Rowell R.M. (ed.) Handbook of Wood Chemistry and Wood Composites

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Hry Temple in Nara, Japan. At over 1300 years old, this temple is one of the oldest remaining wooden

structures in the world.

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Wood and Society

Christopher D. Risbrudt

USDA, Forest Service, Forest Products Laboratory, Madison, WI

Forests, and the wood they produce, have played an important role in human activity since before

recorded history. Indeed, one of the ﬁrst major innovations of humankind was utilizing ﬁre, fueled

by wood, for cooking and heating. It is very likely that early hominids used wood ﬁres for cooking

as long as 1.5 million years ago (Clark and Harris 1985). Clear evidence of this use of wood exists

from sites 400,000 years old (Sauer 1962). Since this ancient beginning, the uses of wood, and the

value of the forest, have expanded dramatically, as the population of humans and their economies

grew. Wood was used in myriad products, including agricultural implements and tools, shelters and

houses, bridges, road surfaces, ships and boats, arrows and bows, spears, shoes, wheelbarrows,

wagons, ladders, and thousands of others. Other important products that forests provided were

food, in the form of berries, nuts, fruits, and wild animals, and, of course, fuel. Wood was the most

important material in early human economies, and though other materials have grown in importance,

wood used for solid products, ﬁber, and chemicals is still the largest single type of raw material

input by weight—with the one exception of crushed stone, sand, and gravel—into today’s economy

(Haynes 2003).

Wood is still the major source of cooking and heating fuel for most of the world. In 2002,

world consumption of fuelwood and charcoal totaled 1,838,218,860 cubic meters. This represents

nearly 54% of the world’s consumption of wood. About 43% of this fuelwood consumption occurs

in Asia, and Africa consumes 31%. The United States consumes only 4% of the world’s total of

fuelwood and charcoal (Food and Agriculture Organization of the United Nations 2004). Total

world consumption of roundwood, which includes fuelwood, charcoal, and industrial wood,

amounted to 3,390,684,310 m

in 2002 (FAO of the UN 2004).

Besides producing fuelwood and wood for construction and other uses, forests have always

been an important part of the American landscape, playing a key role in the social, economic, and

spiritual life of the country. As the American population and economy grew, forests were removed

to make way for farms, cities, and roadways. After the ﬁrst European settlements in North America,

forests were often viewed as an obstacle to farming and travel. Huge acreages were cleared in the

19th century to make way for ﬁelds, pastures, cities, and industry. In 1800, total cropland area in

the United States extended across 20 million acres. By 1850, this had grown to 76 million acres,

with pasture and hayland at perhaps twice that amount. Most of this farmland expansion was at

the expense of forests (MacCleery 1996). The amount of cropland in the United States peaked in

1932, at about 361 million acres. (USDA National Agricultural Statistical Service 2003) However,

although much forestland has been converted to other uses, the net area of forestland has remained

relatively stable since the 1920s (Alig et al. 2003). As shown in Figure 1.1, about 70% of the

original amount of forested land still remains as forest, although much of it is likely modiﬁed from

its structure and composition in 1600. Since 1932, however, as farmed land acreage decreased,

forest area in the United States has been increasing. Forests have been the beneﬁciary of the

conversion from animal power to mechanical power in farming. An estimated 20 million acres of

grain ﬁelds and pastures were no longer needed when gasoline tractors replaced horses and mules.

As agricultural productivity per acre increased, as a result of plant breeding, fertilizers, and

pesticides, forests have reclaimed many acres back from farm ﬁelds.

