Forestry & Forest Sciences · Forest Products & Utilization · also Wood Science, Wood Anatomy, Wood Science and Technology
The structure of wood and how it determines strength, density, durability and use: macroscopic and microscopic identification of timbers and their physical and mechanical properties.
In the exams: ICAR AICE-JRF 59 Unit 7 (wood anatomy); IFoS Forestry Paper II Section B (forest resources and utilization); PRC Forest Engineering and Utilization
Wood is the secondary xylem of trees: the tissue produced inward by the vascular cambium, which conducts water, supports the crown and stores food in the living tree, and which becomes, after felling, one of the most versatile materials people use. Its properties, from density and strength to shrinkage, durability and appearance, follow from its anatomy: the kinds of cells it contains, their size, wall thickness and arrangement, the chemistry of the cell walls and the substances deposited in heartwood. A forester who understands wood anatomy can identify timbers, predict how they will dry, machine, glue and last in service, recognise defects, and grow trees for the qualities markets value.
This subject covers the formation of wood by the cambium and the differences between sapwood and heartwood; the structure of softwoods, with tracheids, rays and resin canals; the structure of hardwoods, with vessels, fibres and parenchyma, and the distinction between ring-porous and diffuse-porous woods; the structure and chemistry of the cell wall; macroscopic and microscopic identification; density and specific gravity; wood-water relations, including moisture content, the fibre saturation point and shrinkage; mechanical properties; natural defects and reaction wood; timber grading and the uses of important timbers; and natural durability. Tree identification in the field, including bark, leaves and habit, is covered in dendrology; harvesting and log handling in forest-operations-harvesting; and drying and protection of wood in wood-seasoning-preservation.
Wood anatomy and properties are examined in Unit 7 of the ICAR AICE-JRF forestry syllabus (wood anatomy, seasoning and preservation, forest products), in forest resources and utilization in Section B of Paper II of the Indian Forest Service forestry optional, and in Forest Engineering and Utilization in the Philippine Foresters licensure examination. Questions ask for the differences between softwoods and hardwoods, the features used to identify timbers, the layers and chemistry of the cell wall, moisture content and shrinkage calculations, the definition and use of density and strength properties, the causes and effects of reaction wood and other defects, and the uses and durability of important timbers.
Values of density, shrinkage and strength quoted in this manual are typical or illustrative; properties vary widely within and between trees, with growth conditions and with moisture content, and design values must come from tested data and national standards.
Wood is produced by the vascular cambium, a thin layer of dividing cells between the bark and the wood. The cambium contains two kinds of initial cells: elongated fusiform initials, which give rise to the axial (longitudinal) cells of wood and phloem, and short ray initials, which give rise to the radial ray cells. Periclinal divisions (parallel to the surface) add cells inward, which become xylem (wood), and outward, which become phloem (inner bark); far more xylem than phloem is produced. Anticlinal divisions (perpendicular to the surface) add new initials so that the cambium can increase its circumference as the stem grows. Each new xylem cell enlarges, forms a secondary wall, becomes lignified and, in most cell types, dies, leaving a hollow cell wall that conducts water or provides support.
In climates with a seasonal pause in growth, the cambium produces a distinct layer of wood each year: a growth ring. Wood formed early in the season (earlywood) usually has larger, thinner-walled cells adapted for conduction; wood formed later (latewood) has smaller, thicker-walled cells and is denser and stronger. The contrast between latewood of one year and earlywood of the next makes the ring boundary visible. In many tropical trees growing without a marked dry or cold season, rings are indistinct or absent. Ring width reflects growth rate and is used to age trees and reconstruct climate (see forest-mensuration). Wood formed near the pith in the first years of growth, called juvenile wood (or core wood), typically has shorter cells, a larger microfibril angle, lower density and higher longitudinal shrinkage than the mature wood formed later; fast-grown plantation timber often contains a high proportion of juvenile wood.
Sapwood is the outer, physiologically active part of the wood, in which water is conducted and living parenchyma cells store starch and other reserves. It is usually lighter in colour than heartwood. Sapwood width varies with species, age and vigour; some species, such as many pines, have wide sapwood, while others, such as teak, have narrow sapwood. Sapwood of all species has low natural durability because it lacks protective extractives and contains starch and sugars that attract fungi and insects, but it is usually easier to treat with preservatives than heartwood.
