Handbook of wood chemistry and wood composites

HANDBOOK OF WOOD CHEMISTRY AND WOOD COMPOSITES © 2005 by CRC Press HANDBOOK OF WOOD CHEMISTRY AND WOOD COMPOSITES ...

Author: Roger M. Rowell

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HANDBOOK OF

WOOD CHEMISTRY AND WOOD COMPOSITES

© 2005 by CRC Press

HANDBOOK OF

WOOD CHEMISTRY AND WOOD COMPOSITES

Edited by

Roger M. Rowell

Boca Raton London New York Singapore

A CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa plc.


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Library of Congress Cataloging-in-Publication Data Handbook of wood chemistry and wood composites / edited by Roger M. Rowell. p. cm. Includes bibliographical references and index. ISBN 0-8493-1588-3 (alk. paper) 1. Wood—Chemistry—Handbooks, manuals, etc. 2. Engineered wood—Handbooks, manuals, etc. I. Rowell, Roger M. TS932.H36 620.1'2—dc22

2004057915

This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher. All rights reserved. Authorization to photocopy items for internal or personal use, or the personal or internal use of specific clients, may be granted by CRC Press, provided that $1.50 per page photocopied is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923 USA. The fee code for users of the Transactional Reporting Service is ISBN 0-8493-1588-3/05/$0.00+$1.50. The fee is subject to change without notice. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. The consent of CRC Press does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from CRC Press for such copying. Direct all inquiries to CRC Press, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe.

Visit the CRC Press Web site at www.crcpress.com © 2005 by CRC Press No claim to original U.S. Government works International Standard Book Number 0-8493-1588-3 Library of Congress Card Number 2004057915 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper


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For a teacher, Success is not measured in the number of students that conform to your way of thinking, It is measured in the number of students in which you have confirmed the excitement of the learning process. Roger Rowell


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Preface Wood has played a major role throughout human history. The earliest humans used wood to make shelters, cook food, construct tools, and make weapons. There are human marks on a climbing pole that were made over 300,000 years ago. We have found wood in the Egyptian pyramids, Chinese temples and tombs, and ancient ships, attesting to the use of wood by past societies. Collectively, society learned very early the great advantages of using a resource that was widely distributed, multifunctional, strong, easy to work, aesthetic, sustainable, and renewable. Wood has been used by people for centuries as a building material; we have accepted its limitations, such as instability toward moisture and degradation due to microorganisms, termites, fire, and ultraviolet radiation, in use. We must accept that wood was designed by nature to perform in a wet environment and that nature is programmed to recycle wood to carbon dioxide and water using the chemistries of biological decay and thermal, ultraviolet, and moisture degradations. By accepting these limitations, however, we also limit our expectations of performance, which, ultimately, limits our ability to accept new concepts for improved materials. As we start the twenty-first century, we are concerned about issues dealing with the environment, sustainability, recycling, energy, sequestering carbon, and the depletion of our natural resources by a growing world population. In many ways, we are rediscovering wood as a material. We will not, however, be able to realize the full potential of the role that wood and wood products can play in our “modern society” as materials and chemical feed stock until we fully understand their chemistry and material properties. That understanding holds the keys to effective utilization. Wood will not reach its highest use potential until we fully describe it, understand the mechanisms that control its performance properties, and, finally, become able to manipulate those properties to elicit the performance we seek. The purpose of this book is to present the latest concepts in wood chemistry and wood composites as understood by the various authors who have written the chapters. I thank them for their time and effort in the preparation of this book. The book is an update of an earlier book of this editor, Chemistry of Solid Wood, Advances in Chemistry Series No. 207, American Chemical Society, Washington, D.C., 1984, which is long out of print. Roger M. Rowell


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About the Editor Roger M. Rowell is a project leader and research chemist at the U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, in Madison, Wisconsin. He is also a professor in the Departments of Forest Ecology and Management and Biological Systems Engineering at the Engineering Center for Plasma Research at the University of Wisconsin, Madison, and in the Department of Chemical Engineering, Chalmers University, Gothenburg, Sweden. He has worked on projects for the United Nations and with many universities, institutes, and companies around the world on bio-based composites. He has taught courses in wood chemistry in several countries and has presented many lectures at international and national scientific meetings. His research specialties are in the areas of carbohydrate chemistry, chemical modification of lignocellulosics for property enhancement, water quality, and sustainable materials. He has been a visiting scholar in Japan, Sweden, the United Kingdom, New Zealand, and China. He is a fellow of the International Academy of Wood Science and the American Chemical Society’s Division of Cellulose, Paper, and Textiles. He received his BS degree in chemistry and mathematics from Southwestern College in Winfield, Kansas, his MS in biochemistry from Purdue University in West Lafayette, Indiana, and his PhD in biochemistry from Purdue University. He has edited 9 books and has over 300 publications and 22 patents. He is married to his wonderful wife, Judith (who has spent many hours helping with this book), and his family includes three grown sons, their wives, and eight grandchildren.


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List of Contributors Lars Bergland Royal Institute of Technology Department of Aeronautical and Vehicle Engineering Stockholm, Sweden Von L. Byrd USDA, Forest Service Forest Products Laboratory Madison, WI Daniel F. Caulfield USDA, Forest Service Forest Products Laboratory Madison, WI Craig Clemons USDA, Forest Service Forest Products Laboratory Madison, WI L. Emilio Cruz-Barba Center for Plasma Aided Manufacturing University of Wisconsin Madison, WI Ferencz S. Denes Department of Biological Systems Engineering and Center for Plasma Aided Manufacturing University of Wisconsin Madison, WI W. Dale Ellis USDA, Forest Service Forest Products Laboratory Madison, WI Charles R. Frihart USDA, Forest Service Forest Products Laboratory Madison, WI James S. Han USDA, Forest Service Forest Products Laboratory Madison, WI © 2005 by CRC Press

Rebecca E. Ibach USDA, Forest Service Forest Products Laboratory Madison, WI Rodney E. Jacobson AJ Engineering Madison, WI Susan L. LeVan-Green USDA, Forest Service Forest Products Laboratory Madison, WI Sorin Manolache Center for Plasma Aided Manufacturing University of Wisconsin Madison, WI Regis B. Miller USDA, Forest Service Forest Products Laboratory Madison, WI Roger Pettersen USDA, Forest Service Forest Products Laboratory Madison, WI Christopher D. Risbrudt USDA, Forest Service Forest Products Laboratory Madison, WI Jeffrey S. Rowell Department of Forest Ecology and Management University of Wisconsin Madison, WI Roger M. Rowell USDA, Forest Service Forest Products Laboratory Department of Biological Systems Engineering University of Wisconsin Madison, WI

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Mandla A. Tshabalala USDA, Forest Service Forest Products Laboratory Madison, WI

R. Sam Williams USDA, Forest Service Forest Products Laboratory Madison, WI

Alex C. Wiedenhoeft USDA, Forest Service Forest Products Laboratory Madison, WI

Jerrold E. Winandy USDA, Forest Service Forest Products Laboratory Department of Bio-Based Products University of Minnesota St. Paul, MN


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Contents Preface Chapter 1

Wood and Society

Christopher D. Risbrudt

PART I Structure and Chemistry Chapter 2

Structure and Function of Wood

Alex C. Wiedenhoeft and Regis B. Miller Chapter 3

Cell Wall Chemistry

Roger M. Rowell, Roger Pettersen, James S. Han, Jeffrey S. Rowell, and Mandla A. Tshabalala

PART II Properties Chapter 4

Moisture Properties

Roger M. Rowell Chapter 5

Biological Properties

Rebecca E. Ibach Chapter 6

Thermal Properties

Roger M. Rowell and Susan L. LeVan-Green Chapter 7

Weathering of Wood

R. Sam Williams Chapter 8

Surface Characterization

Mandla A. Tshabalala

PART III Wood Composites Chapter 9

Wood Adhesion and Adhesives

Charles R. Frihart


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Chapter 10 Wood Composites Lars Bergland and Roger M. Rowell Chapter 11 Chemistry of Wood Strength Jerrold E. Winandy and Roger M. Rowell Chapter 12 Fiber Webs Roger M. Rowell, James S. Han, and Von L. Byrd Chapter 13 Wood Thermoplastic Composites Daniel F. Caulfield, Craig Clemons, Rodney E. Jacobson, and Roger M. Rowell

PART IV Property Improvements Chapter 14 Chemical Modification of Wood Roger M. Rowell Chapter 15 Lumen Modifications Rebecca E. Ibach and W. Dale Ellis Chapter 16 Plasma Treatment of Wood Ferencz S. Denes, L. Emilio Cruz-Barba, and Sorin Manolache


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Ho¯ ryu¯ 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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1

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 m3 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.



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.)

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



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 plantations—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 (CO2), 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. 2. 3. 4. 5. 6.

Conservation of biological diversity Maintenance of productive capacity Maintenance of forest ecosystem health and vitality Maintenance of soil and water resources Maintenance of forest contribution to global carbon cycles 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).



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.

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 institutions, 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 © 2005 by CRC Press


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. Government Printing Office, Washington, DC.


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Part I Structure and Chemistry



and Function 2 Structure of Wood Alex C. Wiedenhoeft and Regis B. Miller USDA, Forest Service, Forest Products Laboratory, Madison, WI

CONTENTS 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11

The Tree Softwoods and Hardwoods Sapwood and Heartwood Axial and Radial Systems Planes of Section Vascular Cambium Growth Rings Cells in Wood Cell Walls Pits 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 identifying the structures in wood that perform them allows insights into the realm of human wood use



(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 largescale 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 differentiated 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. © 2005 by CRC Press


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 temperate 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. © 2005 by CRC Press


in both kinds of wood, most of the cells are dead at maturity even in the sapwood. The cells that are alive at maturity are known as parenchyma cells, and can be found in both softwoods and hardwoods. Additionally, despite what one might conclude based on the names, not all softwoods have soft, lightweight wood, nor do all hardwoods have hard, heavy wood.

2.3 SAPWOOD AND HEARTWOOD In both softwoods and hardwoods, the wood in the trunk of the tree is typically divided into two zones, each of which serves an important function distinct from the other. The actively conducting portion of the stem, in which the parenchyma cells are still alive and metabolically active, is referred to as the sapwood. A looser definition that is more broadly applied is that the sapwood is the band of lighter-colored wood adjacent to the bark. The heartwood is the darker-colored wood found to the interior of the sapwood (Figure 2.1). In the living tree, the sapwood is responsible not only for the conduction of sap, but also for the storage and synthesis of biochemicals. This function is often underappreciated in wood technological discourse. An important storage function is the long-term storage of photosynthate. The carbon that must be expended to form a new flush of leaves or needles must be stored somewhere in the tree, and it is often in the parenchyma cells of the sapwood that this material is stored. The primary storage forms of photosynthate are starch and lipids. Starch grains are stored in the parenchyma cells, and can be easily seen using a microscope. The starch content of sapwood can have important ramifications in the wood industry. For example, in the tropical tree ceiba (Ceiba pentandra), an abundance of starch can lead to the growth of anaerobic bacteria that produce ill-smelling compounds that can make the wood unusable (Chudnoff 1984). In the southern yellow pines of the United States, a high starch content encourages the growth of sap-stain fungi that, though they do not effect the strength of the wood, can nonetheless cause a significant decrease in lumber value for aesthetic reasons (Simpson 1991). The living cells of the sapwood are also the agents of heartwood formation. In order for the tree to accumulate biochemicals, they must be actively synthesized and translocated by living cells. For this reason, living cells at the border between the heartwood and sapwood are responsible for the formation and deposition of heartwood chemicals, one of the important steps leading to heartwood formation (Hillis 1996). Heartwood functions in the long-term storage of biochemicals of many varieties depending on the species in question. These chemicals are known collectively as extractives. In the past it was thought that the heartwood was a disposal site for harmful by-products of cellular metabolism, the so-called secondary metabolites. This led to the concept of the heartwood as a dumping ground for chemicals that, to a greater or lesser degree, would harm the living cells if not sequestered in a safe place. A more modern understanding of extractives indicates that they are a normal and intentional part of the plant’s efforts to protect its wood. Extractives are formed by parenchyma cells at the heartwood-sapwood boundary and are then exuded through pits into adjacent cells (Hillis 1996). In this way it is possible for dead cells to become occluded or infiltrated with extractives despite the fact that these cells lack the ability to synthesize or accumulate these compounds on their own. Extractives are responsible for imparting several larger-scale characteristics to wood. For example, extractives provide natural durability to timbers that have a resistance to decay fungi. In the case of a wood such as teak (Tectona grandis), famed for its stability and water resistance, these properties are conferred by the waxes and oils formed and deposited in the heartwood. Many woods valued for their colors, such as mahogany (Swietenia mahagoni), African blackwood (Diospyros melanoxylon), Brazilian rosewood (Dalbergia nigra), and others, owe their value to the type and quantity of extractives in the heartwood. For these species, the sapwood has little or no value, because the desirable properties are imparted by heartwood extractives. Gharu wood, or eagle wood (Aquilaria malaccensis) has been driven to endangered status due to human harvest of the wood to



FIGURE 2.3 Illustration of the three planes of section. Note that for the tangential plane of section, only the right-hand portion of the cut is perpendicular to the rays; due to the curvature of the rings, the left portion of the cut is out of plane. From Biology of Plants, 4/e, by Peter H. Raven, et. al. © 1971, 1976, 1986 Worth Publishers. Used with permission.

extract valuable resins used in perfume making (Lagenheim 2003). Sandalwood (Santalum spicatum), a wood famed for its use in incenses and perfumes, is only valuable if the heartwood is rich with the desired aromatic extractives. The utility of a wood for a technological application can be directly affected by extractives. For example, if a wood high in hydrophobic extractives is used in a composite bonded with a water-based adhesive, weak or incomplete bonding can result.

2.4 AXIAL AND RADIAL SYSTEMS The distinction between sapwood and heartwood, though important, is a gross feature that is often fairly easily observed. More detailed inquiry into the structure of wood shows that wood is composed of discrete cells that are connected and interconnected in an intricate and predictable fashion to form an integrated system that is continuous from root to twig. The cells of wood are typically many times longer than wide, and are specifically oriented in two separate systems of cells: the axial system and the radial system. The cells of the axial system have their long axes running parallel to the long axis of the organ (e.g., up and down the trunk). The cells of the radial system are elongated perpendicularly to the long axis of the organ, and are oriented like radii in a circle or spokes in a bicycle wheel, from the pith to the bark (Figure 2.3). In the trunk of a tree, the axial system runs up and down, functions in long-distance water movement, and provides the bulk of the mechanical strength of the tree. The radial system runs in a pith-to-bark direction, provides lateral transport for biochemicals, and in many cases performs a large fraction of the storage function in wood. These two systems are interpenetrating and interconnected, and their presence is a defining characteristic of wood as a tissue.

2.5 PLANES OF SECTION Though one could cut wood in any direction and then look at it, such an approach would, in the vast majority of cases, result in perspectives that can provide only a small proportion of the information



that could be gleaned if the wood were properly examined. The organization and interrelationship between the axial and radial systems give rise to three main perspectives from which they can be viewed (Figure 2.3). These three perspectives are the transverse plane of section (the cross-section), the radial plane of section, and the tangential plane of section. The latter planes of section are referred to as longitudinal sections, because they extend parallel to the axial system (along the grain). The transverse plane of section is the face that is exposed when a tree is cut down; looking down at the stump one sees the transverse section. Cutting a board across the grain exposes the transverse section. The transverse plane of section provides information about features that vary both in the pith-to-bark direction (called the radial direction) and also those that vary in the circumferential direction (call the tangential direction). It does not provide information about variations up and down the trunk. The radial plane of section runs in a pith-to-bark direction, and it is parallel to the axial system, so it provides information about longitudinal changes in the stem and from the pith to bark along the radial system. To describe it geometrically, it is parallel to the radius of a cylinder, and extending up and down the length of the cylinder. In a practical sense, it is the face or plane that is exposed when a log is split exactly from pith to bark. It does not provide any information about features that vary in a tangential direction. The tangential plane is at a right angle to the radial plane. Geometrically, it is parallel to any tangent line that would touch the cylinder, and it extends along the length of the cylinder. One way in which the tangential plane would be exposed is if the bark were peeled from a log; the exposed face is the tangential plane. The tangential plane of section does not provide any information about features that vary in the radial direction, but it does provide information about the tangential dimensions of features. All three planes of section are important to the proper observation of wood, and only by looking at each in turn can a holistic and accurate understanding of the three-dimensional structure of wood be gained. The three planes of section are determined by the structure of wood, and the way in which the cells in wood are arrayed. The cells are laid down in these special arrangements by a special part of the trunk.

