Chapter 8: Wood.
After completing this chapter, you should be able to:
* Know the chemical composition of wood
* Learn the difference in cellulose hemicellulose, and lignin
* Describe softwoods and what they are used for
* Describe hardwoods and what they are used for
* Count growth rings in wood
* Understand what reaction wood is
* Gain knowledge of the science of trees, dendrochronology
* Describe the many uses of wood
* Discuss the current and future productivity of the forest
Wood Structure and Secondary Growth
Wood is composed of the secondary xylem tissue laid down by the vascular cambium. Generally, the vessels and tracheids function in water and mineral transport, and fibers are support cells. Additionally, rays provide for lateral transport throughout wood tissue.
Chemical Composition and Properties of Wood
Many of the structural properties of wood depend on the arrangement of the component cells and on the chemical composition of the secondary cell wall. The three major cell wall constituents are cellulose and hemicellulose (polysaccharides and lignin, a chemically complex polymer of phenolic substances). Initially the secondary cell wall consists of cellulose embedded in a hemicellulose matrix. These polysaccharides are later cemented together by the deposition of lignin, which terminates any further cell growth by making the cell wall too rigid for additional expansion.
The most abundant naturally occurring organic compound is cellulose. It is a long molecule of several thousand glucose molecules linked end to end, which provides for the physical organization of the cell wall and approximately half the polysaccharides, found in cell walls. Like cellulose, they are linear molecules, but they may branch to provide a porous matrix around cellulose. Hemicellulose is usually found in greater amounts in woody angiosperms than is gymnosperms.
The second most abundant component in the cell is lignin providing rigidity to the cell wall. Lignin is not found in all plant cell walls, but when present it is especially important and abundant in cells having a supporting function. The deposition of lignin normally occurs after the cell has reached its maximum size, beginning in the middle lamella, then in the primary cell wall, and finally in the secondary cell wall. Although lignin is often present in all three of these layers, lignification is most characteristic of the secondary cell wall.
The relative amounts of each of these materials control some of the physical properties of wood. Since there is more hemicellulose present in angiosperm wood, angiosperms generally contain more moisture. The greater lignin content in gymnosperm wood makes it more stable and less prone to warping.
The commercial value of wood depends on a combination of characteristics that make certain woods better suited to different uses. The specific gravity, figure, grain, cuts, and knots of wood are some of these properties. The following properties are also considerations for the best use of woods.
The resistance of wood to decay, wear, and insect damage is especially desirable in wood that is to be used structurally or that comes in contact with moisture. Since fungal decay is the most common form of wood destruction and since fungi thrive in warm, moist conditions, wood in contact with damp soil or subject to frequent rain and high humidity is more likely to decay. Fence posts, railroad, telephone poles, greenhouse table, coastal, or tropical structures and mine timbers require wood of exceptional durability. The natural preservation found in many trees is often toxic or unpalatable to decaying organisms and insects. This is especially true of tannin, which is found in amounts up to 30% in some woods. The most durable and resistant woods include redwood, cedars, black walnut, junipers, chestnut, bald cypress, black locust, and catalpa.
Color, Luster, and Polish
In wood used for furniture or cabinetry, the color of wood, especially the heartwood, is important. In addition, the natural luster, or ability to reflect light and the ability to take a polish, are functions of cell wall structure and types of cut. Some woods known to polish well are cedar, white pine, cherry, maple, walnut, holly, and some oaks.
Moisture and Shrinkage
The amount of moisture in the wood of a freshly cut tree varies from less than 10% of green weight in some species to over 75% in others. Some moisture is found in the lumens of the vessel elements and evaporates readily without causing any shrinkage in the wood. However, the cell walls of green wood comprise approximately 25% to 30% water, which is removed with more difficulty. As this water is removed from the cell walls, the wood shrinks. Most wood shrinks between 10% and 20% in volume if all the water is removed by oven drying. Nearly all this shrinkage is across the grain and greatest in the tangential direction across the width of flat-saw (tangentially cut) lumber. There is less than 1% shrinkage in the length of the boards.