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Wood has remained an important substance throughout history because of its unique and useful

properties. Wood is recyclable, renewable, and biodegradable. Many species are shock resistant,

bendable, and stable (although all wood changes dimensions as it loses or gains moisture). Density

among species varies greatly; the balsa popular with model-airplane builders can weigh as little

as 6 pounds per cubic foot, and some tropical hardwoods weigh more than 70 pounds per cubic

foot. Wood and lignin can be converted to many useful industrial chemicals, such as ethanol and

plastics. Wood can be treated to resist decay, and with proper construction techniques, and stains

or paint, wood buildings can last hundreds of years. The oldest surviving wood structure is an Asian

temple, built in the 7th century. Today, wood is used in tools, paper, buildings, bridges, guardrails,

railroad ties, posts, poles, mulches, furniture, packaging, and thousands of other products.

Wood’s versatility makes many wood products recyclable. Perhaps the earliest and simplest

recycling was the burning of used wood for heat, whether in a wood stove, ﬁreplace, or furnace.

New technologies are improving the efﬁciency with which used or scrap wood can generate

electricity and heat. The paper and paperboard industry has used recycled paper to augment virgin

wood pulp for decades. At ﬁrst, recycled paper generally found its way into newsprint and other

low-grade products, but recent advances in recycling technology permit used paper to go into the

manufacture of higher-quality papers, where appearance, texture, and consistency are important.

Other products, whether railroad ties or structural timbers from 60-year-old buildings, ﬁnd second

lives as lumber. Afﬂuent consumers, especially in the United States, have long been willing to pay

a premium for the aesthetic pleasure of using 100-year-old barn siding as interior paneling. More

recently, entrepreneurs have recognized the availability and potential value of millions of board-

feet of high-quality lumber in World War II–era buildings sitting on closed (or soon-to-be-closed)

military bases.

FIGURE 1.1 United States forest area. Forests as a percentage of total U.S. land area, 1600–1992. (From

Preliminary data from the 1992 Program Update, USDA Forest Service, 1993.)

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Other recycling opportunities for wood include the manufacture of wood ﬁber and plastic

composite materials, in which wood ﬁbers improve the strength-to-weight ratio over that of plastic

alone—a performance characteristic that has strong appeal in the automotive industry, among others.

The ability of forests to regenerate on abandoned farmland—or after a destructive forest

ﬁre—testiﬁes to the renewable nature of the wood resource. Though consumption of wood outpaces

growth in some parts of the world, in the United States, trees have been growing and producing

wood faster than they have been harvested since the early 1950s. By 1970, U.S. forestlands produced

more than 20 billion cubic feet of wood, some 5 billion more than the harvest. (MacCleery 1996)

Much wood for U.S. home construction and the nation’s paper industry comes from planta-

tions—mostly southern pine—in the southeastern United States. Those plantations depend on the

resource’s renewability, with trees harvested and new ones planted and then harvested 15 to 30

years later. The major inputs are abundant: rain, sun, airborne carbon, and soil nutrients.

Wood’s chemical makeup is largely carbon, hydrogen, and oxygen, arranged as cellulose,

hemicellulose, and lignin. As such, wood presents an appetizing feast for a variety of fungal species

that can metabolize either the sugar-like celluloses or the more complex lignin. With the help of

these fungi in the presence of air and water, wood rots or, in environmental terms, is biologically

degraded.

Wood is renewable, recyclable, and biodegradable—characteristics generally accepted as good

for the environment. At the beginning of the 21st century, however, the most environmentally

friendly aspect of wood may be its role in carbon sequestration. Growing trees soak up great

quantities of carbon from atmospheric carbon dioxide (CO

), widely regarded as a greenhouse gas

that traps heat and affects global climates. Indeed, dry wood is roughly half carbon by weight.

Each cubic foot of wood contains between 11 and 20 pounds of carbon. A single tree can easily

contain a ton or more of carbon. In addition to the carbon in the tree above ground, signiﬁcant

carbon is locked in the roots and surrounding soil. With roughly one third of the U.S. land area,

or some 747 million acres, in forestland, the nation’s forests hold more than 50 billion tons of

carbon out of the atmosphere (U.S. Environmental Protection Agency 2002).

Forests hold unique signiﬁcance in the environments that Americans value (see Table 1.1).