2.6 VASCULAR CAMBIUM The axial and radial systems and their component cells are derived from a special part of the tree called the vascular cambium. The vascular cambium is a thin layer of cells that exists between the inner bark and the wood (Figure 2.1, Figure 2.4A), and is responsible for forming, by means of

FIGURE 2.4 Light microscopic views of the vascular cambium. (A) Transverse section showing wood (w), vascular cambium (vc), inner bark (ib), and outer bark (ob) in Tilia americana. (B) Tangential section through the vascular cambium of Malus sylvestris. Ray initials (r) occur in groups that will give rise to the rays. The vertically oriented cells are fusiform initials, which will give rise to the axial system. Scale bars not available. From Biology of Plants, 4/e, by Peter H. Raven, et. al. © 1971, 1976, 1986 Worth Publishers. Used with permission.



many cell divisions, wood (or secondary xylem) to the inside, and bark (or secondary phloem) to the outside, both of which are vascular conducting tissues (Larson 1994). As the vascular cambium adds cells to the layers of wood and bark around a tree, the girth of the tree increases, and thus the diameter and total surface area of the vascular cambium itself must increase, and this is accomplished by cell division as well. The axial and radial systems are generated in the vascular cambium by two component cells: the fusiform initials and the ray initials (Figure 2.4B). The fusiform initials, named to describe their long, slender shape, give rise to the cells of the axial system, and the ray initials give rise to the radial system. For this reason, there is a direct and continuous link between the most recently formed wood, the vascular cambium, and the inner bark. In most cases, the radial system in the wood is continuous into the inner bark, through the vascular cambium. In this way the wood, a water-conducting tissue, stays connected to the photosynthate-conducting tissue, the inner bark. They are interdependent tissues, because the living cells in wood require photosynthate for respiration and cell growth, and the inner bark requires water in which to dissolve and transport the photosynthate. The vascular cambium is an integral feature that not only gives rise to these tissue systems, but also links them so that they may function in the living tree. In the opening paragraph of this chapter, reference was made to the three functions of wood in the living tree. It is worth reiterating them and their relevance at this point. There is no property of wood, physical, mechanical, chemical, biological, or technological, that is not fundamentally derived from the fact that wood is formed to meet the needs of the living tree. A complementary view is that any anatomical feature of wood can be assessed in the context of the tree’s need for water conduction, mechanical support, and storage of biochemicals. To accomplish any of these functions, wood must have cells that are designed and interconnected in ways suitable to perform these functions.

2.7 GROWTH RINGS Wood is produced by the vascular cambium one layer of cell divisions at a time, but we know from general experience that in many woods there are large cohorts of cells produced more or less together in time, and these cohorts act together to serve the tree. These collections of cells produced together over a discrete time interval are known as growth increments or growth rings. The cells formed at the beginning of the growth increment are called earlywood cells and the cells formed in the latter portion of the growth increment are called latewood cells (Figure 2.5). Springwood and summerwood were terms formerly used to refer to earlywood and latewood, respectively, but their use is anachronistic and not recommended (IAWA Committee1989). In the temperate portions of the world and anywhere else where there is a distinct, regular seasonality, trees form their wood in annual growth increments; that is, all the wood produced in one growing season is organized together into a recognizable, functional entity that many sources refer to as annual rings. Such terminology reflects this temperate bias, so a preferred term is growth increment, or growth ring (IAWA Committee 1989). In many woods in the tropics growth rings are not evident. However, continuing research in this area has uncovered several characteristics whereby growth rings can be correlated with seasonality changes (Worbes 1995, Worbes 1999, Callado et al. 2001). When one looks at woods that form distinct growth rings, and this includes most woods that are likely to be used for wood composites, there are three fundamental patterns within a growth ring: no change in cell pattern across the ring, a gradual reduction of the inner diameter of conducting elements from the earlywood to the latewood, and a sudden and distinct change in the inner diameter of the conducting elements across the ring (Figure 2.6). These patterns appear in both softwoods and hardwoods, but differ in each due to the distinct anatomical structural differences between the two. Many authors use the general term porosity to describe growth rings (recall that vessels and pores are synonymous.)



FIGURE 2.5 Hand-lens views (approximately 14x magnification) of the transverse section showing earlywood and latewood. (A) Distinction in a softwood growth ring between earlywood (ew) and latewood (lw) in Pinus resinosa. (B) Distinction in a hardwood growth ring between earlywood (ew) and latewood (lw) in Quercus rubra.

Nonporous woods (woods without vessels) are softwoods. Softwoods can exhibit any of the three general patterns noted above. Some softwoods such as Western red cedar (Thuja plicata), northern white cedar (Thuja occidentalis), and species of spruce (Picea) and true fir (Abies) have growth increments that undergo a gradual transition from the thin-walled wide-lumined earlywood cells to the thicker-walled, narrower-lumined latewood cells (Figure 2.6B). Other woods undergo an abrupt transition from earlywood to latewood, including Southern yellow pine (Pinus), larch

FIGURE 2.6 Transverse sections of woods showing types of growth rings. Arrows delimit growth rings, when present. (A–C) Softwoods: (A) No transition within the growth ring (growth ring absent) in Podocarpus imbricata. (B) Gradual transition from earlywood to latewood in Picea glauca. (C) Abrupt transition from earlywood to latewood in Pseudotsuga mensiezii. (D–F) Hardwoods: (D) Diffuse porous wood (no transition) in Acer saccharum. (E) Semi-diffuse porous wood (gradual transition) in Diospyros virginiana. (F) Ring porous wood (abrupt transition) in Fraxinus americana. Scale bars = 300 µm.



(Larix), Douglas fir (Pseudotsuga menziesii), bald cypress (Taxodium disticum), and redwood (Sequoia sempervirens) (Figure 2.6C). Since most softwoods are native to the north temperate regions, growth rings are clearly evident. Only in species such as araucaria (Araucaria) and some podocarps (Podocarpus) do you find no transition within the growth ring (Figure 2.6A). Many authors have reported this state as growth rings being absent or only barely evident (Phillips 1948, Kukachka 1960). Porous woods (woods with vessels) are hardwoods, which have two main types of growth rings, and one intermediate form. In diffuse porous woods, the vessels either do not significantly change in size and distribution from the earlywood to the latewood or the change in size and distribution is gradual and no clear distinction between earlywood and latewood can be found (Figure 2.6D). Maple (Acer), birch (Betula), aspen/cottonwood (Populus), and yellow poplar (Liriodendron tulipifera) are examples of diffuse porous species. This pattern is in contrast to ring porous woods in which the transition from earlywood to latewood is abrupt, i.e., the vessels reduce significantly (often by an order or magnitude or more) in diameter and often change their distribution as well. This creates a ring pattern of large, earlywood vessels around the inner portion of the growth increment, alternating with denser, more fibrous tissue in the latewood, as is found in hackberry (Celtis occidentalis), white ash (Fraxinus americana), shagbark hickory (Carya ovata), and northern red oak (Quercus rubra) (Figure 2.6F). Sometimes the vessel size and distribution pattern falls more or less between these two definitions, and this condition is referred to as semi-ring porous (Figure 2.6E). Black walnut (Juglans nigra) and black cherry (Prunus serotina) are temperate-zone semi-ring porous woods. Most tropical hardwoods are diffuse porous except for Spanish cedar (Cedrela) and teak (Tectona grandis), which are generally semi-ring porous. There are no distinctly ring porous species in the tropics and only a very few in the Southern Hemisphere. It is interesting that in genera that span temperate and tropical zones, it is common to have ring porous representatives in the temperate zone and diffuse porous species in the tropics. The oaks (Quercus), ashes (Fraxinus), and hackberries (Celtis) that are native to the tropics are diffuse porous, while their temperate relatives are ring porous. There are numerous detailed texts with more information on growth increments in wood, a few of which are of particular note (Panshin and deZeeuw 1980, Dickison 2000, Carlquist 2001).

2.8 CELLS IN WOOD To understand a growth ring in greater detail, it is essential to begin with an understanding of the structure, function, and variability of the cells that compose the ring. A single plant cell consists of two primary domains: the protoplast and the cell wall. The protoplast is the sum of the living contents that are bounded by the cell membrane. The cell wall is a non-living, largely carbohydrate matrix extruded by the protoplast to the exterior of the cell membrane. The plant cell wall protects the protoplast from osmotic lysis and can provide significant mechanical support to the plant at large (Esau 1977, Raven et al. 1999, Dickison 2000). For cells in wood, the situation is somewhat more complicated than this highly generalized case. In many cases in wood, the ultimate function of the cell is borne solely by the cell wall. This means that many mature wood cells not only do not require their protoplasts, but indeed must completely remove their protoplasts prior to achieving functional maturity. For this reason, it is a common convention in wood literature to refer to a cell wall without a protoplast as a cell. Although this is technically incorrect from a cell biological standpoint, it is a convention common in the literature and will be observed throughout the remainder of the chapter. In the case of a mature cell in wood in which there is no protoplast, the open portion of the cell where the protoplast would have existed is known as the lumen. Thus, in most cells in wood there are two domains: the cell wall and the cell lumen (Figure 2.7). The lumen is a critical component of many cells, whether in the context of the amount of space available for water conduction or in



FIGURE 2.7 Transverse sections of wood showing cell walls and lumina. (A) Softwood: All the rectangular cells are of the same type, some with thicker cell walls and narrower lumina, and others with thinner walls and wider lumina in Pseudotsuga mensiezii. (B) Hardwood: The large round cells have thick cell walls and very large lumina. Other cells have thinner walls and narrower lumina in Quercus rubra. Scale bars = 50 µm.

the context of a ratio between the width of the lumen and the thickness of the cell wall. The lumen has no structure per se, as it is really the void space in the interior of the cell. The relevance of the lumen to the formation of wood composites is the subject of Chapter 15.

2.9 CELL WALLS The cell walls in wood are important structures. Unlike the lumen, which is a void space, the cell wall itself is a highly regular structure, from one cell type to another, between species, and even when comparing softwoods and hardwoods. The cell wall consists of three main regions: the middle lamella, the primary wall, and the secondary wall (Figure 2.8). In each region, the cell wall has three major components: cellulose microfibrils (with characteristic distributions and organization), hemicelluloses, and a matrix or encrusting material, typically pectin in primary walls and lignin in secondary walls (Panshin and deZeeuw 1980). To understand these wall layers and their interrelationships, it is necessary to remember that plant cells generally do not exist singly in nature; instead they are adjacent to many other cells, and this association of thousands of cells, taken together, forms an organ such as a leaf. Each of the individual cells must adhere to others in a coherent way to ensure that the cells can act as a unified whole. This means that they must be interconnected with one another to permit the movement of biochemicals (e.g., photosynthate, hormones, cell signaling agents, etc.) and water. This adhesion is provided by the middle lamella, the layer of cell wall material between two or more cells, a part of which is contributed by each of the individual cells (Figure 2.8). This layer is the outermost layer of the cell wall continuum, and in a non-woody organ is pectin rich. In the case of wood, the middle lamella is lignified. The next layer, formed by the protoplast just interior to the middle lamella, is the primary wall (Figure 2.8). The primary wall is characterized by a largely random orientation of cellulose microfibrils, like thin threads wound round and round a balloon in no particular order, where any microfibril angle from 0 to 90 degrees relative to the long axis of the cell may be present. In cells in wood, the primary wall is very thin, and is generally indistinguishable from the middle lamella. For this reason, the term compound middle lamella is used to denote the primary cell wall of a cell, the middle lamella, and the primary cell wall of the adjacent cell. Even with transmission electron microscopy, the compound middle lamella often cannot be separated unequivocally into its component layers. The compound middle lamella in wood is almost invariably lignified. The remaining cell wall domain, found in virtually all cells in wood (and in many cells in nonwoody plants or plant parts) is the secondary cell wall. The secondary cell wall is composed of three layers (Figure 2.8). As the protoplast lays down the cell wall layers, it progressively reduces the lumen volume. The first-formed secondary cell wall layer is the S1 layer (Figure 2.8), which is adjacent to compound middle lamella (or technically the primary wall). This layer is a thin layer © 2005 by CRC Press


FIGURE 2.8 A cut-away drawing of the cell wall including the structural details of a bordered pit. The various layers of the cell wall are detailed at the top of the drawing, beginning with the middle lamella (ML). The next layer is the primary wall (P), and on the surface of this layer the random orientation of the cellulose microfibrils is detailed. Interior to the primary wall is the secondary wall in its three layers: S1, S2, and S3. The microfibril angle of each layer is illustrated, as well as the relative thickness of the layers. The lower portion of the illustration shows bordered pits in both sectional and face view. The four domains of the pit are illustrated: the pit aperture (pa), the pit chamber (pc), the pit membrane (pm) and the border (b).

and is characterized by a large microfibril angle. That is to say, the cellulose microfibrils are laid down in a helical fashion, and the angle between the mean microfibril direction and the long axis of the cell is large: 50 to 70 degrees. The next wall layer is arguably the most important in determining the properties of the cell and, thus, the wood properties at a macroscopic level (Panshin and deZeeuw 1980). This layer, formed interior to the S1 layer, is the S2 layer (Figure 2.8). This is the thickest secondary cell wall layer and it makes the greatest contribution to the overall properties of the cell wall. It is characterized by a lower lignin percentage and a low microfibril angle: 5 to 30 degrees. There is a strong but not fully understood relationship between the microfibril angle of the S2 layer of the wall and the wood properties at a macroscopic level (Kretschmann et al. 1998), and this is an area of active research. Interior to the S2 layer is the S3 layer, a relatively thin wall layer (Figure 2.8). The microfibril angle of this layer is relatively high and similar to that of S1: 70+ degrees. This layer has the lowest percentage of lignin of any of the secondary wall layers. The explanation of this phenomenon is related directly to the physiology of the living tree. In brief, for water to move up the plant (transpiration), there must be a significant adhesion between the water molecules and the cell walls of the water conduits. Lignin is a hydrophobic macromolecule, so it must be in low concentration in the S3 layer to permit adhesion of water to the cell wall and thus facilitate transpiration. For more detail regarding these wall components and information regarding transpiration and the role of the cell wall, see Chapter 3 or any college-level plant physiology textbook (Kozlowski and Pallardy 1997, Taiz and Zeiger 1991). © 2005 by CRC Press


2.10 PITS Any discussion of cell walls in wood must be accompanied by a discussion of the ways in which cell walls are modified to allow communication between the cells in the living plant. These wall modifications, called pit-pairs (or more commonly just pits), are thin areas in the cell walls between two cells, and are a critical aspect of wood structure too often overlooked in wood technological treatments (Figure 2.8). Pits have three domains: the pit membrane, the pit aperture, and the pit chamber. The pit membrane (Figure 2.8) is the thin semiporous remnant of the primary wall; it is a carbohydrate and not a phospholipid membrane. The pit aperture is the opening or hole leading into the open area of the pit, which is called the pit chamber (Figure 2.8). The type, number, size, and relative proportion of pits can be characteristic of certain types of wood, and furthermore can directly affect how wood behaves in a variety of situations, such as how wood interacts with surface coatings (DeMeijer et al. 1998, Rijkaert et al. 2001). Pits of predictable types occur between different types of cells. In the cell walls of two adjacent cells, pits will form in the wall of each cell separately, but in a coordinated location so that the pitting of one cell will match up with the pitting of the adjacent cell (thus a pit-pair). When this coordination is lacking and a pit is formed only in one of the two cells, it is called a blind pit. Blind pits are fairly rare in wood. Understanding the type of pit can permit one to determine what type of cell is being examined in the absence of other information. It can also allow one to make a prediction about how the cell might behave, particularly in contexts that involve fluid flow. Pits occur in three varieties: bordered, simple, and half-bordered (Esau 1977, Raven et al. 1999). Bordered pits are thus called because the secondary wall overarches the pit chamber and the aperture is generally smaller and/or of a different shape than the pit chamber. The portion of the cell wall that is overarching the pit chamber is called the border (Figure 2.8, Figure 2.9A, and Figure 2.9D). When seen in face view, bordered pits often are round in appearance and look somewhat like a doughnut, with a small round or almond-shaped hole, the pit aperture, in the middle of the pit (Figure 2.9). When seen in sectional view, the pit often looks like a pair of V’s with the open ends of the V’s facing each other (Figure 2.9A and Figure 2.9D). In this case, the long stems of the V represent the borders, the secondary walls that are overarching the pit chamber. Bordered pits always occur between two conducting cells, and sometimes between other cells,

FIGURE 2.9 Light micrographs and sketches of the three types of pits. (A) Longitudinal section of bordered pits in Xanthocyparis vietnamensis. The pits look like a vertical stack of thick-walled letter V’s. (B) Halfbordered pits in Pseudotsuga mensiezii. The arrow shows one half-bordered pit. (C) Simple pits on an end wall in Pseudotsuga mensiezii. The arrow indicates one of five simple pits on the end wall. Scale bars = 20 µm. (D–F) Sketches of the pits shown in A–C.



typically those with thick cell walls. The structure and function of bordered pits, particularly those in softwoods (see next section), are much studied and known to be well suited to the safe and efficient conduction of sap. The status of the bordered pit (whether it is open or closed) has great importance in the fields of wood preservation, wood finishing, and bonding. Simple pits are so called because they lack any sort of border (Figure 2.9C and Figure 2.9F). The pit chamber is straight-walled, and the pits are uniform in size and shape in each of the partner cells. Simple pits are typical between parenchyma cells, and in face view merely look like clear areas in the walls. Half-bordered pits occur between a conducting cell and a parenchyma cell. In this case, each cell forms the kind of pit that would be typical of its type (bordered in the case of a conducting cell and simple in the case of a parenchyma cell) and thus one half of the pit pair is simple and one half is bordered (Figure 2.9B and Figure 2.9E). In the living tree, these pits are of great importance, as they represent the communication link between the conducting cells and the biochemically active cells.