As the tree grows, the inner sapwood dies and is converted to heartwood. The living parenchyma cells die, reserves are used, and extractives (phenolic compounds, tannins, resins, oils and other substances) are deposited in the cell walls and cavities, often darkening the wood. In many hardwoods, tyloses, outgrowths of parenchyma cells that fill the vessels through pits, block the vessels; in softwoods, the pits between tracheids become aspirated (closed). Heartwood no longer conducts water but continues to support the tree. Its extractives give many species their colour, odour and natural durability, and its blocked pathways make it more difficult to dry and to treat with preservatives. Teak, sal, rosewood and deodar heartwoods owe their durability to extractives. Some species have little or no distinct coloured heartwood, and false heartwood, a discoloured central zone caused by injury or infection, occurs in some.
Softwoods are the woods of gymnosperms, mainly conifers such as pines, spruces, firs, deodar and other cedars, larches and Douglas-fir. The term describes botanical origin, not hardness: some softwoods, such as yew, are harder than many hardwoods, and some hardwoods, such as balsa, are much softer than most softwoods. Softwoods have a simpler, more uniform structure than hardwoods, which makes them easier to identify to genus but often difficult to separate at species level.
The dominant cell type in softwoods is the longitudinal tracheid, which makes up about 90% or more of the wood volume. Tracheids are long, narrow cells, commonly about 3 to 5 mm long and about 100 times as long as they are wide, with closed, tapered ends. They perform both conduction and support: earlywood tracheids have wide lumens and thin walls for conduction, latewood tracheids narrow lumens and thick walls for strength. Water passes from one tracheid to the next through bordered pits, whose torus-margo structure in many conifers acts as a valve; when a pit is aspirated, the torus seals the opening, as happens in heartwood and during drying, reducing permeability. Softwoods have no vessels; the absence of pores is the first feature that distinguishes them from hardwoods under a hand lens.
Rays in softwoods are mostly uniseriate (one cell wide) and only a few to tens of cells high, so they are not conspicuous to the naked eye. They consist of ray parenchyma cells and, in some genera such as pines, spruces, larches and Douglas-fir, ray tracheids along the ray margins. The pits connecting ray parenchyma to axial tracheids (cross-field pits) have characteristic shapes (window-like, pinoid, piceoid, cupressoid and taxodioid) that are important microscopic identification features. Axial parenchyma is absent or sparse in most softwoods, though present in some, such as the cypress family.
Resin canals are intercellular spaces lined by epithelial cells that secrete resin. Normal (constitutive) resin canals, both axial and radial (the radial ones within fusiform rays), occur in pines, spruces, larches and Douglas-fir; the size, distribution and epithelium (thin- or thick-walled) help distinguish these genera. In other genera, such as firs, cedars (including deodar) and hemlocks, resin canals are normally absent but traumatic resin canals may form in tangential rows after injury. Resin affects processing (gumming of saws, bleeding through finishes) and use; chir pine and other pines are tapped for resin from which turpentine and rosin are produced. Other softwood features useful in identification include the transition from earlywood to latewood (abrupt or gradual), spiral thickenings in tracheids (for example in Douglas-fir and yew), colour and odour.
Hardwoods are the woods of angiosperm trees, mainly dicotyledons, such as teak, sal, oak, eucalyptus, rosewood and mahogany. Their structure is more complex and varied than that of softwoods, with several specialised cell types, which makes hardwoods more diverse in appearance and properties and generally easier to identify to species groups. Bamboos and palms are monocotyledons; they lack a vascular cambium and secondary xylem and are not wood in the strict sense, although they are used like wood.
Vessels are the main water-conducting cells of hardwoods. Each vessel is a tube made of many short vessel elements joined end to end through openings called perforation plates, which may be simple (a single large opening) or scalariform (a series of bars, like a ladder), or other types. In cross-section, vessels appear as pores, which is why hardwoods are called porous woods. Pore size, arrangement (solitary, in radial multiples, clusters, chains), contents (tyloses, gums, deposits) and distribution across the ring are major identification features.
The distribution of vessels within growth rings is one of the most useful features for identification and has implications for properties. In ring-porous woods, the earlywood vessels are much larger than the latewood vessels and form a distinct band or ring at the start of each growth ring; examples are oak, ash, elm and teak (teak is ring-porous to semi-ring-porous). In diffuse-porous woods, vessels are of fairly uniform size and evenly distributed through the ring; examples are maple, birch, beech, eucalypts and many tropical hardwoods, including sal and other dipterocarps. Semi-ring-porous (semi-diffuse-porous) woods are intermediate, with vessels gradually decreasing in size across the ring. In ring-porous woods, fast growth produces a higher proportion of dense latewood and therefore stronger wood; in diffuse-porous woods, growth rate has less effect on density.
