic)

|Ovendry

|Green

|G12

|Ovendry

|12% MC

|Gx

|Ovendry

|x% MC

|ρ0

|Ovendry

|ρ12

|12% MC

|ρx

|x% MC

The FPL has adopted Gb and G12 for specific gravity, in accordance with the ASTM D2555 standard. These are scientifically useful, but don't represent any condition that could physically occur. The FPL Wood Handbook also provides formulas for approximately converting any of these measurements to any other.

Density

Wood density is determined by multiple growth and physiological factors compounded into "one fairly easily measured wood characteristic" (Elliott 1970).

Age, diameter, height, radial (trunk) growth, geographical location, site and growing conditions, silvicultural treatment, and seed source all to some degree influence wood density. Variation is to be expected. The USDA Forest Service measured a coefficient of variation for the specific gravity of wood as 10%. In other words, about 68% (a standard deviation) of samples will fall within ±10% of the average specific gravity for a given species. Within an individual tree, the variation in wood density is often as great as or even greater than that between different trees (Timell 1986). and Softwoods of North America.

Tables

The following tables list the mechanical properties of wood and lumber plant species, including bamboo. See also Mechanical properties of tonewoods for additional properties.

Wood properties:

{|class="wikitable mw-collapsible mw-collapsed sortable"

!Common name

!Scientific name

!Moisture content

!Density (kg/m3)

!Compressive strength (megapascals)

!Flexural strength (megapascals)

|Balku bans

|Bambusa balcooa

|green

|45

|73.7

|Balku bans

|Bambusa balcooa

|air dry

|54.15

|81.1

|Balku bans

|Bambusa balcooa

|8.5

|820

|69

|151

|Indian thorny bamboo

|Bambusa bambos

|9.5

|710

|61

|143

|Indian thorny bamboo

|Bambusa bambos

|43.05

|37.15

|Nodding Bamboo

|Bambusa nutans

|890

|75

|52.9

|Nodding Bamboo

|Bambusa nutans

|87

|46

|52.4

|Nodding Bamboo

|Bambusa nutans

|12

|85

|67.5

|Nodding Bamboo

|Bambusa nutans

|88.3

|44.7

|88

|Nodding Bamboo

|Bambusa nutans

|14

|47.9

|216

|Clumping Bamboo

|Bambusa pervariabilis

|45.8

|Clumping Bamboo

|Bambusa pervariabilis

|79

|80

|Clumping Bamboo

|Bambusa pervariabilis

|20

|35

|37

|Burmese bamboo

|Bambusa polymorpha

|95.1

|32.1

|28.3

|Bambusa spinosa

|air dry

|57

|51.77

|Indian timber bamboo

|Bambusa tulda

|73.6

|40.7

|51.1

|Indian timber bamboo

|Bambusa tulda

|11.9

|68

|66.7

|Indian timber bamboo

|Bambusa tulda

|8.6

|910

|79

|194

|dragon bamboo

|Dendrocalamus giganteus

|740

|70

|193

|Hamilton's bamboo

|Dendrocalamus hamiltonii

|8.5

|590

|70

|89

|White bamboo

|Dendrocalamus membranaceus

|102

|40.5

|26.3

|String Bamboo

|Gigantochloa apus

|54.3

|24.1

|102

|String Bamboo

|Gigantochloa apus

|15.1

|37.95

|87.5

|Java Black Bamboo

|Gigantochloa atroviolacea

|54

|23.8

|92.3

|Java Black Bamboo

|Gigantochloa atroviolacea

|15

|35.7

|94.1

|Giant Atter

|Gigantochloa atter

|72.3

|26.4

|98

|Giant Atter

|Gigantochloa atter

|14.4

|31.95

|122.7

|Gigantochloa macrostachya

|960

|71

|154