2.11 THE MICROSCOPIC STRUCTURE OF SOFTWOODS AND HARDWOODS As is no doubt clear by now, the fundamental differences between different kinds of woods are founded on the types, sizes, proportions, pits, and arrangements of different cells that compose the wood. Softwoods have a simpler basic structure than do hardwoods due to the presence of only two cell types, and relatively little variation in structure within these cell types. Hardwoods have greater structural complexity because they have both a greater number of basic cell types and a far greater degree of variability within the cell types. In each case, however, there are fine details of structure that could affect the use of a wood, and elucidating these details is the subject of the next portion of this chapter.

2.11.1 SOFTWOODS The structure of a typical softwood is relatively simple. The axial or vertical system is composed mostly of axial tracheids and the radial or horizontal system, the rays, are composed mostly of ray parenchyma cells. 2.11.1.1 Tracheids Tracheids are very long cells, often more than 100 times longer than wide, and they are the major component of softwoods, making up over 90% of the volume of the wood. They provide both conductive and mechanical functions to softwoods. In the transverse view or section (Figure 2.10A), tracheids appear as square or slightly rectangular cells in radial rows. Within one growth ring they can be thin-walled in the earlywood and thicker-walled in the latewood. In order for water to flow between tracheids it must pass through circular bordered pits that are concentrated in the long, tapered ends of the cells. Tracheids overlap with adjacent cells across both the top and bottom 20–30% of their length. Water flow thus must take a slightly zigzag path as it goes from one cell to the next through the pits. Because the pits have a pit membrane, there is a significant resistance to flow. The resistance of the pit membrane coupled with the narrow diameter of the lumina makes tracheids relatively inefficient conduits, compared to the conducting cells of hardwoods. Detailed treatments of the structure of wood in relation to its conductive functions can be found in the literature (Zimmermann 1983, Kozlowski and Pallardy 1997). 2.11.1.2 Axial Parenchyma and Resin Canal Complexes Another cell type that is sometimes present is axial parenchyma. Axial parenchyma cells are similar in size and shape to ray parenchyma cells, but they are vertically oriented and stacked one on top © 2005 by CRC Press


FIGURE 2.10 The microscopic structure of Picea glauca, a typical softwood. (A) Transverse section, scale bar = 150 µm. The bulk of the wood is made of tracheids, the small rectangles of various thicknesses. The three large, round structures are resin canals and their associated cells. The dark lines running from the top to the bottom of the photo are the ray cells of the rays. (B) Radial section showing two rays (arrows) running from left to right. Each cell in the ray is a ray cell, and they are low, rectangular cells. The rays begin on the left in the earlywood (thin-walled tracheids) and continue into and through the latewood (thick-walled tracheids), and into the next growth ring, on the right side of the photo. Scale bar = 200 µm. (C) Tangential section. Rays seen in end-view; they are mostly only one cell wide. Two rays are fusiform rays; there are radial resin canals embedded in the rays, causing them to bulge. Scale bar = 200 µm.

of the other to form a parenchyma strand. In transverse section (Figure 2.11A) they often look like an axial tracheid, but can be differentiated when they contain dark-colored organic substances in the lumen of the cell. In the radial or tangential section (Figure 2.11B) they appear as long strands of cells generally containing dark-colored substances. Axial parenchyma is most common in redwood, juniper, cypress, bald cypress, and some species of Podocarpus, but never makes up even 1% of the cells. Axial parenchyma is generally absent in pine, spruce, larch, hemlock, and species of Araucaria and Agathis. In species of pine, spruce, Douglas fir, and larch structures commonly called resin ducts or resin canals are present vertically (Figure 2.12) and horizontally (Figure 2.12C). These structures are voids or spaces in the wood and are not cells. However, specialized parenchyma cells that function in resin production surround resin canals. When referring to the resin canal and all the associated parenchyma cells, the correct term is axial or radial resin canal (Wiedenhoeft and Miller 2002). In pine, resin canal complexes are often visible on the transverse section to the naked eye, but they are much smaller in spruce, larch, and Douglas fir, and a hand lens is needed to see them. Radial resin canal complexes are embedded in specialized rays called fusiform rays (Figure 2.10C and Figure 2.12C). These rays are much higher and wider than normal rays. Resin canal complexes are absent in the normal wood of other softwoods, but some species can form large tangential clusters of traumatic resin canals in response to significant injury. © 2005 by CRC Press


FIGURE 2.11 Axial parenchyma in Podocarpus madagascarensis. (A) Transverse section showing individual axial parenchyma cells. They are the dark-staining rectangular cells. Two are denoted by arrows, but many more can be seen. (B) Radial section showing axial parenchyma in longitundinal view. The parenchyma cells can be differentiated from the tracheids by the presence of end walls (arrows) in addition to the dark-staining contents. Scale bars = 100 µm.

2.11.1.3 Rays The other cells in Figure 2.10A are ray parenchyma cells that are barely visible and appear as dark lines running in a top-to-bottom direction. Ray parenchyma cells are rectangular prisms or brickshaped cells. Typically they are approximately 15 µm high by 10 µm wide by 150–250 µm long in the radial or horizontal direction (Figure 2.10B). These brick-like cells form the rays, which function primarily in the synthesis, storage, and lateral transport of biochemicals and, to a lesser degree, water. In radial view or section (Figure 2.10B), the rays look like a brick wall and the ray parenchyma cells are sometimes filled with dark-colored substances. In tangential section (Figure 2.10C), the rays are stacks of ray parenchyma cells one on top of the other forming a ray that is only one cell in width, called a uniseriate ray. When ray parenchyma cells intersect with axial tracheids, specialized pits are formed to connect the vertical and radial systems. The area of contact between the tracheid wall and the wall of the ray parenchyma cells is called a cross-field. The type, shape, size, and number of pits in the crossfield is generally consistent within a species and very diagnostic. Figure 2.13 illustrates several types of cross-field pitting. Species that have resin canal complexes also have ray tracheids, which are specialized horizontal tracheids that normally are situated at the margins of the rays. These ray tracheids have bordered pits like vertical tracheids, but are much shorter and narrower. Ray tracheids also occur in a few

FIGURE 2.12 Resin canal complexes in Pseudotsuga mensiezii. (A) Transverse section showing a single axial resin canal complex. In this view the tangential and radial diameters of the canal can be measured accurately. (B) Radial section showing an axial resin canal complex embedded in the latewood. It is crossed by a ray that also extends into the earlywood on either side of the latewood. (C) Tangential section showing the anastomosis between an axial and a radial resin canal complex. The fusiform ray bearing the radial resin canal complex is in contact with the axial resin canal complex. Scale bars = 100 µm.



FIGURE 2.13 Radial sections showing a variety of types of cross-field pitting. All the pits are half-bordered pits, but in some cases the borders are difficult to see. (A) Fenestriform pitting in Pinus strobus. (B) Pinoid pitting in Pinus elliottii. (C) Piceoid pitting in Pseudotsuga mensiezii. (D) Cuppressoid pitting in Juniperus virginiana. (E) Taxodioid pitting in Abies concolor. (F) Araucarioid pitting in Araucaria angustifolia. Scale bars = 30 µm.

species that do not have resin canals. Alaska yellow cedar, (Chamaecyparis nootkatensis), hemlock (Tsuga), and rarely some species of true fir (Abies) have ray tracheids. Additional detail regarding the microscopic structure of softwoods can be found in the literature (Phillips 1948, Kukachka 1960, Panshin and deZeeuw 1980, IAWA Committee 2004).

2.11.2 HARDWOODS The structure of a typical hardwood is much more complicated than that of a softwood. The axial or vertical system is composed of fibrous elements of various kinds, vessel elements in various sizes and arrangements, and axial parenchyma cells in various patterns and abundance. Like softwoods, the radial or horizontal system are the rays, which are composed of ray parenchyma cells, but unlike softwoods, hardwood rays are much more diverse in size and shape. 2.11.2.1 Vessels The unique feature that separates hardwoods from softwoods is the presence of specialized conducting cells in hardwoods called vessels elements (Figure 2.14A). These cells are stacked one on top of the other to form vessels. Where the ends of the vessel elements come in contact with one another, a hole is formed, called a perforation plate. Thus hardwoods have perforated tracheary elements (vessel elements) for water conduction, whereas softwoods have imperforate tracheary elements (tracheids). On the transverse section, vessels appear as large openings and are often referred to as pores (Figure 2.2D). Vessel diameters may be quite small (300 µm), but typically range from 50–200 µm. Their length is much shorter than tracheids and range from 100–1200 µm or



FIGURE 2.14 Vessel elements and vessel features. (A) Macerated cells of Quercus rubra. There are three types of cells labeled. There is a single vessel element (ve); note that it is wider than it is tall, and it is open on both ends. The fiber (f ) is long, narrow, and thick-walled. The hardwood tracheid (t) is shorter than a fiber but longer than a vessel element, and it is contorted in shape. Scale bar = 200 µm. (B) A simple perforation plate in Malouetia virescens. There are two vessel elements (ve), and where they overlap there is an open hole between the cells, the perforation plate (arrow). As the perforation is completely open, it is called a simple perforation plate. (C) A scalariform perforation plate in Magnolia grandiflora. This perforation plate has eight bars crossing it (the eighth is very small), and it is the presence of bars that distinguishes this type of perforation plate from a simple plate. (D) Alternate intervessel pitting in Hevea microphylla. (E) Opposite intervessel pitting in Liriodendron tulipifera. (F) Linear (scalariform) intervessel pitting in Magnolia grandiflora. Note that these intervessel pits are not the same structures as the scalariform perforation plate seen in C. Scale bars in B–F = 30 µm.

0.1–1.2 mm. Vessels are arranged in various patterns. If all the vessels are the same size and more or less scattered throughout the growth ring, the wood is diffuse porous (Figure 2.6D). If the earlywood vessels are much larger than the latewood vessels, the wood is ring porous (Figure 2.6F). Vessels can also be arranged in a tangential or oblique arrangement, in a radial arrangement, in clusters, or in many combinations of these types (IAWA Committee 1989). In addition, the individual vessels may occur alone (solitary arrangement) or in pairs or radial multiples of up to five or more vessels in a row. At the end of the vessel element is a hole or perforation plate. If there are no obstructions across the perforation plate, it is called a simple perforation plate (Figure 2.14B). If bars are present, the perforation plate is called a scalariform perforation plate (Figure 2.14C). Where the vessels elements come in contact with each other tangentially, intervessel or intervascular bordered pits are formed (Figure 2.14D, Figure 2.14E, and Figure 2.14F). These pits range in size from 2–16 µm in height and are arranged on the vessels walls in threes basic ways. The most common arrangement is alternate, in which the pits are more or less staggered (Figure 2.14D). In the opposite arrangement the pits are opposite each other (Figure 2.14E), and in the scalariform arrangement the pits are much wider than high (Figure 2.14F). Combinations of these can also be observed in some species. Where vessel elements come in contact with ray cells, often simple or bordered pits called vessel-ray pits are formed. These pits can be the same size and shape and the intervessel pits or much larger. 2.11.2.2 Fibers Fibers in hardwoods function solely as support. They are shorter than softwood tracheids (200– 1200 µm) and average about half the width of softwood tracheids, but are usually 2–10 times longer than vessel elements (Figure 2.15). The thickness of the fiber cell wall is the major factor governing



FIGURE 2.15 Fibers in Quercus rubra. (A) Transverse section showing thick-walled, narrow-lumined fibers. A ray is passing vertically through the photo, and there are nine axial parenchyma cells, the thin-walled, widelumined cells, in the photo. Scale bar = 30 µm. (B) Macerated wood. There are several fibers (f ), two of which are marked. Also easily observed are parenchyma cells (arrows) both individually and in small groups. Note the thin walls and small rectangular shape compared to the fibers. Scale bar = 300 µm.

density and strength. Species with thin-walled fibers such as cottonwood (Populus deltoides), basswood (Tilia americana), ceiba (Ceiba pentandra), and balsa (Ochroma pyramidale) have a low density and strength, whereas species with thick-walled fibers such as hard maple (Acer saccharum and Acer nigrum), black locust (Robinia pseudoacacia), ipe (Tabebuia serratifolia), and bulletwood (Manilkara bidentata) have a high density and strength. The air-dry (12% moisture content) density of hardwoods varies from 100–1400 kg/m3. The air-dry density of typical softwoods varies from 300–800 kg/m3. Fiber pits are generally inconspicuous and may be simple or bordered. In some woods like oak (Quercus) and meranti/lauan (Shorea), vascular or vasicentric tracheids are present especially near or surrounding the vessels (Figure 2.14A). These specialized fibrous elements in hardwoods typically have bordered pits, are thin-walled, and are shorter than the fibers of the species. The tracheids in hardwoods function in both support and transport. 2.11.2.3 Axial Parenchyma In softwoods, axial parenchyma is absent or only occasionally present as scattered cells, but in hardwoods there is a wide variety of axial parenchyma patterns (Figure 2.16). The axial parenchyma cells in hardwoods and softwoods is essentially the same size and shape, and they also function in the same manner. The difference comes in the abundance and specific patterns in hardwoods. There are two major types of axial parenchyma in hardwoods. Paratracheal parenchyma is associated with the vessels and apotracheal is not associated with the vessels. Paratracheal parenchyma is further divided into vasicentric (surrounding the vessels, Figure 2.16A), aliform (surrounding the vessel and with wing-like extensions, Figure 2.16C), and confluent (several connecting patches of paratracheal parenchyma sometimes forming a band, Figure 2.16E). Apotracheal parenchyma is also divided into diffuse (scattered), diffuse in aggregate (short bands, Figure 2.16B), and banded whether at the beginning or end of the growth ring (marginal, Figure 2.16F) or within a growth ring (Figure 2.16D). Each species has a particular pattern of axial parenchyma, which is more or less consistent from specimen to specimen, and these cell patterns are very important in wood identification. 2.11.2.4 Rays The rays in hardwoods are much more diverse than those found in softwood. In some species, including willow (Salix), cottonwood, and koa (Acacia koa), the rays are exclusively uniseriate and are much like the softwood rays. In hardwoods most species have rays that are more than one cell wide. In oak and hard maple the rays are two-sized, uniseriate and over eight cells wide, and in oak several centimeters high (Figure 2.17A). In most species the rays are 1–5 cells wide and less than 1 mm high (Figure 2.17B) Rays in hardwoods are composed of ray parenchyma cells that are either procumbent or upright. As the name implies, procumbent ray cells are horizontal and are



FIGURE 2.16 Transverse sections of various woods showing a range of hardwood axial parenchyma patterns. A, C, and E show woods with paratracheal types of parenchyma. (A) Vasicentric parenchyma (arrow) in Licaria excelsa. (C) Aliform parenchyma in Afzelia africana. The parenchyma cells are the light-colored, thinwalled cells, and are easily visible. (E) Confluent parenchyma in Afzelia cuazensis. B, D, and F show woods with apotracheal types of parenchyma. (B) Diffuse-in-aggregate parenchyma in Dalbergia stevensonii. (D) Banded parenchyma in Micropholis guyanensis. (F) Marginal parenchyma in Juglans nigra. In this case, the parenchyma cells are darker in color, and they delimit the growth rings (arrows). Scale bars = 300 µm.

similar in shape and size to the softwood ray parenchyma cells (Figure 2.17C). The upright ray cells are ray parenchyma cells turned on end so that their long axis is vertical (Figure 2.17D). Upright ray cells are generally shorter and sometimes nearly square. Rays that have only one type of ray cell, typically only procumbent cells, are called homocellular rays. Those that have procumbent and upright cells are called heterocellular rays. The number of rows of upright ray cells varies from one to more than five. The great diversity of hardwood anatomy is treated in many sources throughout the literature (Metcalfe and Chalk 1950, Metcalfe and Chalk 1979, Panshin and deZeeuw 1980, Metcalfe and Chalk 1987, IAWA Committee 1989, Gregory 1994, Cutler and Gregory 1998, Dickison 2000, Carlquist 2001).