To prevent uneven shrinkage and resulting warping, most commercial lumber is "seasoned" or "cured" by drying the wood under controlled conditions. This is especially important in hardwoods because of their use in furniture. In addition to prevent warping, proper drying reduces shipping weight and cost, increases strength, and improves the wood's ability to be glued, painted, stained, and polished.
Drying is done either in the open air, in drying sheds, or in kilns. The control of humidity and the application of heat are important variables in preventing uneven drying, which causes warping, cracking, twisting, and other distortions. Rapid air circulation helps regulate these factors, as does proper stacking with spacers to keep the boards straight and to allow for proper air circulation. Although it is economically important to dry wood as efficiently as possible, too rapid drying can result in stress, which may cause checks to form on the surface or allow distortions during manufacture or in use. Properly dried hardwoods have a moisture content of 6% to 8%, whereas most softwood for construction has from 15% to 19%.
As amazing as it might sound, fire protection often involves the use of wood. Solid wooden doors conduct heat slowly and help prevent the spread of a fire from one room to the next. Dry wood is a poor conductor of heat, and generally lighter woods conduct heat more slowly than heavy woods.
For wood used in musical instruments, the resonance depends on a combination of elasticity, density, thickness, and cut. The soundboard is a piano, responsible for resonance and tonal quality, and is best made of spruce; laminated hard maple holds the metal tuning pegs tightly. The various woodwind instruments, such as clarinet, oboe, and bassoon, and the string instruments, such as violin, guitar, mandolin, and bass, all depend on the acoustical resonance properties of the woods used in their construction. A master instrument craftsman must know woods as well as music. In addition, the reeds of the clarinet, saxophone, and oboe are tapered strips of the woody cane from a large tropical grass, Arundo donax. Reeds from this plant have been used for woodwind instruments since at least 3000 BC.
Hardwoods and Softwoods
One of the most commonly used distinctions made when discussing wood is the categorization of all woody dicots as hardwoods and all coniferous gymnosperm as softwoods. Although there is some justification for such an oversimplification, there are a number of exceptions. One of the softest (lightest) of woods is balsa (Ochroma lagopus), a dicot; whereas slash pine (Pinus elliottii), a conifer, is harder than many hardwoods. What is actually being measured is relative density. Generally, the less dense or lighter a wood the softer and weaker it is.
Relative density is determined by measuring the specific gravity of the wood, which depends on cell size, cell wall thickness, and the number of different kinds of cells. For example, fibers can be thick walled and have small lumens and can be packed closely together, providing for a very dense (high specific gravity) wood with little air space. Conversely, fibers with thin walls and large lumens produce a wood with lower specific gravity. Vessel elements have relatively thin walls and large lumens, so a high vessel volume results in decreased specific gravity.
Specific gravity is determined by weighting a paraffin-coated block of wood (to prevent water absorption), immersing it in water, and weighing the displaced volume of water (1 cubic centimeter of water = 1gram).
Specific gravity = Dry weight of wood/Weight of displaced volume of water
FAMOUS TREES Giant redwoods (Sequoia sempervirens) from Humboldt County, California, measured at 111.6 m. Although giant redwoods are generally considered to be the tallest tree species, unsubstantiated claims exist of a eucalyptus tree (Eucalyptus reganus) in Australia measuring over 140 m. Certainly there are a number of eucalyptus trees validated to be taller than 91 m, so they are easily the second tallest tree species in the world. The tule tree (Taxodium mucronatum) found outside the city of Oaxaca, Mexico, and as shown in Figure 8-1, is believed to have the world's largest trunk circumference (42 m). Only 40 m wide this tree is over 2,000 years old and was visited by Cortez, the Spanish Explorer that defeated and conquered the Aztec Empire, after he heard stories of its size. The tule tree is a youngster compared with the bristle cone pine (Pinus longaeva) found in the White Mountains of California, and in Nevada, Utah, and Colorado. Several of these remarkable trees are known to be over 3,500 years old; one of the oldest living ones, named Methuselah, is approximately 5,000 years old.