Not only is the United States the world’s largest importer and exporter of wood products, the

forests of America are also highly regarded for their recreational, aesthetic, spiritual, and natural

values. Forests are valued for providing wildlife and ﬁsh habitat, clean water, and clean air. Forests

are further shown to be important sinks of carbon, slowing and ameliorating global warming.

These concerns are all melded into the concept of sustainability. This concept, expressed through

worldwide concern in the 1992 Rio de Janeiro agreement, has resulted in a multinational effort

to measure forest sustainability. The Montreal Process, resulting from the Rio agreement, lists

seven criteria and 67 indicators of factors and conditions that can help in the judgment of

sustainability:

1. Conservation of biological diversity

2. Maintenance of productive capacity

3. Maintenance of forest ecosystem health and vitality

4. Maintenance of soil and water resources

5. Maintenance of forest contribution to global carbon cycles

6. Maintenance and enhancement of long-term multiple socio-economic beneﬁts to meet

the needs of societies

7. Legal, institutional, and economic framework; capacity to measure and monitor changes;

and capacity to conduct and apply research and development for forest conservation and

sustainable management.

The U.S. has just completed its ﬁrst assessment of these criteria and indicators (USDA Forest

Service 2004).

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Not surprisingly, the assessment produces mixed results in almost every category. Although the

total area of forest in the United States has remained stable for the past 80 years, the location of

forests is changing and the nature of forests and how they are used is changing. Though much

forest habitat has remained stable over recent decades, some forest plants, birds, and other animals

are at risk of extinction. Net growth on timberland continues to exceed harvest removals. The

downside of this is that the net gain can represent overcrowding, which increases risk of wildﬁre

and of susceptibility to insects and disease, serious threats to the forest ecosystems.

Healthy forests function well as sources of water for towns and cities, especially in the Western

United States. The ﬁrst sustainability assessment indicates, however, that at least 10% of the forested

counties in each region have areas where forest conditions have deteriorated and water and soil

quality have been compromised through reduced oxygen levels, higher sediment, dissolved salts,

or acidity.

The assessment of long-term socioeconomic beneﬁts to society reveals the interrelatedness and

interdependence of many indicators. Recreational use of the nation’s forests is increasing. At the

same time, the increased demand for wood and wood products has not led to increased harvest of

U.S. trees. It appears that increased wood imports and increased recovery of paper for recycling

have enabled Americans to use more wood and paper without cutting more trees from America’s

forests. The global implications—economic, social, political, and environmental—can only be

guessed.

The United States, through government agencies, nongovernmental organizations and institu-

tions, industry, and academia, conducts extensive research regarding forests, wood, recycling, and

related topics. Nonetheless, the national debate about the proper care and nurture of forests appears

in many cases to be rooted in emotion and politics rather than science. The debate can be shrill at

times, which reﬂects perhaps the intensity with which our culture regards the forests. Perhaps the

TABLE 1.1

Indicator Variables for Outputs from FS- and BLM-Administered

Lands Suitable for an Ecosystem Market Basket

Indicator Variable

Carbon storage

Ecosystem health

Fire risk to life and property

Fish

Game

Minerals

Passive-use values:

Existence of salmon

Existence of other threatened and endangered species

Existence of unroaded areas

Range

Recreation:

Access (roads)

Access to riparian areas

Science integrity

Soil productivity

Special forest and range products

Timber

Visibility

Water quality

Source: Haynes and Horne, 1997.

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greatest challenge to ensuring the long-term sustainability of healthy forests that provide our society

with valued resources and recreational and aesthetic opportunities, as well as environmentally vital

carbon sequestration, lies not in science but in ﬁnding a way to sit down at the table together and

agree on some goals and the continued need to explore alternative ways to achieve those goals.

REFERENCES

Alig, R.J., Pantinga, A.J., Ahn, S.E., and Kline, J.D. (2003). Land use changes involving forestry in the United

States: 1952 to 1997, with projections to 2050. General Technical Report PNW-GTR-587. U.S.