2.12 WOOD TECHNOLOGY Though it is necessary to speak briefly of each kind of cell in isolation, the beauty and complexity of wood are found in the interrelationship between many cells at a much larger scale. The macroscopic properties of wood such as density, hardness, and bending strength, among others, are properties derived from the cells that compose wood. Such larger-scale properties are really the product of a synergy in which the whole is indeed greater than the sum of its parts, but are nonetheless based on chemical and anatomical details of wood structure (Panshin and deZeeuw 1980). The cell wall is largely made up of cellulose and hemicellulose, and the hydroxyl groups on these chemicals make the cell wall very hygroscopic. Lignin, the agent cementing cells together,



FIGURE 2.17 Rays in longitudinal sections. A and B show tangential sections, scale bars = 300 µm. (A) Quercus rubra showing very wide multiseriate ray (arrow) and many uniseriate rays. (B) Swietenia macrophylla showing numerous rays ranging from one to four cells wide. Note that in this wood the rays are arranged roughly in rows from side to side. C and D show radial sections, scale bars = 200 µm. (C) Homocellular ray in Fraxinus americana. All the cells in the ray are procumbent cells; they are longer radially than they are tall. (D) A heterocellular ray in Khaya ivorensis. The central portion of the ray is composed of procumbent cells, but the margins of the ray, both top and bottom, have two rows of upright cells (arrows), which are as tall as or taller than they are wide.

is a generally hydrophobic molecule. This means that the cell walls in wood have a great affinity for water, but the ability of the walls to take up water is limited, in part by the presence of lignin. Water in wood has a great effect on wood properties, and wood-water interactions greatly affect the industrial use of wood in wood products. Often it is useful to know how much water is contained in a tree or a piece of wood. This relationship is called moisture content and is the weight of water in the cell walls and lumina expressed as a percentage of the weight of wood with no water (oven-dry weight). Water exists in wood in two forms: free water and bound water. Free water is the liquid water that exists within the lumina of the cells. Bound water is the water that is adsorbed to the cellulose and hemicellulose in the cell wall. Free water is only found when all sites for the adsorption of water in the cell wall are filled; this point is called the fiber saturation point (FSP). All water added to wood after the FSP has been reached exists as free water. Wood of a freshly cut tree is said to be green; the moisture content of green wood can be over 100%, meaning that the weight of water in the wood is more than the weight of the dried cells. In softwoods the moisture content of the sapwood is much higher than that of the heartwood, but in hardwoods, the difference may not be as great and in a few cases the heartwood has a higher moisture content than the sapwood. When drying from the green condition to the FSP (approximately 25–30% moisture content), only free water is lost, and thus no change in the cell wall volumes occurs. However, when the wood is dried further, bound water is removed from the cell walls and shrinkage of the wood begins.



Some of the shrinkage that occurs from green to dry is irreversible; no amount of rewetting can swell the wood back to its original dimensions. After this process of irreversible shrinkage has occurred, however, shrinkage and swelling is reversible and essentially linear from 0% moisture content up to the FSP. Controlling the rate at which bound water is removed from green wood is the subject of entire fields of research. By properly controlling the rate at which wood dries, drying defects such as cracking, checking, honeycombing, and collapse can be minimized (Hillis 1996). Density or specific gravity is one of the most important physical properties of wood (Desch and Dinwoodie 1996, Forest Products Laboratory 1999, Bowyer et al. 2003). Density is the weight of wood divided by the volume at a given moisture content. Thus the units for density are typically expressed as pounds per cubic foot (lbs/ft3) or kilograms per cubic meter (kg/m3). When density values are reported in the literature it is critical that the moisture content of the wood is also given. Often density values are listed as air-dry, which means 12% moisture content in North America and Europe, but air-dry sometimes means 15% moisture content in tropical countries. Specific gravity is similar to density and is defined as the ratio of the density of wood to the density of water. Since 1 cm3 of water weighs 1 g, density in g/cm3 is numerically the same as specific gravity. Density in kg/m3 must be divided by 1000 to get the same numerical number as specific gravity. Since specific gravity is a ratio, it does not have units. The term basic specific gravity (sometimes referred to as basic density) is defined as the oven-dry weight of wood divided by the volume of the wood when green (no shrinkage). Basic specific gravity =

Density of wood (oven-dry weight /volume when green) Density of water

Specific gravity can be determined at any moisture content, but typically it is based on weight when oven-dry and when the volume is green or at 12% moisture content (Forest Products Laboratory 1999). However, basic specific gravity is generally the standard used throughout the world. The most important reason for measuring basic specific gravity is repeatability. The weight of wood can be determined at any moisture content, but conditioning the wood to a given moisture content consistently is difficult. The oven-dry weight (at 0% moisture content) is relatively easy to obtain on a consistent basis. Green volume is also relatively easy to determine using the water displacement method (ref). The sample can be large or small and nearly any shape. Thus basic specific gravity can be determined as follows: Basic specific gravity =

Oven-dry weight Weight of displaced water

Specific gravity and density are strongly dependent on the weight of the cell wall material in the bulk volume of the wood specimen. In softwoods where the latewood is abundant (Figure 2.5A) in proportion to the earlywood, the specific gravity is high (e.g., 0.54 in longleaf pine, Pinus palustris). The reverse is true when there is much more earlywood than latewood (Figure 2.6B) (e.g., 0.34 in eastern white pine, Pinus strobus). To say it another way, specific gravity increases as the proportion of cells with thick cell walls increases. In hardwoods specific gravity is not only dependent on fiber wall thickness, but also on the amount of void space occupied by the vessels and parenchyma. In balsa the vessels are large (typically >250 µm in tangential diameter), and there is an abundance of axial and ray parenchyma. The fibers that are present are very-thin-walled and the basic specific gravity may be less than 0.20. In very dense woods the fibers are very-thickwalled, the lumina are virtually absent, and the fibers are abundant in relation to the vessels and parenchyma. Some tropical hardwoods have a basic specific gravity of greater than 1.0. In a general sense in all woods, the specific gravity is the relation between the volume of cell wall material to the volume of the lumina of those cells in a given bulk volume.



2.13 JUVENILE WOOD AND REACTION WOOD Two key examples of the biology of the tree affecting the quality of the wood can be seen in the formation of juvenile wood and reaction wood. They are grouped together because they share several common cellular, chemical, and tree physiological characteristics, and each may or may not be present in a certain piece of wood. Juvenile wood is the first-formed wood of the young tree, the rings closest to the pith. If one looks at the growth form of a tree, based on the derivation of wood from the vascular cambium, it quickly becomes evident that the layers of wood in a tree are concentric cones. In a tree of large diameter, the deflection of the long edge of the cone from vertical may be very close to zero, but in narrower-diameter trees, or narrower-diameter portions of a large tree, the angle of deflection is considerably greater. These areas of narrower diameter are typically chronologically younger portions of the tree, for example, the first 15–20 years of growth in softwoods are the areas where juvenile wood may form. Juvenile wood in softwoods is in part characterized by the production of axial tracheids that have a higher microfibril angle in the S2 wall layer (Larson et al. 2001). A higher microfibril angle in the S2 is correlated with drastic longitudinal shrinkage of the cells when the wood is dried for human use, resulting in a piece of wood that has a tendency to warp, cup, and check. The morphology of the cells themselves is often altered so that the cells, instead of being long and straight, are shorter and angled, twisted, or bent. The precise functions of juvenile wood in the living tree are not fully understood, but it must confer certain little-understood advantages. Reaction wood is similar to juvenile wood in several respects, but is formed by the tree for different reasons. Almost any tree of any age will form reaction wood when the woody organ

FIGURE 2.18 Macroscopic and microscopic views of reaction wood in a softwood and a hardwood. (A) Compression wood in Pinus sp. Note that the pith is not in the center of the trunk, and the growth rings are much wider in the compression wood zone. (B) Tension wood in Juglans nigra. The is nearly centered in the trunk, but the growth rings are wider in the tension wood zone. (C) Transverse section of compression wood in Picea engelmannii. The tracheids are thick-walled and round in outline, giving rise to prominent intercellular spaces in the cell corners (arrow). (D) Tension wood fibers (between the brackets) showing prominent gelatinous layers in Hevea microphylla. Rays run from top to bottom on either side of the tension wood fibers, and below them is a band of normal fibers with thinner walls. Scale bars (in C and D) = 50 µm.



(whether a twig, a branch, or the trunk) is deflected from the vertical by more than one or two degrees. This means that all nonvertical branches form considerable quantities of reaction wood. The type of reaction wood formed by a tree differs in softwoods and hardwoods. In softwoods, the reaction wood is formed on the underside of the leaning organ, and is called compression wood (Figure 2.18A) (Timmel 1986). In hardwoods, the reaction wood forms on the top side of the leaning organ, and is called tension wood (Figure 2.18B) (Desch and Dinwoodie 1996, Bowyer et al. 2003). As just mentioned, the various features of juvenile wood and reaction wood are similar. In compression wood, the tracheids are shorter, misshapen cells with a large S2 microfibril angle, a high degree of longitudinal shrinkage, and high lignin content (Timmel 1986). They also take on a distinctly rounded outline (Figure 2.18C). In tension wood, the fibers fail to form a proper secondary wall and instead form a highly cellulosic wall layer called the G layer, or gelatinous layer (Figure 2.18D).

2.14 WOOD IDENTIFICATION The identification of wood can be of critical importance to primary and secondary industrial users of wood, government agencies, and museums, as well as to scientists in the fields of botany, ecology, anthropology, forestry, and wood technology. Wood identification is the recognition of characteristic cell patterns and wood features, and is generally accurate only to the generic level. Since woods of different species from the same genus often have different properties and perform differently under various conditions, serious problems can develop if species or genera are mixed during the manufacturing process and in use. Since foreign woods are imported into the U.S. market, it is imperative that both buyers and sellers have access to correct identifications and information about their properties and uses. Lumber graders, furniture workers, and those working in the industry, as well as hobbyists, often identify wood with their naked eye. Features often used are color, odor, grain patterns, density, and hardness. With experience these features can be used to identify many different woods, but the accuracy of the identification is dependent on the experience of the person and the quality of the unknown wood. If the unknown wood is atypical, decayed, or small, often the identification is incorrect. Examining woods, especially hardwoods, with a 10–20X hand lens greatly improves the accuracy of the identification (Panshin and deZeeuw 1980, Hoadley 1990, Brunner et al. 1994). Foresters and wood technologists armed with a hand lens and sharp knife can accurately identify lumber in the field. They make a cut on the transverse surface and examine the vessel and parenchyma patterns to make an identification. Scientifically rigorous accurate identifications require that the wood be sectioned and examined with a light microscope. With the light microscope even with only a 10X objective, many more features are available for use in making the determination. Equally as important as the light microscope in wood identification is the reference collection of correctly identified specimens to which unknown samples can be compared (Wheeler and Baas 1998). If a reference collection is not available, books of photomicrographs or books or journal articles with anatomical descriptions and dichotomous keys can be used (Miles 1978, Schweingruber 1978, Core et al. 1979, Gregory 1980, Ilic 1991, Miller and Détienne 2001). In addition to these resources, several computer-assisted wood identification packages are available and are suitable for people with a robust wood anatomical background. Wood identification by means of molecular biological techniques is a field that is still in its infancy. Though technically feasible, there are significant population-biological limits to the statistical likelihood of a robust and certain identification for routine work (Canadian Forest Service 1999). In highly limited cases of great financial or criminal import and a narrowly defined context, the cost and labor associated with rigorous evaluation of DNA from wood can be warranted (Hipkins 2001). For example, if the question were, “Did this piece of wood come from this individual tree?” or, “Of the 15 species present in this limited geographical area, which one



produced this root?” it is feasible to analyze the specimens with molecular techniques (Brunner et al. 2001). If, however, the question were, “What kind of wood is this, and from which forest did it come?” it would not be feasible at this point in time to analyze the specimen. Workers have shown that specific identification among six species of Japanese white oak can be accomplished using DNA (Ohyama et al. 2001), but the broad application of their methods is not likely for some time. As technological advances improve the quality, quantity, and speed with which molecular data can be collected, the difficulty and cost of molecular wood identification will decrease. At some point in the indefinite future it is reasonable to expect that molecular tools will be employed in the routine identification of wood, and that such techniques will greatly increase the specificity and accuracy of identification, but until that day comes, scientific wood identifications will rely on microscopic evaluation of wood anatomical features.

REFERENCES Bowyer, J., Shmulsky, R., and Haygreen, J.G. (2003). Forest Products and Wood Science: An Introduction (4th ed.). Iowa State University Press, Des Moines. Brunner, I., Brodbeck, S., Buchler, U., and Sperisen, C. (2001). Molecular identification of fine roots from trees from the Alps: Reliable and fast DNA extraction and PCR-RFLP analyses of plastid DNA. Mol. Ecol. 10:2079–2087. Brunner, M., Kucera, L.J., and Zürcher, E. (1994). Major Timber Trees of Guyana: A Lens Key. Tropenbos Series 10. The Tropenbos Foundation, Wageningen, Netherlands. Callado, C.H., Neto, A.J.d.S., Scarano, F.R., and Costa, C.G. (2001). Periodicity of growth rings in some flood-prone trees of the Atlantic rain forest in Rio de Janeiro, Brazil. Trees 15:492–497. Canadian Forest Service, Pacific Forestry Centre. (1999). Combating Tree Theft Using DNA Technology. [Breakout session consensus.] Author, Victoria, BC, Canada. Carlquist, S. (2001). Comparative Wood Anatomy (2nd ed.). Springer. Chudnoff, M. (1984). Tropical Timbers of the World. USDA Agriculture Handbook # 607. U.S. Government Printing Office, Washington, DC. Core, H.A., Côte, W.A., and Day, A.C. (1979). Wood Structure and Identification (2nd ed.). Syracuse University Press, Syracuse, NY. Cutler, D.F. and Gregory, M. (1998). Anatomy of the Dicotyledons (2nd ed.). Vol. IV. Oxford University Press, New York. DeMeijer, M., Thurich, K., and Militz, H. (1998). Comparative study on penetration characteristics of modern wood coatings. Wood Sci. and Tech. 32:347–365. Desch, H.E. and Dinwoodie, J.M. (1996). Timber Structure, Properties, Conversion and Use (7th ed.). Macmillan Press, London. Dickison, W. (2000). Integrative Plant Anatomy. Academic Press, New York. Esau, K. (1977). Anatomy of the Seed Plants (2nd ed.). John Wiley & Sons, New York. Forest Products Laboratory. (1999). Wood Handbook: Wood as an Engineering Material. USDA General Technical Report FPL-GTR-113. U.S. Department of Agriculture Forest Service, Madison, WI. Gregory, M. (1980). Wood identification: An annotated bibliography. IAWA Bull. n.s. 1(1):3–41. Gregory, M. (1994). Bibliography of systematic wood anatomy of dicotyledons. IAWA J. Suppl. 1. Hillis, W.E. (1996). Formation of robinetin crystals in vessels of Intsia species. IAWA J. 17(4):405–419. Hipkins, V. (2001). DNA profiling and identity analysis of Ponderosa pine evidence samples, in NFGEL Annual Report. Hoadley, R.B. (1990). Identifying Wood: Accurate Results with Simple Tools. Taunton Press, Newtown, CT. Hoadley, R.B. (2000). Understanding Wood: A Craftsman’s Guide to Wood Technology (2nd ed.). Taunton Press, Newtown, CT. IAWA Committee. (1989). IAWA list of microscopic features for hardwood identification, Wheeler, E.A., Baas, P., and Gasson, P. (Eds.). IAWA Bull. n.s. 10(3):219–332. IAWA Committee. (2004). IAWA list of microscopic features of softwood identification. Richter, H.G., Grosser, D., Heinz, I., and Gasson, P. (Eds.). IAWA J. 25(1):1–70. Ilic, J. (1991). CSIRO Atlas of Hardwoods. Crawford House Press, Bathurst, Australia.