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Generally, a specific gravity of 0.41 or less is considered softwood, and above 0.41 hardwood. Most coniferous gymnosperms are softwoods and most dicots are hardwoods, but there is overlap. Several woods are above a specific gravity of 1.0, which makes them denser than water, so they sink.
Several structural differences exist between coniferous wood (softwood) and dicot wood (hardwood). Conifers are more homogenous and less complex than dicots. They contain no vessels or fibers, and approximately 90% of the woody tissue is composed of tracheids with little parenchyma. Some of the parenchyma present is associated with resin canals; long intercellular spaces present in the longitudinal system of cells and in some of the rays. Resin canals are found only in pine, spruce, larch, and Douglas fir. They are absent in the other conifers. Parenchymal cells surround the resin canal and produce the resin, which is secreted into the canal. Most parenchymal cells in coniferous woods are in the rays.
The rays, which provide the lateral or radial transportation capabilities of the wood, are uniformly smaller in conifer, being normally 1 cell wide and 1 to 20 cells high. Dicot rays are usually larger, up to 20 or more cells wide, and can be up to several hundred cells high. In addition, the ray volume in hardwoods ranges from 5% to 30% of the wood tissue; softwood ranges from 5% to 10%. The variability of the rays is also much greater in hardwoods, adding to their more heterogeneous structure.
Dicot wood has not only more and larger rays, but also a greater variety of cell types in the longitudinal system. In general, the presence of vessel elements, which are the primary water conducting cells, is the main difference, although certain dicots do contain some tracheids (oaks, for example). Dicot woods also contain fibers and parenchymal cells, and different species contain varying amounts and kinds of fibers. Thus, the kinds of cells, the relative amounts of each kind, and, most important, the presence of vessels distinguish the more complex structure of dicot wood from conifer wood.
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Although in a cross section of a log the phloem does not normally appear as discernible rings (because of the compacting of the soft cells), the hard-walled xylem cells form a new growth ring of wood each time the vascular cambium becomes active. In temperate regions, such activity usually occurs only once during the growing season each calendar year, and thus these are termed annual rings, as seen in Figure 8-2. The rings are a sectional view of the annual growth layer that form the full length of the tree.
Since water availability is one of the most important environmental factors controlling plant growth, a drought may cause early cessation of growth followed by a second burst of growth after subsequent rainfall. Infrequently, then, false rings may form, resulting in two or more apparent growth rings in one year. Such sporadic growth patterns are generally restricted to areas with unpredictable climatic patterns, such as arid and semiarid regions.
Woody growth appears as visible concentric rings because of the contrast in cell diameter and cell wall thickness from the early to the late part of the growing season. In typical years in temperate regions, growth begins in the early spring, while plenty of soil moisture is available from winter rains and snow. The cells produced are large and thin walled, making them less dense than the xylem produced in the summer. The less dense early wood, therefore, appears lighter than the smaller, thick-walled latewood. Whereas the morphology of cells formed from the vascular cambium change gradually with the growing season and does not present a sharp contrast from early wood to latewood, the interface between the latewood of one year's ring and the early wood of the next year's ring is clearly delineated. The annual rings are therefore distinctly visible.
A complex of environmental factors besides total water availability affects the width of the growth rings. Temperature, length of growing season, time of precipitation, disease, and soil fertility are among the variables that work together to control the patterns of lateral growth from year to year. A wide ring generally reflects a long, wet, and moderate growing season, whereas narrow rings usually reflect some kind of environmental stress. An extremely dry, cold winter followed by a hot dry summer could even prevent any lateral growth for that year.