Department of Agriculture, Forest Service, Paciﬁc Northwest Research Station, Portland, OR.

Clark, J.D. and Harris, J.W.K. (1985). Fire and its roles in early hominid lifeways. The African Archaeological

Review 3:3–27.

Food and Agriculture Organization of the United Nations. (2004). FAO statistical data 2004: Forestry data,

roundwood, sawnwood, woodbased panels. Available at www.faostat.fao.org (accessed May 14, 2004).

Haynes, R.W. (Tech. Coord.) (2003). An analysis of the timber situation in the United States: 1952–2050.

General Technical Report PNW-GTR-560. USDA Forest Service, Portland, OR.

MacCleery, D.W. (1996). American Forests: A History of Resiliency and Recovery (rev. ed.). Forest History

Society Issues Series. Forest History Society.

Sauer, C.O. (1962). Fire and early man. Paideuma 7:399–407.

U.S. Environmental Protection Agency. (2002). Inventory of U.S. Greenhouse Gas Emissions and Sinks:

1990–2000. EPA 430-R-02-003. U.S. Government Printing Ofﬁce, Washington, DC.

USDA National Agricultural Statistical Service. (2003). Historical track records. U.S. Government Printing

Ofﬁce, Washington, DC.

USDA Forest Service. (2004). National Report on Sustainable Forests—2003. Report FS-766. U.S. Govern-

ment Printing Ofﬁce, Washington, DC.

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Part I

Structure and Chemistry

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Structure and Function

of Wood

Alex C. Wiedenhoeft and Regis B. Miller

USDA, Forest Service, Forest Products Laboratory, Madison, WI

CONTENTS

2.1 The Tree

2.2 Softwoods and Hardwoods

2.3 Sapwood and Heartwood

2.4 Axial and Radial Systems

2.5 Planes of Section

2.6 Vascular Cambium

2.7 Growth Rings

2.8 Cells in Wood

2.9 Cell Walls

2.10 Pits

2.11 The Microscopic Structure of Softwoods and Hardwoods

2.11.1 Softwoods

2.11.1.1 Tracheids

2.11.1.2 Axial Parenchyma and Resin Canal Complexes

2.11.1.3 Rays

2.11.2 Hardwoods

2.11.2.1 Vessels

2.11.2.2 Fibers

2.11.2.3 Axial Parenchyma

2.11.2.4 Rays

2.12 Wood Technology

2.13 Juvenile Wood and Reaction Wood

2.14 Wood Identiﬁcation

References

Despite the many human uses to which various woods are suited, at a fundamental level wood is

a complex biological structure, itself a composite of many chemistries and cell types acting together

to serve the needs of the plant. Although humans have striven to understand wood in the context

of wood technology, we have often overlooked the key and basic fact that wood evolved over the

course of millions of years to serve three main functions in plants: the conduction of water from

the roots to the leaves, the mechanical support of the plant body, and the storage of biochemicals.

The need for these three functions has driven the evolution of approximately 20,000 different extant

species of woody plants, each with unique properties, uses, and capabilities, in both plant and

human contexts. Understanding the basic requirements dictated by these three functions and iden-

tifying the structures in wood that perform them allows insights into the realm of human wood use

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(Hoadley 2000). A scientist with a robust understanding of the interrelationships between form and

function can predict the usefulness of a speciﬁc wood in a new context.