Kozlowksi, T.T. and Pallardy, S.G. (1997). Physiology of Woody Plants (2nd ed.). Academic Press, San Diego, CA. Kretschmann, D.E., Alden, H.A., and Verrill, S. (1998). Variations of microfibril angle in loblolly pine: Comparison of iodine crystallization and x-ray diffraction techniques, in Microfibril Angle in Wood, Butterfield, B.G. (Ed.). University of Canterbury, pp. 157–176. Kukachka, B.F. (1960). Identification of coniferous woods. Tappi 43(11):887–896. Lagenheim, J.H. (2003). Plant Resins: Chemistry, Evolution, Ecology, and Ethnobotany. Timber Press, Portland, OR. Larson, P.R. (1994). The Vascular Cambium, Development and Structure. Springer-Verlag, Berlin. Larson, P.R., Kretschmann, D.E., Clark, A., III, and Isenbrands, J.G. (2001). Formation and Properties of Juvenile Wood in Southern Pines: A Synopsis. USDA General Technical Report FPL-GTR–129. U.S. Government Printing Office, Washington, DC. Metcalfe, C.R. and Chalk, L. (1950). Anatomy of the Dicotyledons, 2 vols. Clarendon Press, Oxford, UK. Metcalfe, C.R. and Chalk, L. (1979). Anatomy of the Dicotyledons (2nd ed.). Vol. I. Oxford University Press, New York. Metcalfe, C.R. (1987). Anatomy of the Dicotyledons (2nd ed.). Vol. III. Oxford University Press, New York. Miles, A. (1978). Photomicrographs of World Woods, Building Research Establishment, Her Majesty’s Stationery Office, London. Miller, R.B. and Détienne, P. (2001). Major Timber Trees of Guyana: Wood Anatomy. Tropenbos Series 20. Tropenbos International, Wageningen, Netherlands. Ohyama, M., Baba, K., and Itoh, T. (2001). Wood identification of Japanese Cyclobalanopsis species (Fagaceae) based on DNA polymorphism of the intergenic spacer between trnT and trnL 5’ exon. J. Wood Sci. 47:81–86. Panshin, A.J. and deZeeuw, C. (1980). Textbook of Wood Technology (4th ed.). McGraw-Hill, New York. Phillips, E.W.J. (1948). Identification of softwoods by microscopic structure. For. Prod. Res. Bull. 22. Raven, P., Evert, R., and Eichhorn, S. (1999). Biology of Plants (6th ed.). W.H. Freeman, New York. Rijkaert, V., Stevens, M., de Meijer, M., and Militz, H. (2001). Quantitative assessment of the penetration of water-borne and solvent-borne wood coatings in Scots pine sapwood. Holz als Roh- und Werkstoff 59:278–287. Schweingruber, F. (1978). Microscopic Wood Anatomy. Swiss Federal Institute for Foreign Research, Birmensdorf. Simpson, W.T. (Ed.). (1991). Dry Kiln Operator’s Manual. USDA Agriculture Handbook AH-188. Taiz, L. and Zeiger, E. (1991). Plant Physiology. Benjamin/Cummings, Redwood City, CA. Timmel, T.E. (1986). Compression Wood in Gymnosperms. Springer, Heidelberg, Germany. Wheeler, E.A., and Baas, P. (1998). Wood Identification—A Review. IAWA J. 19(3):241–264. Wiedenhoeft, A.C., and Miller, R.B. (2002). Brief comments on the nomenclature of softwood axial resin canals and their associated cells. IAWA J. 23(3):299–303. Worbes, M. (1995). How to measure growth dynamics in tropical trees: a review. IAWA J. 16(4):337–351. Worbes, M. (1999). Annual growth rings, rainfall-dependent growth and long-term growth patterns of tropical trees in the Capar Forest Reserve in Venezuela. J. Ecol. 87:391–403. Zimmermann, M.H. (1983). Xylem Structure and the Ascent of Sap. Springer-Verlag, New York.



3 Cell Wall Chemistry Roger M. Rowell1,3, Roger Pettersen1, James S. Han1, Jeffrey S. Rowell2, and Mandla A. Tshabalala 1

USDA, Forest Service, Forest Products Laboratory, Madison, WI Department of Forest Ecology and Management, University of Wisconsin, Madison, WI 3Department of Biological Systems Engineering, University of Wisconsin, Madison, WI 2

CONTENTS 3.1

3.2 3.3 3.4

3.5 3.6 3.7 3.8

Carbohydrate Polymers 3.1.1 Holocellulose 3.1.2 Cellulose 3.1.3 Hemicelluloses 3.1.3.1 Hardwood Hemicelluloses 3.1.3.2 Softwood Hemicelluloses 3.1.4 Other Minor Polysaccharides Lignin Extractives Bark 3.4.1 Extractives 3.4.1.1 Chemical Composition of Extractives 3.4.2 Hemicelluloses 3.4.3 Cellulose 3.4.4 Lignin 3.4.5 Inorganics and pH Inorganics Distribution of Chemical Components in the Cell Wall Juvenile Wood and Reaction Wood Analytical Procedures 3.8.1 Sampling Procedure 3.8.2 Extraction 3.8.2.1 Scope and Summary 3.8.2.2 Sample Preparation 3.8.2.3 Apparatus 3.8.2.4 Reagents and Materials 3.8.2.5 Procedures 3.8.3 Ash Content (ASTM D-1102-84) 3.8.3.1 Scope 3.8.3.2 Sample Preparation 3.8.3.3 Apparatus 3.8.3.4 Procedure



3.8.4

3.8.5

3.8.6

3.8.3.5 Report 3.8.3.6 Precision Preparation of Holocellulose (Chlorite Holocellulose) 3.8.4.1 Scope 3.8.4.2 Sample Preparation 3.8.4.3 Apparatus 3.8.4.4 Reagents 3.8.4.5 Procedure Preparation of Alpha-Cellulose (Determination of Hemicelluloses) 3.8.5.1 Scope 3.8.5.2 Principle of Method 3.8.5.3 Apparatus 3.8.5.4 Reagents 3.8.5.5 Procedure 3.8.5.6 Calculation and Report Preparation of Klason Lignin 3.8.6.1 Scope 3.8.6.2 Apparatus 3.8.6.3 Reagent 3.8.6.4 Procedure 3.8.6.5 Additional Information

3.8.7 Determination of Methoxyl Groups 3.8.7.1 Scope 3.8.7.2 Principle of Method 3.8.7.3 Sample Preparation 3.8.7.4 Apparatus 3.8.7.5 Reagents 3.8.7.6 Procedure 3.8.7.7 Calculation and Report 3.8.8 Determination of Acetyl by Gas Liquid Chromatography 3.8.8.1 Scope 3.8.8.2 Reagents 3.8.8.3 Sample Preparation 3.8.8.4 Gas Chromatography 3.8.8.5 Reporting References

In chemical terms, wood is best defined as a three-dimensional biopolymer composite composed of an interconnected network of cellulose, hemicelluloses, and lignin with minor amounts of extractives and inorganics. The major chemical component of a living tree is water, but on a dryweight basis, all wood cell walls consist mainly of sugar-based polymers (carbohydrates, 65–75%) that are combined with lignin (18–35%). Overall, dry wood has an elemental composition of about 50% carbon, 6% hydrogen, 44% oxygen, and trace amounts of inorganics. Simple chemical analysis can distinguish between hardwoods (angiosperms) and softwoods (gymnosperms) but such techniques cannot be used to identify individual tree species because of the variation within each species and the similarities among species. In general, the coniferous species (softwoods) have a higher cellulose content (40–45%), higher lignin (26–34%), and lower pentosan (7–14%) content as compared to deciduous species (hardwoods) (cellulose 38–49%, lignin 23–30%, and pentosans 19–26%). Table 3.1 shows a summary of the carbohydrates, lignin, and ash content of hardwoods and softwoods in the United States (Pettersen 1984). © 2005 by CRC Press


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TABLE 3.1 Summary of Carbohydrate, Lignin, and Ash Compositions for U.S. Hardwoods and Softwoods Species

Holocellulose

Alpha Cellulose

Pentosans

Klason Lignin

Ash

Hardwoods Softwoods

71.7 ± 5.7 64.5 ± 4.6

45.4 ± 3.5 43.7 ± 2.6

19.3 ± 2.2 −9.8 ± 2.2

23.0 ± 3.0 28.8 ± 2.6

0.5 ± 0.3 0.3 ± 0.1

Source: Pettersen, 1984.

A complete chemical analysis accounts for all of the components of wood. Vast amounts of data are available on the chemical composition of wood. The tables at the end of this chapter (Table 3.13 and Table 3.14) summarize data for wood species in North America (Pettersen 1984).

3.1 CARBOHYDRATE POLYMERS 3.1.1 HOLOCELLULOSE The major carbohydrate portion of wood is composed of cellulose and hemicellulose polymers with minor amounts of other sugar polymers such as starch and pectin (Stamm 1964). The combination of cellulose (40–45%) and the hemicelluloses (15–25%) is called holocellulose and usually accounts for 65–70 percent of the wood dry weight. These polymers are made up of simple sugars, mainly, D-glucose, D-mannose, D-galactose, D-xylose, L-arabinose, D-glucuronic acid, and lesser amounts of other sugars such as L-rhamnose and D-fucose. These polymers are rich in hydroxyl groups that are responsible for moisture s rganic chemical on the face of the earth. It is a glucan polymer of D-glucopyranose units, which are linked together by β-(1→ 4)-glucosidic bonds. The building block for cellulose is actually cellobiose, since the repeating unit in cellulose is a two-sugar unit (Figure 3.1). The number of glucose units in a cellulose molecule is referred to as the degree of polymerization (DP). Goring and Timell (1962) determined the average DP for native celluloses from several sources using a nitration isolation procedure that minimizes depolymerization and maximizes yield. These molecular weight determinations, done by light-scattering experiments, indicate that wood cellulose has an average DP of at least 9,000–10,000 and possibly as high as 15,000. An average DP of 10,000 would correspond to a linear chain length of approximately 5 µm in wood. This would mean an approximate molecular weight for cellulose ranging from about 10,000 to 150,000. Figure 3.2 shows a partial structure of cellulose.

H

H

H

HO

H

CH2OH

H O

O CH2OH

O H

FIGURE 3.1 Chemical structure of cellobiose.


H

OH

O

HO

OH H

H

H


38

Handbook of Wood Chemistry and Wood Composites

H

H H CH2OH

O

HO

O OH H

H

H

H

CH2OH

H O

H

CH2OH

O H

H

H

OH H

H

H

N

CH2OH

H O

O

O HO

H

OH

HO

O

O HO

H

H OH

H

CH2OH

O H

O HO

OH H

H

H

FIGURE 3.2 Partial structure of cellulose.

Cellulose molecules are randomly oriented and have a tendency to form intra- and intermolecular hydrogen bonds. As the packing density of cellulose increases, crystalline regions are formed. Most wood-derived cellulose is highly crystalline and may contain as much as 65% crystalline regions. The remaining portion has a lower packing density and is referred to as amorphous cellulose. X-ray diffraction experiments indicate that crystalline cellulose (Valonia ventricosa) has a space group symmetry where a = 16.34 Å and b = 15.72 Å (Figure 3.3, Gardner and Blackwell 1974). The distance of one repeating unit (i.e., one cellobiose unit) is c = 10.38 Å (Figure 3.4). The unit cell contains eight cellobiose moieties. The molecular chains pack in layers that are held together by weak van der Waals forces. The layers consist of parallel chains of anhydroglucopyranose units and the chains are held together by intermolecular hydrogen bonds. There are also intramolecular hydrogen bonds between the atoms of adjacent glucose residues (Figure 3.4). This structure is referred to as cellulose I or native cellulose. There are several types of cellulose in wood: crystalline and noncrystalline (as described in the preceding paragraph) and accessible and nonaccessible. Accessible and nonaccessible refer to the availability of the cellulose to water, microorganisms, etc. The surfaces of crystalline cellulose are accessible but the rest of the crystalline cellulose is nonaccessible. Most of the noncrystalline cellulose is accessible but part of the noncrystalline cellulose is so covered with both hemicelluloses and lignin that it becomes nonaccessible. Concepts of accessible and nonaccessible cellulose are very important in moisture sorption, pulping, chemical modification, extractions, and interactions with microorganisms. Cellulose II is another important type of cellulose used for making cellulose derivatives. It is not found in nature. Cellulose II is obtained by mercerization and regeneration of native cellulose. Mercerization is treatment of cellulose I with strong alkali. Regeneration is treatment with carbon disulfide to form a soluble xanthate derivative. The derivative is converted back to cellulose and reprecipitated as cellulose II. Cellulose II has space group a = 8.01 Å, b = 9.04 Å (Figure 3.3), and c = 10.36 Å (Figure 3.4). There is also a cellulose III structure, which is formed by treatment of cellulose I with liquid ammonia at about –80ºC followed by evaporation of the ammonia. Alkali treatment of cellulose III gives cellulose II. Cellulose IV is formed by heating cellulose III in glycerol at 260ºC.

FIGURE 3.3 Axial projection of the crystal structure of cellulose I.



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39

FIGURE 3.4 Planar projection of two cellulose chains showing some of the hydrogen bond between cellulose chains and within a single cellulose chain.

Another type of cellulose (based on the method of extraction from wood) often referred to in the literature is Cross and Bevan cellulose. It consists largely of cellulose I but also contains some hemicellulose. It is obtained by chlorination of wood meal, followed by washing with aqueous solutions of 3% sulfur dioxide (SO2) and 2% sodium sulfite (NaSO3). Finally, there is another structure of cellulose referred to as Kürschner cellulose (also based on the method of isolation). Kürschner cellulose is obtained by refluxing wood meal three times for 1 hour with a 1:4 (v/v) mixture of nitric acid and ethyl alcohol. The water-washed and dried cellulose is referred to as Kürschner cellulose, which also contains some hemicelluloses. This method of cellulose isolation is not often used because it destroys some of the cellulose and the nitric acid–ethanol mixture is potentially explosive. Cellulose I is insoluble in most solvents including strong alkali. Alkali will swell cellulose but not dissolve it. Cellulose dissolves in strong acids such as 72% sulfuric acid, 41% hydrochloric acid, and 85% phosphoric acid, but degradation occurs rapidly. It is difficult to isolate cellulose from wood in a pure form because it is intimately associated with lignin and hemicellulose. The analytical method for isolating cellulose is given in naly tic l ection of polysaccharide polymers with a lower DP than cellulose (average DP of 100–200) and containing mainly the sugars Dxylopyranose, D-glucopyranose, D-galactopyranose, L-arabinofuranose, D-mannopyranose, Dglucopyranosyluronic acid, and D-galactopyranosyluronic acid with minor amounts of other sugars. The structure of hemicelluloses can be understood by first considering the conformation of the monomer units. There are three entries under each monomer in Figure 3.5. In each entry, the letter designations D and L refer to the standard configurations for the two optical isomers of glyceraldehyde,



40


FIGURE 3.5 Sugar monomer components of wood hemicellulose.

the simplest carbohydrate, and designate the conformation of the hydroxyl group at carbon 4 (C-4) for pentoses (xylose and arabinose) and C-5 for hexoses (glucose, galactose, and mannose). The Greek letters α and β refer to the configuration of the hydroxyl group on C-1. The two configurations are called anomers. The first name given for each structure is a shortened form of the sugar name. The second name given for each structure explicitly indicates the ring structure: Furanose refers to a five-membered ring and pyranose refers to a six-membered ring. The six-membered ring is usually in a chair conformation. The third name given for each structure is an abbreviation commonly used for a sugar residue in a polysaccharide (Whistler et al. 1962, Timell 1964, Timell 1965, Whistler and Richards 1970, Jones et al. 1979). Hemicelluloses are intimately associated with cellulose and contribute to the structural components of the tree. Some hemicelluloses are present in very large amounts when the tree is under stress, e.g., compression wood has a higher D-galactose content as well as a higher lignin content (Timell 1982). They usually contain a backbone consisting of one repeating sugar unit linked β-(1→4) with branch points (1→2), (1→3), and/or (1→6). Hemicelluloses usually consists of more than one type of sugar unit and are sometimes referred to by the sugars they contain, for example, galactoglucomanan, arabinoglucuronoxylan, arabinogalactan, glucuronoxylan, glucomannan, etc. The hemicelluloses also contain acetyl- and methyl-substituted groups. Hemicelluloses are soluble in alkali and are easily hydrolyzed by acids. A gradient elution at varying alkali concentrations can be used for a crude fractionation of the hemicelluloses from



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FIGURE 3.6 Partial molecular structure (top) and structure representation (bottom) of O-acetyl-4-O-methylglucuronoxylan.

wood. The hemicelluloses can then be precipitated from the alkaline solution by acidification using acetic acid. Further treatment of the neutralized solution with a neutral organic solvent such as ethyl alcohol results in a more complete precipitation (Sjöström 1981). The detailed structures of most wood hemicelluloses have not been determined O-methyl-glucuronoxylan from a hardwood. This class of hemicelluloses is usually referred to as glucuronoxylans. This polysaccharide contains a xylan backbone of D-xylopyranose units linked β-(1→ 4) with acetyl groups at C-2 or C-3 of the xylose units on an average of seven acetyls per ten xylose units (Sjöström 1981). The xylan is substituted with sidechains of 4-O-methylglucuronic acid units linked to the xylan backbone α(1→2) with an average frequency of approximately one uronic acid group per ten xylose units. The sidechains are quite short. Hardwoods also contain 2–5% of a glucomannan composed of β-D-glucopyranose and β-Dmannopyranose units linked (1→4). The glucose:mannose ratio varies between 1:2 and 1:1 depending on the wood species. Table 3.2 shows the major hemicelluloses found in hardwoods.