Sapwood and Heartwood
With increased age, the center of the trunk ceases to function in the transport of water and minerals because it accumulates resins, tannins, oils, gums, and other metabolic by-products. Since trees do not have the ability to remove these compounds, they remain in the plant body. The centralization of all these materials allows the outer wood to remain unclogged and functional. These central wood rings, as shown in Figure 8-3, are often darker in color, more resistant to decay, and sometimes aromatic. This central heartwood is usually visibly distinguishable from the lighter, functional sapwood in fresh cut or cured and processed wood.
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The natural preservation properties of some heartwoods, such as cedar, cypress, redwood, black walnut, and mahogany, makes them especially valuable for fine furniture. The ratio of heartwood to sapwood varies from one species to the next, as does the degree of visible differences between the two woods. Some characteristics of heartwood increase its commercial applications.
An interesting response of trees that have lost their vertical position is the production of reaction wood. For instance, if a tree has been bent by another tree falling against it or by a boulder rolling down on one side of it, reaction wood will form along one side of the trunk and bend the trunk back to a vertical position again. In young seedlings this can occur in a single growing season, whereas it takes many years in older trees. This phenomenon of plant movement is completely different from phototropic response because it involves the lateral meristems in the already elongated cells of the trunk, not the apical meristematic growth of the plant tip.
The reaction wood of dicotyledonous angiosperms is called tension wood and is produced along the upper sides of leaning stems and branches, causing reorientation by contraction on the upper side pulling the stem or branch back into a normal growth position. The fibers of tension wood resist cutting and project from the surface of boards sawed from hardwoods. This wooliness is an even greater problem in the planning of the boards during the finishing process. Some of these hardwood trees split at the cut end immediately after felling due to the release of the internal stresses caused by this tissue. Less of the particular log can be used in the production of board lumber than a log from trees without tension wood.
The reaction wood of gymnosperms is called compression wood and is produced along the lower side of the leaning stems or branches, causing straightening by expanding and pushing the trunk upright again. Compression wood is denser (harder) than normal wood. This wood is undesirable for commercial lumber because of its poor nailholding characteristic.
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Dendrochronology is the science of interpreting past conditions by studying the growth of wood. Since climatic conditions, especially water availability, influence the width of growth rings, and since there is normally only one ring produced each year, patterns of annual ring production directly reflect past climates. By counting the number of rings a given tree has, the age of that tree can be determined. The oldest trees are the bristlecone pine (Pinus longaeva) found in the White Mountains of California at above 3000 m elevation. Because of the short growing season and small amount of rainfall at elevation, bristlecone pines grow very slowly and have narrow rings, as many as 1,000 or more in less than 13 cm of lateral growth. Because of this slow growth, bristlecone pines are much smaller in both trunk diameter and height than the massive giant redwoods (Sequoia sempervirens), shown in Figure 8-4.
By matching ring patterns from the wood of a living tree with the wood of an older tree containing a partially overlapping growth ring sequence and continuing with the partially overlapping older woods from trees long dead, a continuous chronology of climatic patterns can be established for a particular region. Such a chronology has been established for the White Mountains, extending back over 8,000 years. Because bristlecone pine wood, dating back about 9,000 years, has been found, it is probable that this chronology can be extended even further back. Interestingly, by using radiocarbon dating techniques on wood that has a known age through ring sequencing, it has been determined that radiocarbon dating is increasingly inaccurate for material over 1,000 years old. Carbon-14 dating of 1,600 and 3,300 years were found for bristlecone pine specimens having 2,000 and 4,000 annual rings, respectively. With continued cross dating, a more accurate age estimate for archaeological studies could be made, thus, improving our understanding of their cultural beginnings. Wooden utensils, tools, ornaments, and building timbers from archaeological digs can help provide information about the climate as well as the age in which a particular culture existed.