Fibres are long, thick-walled cells that provide mechanical support; they include libriform fibres and fibre-tracheids. Their wall thickness and proportion largely determine the density and strength of hardwoods. Axial parenchyma consists of living storage cells arranged in patterns that are important for identification: paratracheal parenchyma is associated with vessels (scanty, vasicentric around vessels, aliform with wing-like extensions, or confluent linking several vessels), while apotracheal parenchyma is independent of vessels (diffuse, diffuse-in-aggregates, or banded, including marginal bands at ring boundaries). Rays in hardwoods range from uniseriate to very wide multiseriate rays, as in oak, whose broad rays produce the decorative 'silver figure' on quarter-sawn surfaces. In some hardwoods, such as species of Dalbergia and Pterocarpus, rays and other elements are arranged in horizontal tiers (storied structure), producing fine ripple marks on tangential surfaces. Some hardwoods, including dipterocarps such as sal, have axial intercellular (resin) canals, often in tangential lines.
The wood cell wall is built in layers. Between adjacent cells lies the middle lamella, rich in lignin and pectins, which bonds cells together. Each cell has a thin primary wall, formed while the cell is enlarging, with a loose network of cellulose microfibrils. Inside it, the secondary wall is deposited after enlargement in three layers: a thin outer layer (S1), a thick middle layer (S2) and a thin inner layer (S3). The layers differ in the angle at which cellulose microfibrils are wound around the cell: in S1 and S3 the microfibrils are nearly perpendicular to the cell axis, while in S2, which forms most of the wall thickness, they are nearly parallel to it, at a microfibril angle commonly of about 10° to 30°. The S2 layer therefore largely determines the cell's stiffness, strength and shrinkage behaviour. A small microfibril angle gives high stiffness and low longitudinal shrinkage; juvenile wood and compression wood have larger angles.
Cellulose, the main structural component, makes up roughly 40–50% of dry wood. It is a linear polymer of glucose units linked by beta-1,4 bonds, with long chains aggregated into microfibrils that are partly crystalline and partly amorphous. Cellulose gives wood its tensile strength and is the raw material for paper, cellulose derivatives and textile fibres such as viscose. Its hydroxyl groups attract water, which is the basis of wood's hygroscopic behaviour.
Hemicelluloses, about 20–30% of wood, are branched polysaccharides of various sugars that bind to cellulose microfibrils; softwoods are rich in galactoglucomannans and hardwoods in xylans. They are more easily degraded than cellulose by heat, chemicals and fungi. Lignin, roughly 20–35% of wood, is a complex aromatic polymer that fills spaces between microfibrils and in the middle lamella, cementing the cells together and providing compressive strength, stiffness and resistance to decay. Softwoods generally contain more lignin than hardwoods, and their lignin differs in chemical units (guaiacyl lignin in softwoods; guaiacyl-syringyl lignin in hardwoods). Lignin must be removed in chemical pulping to free the fibres for paper.
Extractives are compounds that can be removed with water or organic solvents: tannins, resins, fatty acids, terpenes, phenolic compounds, gums and others. Usually a few percent of the wood, they may exceed 10% in some heartwoods. They give colour, odour and natural durability, may affect gluing, finishing, drying and pulping, and some are commercially valuable (resin, tannins, sandalwood oil). Some cause health effects such as dermatitis or allergic reactions in woodworkers. Inorganic ash content is usually below 1%, though some tropical woods contain silica, which blunts cutting tools.
Wood identification is needed to check that timber is of the species claimed in trade, to enforce laws on protected species and illegal logging (see forest-policy-law-governance), to choose suitable timber for a use, to diagnose problems in service, to study archaeological and historical objects and to verify claims in certification and due diligence. Identification uses gross features visible to the naked eye or with a hand lens, microscopic features, and increasingly chemical and genetic methods.