TABLE 3.2 Major Hemicelluloses in Hardwoods Hemicellulose Type DP Glucuronoxylan 200

Percent in Wood 15–30

Units β-D-Xylp 4-O-Me-α-DGlupA Acetyl

Glucomannan 200


2–5

β-D-Manp β-D-Glup

Molar Ratio

Linkage

10

1→4

1 7

1→2

1-2 1

1→4 1→4


42


TABLE 3.3 Major Hemicelluloses in Softwoods Hemicellulose Type DP

Percent in Wood

Galactoglucomannan 100

5–8

Galactoglucomannan 100

10–15

Arabinoglucuronoxylan 100

Molar Ratio

Linkage Avg

β-D-Manp

3

1→4

β-D-Glup α-D-Galp

1 1

1→4 1→6

β-D-Manp

4

1→4

β-D-Glup α-D-Galp Acetyl

1 0.1 1

1→4 1→6

β-D-Xylp

7–10

Arabinogalactan 200 (Larch wood)

Units

5–35

10

1→4

4-O-Me-α-DGlupA α-L-Araf

2

1→2

1.3

1→2

β-D-Galp

6

1→4

2–3 1–3 trace

1→6 1→6 1→3 1→6

α-L-Araf β-D-Arap β-D-GlupA

3.1.3.2 Softwood Hemicelluloses Table 3.3 shows the major hemicelluloses from softwoods. One of the main hemicelluloses from softwoods contains a backbone polymer of D-galactose, D-glucose, and D-mannose (Sjöström 1981). The galactoglucomannan is the principal hemicellulose (approximately 20%), with a linear or possibly slightly branched chain with β-(1→4) linkages (Figure 3.7). Glucose and mannose make up the backbone polymer with branches containing galactose. There are two fractions of these polymers, which differ in galactose content. The low-galactose fraction has a ratio of galactose: glucose:mannose of about 0.1:1:4 and the high-galactose fraction has a ratio of 1:1:3. The Dgalactopyranose units are linked as a single-unit sidechain by α-(1→6) bonds. The C-2 and C-3 positions of the backbone polymer have acetyl groups substituted on them an average of every three to four hexose units. →4-β-D-Xylop-1→4-β-D-Xylop-1→4-β-D-Xylop-1→4-β-D-Xylop-1→4-β-D-Xylop-1→ 2 3 ↓

↓

1 4-O-Me –α-D-GlupA

1 α-L-Araf

FIGURE 3.7 Partial structure of a softwood arabino 4-O-methylglucuronoxylan.


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43

→4-β-D-Manop-1→4-β-D-Glup-1→4-β-D-Manop-1→4-β-D-Manop-1→4-β-D-Glup-1→ 6 ↓ 1 β-D-Galp

2 or 3 ↓ 1 Acetyl

FIGURE 3.8 Partial structure of a softwood O-acetyl-galacto-glucomannan.

Another major hemicellulose polymer in softwoods (5–10%) is an arabinoglucuronoxylan consisting of a backbone of β-(1→4) xylopyranose units with α-(1→2) branches of D-glucopyranosyluronic acid on an average of every two to ten xylose units and α-(1→3) branches of L-arabinofuranose, on average, every 1.3 xylose units (Figure 3.8). Another hemicellulose that is found mainly in the heartwood of larches is an arabinogalactan. Its backbone is a β-(1→3)-linked D-galactopyranose polymer with almost every unit having a branch attached to C-6 of β-D-galactopyranose residues. In some cases this sidechain is β-Larabinofuranose linked (1→3) or β-D-arabinopyranose linked (16). There are other minor hemicelluloses in softwoods that mainly contain L-arabinofuranose, Dgalactopyranose, D-glucopyranouronic acid, and D-galactopyroanuronic acid (Sjöström 1981).

3.1.4 OTHER MINOR POLYSACCHARIDES Both softwoods and hardwoods contain small amounts of pectins, starch, and proteins. Pectin is a polysaccharide polymer made up of repeating units of D-galacturonic acid linked α-(1→4). Pectin is found in the membranes in the bordered pits between wood cells and in the middle lamella. Degradation of this membrane by microorganisms increases permeability of wood to water-based treatment chemicals such as fire retardants and wood preservatives. Pectins are found in high concentration in the parenchyma cell walls in the inner bark where they may act as a binder. LArabinofuranose and D-galactopyranose are often found as a minor part of the pectic substance. Pectin is also found as the methyl ester. Starch is the principal reserve polysaccharide in plants. Small amount of starch can also be found in the wood cell wall. Starch normally occurs as granules and is composed of D-glucopyranose units linked α-(1→4) (amylose) or α-(1→4) with branches about every 25 glucopyraosyl units at α-(1→6) (amylopectin). Amylose occurs as a helix structure in the solid state due to the α-configuration in the polymer. Amylopectin is highly branched.

3.2 LIGNIN Lignins are amorphous, highly complex, mainly aromatic polymers of phenylpropane units (Figure 3.9) that are considered to be an encrusting substance. The three-dimensional polymer is made up of C–O–C and C–C linkages. The precursors of lignin biosynthesis are p-coumaryl alcohol (Figure 3.9, structure 1), coniferyl alcohol (Figure 3.9, structure 2), and sinapyl alcohol (Figure 3.9, structure 3). Structure 1 is a minor precursor of both softwood and hardwood lignins, structure 2 is the predominate precursor of softwood lignin, and structures 2 and 3 are both precursors of hardwood lignin (Alder 1977). Softwood lignin has a methoxyl content of 15–16%; hardwood lignin has a methoxyl content of 21%. Lignin does not have a single repeating unit of the hemicelluloses like cellulose does, but instead consists of a complex arrangement of substituted phenolic units.



44


FIGURE 3.9 Chemical structures of lignin precursors: (1) p-coumaryl alcohol, (2) coniferyl alcohol, and (3) sinapyl alcohol.

Lignins can be classified in several ways, but they are usually divided according to their structural elements (Sjöström 1981). All wood lignins consist mainly of three basic building blocks of guaiacyl, syringyl, and p-hydroxyphenyl moieties, although other aromatic units also exist in many different types of woods. There is a wide variation of structures within different wood species. The lignin content of hardwoods is usually in the range of 18–25%, whereas the lignin content of softwoods varies between 25 and 35%. The phenylpropane can be substituted at the α, β, or γ positions into various combinations linked together both by ether and carbon to carbon linkages (Sakakibara 1991). Lignins from softwoods are mainly a polymerization product of coniferyl alcohol and are called guaiacyl lignin. Hardwood lignins are mainly syringyl-guauacyl lignin, because they are a copolymer C C C

HC = O [CH2OH]

CH2OH HC-O— HCOH

CH2OH

HC CH3O

CH

HCO CH

O

O

CH3O HOCH2

CH CHOH

OH

CH CHOH

H2OOH O

CH CH O HC 3 HC

CH3O

O

O

CH

CH3O OH

OCH3

OCH3

HOH2C CH O 3 HC HC-OH H2COH CH3O HC C=O

OCH3 OH[O-C]

FIGURE 3.10 Partial structure of a softwood lignin.

O

H

HOH2C-C-C

OCH2

OCH3

CH2OH O

O

OH

HCOH


CH3O

O

O

HOCH2 HC

OCH2

CH CHOH

CH2OH

CH3O

OH

HOCH2

CH2OH CH HCOH

H2C HC HC

O O

O

O

CH CH CH2

OCH2


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45

of coniferyl and sinapyl alcohols. The ratio of these two types varies in different lignins from about 4:1 to 1:2 (Sarkanen and Ludwig 1971). A proposed structure for a hardwood lignin (Fagus silvatica L.) is shown in Figure 3.10 (Adler 1977). Lignins found in woods contain significant amounts of constituents other than guaiacyl- and syringylpropane units (Sarkanen and Ludwig 1971). Lignin is distributed throughout the secondary cell wall, with the highest concentration in the middle lamella. Because of the difference in the volume of middle lamella to secondary cell wall, about 70% of the lignin is located in the cell wall. Lignin can be isolated from wood in several ways. So-called Klason lignin is obtained after hydrolyzing the polysaccharides with 72% sulfuric acid. It is highly condensed and does not truly represent the lignin in its native state in the wood. The polysaccharides can be removed using enzymes to give an “enzyme lignin” that is much closer to a native lignin than Klason lignin. “Milled wood lignin” or Björkman lignin can be isolated by using a vibratory ball mill on fine wood flour and then extracting with suitable organic solvents (Björkman 1956, 1957). Approximately 30–50% of the native lignin is isolated using this procedure. This procedure is tedious but does isolate a lignin closer to a native lignin. The molecular weight of lignin depends on the method of extraction. Klason lignin, since it is highly condensed, has molecular weights as low as 260 and as high as 50 million (Goring 1962). Björkman lignin has a molecular weight of approximately 11,000. Lignins are associated with the hemicelluloses forming, in some cases, lignin–carbohydrate complexes that are resistant to hydrolysis even under pulping conditions (Obst 1982). There is no evidence that lignin is associated with cellulose.

3.3 EXTRACTIVES As the name implies, extractives (also referred to as natural products) are chemicals in the wood that can be extracted using solvents. In some cases, the extractives are classified by the solvent used to extract them. For example, water-soluble or toluene-ethanol–soluble or ether-soluble extractives. Hundreds of extractives have been identified and in some cases their role in the tree is well understood. In other cases, it is not clear why they are present (Rowe 1989). Extractives, such as pine pitch and resins, have been used for centuries to waterproof wooden boats, in torches, and as a binder. They have also found application in medicine, cosmetics, and as a preservative (Hillis 1989). Some of the extractives in wood are precursors to other chemicals, some are formed in response to wounds, and some act as part of a defense mechanism. The extractives are a group of cell wall chemicals mainly consisting of fats, fatty acids, fatty alcohols, phenols, terpenes, steroids, resin acids, rosin, waxes, and many other minor organic compounds. These chemicals exist as monomers, dimers, and polymers. In general, softwoods have a higher extractives content than hardwoods. Most of the extractives in both softwoods and hardwoods are located in the heartwood, and some are responsible for the color, smell, and durability of the wood. The qualitative difference in extractive content from species to species is the basis of chemotaxonomy (taxonomy based on chemical constituents). Resins and fats are made up of resin acids and fatty acids, respectively. Fatty acids are esters with alcohols, such as glycerol, and mainly occur in sapwood. Resin acids have a free carboxylic acid functional group and are mainly found in heartwood (Kai 1991). Abietic acid (Figure 3.11, structure 1) is a common type of resin acid. The most common terpenes in softwoods are pinene (Figure 3.11, structure 2) and other similar chemical structures. One of the most important polyphenols is pinosylvin (Figure 3.11, structure 3), which is very toxic and found in pine heartwood. Lignans are a combination of two phenylpropane units and are common in softwoods (Gottlieb and Yoshida 1989). Conidendrin (Figure 3.11, structure 4) is found in spruce and hemlock. Tannins in wood can be classified into three classes: gallotannins, ellagtannins, and condensed tannins (Hemingway 1989, Porter 1989). Gallotannins are polymeric esters of gallic acid (Figure 3.11, structure 5) and are usually associated with sugars



46


FIGURE 3.11 Chemical structures of some of the extractives in wood: (1) abietic acid, (2) α-pinene, (3) pinosylvin, (4) pineresinol, (5) gallic acid, (6) α-, β-, and γ-thujaplicin.

(Haslam 1989). Tropolones are responsible for the durability of cedar wood. Examples of this class of extractives include α-, β-, and γ-thujaplicin (Figure 3.11, structure 6) (Kollmann and Côté 1968).

3.4 BARK Bark is a very complex tissue that is composed of two principal zones: the inner bark and the outer bark. The outer bark, which is sometimes referred to as rhytidome and is also known as the periderm, is made up of three layers: the phellem (cork cells), phellogen (cork cambium), and the phelloderm (cork skin). The thickness of the periderm varies greatly between and within species and with the age of the bark. The inner bark, which is referred to as the phloem or bast, is complex in structure and is composed of several types of cells including sieve tubes, fiber cells, albuminose cells, companion cells, parenchyma cells, ideoblasts, and lactifers. Not all cell types occur in every bark. The bark is divided from the wood or xylem by the vascular cambium layer (Sandved et al. 1992). The chemical composition of bark is complex and varies between and within species, and also between the inner and outer bark. Proximate chemical analysis of bark from different species indicates that the chemical constituents of bark can be classified into four major groups: polysaccharides (cellulose, hemicellulose, and pectic materials); lignin and polyphenols; hydroxy acid complexes (suberin); and extractives (fats, oils, phytosterols, resin acids, waxes, tannins, terpenes, phlobaphenes, and flavonoids). Table 3.4 illustrates the variability of the chemical composition of bark between softwood and hardwood species, Pinus pinaster and Quercus suber, respectively.