The study of tree rings also can tell a much more complete story about an individual tree's history than just its age and the general climatic conditions during its lifetime, especially when ring patterns from other trees in the region tell a slightly different story for the area in general. Damage due to fire, landslide, insects, and other natural occurrences can be seen in the patterns of annual ring formation.
There are many uses of wood, some obvious and some less well known. The uses of wood and wood products are expanding because trees are a renewable resource. With proper management and use, the forests of the world will be able to provide this valuable raw material in more than sufficient quantities as long as it is needed.
One of the most obvious uses for trees is the production of lumber for building and furniture. Many millions of board feet of certain softwoods are used each year for home construction because of their physical properties (a board foot is 1 foot long, 1 foot wide, and 1 foot thick). Trees such as white pine have a soft, uniform texture and an even grain that can be machined easily; does not shrink, swell, or warp significantly; is strong; and holds nails well. Not all gymnosperms have equally desirable qualities, although several other species are used for construction because of the ease with which nails can be driven into them. Framing a house out of oak, walnut, maple, or hickory, on the other hand, would be quite a task because their wood is so hard that boards would have to be drilled and screwed together instead of nailed.
Because of the grains, colors, and durability of hardwoods, they are most often used in furniture making. Some softwood, especially certain pines, is also used for furniture. The use of different woods depends on several properties, which are partially a function of the type of cut and the part of the tree used.
Cuts and Grains
In addition to the kinds of cells and the width of annual rings in a given wood, the appearance of a finished wood surface is a function of how the log has been cut. In a transverse cut or cross section, the annual rings appear as concentric circles. This cut is not commonly used to produce a commercial piece of lumber, although transverse slices that included the bark can make a unique and beautiful tabletop. A radical cut is made longitudinally through the center of the log. In a radial section of wood, the annual rings appear as parallel lines running the length of the flat board surface with the rays running at right angles across them. Since this cut is made through the center of the log, only a few boards can be cut from each log. Radial cuts are also said to be quarter-sawed.
The most common board surface is a log cut longitudinally but not through the center. This tangential cut results in the annual rings appearing as wavy bands with ends of the rays scattered out throughout. Tangential cut lumber is also said to be flat-cut or plain-sawed, and it is the most common cut because so many more boards can be sawed from a log than with the radial cut. The design resulting from these different cuts is the figure of the wood, not the grain. The grain of a wood technically refers to the direction of the fibers, tracheids, and vessel members. The density and size of the cells and the way they are grouped affect the texture of the wood surface more than does the surface pattern of the growth rings.
Another textural and structural consideration in woods is the presence of knots, which are the bases of branches that subsequent lateral growth has covered over. A higher proportion of knots are in the center of the trunk because the tree was younger and smaller with lower branches then. As the tree grows and ages, lower branches cease to be formed, and the old ones die and eventually break off, as shown in Figure 8-5. Just as the lateral growth will eventually cover the base of old branches, so will it cover scars from fire and other injury. You may have noticed that a nail or eye screw put in a tree trunk to hold a clothesline or bird feeder many years ago is not only partially, if at all, showing. It is the same lateral growth that ultimately turns two closely adjacent young trees into a "single" double-trunked tree. Occasionally, two different species growing in proximity results in the faster growing tree gradually enveloping the lower trunk of the other. Tree trunks have even been found with rocks in them. In many of the hardwood forests of France, no tree old enough to have been present during World War II is harvested because the metal bomb fragments and bullets that are often hidden well within the wood will quickly destroy a sawmill blade.
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More than one-third of the world's total population depends on wood for heating and cooking. The significance of wood as a fuel is greatest in developing countries, where more than 86% of all wood consumption is for fuel.
Approximately 1.5 billion people derive at least 90% of their energy requirements for cooking and heating from wood and charcoal, and another billion people depend on wood for at least 50% of their needs. These are impressive statistics, especially since these are also the countries with the fastest growing populations.