Gross features include colour (of heartwood and sapwood and their contrast), lustre, odour and taste, weight and hardness, grain (straight, interlocked, spiral, wavy), texture (fine or coarse, related to the size of pores and cells) and figure (the pattern on a surface). With a 10× hand lens on a cleanly cut end-grain surface, the examiner records whether pores are present (hardwood) or absent (softwood); in hardwoods, the porosity (ring-, semi-ring- or diffuse-porous), pore size, arrangement and contents (tyloses, gum, white deposits), the type and distribution of axial parenchyma, ray width and visibility, storied structure (ripple marks), and intercellular canals; in softwoods, resin canals and the transition from earlywood to latewood. Dichotomous keys and multiple-entry keys, including computer keys, lead from features to probable identifications.
Microscopic identification uses thin sections cut in three planes, transverse (cross-section), radial and tangential, observed with a light microscope. Features include vessel perforation plates and intervessel pitting, vessel-ray pitting, spiral thickenings, fibre types, the detailed arrangement of parenchyma, ray composition (homocellular or heterocellular, with procumbent, upright or square cells), crystals and silica, and in softwoods cross-field pit types, ray tracheids and their wall sculpturing, and resin canal epithelium. The International Association of Wood Anatomists (IAWA) has published standard lists of microscopic features for hardwood identification (1989) and softwood identification (2004), used in descriptions, databases and computer-aided identification.
Anatomy can often identify wood only to genus, which is insufficient for enforcing rules on particular species, such as those listed under CITES. Complementary methods include DNA analysis (barcoding and population genetics, which can sometimes determine geographic origin), mass spectrometry (such as DART-TOFMS, which identifies characteristic chemical profiles), near-infrared spectroscopy, stable isotope analysis for provenance, and machine-vision systems that identify wood from images of end-grain. Reference collections of authenticated specimens (xylaria), such as that of the Forest Research Institute, Dehradun, are essential for all methods.
Density is mass per unit volume. Because both the mass and the volume of wood change with moisture content, the moisture conditions must be stated. Common measures are: green density (green mass ÷ green volume), air-dry density (mass ÷ volume at a stated moisture content, commonly 12%), oven-dry density (oven-dry mass ÷ oven-dry volume) and basic density (oven-dry mass ÷ green volume). Basic density is the most stable measure for comparing species and trees and for converting green volume measured in inventory to dry biomass (see forest-mensuration). Specific gravity is the ratio of the density of wood to the density of water, so it has no units; basic specific gravity uses oven-dry mass and green volume. A wood with a basic density of 600 kg/m³ has a basic specific gravity of 0.6.
Wood densities vary enormously: from about 0.1–0.2 for balsa, one of the lightest commercial woods, through about 0.4–0.6 for many softwoods and medium hardwoods, to above 1.0 for the heaviest woods, such as lignum vitae and some ebonies, which sink in water. Density depends mainly on the proportion of cell wall material, since the cell wall substance itself has a density of about 1.5 g/cm³ in almost all woods; dense woods have thick-walled fibres or tracheids with small lumens. Within a species, density varies with growth rate (in ring-porous hardwoods and many softwoods), with position in the tree (juvenile wood near the pith is usually lighter), with site and with genetics, which allows breeding for density.
Density is measured by weighing specimens and determining their volume, either from dimensions of regular samples or by water displacement for irregular ones; increment cores are used for basic density in standing trees. X-ray densitometry of thin strips records density variation across growth rings at fine resolution. Non-destructive tools such as resistance drills and acoustic methods give indirect estimates.
Density is the best single predictor of many wood properties. Strength and stiffness generally increase with density; so do hardness, nail and screw holding, and calorific value per unit volume. Denser woods usually shrink more, are harder to dry without defects, harder to work and often harder to treat with preservatives. For pulp, density affects yield per cubic metre and paper properties; for fuel, energy per cubic metre. Density is therefore a key target in tree improvement and a key variable in grading and design.
The moisture content (MC) of wood is the mass of water it contains expressed as a percentage of its oven-dry mass: MC = (wet mass − oven-dry mass) ÷ oven-dry mass × 100. Because it is based on dry mass, moisture content can exceed 100% in green wood of light species. A sample weighing 150 g wet and 100 g after oven-drying at 103 ± 2 °C has an MC of 50%. Water in wood occurs as free water in the cell cavities and as bound water held by hydrogen bonds within the cell walls. Moisture content is measured by the oven-dry method (the reference), by electrical resistance or capacitance meters (convenient but needing corrections for species and temperature) and by other methods.