3.4.1 EXTRACTIVES The extractives content of bark is quite high compared to wood, but values reported in the literature can be very different even for the same species. These apparent differences depend on the method of extraction. For example, McGinnis and Parikh (1975) reported 19.9% extractives for loblolly pine bark using petroleum ether, benzene, ethanol, and cold and hot water. Labosky (1979) extracted loblolly pine bark with hexane, benzene, ethyl ether, ethanol, water, and 1% sodium hydroxide and reported 27.5% extractives. The analysis methods developed for wood cannot be used for bark directly. There are many compounds in bark that are not found in wood that interfere with these analysis methods. For example,



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TABLE 3.4 Average Chemical Composition of Softwood and Hardwood Bark Percent Oven-Dry Weight Component Polysaccharides Lignin and polyphenols Suberin Extractives Ash a b

Pinus pinaster a 41.7 43.7 1.5 11.4 1.2

± ± ± ± ±

0.9 2.4 0.2 2.2 0.6

Quercus suber b 19.9 23.0 39.4 14.2 1.2

± ± ± ± ±

2.6 0.5 1.7 1.1 0.2

Data obtained from Nunes et al. 1996. Data obtained from Pereira, 1988.

the presence of suberin in bark tends to limit access of delignification reagents to the lignin in the bark, and therefore may lead to a holocellulose that is not pure enough to permit fractionation of individual bark polysaccharides. Suberin, polyflavonoids, and other high-molecular-weight condensed tannins can also complicate analysis of bark lignin, resulting in false high values of lignin content in bark. Because of the interference of the extractives in polysaccharide and lignin analysis, procedures for elucidation of the chemical composition of bark begin with an extraction protocol that consists of sequential extraction solvents of increasing polarity. A common protocol begins with a diethyl ether extraction step that yields waxes, fatty acids, fats, resin acids, phytosterols, and terpenes. This is followed by an ethyl alcohol extraction step that yields condensed tannins, flavonoids, and phenolics. The third step uses hot water, and yields condensed tannins and water-soluble carbohydrates. To release phenolic acids, hemicelluloses, and suberin monomers from the residue from the third step, 1% aqueous sodium hydroxide is used (Holloway and Deas 1973, Kolattukudy 1984). The extract fractions from the above-mentioned steps are then subjected to further workup to separate each into easy-to-analyze mixtures of compounds. For example, partitioning the diethyl ether fraction against aqueous sodium bicarbonate separates the fatty acids and resin acids from the neutral components, tannins, terpenes, and flavonoids. The neutral fraction is then saponified to give the alcohols and salts of fatty acids, dicarboxylic, hydroxy-fatty, and ferulic acids. Ethanol extraction followed by hot water extraction of the insoluble ether fraction yields soluble simple sugars and condensed tannins. Sodium hydroxide extraction of the insoluble residue gives soluble suberin monomers, phenolic acids, and hemicelluloses. Sulfuric acid treatment of the insoluble fraction yields lignin (Chang and Mitchell 1955, Hemingway 1981, Laks 1991). 3.4.1.1 Chemical Composition of Extractives The waxes in bark are esters of high-molecular-weight long-chain monohydroxy-alcohol fatty acids. A lot of research has been done on softwood waxes, but very little on hardwood waxes. At one time, hardwood waxes were produced commercially for use in polishes, lubricants, additives to concrete, carbon paper, fertilizers, and fruit coatings (Hemingway 1981). Terpenes are a condensation of two or more five-carbon isoprene (2-methy-1,3-butadiene) units in a linear or cyclic structure. They can also contain various functional groups. The most common of the monoterpenes are α- and β-pinenes found in firs and pines. Birch bark can contain up to 25% terpenes, by total dry weight (Seshadri and Vedantham 1971).



48


Flavonoids are a group of compounds based on a 15-carbon hydroxylated tricyclic unit (Laks 1991). They are often found as glycosides. Many tree barks are rich in mono- and polyflavonoids (Hergert 1960, 1962). Their function seems to be as an antioxidant, pigment, and growth regulator (Laks 1991). Hydrolyzable and condensed tannins are also major extractives from bark. The hydrolyzable tannins are esters of carboxylic acids and sugars that are easily hydrolyzed to give benzoic acid derivatives and sugars. Over 20 different hydrolyzable tannins have been isolated from oaks (Nonaka et al. 1985). The condensed tannins are a group of polymers based on a hydroxylated C-15 flavonoid monomer unit. Low degree of polymerization tannins are soluble in polar solvents, whereas the high degree of polymerization tannins are soluble in dilute alkali solutions (Hemingway et al. 1983). It is difficult to isolate pure fractions of tannins and the structure can be altered by the extraction procedure. Free sugars are also extracted from bark. Hot water extraction yields about 5% free sugar fraction, which is mainly composed of glucose and fructose; this amount varies depending on the growing season. For example, the free sugar content is low in early spring and increases during the growing season, reaching a maximum in the fall (Laks 1991). Other minor free sugars found in bark include galactose, xylose, mannose, and sucrose. Hydrolysis of the hot water extract of bark yields more free sugars, the most abundant one being arabinose. These sugars are tied up as glycosides or in the hemicelluloses. Other sugars released during hydrolysis are glucose, fructose, galactose, xylose, mannose, and rhamnose.

3.4.2 HEMICELLULOSES The hemicellulose content of different barks varies from 9.3% for Quercus robur to 23.1% for Fagus sylvatica (Dietrichs et al. 1978). The main hemicellulose in conifer barks is a galactoglucomannan. Arabino-4-O-methyl-glucuronoxylan is the main hemicellulose in deciduous barks. In general, bark xylans and glucomannans are similar to ones found in wood. Other hemicelluloses that have been isolated from barks include 4-O-methy-glucuronoxylans, glucomannans, O-acetyl-galactoglucomannan, and O-acetyl-4-O-methyl-glucuronoxylan (Painter and Purves 1960, Jiang and Timell 1972, Dietrichs 1975). In the xylans, the xylose units are connected β-(1→4) and the glucuronic acid groups are attached to the xylan backbone α-(1→2). The ratio of xylose to GluU is 10:1 with a degree of polymerization of between 171 and 234 (Mian and Timell 1960). Glucomannans from deciduous barks contain mannose and glucose units in a ratio of from 1:1 to 1.4:1 (Timell 1982). In the mannans from the barks of aspen and willow, galactose units were found as sidechains. The ratio of mannose: glucose:galactose was 1.3:1:0.5 with an average degree of polymerization of 30 to 50 (Timell 1982). Arabinans have been reported in the barks of aspen, spruce, and pine (Painter and Purves 1960). The backbone is α-(1→5)-arabinofuranose units and, in the case of pine, the average degree of polymerization is 95 (Timell 1982). A group of galacturonic acid polymers has been isolated from birch. One is a galacturonic acid backbone linked α-(1→4) with arabinose sidechains in a ratio of galacturonic acid to arabinose of 9:1, and another consists of galacturonic acid, arabinose, and galactose in a ratio of 7:3:1. Small amounts of glucose, xylose, and rhamnose were also found in these polymers (Mian and Timell 1960, Timell 1982). A pectic substance that contains either galactose alone or galactose and arabinose units has also been isolated from barks (Toman et al. 1976). The pure galactan is water-soluble and consists of 33 β-(1→4)-linked galactose units with a sidechain at C-6 of the backbone. A highly branched arabinogalactan was found in the bark of spruce with a ratio of galactose to arabinose of 10:1 (Painter and Purves 1960). In almost all cases, the hemicelluloses found in bark are similar to those found in wood, with some variations in composition. Table 3.5 shows the sugars present after hydrolysis of the polysaccharides in bark.



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TABLE 3.5 Sugars Present in Hydrolyzates of Some Tree Barks Species

Glu

Man

Gal

Xyl

Ara

Rha

UrA

Ac

Abies amabills Picea abies Picea engelmannii Pinus contoria Inner bark Outer bark Pinus sylvestris Pinus taeda Inner bark Outer bark Betula papyrifera Inner bark Fagus sylvatica Quercus robur

37.4 36.6 35.7

8.0 6.5 2.9

1.6 1.3 2.4

3.2 4.8 3.8

3.2 1.8 3.3

— 0.3 —

5.6 — 8.0

0.8 — 0.5

40.9 26.8 30.2

2.5 2.5 5.4

4.3 4.2 2.4

3.7 3.4 5.8

10.6 5.5 2.1

— — 0.3

9.9 7.7 —

0.2 0.8 —

21.3 15.8

2.5 2.6

3.1 2.5

2.1 3.8

5.6 1.8

0.3 0.1

4.6 2.1

— —

28.0 29.7 32.3

0.2 0.2 0.5

1.0 3.1 1.3

21.0 20.1 16.4

2.7 3.1 2.0

— 1.2 0.5

2.2 — —

— — —

Source: Fengel and Wegener, 1984.

3.4.3 CELLULOSE The cellulose content of barks ranges from 16–41% depending on the method of extraction. In unextracted bark, the cellulose content was between 20.2% for pine and 32.6% for oak (Dietrichs et al. 1978). The high extractives content, especially of suberin, requires harsh conditions to isolate the cellulose, so the cellulose content is usually low and the cellulose is degraded during the isolation process. The outer bark usually contains less cellulose than the inner bark (Harun and Labosky 1985). Timell (1961a,b) and Mian and Timell (1960), found a number average degree of polymerization for bark cellulose of 125 (Betula papyrifera) to 700 (Pinus contorta), and a weight average of 4000 (Abies amabilis, Populus grandidentata) to 6900 (Pinus contorta). Bark cellulose has the same type of crystalline lattice (cellulose I) as normal wood, but the degree of crystallinity is less.

3.4.4 LIGNIN As with other analyses involving bark components, literature values for lignin content can vary depending on the method of extraction (Kurth and Smith 1954, Higuchi et al. 1967). Bark contains high contents of condensed and hydrolyzable tannins and sulfuric acid-insoluble suberin that can give false high values of lignin content. For example, the Klason lignin from Pinus taeda bark is 46.0% when including both lignin and condensed tannins but only 20.4% when the bark is first extracted with alkali (McGinnis and Parikh 1975). Other researchers have found lignin contents from 38–58% (Labosky 1979). The elemental composition and functional group content of bark lignins are similar to those of the lignin from the wood of the same species (Sarkanen and Hergert 1971, Hemingway 1981). There is less lignin in the inner bark as compared to the outer bark. There is a lower ratio of OCH3 groups in aspen bark than in aspen wood and a higher ratio of phenolic OH groups to OCH3 (Clermont 1970). There are more guaiacyl units in deciduous bark and more p-hydroxyphenyl units in coniferous bark as compared to the wood of the same species (Andersson et al. 1973). While there are some differences in the ratios of components, no structural difference have been found between most bark lignins and the corresponding wood. © 2005 by CRC Press


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3.4.5 INORGANICS

Handbook of Wood Chemistry and Wood Composites AND PH

Bark is generally higher in inorganics than normal wood. The inorganic (ash) content can be as high as 13% and, in general, the inner bark contains more inorganics as compared to the outer bark (Young 1971, Choong et al. 1976, Hattula and Johanson 1978, Harper and Einspahr 1980). For example, the outer bark of willow contains 11.5% ash, the inner bark 13.1%, and the sapwood 0.9%; sweetgum outer bark is 10.4%, inner bark 12.8%, and sapwood 0.5% ash; red oak outer bark is 8.9%, inner bark 11.1%, and sapwood 0.9% ash; and ash outer bark is 12.3%, inner bark 12.1%, and sapwood 0.9% ash. The major inorganic elements in bark are Na, K, Ca, Mg, Mn, Zn, and P (Choong et al. 1976). There is more Na, K, Mg, Mn, Zn, and P in sapwood than in bark and more Ca in bark than in sapwood. In general, the pH of bark is lower than that of the corresponding wood due to the higher inorganic content of bark compared to wood. For example, Martin and Gray (1971) reported pH values of southern pines ranging from about 3.1–3.8 with an average of 3.4–3.5 compared to a pH of 4.4–4.6 for sapwood. The outer bark has a lower pH than the inner bark, presumably due to a higher content of Ca in the outer bark (Volz 1971). The pH of bark decreases slightly with the age of the tree.

3.5 INORGANICS The inorganic content of a wood is usually referred to as its ash content, which is an approximate measure of the mineral salts and other inorganic matter in the fiber after combustion at a temperature of 575 ± 25°C. The inorganic content can be quite high in woods containing large amounts of silica; however, in most cases, the inorganic content is less than 0.5% (Browning 1967). This small amount of inorganic material contains a wide variety of elements (Ellis 1965, Young and Guinn 1966). Ca, Mg, and K make up 80% of the ash in wood. These elements probably exist in the wood as oxalates, carbonates, and sulfates, or bound to carboxyl groups in pectic materials (Hon and Shiraishi 1991). Other elements present are Na, Si, B, Mn, Fe, Mo, Cu, Zn, Ag, Al, Ba, Co, Cr, Ni, Pb, Rb, Sr, Ti, Au, Ga, In, La, Li, Sn, V, and Zr (Ellis 1965). Some of these are essential for wood growth. Inorganic ions are absorbed into the tree through the roots and transported throughout the tree. Young and Guinn (1966) determined the distribution of 12 inorganic elements in various part of a tree (roots, bark, wood, and leaves) and concluded that both the total inorganic content and concentration of each element varied widely both within and between species. The inorganic content varies depending on the environmental conditions in which the tree lives. See Table 3.12 for a partial list of the inorganic content of some woods. Saka and Goring (1983) studied the distribution of inorganics from the pith to the outer ring of black spruce (Picea mariana Mill) using EDXA. They found 15 different elements including Na, Mg, Al, S, K, Ca, Fe, Ni, Cu, Zn, and Pb. They also found that the inorganic content was higher in earlywood as compared to latewood. The pH of wood varies from 4.2 (Pinus sylvestris) to 5.3 (Fagus sylvatica) with an average of approximately 4.7.

3.6 DISTRIBUTION OF CHEMICAL COMPONENTS IN THE CELL WALL The content of cell wall components depends on the tree species and where in the tree the sample is taken. Softwoods are different from hardwoods, heartwood from sapwood, and latewood from earlywood. Table 3.6 shows the cell wall polysaccharides in earlywood compared to latewood (Saka 1991). Latewood contains more glucomannans as compared to earlywood, but earlywood contains more glucuronoarabinoxylans. Heartwood contains more extractives than sapwood, and as sapwood is transformed into heartwood, aspiration of the bordered pits takes place in softwoods



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TABLE 3.6 Cell Wall Polysaccharides in Earlywood and Latewood in Pine Cell Wall Component Cellulose Galactan Glucomannan Arabinan Glucuronoarabinoxylan

Earlywood% 56.7 3.4 20.3 1.0 18.6

Latewood% 56.2 3.1 24.8 1.8 14.1

Source: Saka, 1991.

and encrustation of pit membranes with the formation of tyloses occurs in hardwoods. Earlywood contains more lignin than latewood. Figure 3.12 shows the distribution of components across the cell wall of scotch pine. The middle lamella and primary wall is mainly composed of lignin (84%) with lesser amounts of hemicelluloses (13.3%) and even less cellulose (07%). The S1 layer is composed of 51.7% lignin, 30.0% cellulose, and 18.3% hemicelluloses. The S2 layer is composed of 15.1% lignin, 54.3% cellulose, and 30.6% hemicelluloses. The S3 layer has little or no lignin, 13% cellulose, and 87% hemicelluloses. The content of xylan is lowest in the S2 layer and higher in the S1 and S3 layers. The concentration of galactoglucomannan is higher in the S2 than in the S1 or S3 layers. On a percentage basis, the middle lamella and primary wall contain the highest concentration of lignin but there is more lignin in the S2 because it is a much thicker layer as compared to the middle lamella and primary wall. The lignin in the S2 layer is evenly distributed throughout the layer. The angle of the cellulose microfibrils in the various cell wall layers, in relation to the fiber axis, is known as the fibril angle. It is one of the most important structural parameters determining mechanical properties of wood. For normal wood, the microfibril angle of the cellulose in the S2

FIGURE 3.12 Chemical composition of the cell wall of scots pine.



52


layer is 14–19 degrees. It is because this angle is so low in the thick S2 layer that wood does not swell or shrink to as large an extent in the longitudinal direction (0.1–0.3%). A further discussion of the distribution of the hemicelluloses in the cell wall can be found in Chapter 15. Strength properties of wood are related to the distribution of hemicelluloses in the cell wall.

3.7 JUVENILE WOOD AND REACTION WOOD Juvenile wood is the wood that develops in the early stages of tree growth. It physical properties are described in Chapter 2 part 13. Juvenile wood cells are shorter, have smaller cell diameter, larger microfibril angle (up to 55 degrees) and have a high content of compression wood as compared to mature wood. Juvenile wood has a lower density and strength than mature wood. Juvenile wood has less cellulose, more hemicelluloses and lignin compared to mature wood. There is a gradual increase in cellulose content as the cells mature and a gradual decrease in hemicellulose content. The lignin content decreases more rapidly as the cell mature. Normal wood growth is erect and vertical. When a tree is forced out of this pattern either by wind or gravitational forces, abnormal woody tissue is formed in different parts of the tree to compensate for the abnormal growing conditions. The wood cells that are formed when softwoods and hardwoods are out of vertical are called reaction wood since these cells are reacting to the stressful conditions. In softwoods, irregular cells develop on the underside of a stem or branch and are referred to as compression wood. In hardwoods, irregular cells develop on the upper side of a stem or branch and are referred to as tension wood. Table 3.7 shows the chemical composition of softwood compression wood (Panshin and de Zeeuw 1980, Timell 1982). Compression wood has a higher lignin content and a lower cellulose content as compared to normal wood. The cellulose in the S2 layer has a lower degree of crystallization than normal wood and the lignin is largely concentrated in the S2 layer as compared to normal wood. Forty percent of the lignin is is in the outer zone of the S2 layer and an additional 40% is uniformly distributed over the remaining part of the S2 layer (Panshin and de Zeeuw 1980). There are more galactoglucomannans in normal wood and more 1 → 3 linked glucans and galactans in compression wood. The midrofibril angle in the modified S2 layer in compression wood is quite high (44–47º) and have more rounded tracheids that are 10 to 40% shorter than normal tracheids. Compression wood is weaker than normal wood and lower elastic properties. The reduced cellulose content and high microfibril angle is probably responsible for the reduction in mechanical properties (Panshin and de Zeeuw 1980).