Because of the dependence on firewood in these countries, it is estimated that 50% of all wood used worldwide each year goes to fuel. In developed countries with stabilized population sizes and appropriate management and use practices, wood is a renewable resource that should never be in short supply. Theoretically, the developing countries, which depend much more heavily on wood, could produce enough through replanting and appropriate management to meet their needs. Practically, however, it may already be too late in some areas, since this resource has already been depleted to a critical level. The people are now depending on dried animal dung and crop residue for fuel. This removes much-needed nutrients from the soil, which will in turn be less productive. The vicious cycle continues as new land is cleared for more crop production to feed the growing population in these countries. Clearing without replanting removes even more of the wood that could have been used in the future for fuel.
In the United States, although only slightly over 10% of our total wood use is for fuel, over 1 million homes now use wood for their primary heat source. Since 1933, the wood fuel use growth rate has been about 15% annually. More recent trends are even greater because of the renewed interest in wood-burning stoves.
Even at this rate of increase, wood use for fuel is critical only in the developing countries, where the "energy crisis" is much more serious than ours, even with our petroleum shortages. If the developed countries fail to solve the energy crisis, however, we could well join the rest of the world in our dependence on wood as a primary fuel.
Paper production accounts for over half of the wood that is pulped through mechanical or chemical means. The balance of the wood pulp produced each year goes into cardboard and fiberwood. Paper is basically composed of separate plant fiber cells that have been slurred and then spread into thick sheets and dried. True paper was first produced around 105 AD in China. Before the Chinese developed a way of making paper, they wrote on strips of wood with a stylus and later on woven cloth, especially silk. Also in pre-Christian times, Egyptians made a type of paper by beating laminated stems of papyrus until they were very thin. The only other nonpaper material used prior to the second century was parchment, which is made from animal skins. The Chinese guarded the secret of paper making for over 500 years, establishing themselves as the only supplier of paper. Papermaking was a slow hand-labor process until the first machines were invented around 1800.
In 1865, the sulfite process for removing unwanted lignins from the pulp was developed. Lignin causes paper to turn yellow and become brittle with age. For economy, newsprint production does not include this lignin-removing process, and so newsprint eventually becomes yellow and brittle. To add weight and body to paper and to produce a smooth ink-impervious or "hard" surface, given fillers are mixed with the pulp while it is in the liquid slurry stage. Clay, alum, and talc are common fillers to add weight and stiffness to hard surfaces for writing purposes. To make colored paper, dyes are added at this pulp-slurry stage. Over 90% of all paper comes from wood; the rest is made from the fiber of other plants such as flax (paper money and cigarette paper), cotton, and hemp.
Charcoal is made by partially combusting hardwood blocks in the presence of very little air. Today charcoal is made by destructive distillation of hardwoods, and vapors are collected. From these distillation vapors wood alcohol (methanol), methane gas, wood tar, acetic acid, acetone, and hydrogen gas are separated. The distillation of softwoods, especially pines, yields turpentine and rosin, both valuable in the paint industry. Rosin is also used on musicians' bows and baseball pitchers handle a rosin bag to gain a better grip on the ball. Boxers and ballerinas shuffle their feet in a low-sided square rosin box to improve their footing.
Not all synthetics are made from petroleum by-products. Rayon was the first commercial synthetic fiber and is made from dissolved cellulose material of wood pulp. Other products include cellophane, acetate plastics, photographic film, and other molded plastics used as handles for tools. Although synthetics from petrochemicals are much more common and popular now, because wood is a renewable resource, rayon and other wood-pulp synthetics will be revitalized in the future. In fact, rayon blends are increasingly common in clothing.
Cork is a nonwood product of trees, specifically the "bark" of the cork oak (Quercus suber). Native to Europe, most cork is produced in Portugal because the climate conditions there are most conducive to rapid outer periderm growth. The cork cells are naturally impregnated during growth with a wax suberin, which makes them watertight and produces incredible buoyancy. A cork oak can be stripped of its outer bark after the first 20 years of growth and then every 10 years thereafter until the tree is approximately 150 years old. However, cork is being replaced by plastics for various items that at one time were unheard of (wine corks, fishing floats).