As green wood dries, free water leaves first. The fibre saturation point (FSP) is the moisture content at which the cell walls are saturated with bound water but the cavities contain no free water; it is commonly about 25–30% for most woods, lower for some with high extractive content. Above the FSP, properties change little with moisture; below it, removing bound water causes shrinkage and increases strength and stiffness. Wood exchanges moisture with the surrounding air until it reaches its equilibrium moisture content (EMC), which depends on relative humidity and temperature: for example about 12% at 65% relative humidity and 20 °C. Wood should be dried to the EMC of its service environment, roughly 6–10% in heated buildings and higher outdoors or in humid climates, to minimise later shrinkage and swelling.
Wood shrinks as it loses bound water below the FSP and swells as it gains it. Shrinkage is anisotropic: it is greatest in the tangential direction (along the growth rings), about half as great in the radial direction, and very small, usually about 0.1–0.2%, in the longitudinal direction for normal wood. Total tangential shrinkage from green to oven-dry is commonly about 5–10% and radial about 2–6%, depending on species and density. The difference between tangential and radial shrinkage causes cupping of flat-sawn boards and distortion of squares to diamonds; the T/R ratio is a useful index of dimensional stability, lower values being more stable. Quarter-sawn (radial) boards shrink less in width than flat-sawn (tangential) boards. Teak is valued for its low shrinkage and stability.
Shrinkage is usually expressed as a percentage of green dimension. Between the FSP and oven-dry, shrinkage is approximately proportional to the loss of moisture, so partial shrinkage to a given MC can be estimated by: shrinkage to MC = total shrinkage × (FSP − MC) ÷ FSP. For a wood with total tangential shrinkage of 8% and FSP of 28%, drying to 12% MC gives a tangential shrinkage of about 8 × (28 − 12) ÷ 28 = 4.6%. A flat-sawn board 200 mm wide green would then be about 190.9 mm wide at 12% MC. Movement in service (the change in dimension with changes in humidity) is the practical concern for joinery and flooring and differs between species with similar total shrinkage.
Wood is orthotropic: its mechanical properties differ along three axes, longitudinal (parallel to the grain), radial and tangential. It is far stronger and stiffer parallel to the grain than perpendicular to it. Properties are measured on small, clear, straight-grained specimens under standard test methods, such as those of the Indian Standard IS 1708, ASTM D143 and ISO standards, and on structural-size members for design. Results are reported at a stated moisture content, commonly green and 12%.
Static bending tests load a beam until it fails. The modulus of rupture (MOR) is the maximum bending stress, a measure of bending strength; for a simple beam loaded at the centre, MOR = 3PL ÷ (2bd²), where P is the maximum load, L the span, b the width and d the depth. For example, a 20 × 20 mm clear specimen on a 300 mm span that fails at 2000 N has MOR = 3 × 2000 × 300 ÷ (2 × 20 × 20²) = 112.5 N/mm² (MPa). The modulus of elasticity (MOE) is a measure of stiffness, calculated from the slope of the load-deflection curve within the elastic range: MOE = PL³ ÷ (4bd³Δ) for centre loading, where Δ is the deflection at load P. MOE governs deflection and buckling and is the main property used for machine stress grading.
Compression parallel to the grain measures the strength of short columns and is important for posts and piles; compression perpendicular to the grain measures resistance to crushing at bearings, such as where beams rest on supports or rails on sleepers. Tension parallel to the grain is very high in clear wood but greatly reduced by knots and sloping grain, so it is seldom the governing property of small clear specimens. Shear strength parallel to the grain is relatively low and governs the design of short, deep beams and joints. Hardness measures resistance to indentation and wear: the Janka hardness test records the force needed to embed a steel ball of 11.28 mm (0.444 inch) diameter to half its diameter into the wood, and is used to compare woods for flooring and other wearing surfaces. Impact bending and toughness measure resistance to sudden loads, important for tool handles and sports goods.
Strength and stiffness increase with density and decrease as moisture content rises up to the fibre saturation point, above which they change little; dry wood at 12% MC may be more than half again as strong in bending as green wood. Slope of grain, knots and other defects reduce strength, often greatly, so structural timber is graded. Duration of load matters: wood carries higher loads for short periods than for long periods (creep and duration-of-load effects). High temperature, decay and some chemical treatments reduce strength. Design values in structural codes are derived from tests and adjusted for grade, moisture, duration of load and safety.