To begin, it is necessary to deﬁne and delimit the component parts of wood at a variety of

scales. There is a signiﬁcant difference in the quality and quantity of wood anatomical expertise

necessary for a researcher who is using a solid wood beam compared to the knowledge necessary

for an engineer designing a glued-laminated beam, and these are in turn different compared to the

knowledge required for making a wood-resin composite with wood ﬂour. In the ﬁrst case, a large-

scale anatomical understanding may help to explain and quantify the mechanical properties of the

beam. In the second case, an understanding of anatomical effects on mechanical properties must

be coupled with chemical knowledge about the efﬁcacy of various adhesives. In the third case, an

understanding of particle size distribution and wood cell wall chemistry will be key pieces of

knowledge. The differences in the kinds of knowledge in these three cases are related to the scale

at which one intends to interact with wood, and in all three cases the technologically different

properties are derived from the biological needs of the living tree. For this reason, the structure of

wood will be explained in this chapter at decreasing scales, and in ways that demonstrate the

biological rationale for a plant to produce wood with such features. Such background will permit

the reader to access primary literature related to wood structure with greater ease.

Although shrubs and many vines form wood, the remainder of this chapter will focus on the

wood from trees. As trees are the predominant source of wood for commercial applications and

provide examples of virtually all features that merit discussion, this restriction of scope is warranted.

2.1 THE TREE

The general body plan of a tree must be brieﬂy outlined so that all subsequent information can be

understood in its proper context within the living organism. A living, growing tree has two main

domains, the shoot and the roots. The roots are the subterranean structures responsible for water

uptake, mechanical support of the shoot, and storage of biochemicals. The shoot comprises the

FIGURE 2.1 Macroscopic view of a transverse section of a Quercus alba trunk. Beginning at the outside of

the tree, there is the outer bark (ob), the inner bark (ib), and then the vascular cambium (vc), which is too

narrow to see at this magniﬁcation. Interior to the vascular cambium is the sapwood, which is easily differ-

entiated from the heartwood that lies to the interior. At the center of the trunk is the pith (p), which is barely

discernible in the center of the heartwood.

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trunk or bole of the tree, the branches, and the leaves (Raven et al. 1999). It is with the trunk of

the tree that the remainder of the chapter will be concerned.

If one cuts down a tree and looks at the stump, there are several gross observations that can

be easily made. The trunk is composed of various materials present in concentric bands. From the

outside of the tree to the inside there are six layers: outer bark, inner bark, vascular cambium,

sapwood, heartwood, and the pith (Figure 2.1). Outer bark provides mechanical protection to the

softer inner bark, and also helps to limit evaporative water loss. Inner bark (phloem) is the tissue

through which sugars produced by photosynthesis (photosynthate or “food”) are translocated from

the leaves to the roots or growing portions of the tree. The vascular cambium is the layer between

the bark and the wood that is responsible for producing both these tissues. The sapwood is the

active, “living” wood that is responsible for conducting the water (or sap) from the roots to the

leaves. It has not yet accumulated the often-colored chemicals that set apart the nonconductive

heartwood found as a core of darker-colored wood in the middle of most trees. The pith at the very

center of the trunk is the remnants of the early growth of the trunk, before wood was formed.

2.2 SOFTWOODS AND HARDWOODS

To deﬁne them botanically, softwoods are those woods that come from gymnosperms (mostly

conifers), and hardwoods are woods that come from angiosperms (ﬂowering plants). In the tem-

perate portion of the Northern Hemisphere, softwoods are generally needle-leaved evergreen trees

such as pine (Pinus) and spruce (Picea), whereas hardwoods are typically broadleaf, deciduous

trees such as maple (Acer) and birch (Betula). Not only do softwoods and hardwoods differ in

terms of the types of trees from which they are derived, but they also differ in terms of their

component cells. The single most important distinction between the two general kinds of wood is

that hardwoods have a characteristic type of cell called a vessel element (or pore), whereas softwoods

lack these (Figure 2.2). An important cellular similarity between softwoods and hardwoods is that

FIGURE 2.2 Softwood and hardwood. (A) The general form of a generic softwood tree. (B) The general form

of a generic hardwood tree. (C) Transverse section of Pseudotsuga mensiezii, a typical softwood. The three round

white spaces are resin canals. (D) Transverse section of Betula allegheniensis, a typical hardwood. The many

large, round white structures are vessels or pores, the characteristic feature of a hardwood. Scale bars = 300 µm.

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