Other Wood Uses
The application of wood as lumber includes plywood, particleboard, veneers, and paneling as well as board lumbers for construction and furniture. In fact, the use of plywood for covering large surface areas is now preferred over board lumber in construction framing. Particleboard is especially popular in Europe, as is fiberboard, made in a similar manner. The manufacture of these materials was not economically feasible until the development of modern wood processing techniques and glues that would permanently bond wood and wood particles.
A veneer is a sheet of wood sliced from a log. Most commonly, the log is rotated against a stationary blade that moves inward, producing a continuous sheet of desired thickness. Plywood is made by cutting the continuous sheets to desired size and bonding them together under pressure. Alternate sheets are laid with their grains at right angles to each other to increase strength and reduce warping. When three or seven sheets are used, the center sheet is twice as thick as the outer ones so an equal amount of wood has the grain running in each direction. Different veneering techniques allow the thinner and smoother slices, which can be used as the "paneled" surface for a finished wall or for the facing on furniture.
Master craftsmen have produced veneered furniture for hundreds of years, and veneer has been known since 1500 BC: evidence of this has been found in the tombs of Egyptian pharaohs. Undoubtedly the items painstakingly made by hand were highly prized because of the beautiful figures on the surface of the fine furniture. Although thickness of as little as 0.23 cm (1/110 inch) is possible, most veneer is about 0.084 cm (1/30 inch) thick.
Gluing wood chips together under pressure makes particleboard. Fiberboard is made by the same treatment, but the wood fiber is obtained by various chemical or mechanical pulping methods. Their use is primarily as insulation board or for building containers. However, when faced with a finished veneer or vinyl overlay, they are used for paneling and furniture.
Raw wood treated with preservatives is used extensively for fence post, telephone poles, mine support timbers, boat dock pilings, and timbers that are buried in unstable soils under building foundations. Most of the preservatives used are inorganic chemicals toxic to decaying organisms or toxic organic oils that prevent bacteria and fungi from decomposing (rotting) the wood tissues. If free from such decomposition, wooden post, poles, and timbers are incredibly strong and durable.
Forests and Forestry
The total U.S. land surface area of the 50 states is approximately 2.3 billion acres. About one-third of that amount, 760 million acres, is forested, and almost two-thirds of that forestland, 482 million acres, is classed as commercial timberland.
Proper management of existing forest includes thinning inferior trees to allow for more rapid growth of healthy ones, removing litter and diseased trees, which increase fire hazards, and employing the most efficient harvesting and restocking methods. In addition, studies show that some 168 million acres of commercial timberland would yield greater returns from the application of such intensive management techniques than other lands (see Figure 8-6). Economically, it would make sense to concentrate improvement efforts on the most productive lands.
With cost-effective technologies developed, those parts of harvested trees relegated in the past to waste could be used for pulp fuel biomass or synthetic extraction. Inferior, diseased, and damaged trees that are now culled from the harvestable strands could also become a resource instead of a liability.
An average of over 4 billion cubic feet of timber is lost each year to such destructive agents as insects, diseases, storms, and fires. Biological control of insect pests and fungi may effectively supplement existing efforts of controlling these destructive organisms. Controlled burning to remove ground fuel in the form of dead trees, dry underbrush, and dead limbs has been used as a management tool in many of the federally controlled forest lands. However, after several summers of devastation from forest fires started by controlled burning, the U.S. government is rethinking this management practice.
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New "super strains" of trees are being produced that will increase productivity in many future forests. By using cuttings, grafts, and tissue cultures to speed up the reproductive cycle, new strains are being developed that might be twice as productive as wild genetic strains. A concern with creating a monoculture that might be wiped out by a single catastrophic event is voiced by some researchers, but others are convinced that the development of super trees is the only way production will keep up with demands of the future. The United States is already the largest importer of timber in the world, and many economic experts warn against any increase in the dependence on imports.