A knot is the base of a branch enclosed in the stem. Intergrown (live, tight) knots form while the branch is alive and the wood of branch and stem is continuous; encased (dead, loose) knots form after the branch dies and the stem grows around the dead stub, often with bark between, so they may fall out of sawn boards. Knots distort the grain around them and reduce strength, especially in tension and bending, and are the most important grading defect in structural timber. Their size, number, location (edge knots are more serious than centre knots) and soundness are recorded in grading. Pruning young trees produces clear wood outside a knotty core.
Straight grain, with fibres parallel to the axis of the piece, gives the best strength and machining. Spiral grain, in which fibres wind helically around the stem, causes twist when wood dries and reduces strength. Interlocked grain, in which the direction of the spiral alternates in successive layers, is common in tropical hardwoods such as many dipterocarps, sapele and eucalypts; it produces attractive ribbon figure on quarter-sawn surfaces but makes splitting difficult and causes tearing in planing. Cross grain and slope of grain resulting from sawing not parallel to the stem axis also reduce strength, and maximum permissible slope of grain is specified in grading rules.
Shakes are separations along or across the growth rings in the standing tree or after felling: ring shake (separation along a growth ring), heart shake (radial cracks starting at the pith), and star shake (several radial cracks from the pith). They can result from wind stress, frost, growth stresses or felling damage. Brittleheart is weak, brittle wood near the centre of large logs of some tropical hardwoods, caused by long-term compressive growth stress. Other natural defects include resin pockets and streaks, bark pockets, included sapwood, pith, compression failures, mineral streaks and insect holes, and damage from fungal decay and stain (see forest-pathology).
Reaction wood forms in leaning stems and branches as the tree attempts to restore their position. Compression wood forms in softwoods on the lower (compression) side of a leaning stem; it is darker, denser and harder than normal wood, has rounded tracheids with intercellular spaces, a large microfibril angle and higher lignin content, and shows high longitudinal shrinkage, which causes bowing and crook in drying, as well as brittleness. Tension wood forms in hardwoods on the upper (tension) side; it contains gelatinous fibres with an inner, cellulose-rich gelatinous layer (G-layer), and produces woolly or fuzzy surfaces when sawn, collapse and high longitudinal shrinkage on drying, and lower strength in some properties. Reaction wood is common in trees grown on slopes, exposed to wind or with asymmetric crowns, and is a significant cause of distortion in plantation timber.
Grading sorts sawn timber into classes of similar quality for particular uses. Appearance grading, used for joinery, furniture and decorative uses, considers clear area, colour and appearance defects. Structural (stress) grading assigns timber to classes with defined strength properties so that engineers can design with it. Visual stress grading assesses defects that reduce strength, such as knot size and position, slope of grain, wane, fissures, distortion and rate of growth, according to grading rules. Machine stress grading measures stiffness (MOE) non-destructively, by bending or acoustic methods, and assigns grades from its correlation with strength, usually combined with a visual check. Grades are combined with species into strength classes; in Europe, EN 338 defines C classes for softwoods and D classes for hardwoods. In India, IS 883, the code of practice for design of structural timber, groups timbers into groups A, B and C according to their stiffness and gives permissible stresses.
Teak (Tectona grandis) is renowned for its durability, dimensional stability, workability and appearance, and is used for high-quality furniture, joinery, doors, panelling, decking and shipbuilding. Sal (Shorea robusta) is heavy, strong and durable, used for heavy construction, railway sleepers, beams and posts, though it is difficult to season. Indian rosewood or shisham (Dalbergia sissoo and D. latifolia) is valued for furniture, cabinetry and carving. Deodar (Cedrus deodara) is the most important Himalayan softwood, durable and aromatic, used for construction, sleepers and joinery. Chir pine (Pinus roxburghii) and blue pine are used for packaging, construction, poles (after treatment) and pulp. Eucalypts and poplars, grown widely in plantations and on farms, supply pulpwood, poles, plywood (poplar veneer) and small timber. Mango, semul (Bombax ceiba) and other light woods are used for packing cases, matchwood and plywood. Sandalwood (Santalum album) is valued for its fragrant heartwood oil and carving.
Choosing timber for a use considers strength and stiffness (structural members), density and hardness (flooring, tool handles), durability and treatability (exterior use, ground contact, marine use), dimensional stability (joinery, furniture, flooring), workability and finishing, appearance, acoustic properties (spruce for soundboards of musical instruments), resilience and toughness (sports goods such as cricket bats from willow), availability and cost, and legality and sustainability of supply. Lesser-used species can often substitute for scarce traditional timbers when their properties are known and when they are correctly dried and treated, which reduces pressure on overexploited species.