TABLE 3.7 Chemical Composition of Compression Wood in Softwoods Normal Wood

Compression Wood

Cell Wall Component

Range%

Average%

Range%

Lignin Cellulose Galactoglucomannan 1,3-Glucan Galactan Glucuronoarabinoxylan Other polysaccharides

24.2–33.3 37.7–60.6 — — 1.0–3.8 — —

28.8 44.6 18 Trace 2.2 8 2

30.9–40.9 27.3–53.7 — — 7.1–12.9 — —

Data from Panshin and Zeeuw, 1980, and Timell, 1982.


Average% 37.7 34.9 9 2 10.0 8 2


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TABLE 3.8 Chemical Composition of Tension Wood in Hardwoods Cell Wall Component Lignin Cellulose Pentosans Acetyl Galactosans

Normal Wood 29% 44% 15% 3% 2%

Compression Wood 14% 57% 11% 2% 7%

Source: Schwerin, 1958.

Table 3.8 shows the chemical composition of hardwood tension wood (Schwerin 1958). Tension wood has a lower lignin content and a higher cellulose content as compared to normal wood. There is a lower content of pentosans (xylans) and acetyls than in normal wood and more galactosans in tension wood. There is no S3 layer in tension wood but rather what is known as a G layer or gelatinous layer. This layer is approximately 98% cellulose. The cellulose in the G layer is highly crystalline with a microfibril angle of only 5% and contains very little hemicelluloses or lignin. The G layer is as thick or thicker than the S2 layer in normal wood and contains about the same quantity of inorganics. Tension wood has lower mechanical properties as compared to normal wood (Panshin and de Zeeuw 1980). For example, compression parallel and perpendicular to the grain, modulus of elasticity in bending, modulus of rupture in static bending and longitudinal shear are all reduced in tension wood as compared to normal wood.

3.8 ANALYTICAL PROCEDURES Chemical composition varies from species to species and within different parts of the same wood species. Chemical composition also varies within woods from different geographic locations, ages, climates and soil conditions. There are hundreds of reports on the chemical composition of wood material. In reviewing this vast amount of data, it becomes apparent that the analytical procedures used, in many cases, are different from lab to lab and a complete description of what procedure was used in the analysis is not clear. For example, many descriptions do not describe if the samples were pre-extracted with some solvent before analysis. Others do not follow a published procedure so comparison of data is not possible. The following section is composed of standard procedures used in many laboratories to determine the chemical components of the wood cell wall. Tables 3.9 through 3.12 give summaries of various types of chemical compositions of hardwoods and softwoods in the United States. This data has been collected from the analytical laboratories of the USDA, Forest Service, Forest Products Laboratory from 1927 to 1968.

3.8.1 SAMPLING PROCEDURE In reporting the chemical content of a wood, it is very important to report as much information about the samples as possible. Since the chemical content of a given species may vary depending upon the growing conditions, harvesting times of the year, etc., it is critical to report these conditions along with the chemical analysis. It is also important to report the exact analytical conditions and procedures used. This way, it may be possible to reproduce the results by other workers in different laboratories. Without this information, it is not possible to compare data from different laboratories.



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TABLE 3.9 Methoxyl Content of Some Common Hardwoods and Softwoods Type of Wood

Methoxy Content (%)

Hardwoods Balsa Basswood Yellow birch Shellbark hickory Sugar maple Mesquite Tanoak

5.68 6.00 6.07 5.63 7.25 5.55 5.74

Softwoods Incense cedar Alaska cedar Douglas fir Western larch Longleaf pine Western white pine Redwood White spruce

6.24 5.25 4.95 5.03 5.05 4.56 5.21 5.30

Source: Moore and Johnson, 1967.

The following information should accompany each chemical analysis: 1. Source of the wood a. Geographic location b. Part of the tree sampled c. Date sample was taken 2. Sampling a. Different anatomical parts b. Degree of biological deterioration, if any c. Sample size d. Drying method applied 3. Analytical procedure used 4. Calculations and reporting technique All of the above-mentioned criteria could contribute in one way or another toward variations in chemical analyses.

3.8.2 EXTRACTION 3.8.2.1 Scope and Summary This method describes a procedure for extraction of wood for further analysis, such as holocellulose, hemicellulose, cellulose, and lignin analysis. Wood materials = Extractives + holocellulose + lignin + inorganics (ash)



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TABLE 3.10 Acetyl Content of Some Common Hardwoods and Softwoods Type of Wood

Acetyl Content (%)

Hardwoods Aspen Balsa Basswood Beech Yellow birch White birch Paper birch American elm Shellbark hickory Red maple Sugar maple Mesquite Overcup oak Southern red oak Tanoak

3.4 4.2 4.2 3.9 3.3 3.1 4.4 3.9 1.8 3.8 3.2 1.5 2.8 3.3 3.8

Softwoods Eastern white-cedar Incense-cedar Western red-cedar Alaska-cedar Douglas-fir Balsam fir Eastern hemlock Western hemlock Western larch Jack pine Loblolly pine Longleaf pine Western white pine Redwood White spruce Tamarack

1.1 0.7 0.5 1.1 0.7 1.5 1.7 1.2 0.5 1.2 1.1 0.6 0.7 0.8 1.3 1.5

Source: Moore and Johnson, 1967.

Neutral solvents, water, toluene or ethanol, or combinations of solvents are employed to remove extractives in wood. However, other solvents ranging from diethyl ether to 1% NaOH, etc. could be applied according to the nature of extractives and sample type, i.e., bark, leaves, etc. 3.8.2.2 Sample Preparation It is highly recommended to have a fresh sample. If this is not possible, keep the sample frozen or in a refrigerator to avoid fungal attack. Peel off the bark from the stem and separate the sample into component parts. Dry samples are oven dried for 24 hours (usually at 105ºC) prior to milling. Wet samples can be milled while frozen in order to prevent oxidation or other undesirable chemical reactions. Samples are ground to pass 40 mesh (0.40 mm) using a Wiley Mill.


Solubility Botanical Name

Bigleaf maple Boxelder Red maple Silver maple Sugar maple Red alder Pacific madrone Yellow birch River birch Paper birch Bitternut hickory Pignut hickory Shagbark hickory Sand hickory Mockernut hickory Sugarberry Alpine ash American beech White ash Green ash Honey locust White mangrove Sweetgum Yellow poplar

Holo Cellulose

Alpha Cellulose

Pentosans

Klason Lignin

1% NaOH

Hot Water

EtOH/ Benzene

Ether

Ash

— — 77.0 — — 74.0 — 73.0 — 78.0 — 71.0 71.0 69.0 71.0 — 72.0 77.0 — — — — — —

46.0 45.0 47.0 42.0 45.0 44.0 44.0 47.0 41.0 45.0 44.0 49.0 48.0 50.0 48.0 40.0 49.0 49.0 41.0 40.0 52.0 40.0 46.0 45.0

22.0 20.0 18.0 19.0 17.0 20.0 23.0 23.0 23.0 23.0 19.0 17.0 18.0 17.0 18.0 22.0 14.0 20.0 15.0 18.0 22.0 19.0 20.0 19.0

25.0 30.0 21.0 21.0 22.0 24.0 21.0 21.0 21.0 18.0 25.0 24.0 21.0 23.0 21.0 21.0 22.0 22.0 26.0 26.0 21.0 23.0 21.0 20.0

18.0 10.0 16.0 21.0 15.0 16.0 23.0 16.0 21.0 17.0 16.0 17.0 18.0 18.0 17.0 23.0 16.0 14.0 16.0 19.0 19.0 29.0 15.0 17.0

2.0 — 3.0 4.0 3.0 3.0 5.0 2.0 4.0 2.0 5.0 5.0 5.0 7.0 5.0 6.0 7.0 2.0 7.0 7.0 — 15.0 3.0 2.0

3.0 — 2.0 3.0 3.0 2.0 7.0 2.0 2.0 3.0 4.0 4.0 3.0 4.0 4.0 3.0 4.0 2.0 5.0 5.0 — 6.0 2.0 1.0

0.7 0.4 0.7 0.6 0.5 0.5 0.4 1.2 0.5 1.4 0.5 0.4 0.4 0.4 0.4 0.3 0.3 0.8 0.5 0.4 0.4 2.1 0.7 0.2

0.5 — 0.4 — 0.2 0.3 0.7 0.7 — 0.3 — 0.8 0.6 1.0 0.6 — 0.2 0.4 — — — — 0.3 1.0


Hardwoods Acer macrophyllum Acer negundo Acer rubrum Acer saccharinum Acer saccharum Alnus rubra Arbutus menziesii Betula alleghaniensis Betula nigra Betula papyrifera Carya cordiformus Carya glaubra Carya ovata Carya pallida Carya tomentosa Celtis laevigata Eucalyptus gigantea Fagus grandifolia Fraxinus americana Fraxinus pennsylvanica Gleditsia triacanthos Laguncularia racemosa Liquidambar styraciflua Liriodendron tulipifera

Common Name


56


TABLE 3.11 Chemical Composition of North American Hardwoods and Softwoods

71.0 — — 72.0 — — 78.0 —

46.0 43.0 45.0 45.0 52.0 47.0 49.0 49.0

20.0 19.0 16.0 17.0 23.0 18.0 19.0 19.0

19.0 27.0 24.0 27.0 16.0 23.0 19.0 21.0

20.0 21.0 16.0 15.0 20.0 15.0 18.0 18.0

5.0 4.0 4.0 3.0 4.0 2.0 3.0 3.0

3.0 2.0 3.0 2.0 5.0 2.0 3.0 3.0

0.4 0.5 0.6 0.4 0.9 0.8 1.2 0.7

0.7 — 0.6 0.5 — 0.4 0.4 0.5

Prunus serotina

Black cherry

85.0

45.0

20.0

21.0

18.0

4.0

5.0

0.9

0.1

Quercus alba

White oak

67.0

47.0

20.0

27.0

19.0

6.0

3.0

0.5

0.4

Quercus coccinea

Scarlet oak

63.0

46.0

18.0

28.0

20.0

6.0

3.0

0.4

—

Quercus douglasii

Blue oak

59.0

40.0

22.0

27.0

23.0

11.0

5.0

1.4

1.4

Quercus falcata

Southern red oak

69.0

42.0

20.0

25.0

17.0

6.0

4.0

0.3

0.4

Quercus kelloggii

California black oak

60.0

37.0

23.0

26.0

26.0

10.0

5.0

1.5

0.4

Quercus lobata

Valley oak

70.0

43.0

19.0

19.0

23.0

5.0

7.0

1.0

0.9

Quercus lyrata

Overcup oak

—

40.0

18.0

28.0

24.0

9.0

5.0

1.2

0.3

Quercus marylandica

Blackjack oak

—

44.0

20.0

26.0

15.0

5.0

4.0

0.6

—

Quercus prinus

Chestnut oak

76.0

47.0

19.0

24.0

21.0

7.0

5.0

0.6

0.4

Quercus rubra

Northern red oak

69.0

46.0

22.0

24.0

22.0

6.0

5.0

1.2

0.4

Quercus stellata

Post oak

Quercus velutina

Black oak

Salix nigra

Black willow

—

46.0

19.0

Tilia heterophylla

Basswood

77.0

48.0

17.0

Ulmus americana

American elm

73.0

50.0

17.0

Ulmus crassifolia

Cedar elm

—

50.0

19.0

—

41.0

18.0

24.0

21.0

8.0

4.0

0.5

1.2

71.0

48.0

20.0

24.0

18.0

6.0

5.0

0.2

0.2

21.0

19.0

4.0

2.0

0.6

—

20.0

20.0

2.0

4.0

2.1

0.7

22.0

16.0

3.0

2.0

0.5

0.4

27.0

14.0

—

—

0.3

—

(Continued )


Tanoak Cajeput Water tupelo Black tupelo White poplar Eastern cottonwood Quaking aspen Black cottonwood

Cell Wall Chemistry


Lithocarpus densiflorus Milalenca quinquenervia Nyssa aquatica Nyssa sylvatica Populus alba Populus deletoides Populus tremoides Populus trichocarpa

57

Solubility Botanical Name

Common Name

Holo Cellulose

Alpha Cellulose

Pentosans

Klason Lignin

1% NaOH

Hot Water

EtOH/ Benzene

Ether

Ash

Softwoods Abies amabilis

Pacific silver fir

—

44.0

10.0

29.0

11.0

3.0

3.0

0.7

0.4

Abies balsamea

Balsam fir

—

42.0

11.0

29.0

11.0

4.0

3.0

1.0

0.4

Abies concolor

White fir

66.0

49.0

6.0

28.0

13.0

5.0

2.0

0.3

0.4

Abies lasiocarpa

Subalpine fir

67.0

46.0

9.0

29.0

12.0

3.0

3.0

0.6

0.5

61.0

43.0

9.0

29.0

10.0

2.0

3.0

0.6

0.4

—

41.0

9.0

33.0

16.0

3.0

6.0

2.4

—

Noble fir Atlantic white cedar

Juniperus deppeana

Alligator juniper

57.0

40.0

5.0

34.0

16.0

3.0

7.0

2.4

0.3

Larix larcina

Tamarack

64.0

44.0

8.0

26.0

14.0

7.0

3.0

0.9

0.3

Larix occidentalis

Western larch

65.0

48.0

9.0

27.0

16.0

6.0

2.0

0.8

0.4

Libocedrus decurrens

Incense cedar

56.0

37.0

12.0

34.0

9.0

3.0

3.0

0.8

0.3

Picea engelmanni

Engelman spruce

69.0

45.0

10.0

28.0

11.0

2.0

2.0

1.1

0.2

Picea glauca

White spruce

—

43.0

13.0

29.0

12.0

3.0

2.0

1.1

0.3

Picea mariana

Black spruce

—

43.0

12.0

27.0

11.0

3.0

2.0

1.0

0.3

Picea sitchensis

Sitka spruce

—

45.0

7.0

27.0

12.0

4.0

4.0

0.7

—

Pinus attenuata

Knobcone pine

—

47.0

14.0

27.0

11.0

3.0

1.0

—

0.2

Pinus banksiana

Jack pine

66.0

43.0

13.0

27.0

13.0

3.0

5.0

3.0

0.3

Pinus clausa

Sand pine

—

44.0

11.0

27.0

12.0

2.0

3.0

1.0

0.4

Pinus contorta

Lodgepole pine

68.0

45.0

10.0

26.0

13.0

4.0

3.0

1.6

0.3

Pinus echinata

Shortleaf pine

69.0

45.0

12.0

28.0

12.0

2.0

4.0

2.9

0.4

Pinus elliottii

Slash pine

64.0

46.0

11.0

27.0

13.0

3.0

4.0

3.3

0.2

Pinus monticola

Western white pine

69.0

43.0

9.0

25.0

13.0

4.0

4.0

2.3

0.2


Abies procera Chamaecyparis thyoides


58


TABLE 3.11 Chemical Composition of North American Hardwoods and Softwoods (Continued )

—

44.0

12.0

30.0

12.0

3.0

4.0

1.4

—

Ponderosa pine

68.0

41.0

9.0

26.0

16.0

4.0

5.0

5.5

0.5

Pinus resinosa

Red pine

71.0

47.0

10.0

26.0

13.0

4.0

4.0

2.5

—

Pinus sabiniana

Digger pine

—

46.0

11.0

27.0

12.0

3.0

1.0

—

0.2

Pinus strobes

Eastern white pine

68.0

45.0

8.0

27.0

15.0

4.0

6.0

3.2

0.2

Pinus sylvestris

Scotch or Scots pine

—

47.0

11.0

28.0

—

1.0

—

1.6

0.2

Pinus taeda

Loblolly pine

68.0

45.0

12.0

27.0

11.0

2.0

3.0

2.0

—

Pseudotsuga menziesii

Douglas fir

66.0

45.0

8.0

27.0

13.0

4.0

4.0

1.3

0.2

Sequoia sempervirens

Redwood old growth

55.0

43.0

7.0

33.0

19.0

9.0

10.0

0.8

0.1

61.0

46.0

7.0

33.0

14.0

5.0

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