Obviously, wood is incredibly important especially since it is renewable. With the current and future emphasis on wood and wood products, intelligent long-range planning is a must if we are to keep supply and demand in balance.
1. Over 4,500 different commercial products come from wood or secondary xylem. The giant redwood is the world's tallest tree, and the bristlecone pine is the oldest living organism. Wood is composed of cellulose, hemicellulose, and lignin. Properties of wood that depend on the relative composition of these materials include durability, color, luster, polishability, shrinkage, heat conduction, and acoustics.
2. The terms softwood and hardwood, although somewhat misleading, refer to coniferous gymnosperms and woody dicots, respectively. Hardness actually refers to specific gravity or density.
3. In most trees, a single annual growing season results in the formation of a growth ring. These rings are xylem tissues, and as the tree ages, the inner (earliest) rings usually accumulate metabolic by-products and take on a different appearance. This heartwood no longer conducts water and minerals as the outer sapwood layers do.
4. Reaction wood allows trunks or branches to straighten up from a leaning position. Tension wood of dicots and compression wood of conifers differs in their mode of action, but they both produce the same results.
5. Although not wood, bark is an important part of most tree trunks. Cork is the bark of the cork oak (Quercus suber). The study of the past by using tree rings is called dendrochronology. The number of annual growth rings establishes the age, and the widths of the rings reveal the climatic conditions for that growing season. A continuous chronology for over 8,000 years has been established using dead tree trunks of bristlecone pines.
6. Building lumber can be categorized according to density, texture, and the part of the tree used. Transverse, radial, and tangential cuts result in different grains. The presence of knots is also a consideration when selecting building lumber. Knots are old branches covered over by the expanding trunk girth or diameter.
7. Wood used for fuel is an essential natural resource in most of the developing countries. The use of wood as fuel is increasing in the United States and in several other developing countries as well.
8. Paper production is a significant commercial use of wood, as is charcoal. Rayon is a synthetic fiber produced from wood, not petroleum by-products.
9. Other uses for wood include veneer making and plywood manufacture. In addition, some woods have natural preservatives that allows them to be used as timbers and pilings in wet environments.
10. Although the United States has adequate wood resources, some countries are not properly harvesting and caring for their forests. Proper management policies mean adequate timber production in the United States for many years to come.
Something to Think About
1. What is the world tallest tree?
2. List 5 softwood trees.
3. List 5 hardwood trees.
4. What are growth rings made up from?
5. Compare tension wood of dicots and compression wood of monocots.
6. Explain what dendrochronology is.
7. How are knots in wood made?
8. Rayon is made from what?
9. List some commercial uses of wood.
10. Proper management polices of the forest will mean what for the United States?
Bowyer, J. L., R. Shmulsky, and L. Haggreen. 2002. Forest products and wood science. Malden, MA: Blackwell Publishing.
Core, H. A., W. A. Cote, and A. C. Day. 1979. Wood structure and identification. Syracuse, NY: Syracuse University Press.
Wood and Fiber Science. 2005.
Internet sites represent a vast resource of information. The URLs for Web sites can change. Using one of the search engines on the Internet, such as Google, Yahoo!, Ask.com, and MSN Live Search, find more information by searching for these words or phrases: chemical composition of wood, tule tree, cellulose, lignin, heartwood, sapwood, hardwood, softwoods, relative density, specific gravity of wood, growth rings, reaction wood, compression wood, dendrochronology, radial cut lumber, knots, renewable resource, paper, fiber board, plywood, and forest management.
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|Title Annotation:||PART 2: Form and Structure|
|Publication:||Fundamentals of Plant Science|
|Date:||Jan 1, 2009|
|Previous Article:||Chapter 7: Growth: cells to tissue.|
|Next Article:||Chapter 9: Plant-soil-water relationships.|