Modern construction increasingly uses engineered wood products, such as glued laminated timber, laminated veneer lumber and cross-laminated timber, which combine smaller or lower-grade pieces into large, reliable members (see wood-products-industries). These products make better use of plantation timber, including fast-grown species with more juvenile wood, but depend on knowledge of the anatomy and properties of the raw material for grading, gluing and design.
Natural durability is the inherent resistance of wood to attack by decay fungi, insects (especially termites) and marine borers, without preservative treatment. It is a property of heartwood: the sapwood of all species is non-durable. Durability comes mainly from the toxic or deterrent extractives deposited in heartwood, such as the tectoquinones of teak, the resins of deodar and other compounds, and is influenced by density and the presence of silica. Durability varies between species, between trees of the same species (fast-grown plantation trees may have less durable heartwood than slow-grown natural trees), and within a tree (outer heartwood is often more durable than inner heartwood near the pith).
Durability is assessed by field (graveyard or stake) tests, in which stakes of heartwood are placed in the ground and inspected over years until they fail; by laboratory decay tests with specific fungi; by termite tests; and by marine exposure tests. Species are then grouped into durability classes. In India, the Forest Research Institute's classification, based on graveyard tests, places timbers with an average life of 120 months (10 years) or more in Class I (highly durable), 60 to 120 months in Class II (moderately durable), and less than 60 months in Class III (non-durable). European standard EN 350 uses classes from 1 (very durable) to 5 (not durable). Class assignments are averages and actual service life depends on the hazard of the use.
Among Indian timbers, teak, sal, deodar, Indian rosewood, padauk (Pterocarpus), Hardwickia and some others are durable heartwoods. Many species widely used today, such as mango, semul, poplar, rubberwood, chir pine sapwood and most plantation eucalypts' sapwood, are non-durable or perishable, and need preservative treatment for any use where they may become wet or be exposed to termites. Durability must be matched to the hazard: a moderately durable timber may last indefinitely in dry indoor use but fail quickly in ground contact in a warm, termite-rich climate.
Where durable species are available legally and sustainably, they can be used without chemical treatment in hazardous situations, which avoids the environmental and health concerns of some preservatives. However, many durable tropical timbers are scarce or overexploited, and some are protected. Using preservative-treated or modified non-durable timber, designing to keep wood dry, and removing sapwood are therefore important complementary approaches, treated in wood-seasoning-preservation.
Learn to describe and distinguish softwood and hardwood structure, the cell wall layers and chemistry, and the features used with a hand lens. Practise the calculations: moisture content on the oven-dry basis, partial shrinkage below FSP, density and specific gravity, and MOR and MOE from test data. Know the causes and effects of reaction wood and other defects, the principles of grading and the durability classes, and the properties and uses of major Indian timbers. Practical identification with a hand lens on end grain is the best way to learn anatomy.
A forester five years in identifies timber in depots and checks consignments for species and legality, measures moisture content before sale or treatment, grades logs and sawn timber, recommends species and treatments for local uses such as poles, sleepers and construction, and advises growers on silviculture that improves wood quality (pruning for clear wood, avoiding reaction wood, rotation length and juvenile wood). Practical skills include using a hand lens and moisture meter and reading grading rules.
A senior forester shapes timber supply and utilisation policy: promoting lesser-used and plantation species, setting standards and grading systems, supporting wood identification for enforcement, and linking tree improvement to wood quality. The senior forester works with industry, standards bodies and research institutes to make plantation timber suitable for engineered products and to reduce pressure on scarce natural timbers.
A senior researcher studies wood formation and its genetic and environmental control, cell wall structure and chemistry, wood quality of plantation species, non-destructive evaluation, wood identification technologies and the relationships between anatomy and properties; maintains xylaria and databases; and contributes to standards and to forest products policy.
No species property tables are reproduced; all numerical values for density, shrinkage and strength are typical or illustrative, and design values must come from national standards and tested data. Grade rules, permissible stresses and the stiffness limits of IS 883 groups are not given. Detailed identification keys and descriptions of individual timbers are not included; use xylaria, the IAWA lists and regional atlases. Bamboo and palm anatomy are mentioned only in passing. Field identification of trees is in dendrology, harvesting in forest-operations-harvesting and drying and preservation in wood-seasoning-preservation.
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