Wood Properties Growth and Structure

The aim of this is to describe how wood grows in the tree and what wood consists of.

This describes features and functions of whole trees, then discusses the structure of wood and finally explains the microscopic and chemical structure of cell walls. Understanding the structure of wood is essential in understanding the pathways that preservatives follow when wood is treated.

Wood Is Our Most Valuable Natural Resource

Throughout recorded history, the unique characteristics and relative abundance of wood have made it one of mankind's most valuable and useful natural resources. Today literally thousands of products that we take for granted come from solid wood, wood pulp and chemicals derived from wood. Why is wood man's most important building material? First, only wood is a renewable resource. No other building material-- steel, aluminum, brick, concrete, plastics, glass, ceramics--can be regenerated as can trees. And trees also provide wildlife habitat and recreational areas while they grow.

Advantages of wood
When compared with competing construction materials, wood has many other advantages.

· Wood is available in many species, sizes, shapes and conditions and can suit almost every demand.

· Wood is readily available and is a material most people are familiar with.

· In comparison to other raw materials, wood requires far less energy to process into products.

· Wood has a high strength-to-weight ratio and therefore performs well as a structural material.

· Wood is easily cut and shaped with tools and fastened with adhesives, nails, screws, bolts and dowels.

· Wood is lightweight and easy to install.

· Wood, when dry, has good insulating properties against heat, cold, sound and electricity.

· Wood has good shock resistance and absorbs and dissipates vibrations.

· Because of the variety of grain patterns and colors, wood is an esthetically pleasing material and its appearance can be enhanced by many finishes.

· Wood is easily repaired and wood structures are easily remodeled.

· Wood combines with almost any other material for both functional and esthetic uses.

· Wood can be highly durable if properly protected or treated.

Disadvantages of wood
Biological deterioration and fire are two obvious threats or disadvantages to wood use.

· Biological deterioration. Because of the sugars and starch in untreated wood, it is a source of food for a variety of fungi, insects and other organisms. Given the right circumstances, they can break down and consume the cellulose, lignin and other components of wood and damage the wood members of a structure. Wood preservation is used to prevent this kind of damage. In Lesson 3 we will look more closely at wood decay, decay fungi and harmful insects, and in Lessons 4 and 5 we'll see how preservative treatment can deter these destructive agents.

· Fire. Wood is combustible when provided with adequate heat and oxygen. In fact, wood is the most widely used fuel in many parts of the world. Wood's combustibility often limits the use of lumber products to light-frame construction such as housing and similar structures. However, some commercial building designs call for and permit the use of heavy timber construction. Untreated large wooden beams are often safer in a fire than unprotected steel beams. When subjected to high temperatures, steel rapidly loses its strength and rigidity. This can lead to the sudden collapse of a building with great risk to life and property. Large cross- sectional timbers, on the other hand, burn slowly from the outside in, often retaining a good proportion of their strength during a fire and after it has been extinguished. For some uses, building codes or standards require wood to be protected by fire retardant treatment.

Wood: Many Varieties Create Wide Variations in Properties

Wood may appear to be a very simple material, but its make-up is quite complex. All wood is composed of four chemical components: cellulose, lignin, hemicellulose and extractives, which combine to form a cellular structure. Variations in the characteristics and volume of the four components and differences in cellular structure result in some woods being hard and heavy and some light and soft, some strong and some weak, some naturally durable and some prone to decay. Four primary reasons account for the great variation in wood and its properties.

First, there are many varieties of trees. Each variety, such as red oak, loblolly pine and Douglas fir, is known as a species. There are approximately 50,000 species of trees in the world and the properties and characteristics of these various woods differ markedly. Within a single species, physical and chemical properties are relatively constant; therefore, selection of wood by species alone may often be adequate. Thousands of different tree species grow in North America; however, only 60 or so have commercial use and even fewer are suitable for treating.

A second reason for variation between pieces of wood occurs within each tree. For instance, it is common for the wood found toward the center of a tree trunk (theheartwood) to be quite different from that found toward the outside (the sapwood).

Another reason for differences within a wood species results from where the tree grows. We could expect radiata pine grown in New Zealand, South Africa and Brazil to be affected by differences in sunlight, latitude, rainfall and wind. The same tree species growing high on a mountain will produce quite different wood characteristics from its twin planted at the same time in a nearby fertile valley.

Finally, after a tree is harvested, the different ways that wood is processed (sawn, seasoned, chemically treated, machined, etc.) will also affect the characteristics of the final wood product. For reasons like these, wood is a variable and complex material, whose properties can never be precisely predicted. Satisfactory treatment must take into consideration the various characteristics of different species and their intended uses.

Names for trees
People who process, distribute or use wood products on a daily basis refer to tree species or wood by a "common" name. However, sometimes the same name is used to describe wood from several completely different tree species, which may or may not have similar properties or appearance. And sometimes different common names are used for the same tree; for example, yellow poplar may also be called tulip tree or just poplar. This can be confusing and create problems for buyers, sellers and processors.

The only way to be certain of a wood species is to refer to it by its scientific (or Latin) name. As an example, Eastern white pine and Western white pine may sound like the same tree growing in different areas of the country. In fact, they are different species of trees, which can be distinguished by studying the needles, cones, bark, flowers and wood structure. The scientific name of the former is Pinus strobus L. and the latter is Pinus monticola (Dougl.).

Softwood and hardwood trees
A tree is usually defined as a woody plant which, when mature, is at least 20 feet tall, has a single trunk, unbranched for at least several feet above the ground and has a definite crown. Trees are divided into two biological categories: softwoods and hardwoods. The terms softwood and hardwood do not refer to the hardness or density of the wood. Softwoods are not always soft, nor are hardwoods always hard. Mountain-grown Douglas fir, for example, produces an extremely hard wood although it is classified as a "softwood," and balsawood, so useful in making toy models, is classified a "hardwood" although it is very soft.

In biological terms, softwoods are called gymnosperms, which are trees that produce "naked seeds." The most important group of softwoods are the conifers or cone-bearing trees, which have seeds that are usually visible inside opened cones. All species of pine, spruce, hemlock, fir, cedar, redwood and larch are softwoods. Nearly all softwood trees have another common characteristic: their leaves are actually needles or scales and they remain on the tree throughout the winter, which is why they are also called evergreen trees. Exceptions are larch (or tamarack) and cypress whose needles drop in the fall, leaving the tree bare during winter.

Hardwoods are biologically called angiosperms, which are trees that produce seeds enclosed in a fruit or nut. The hardwood category includes the oaks, ashes, elms, maples, birches, beeches and cottonwoods. In contrast to softwoods, hardwood trees have broad leaves and nearly all North American hardwoods are deciduous, which means they drop their leaves in the fall. However, there are exceptions: holly, magnolia and live oak are hardwoods that retain their leaves year- round.

Though there are many more hardwood species than there are softwoods, the softwoods produce a larger share of commercial wood products, particularly those used for structural applications. This is evident by the dominant use of a few softwood species such as the southern yellow pines, indigenous to the south, and Douglas fir, hemlocks, spruces, other pines and true firs from the west, all of which play crucial roles in construction.

Growth Process of Trees

Tree growth is a miraculous process. Water and nutrients are absorbed by roots and transported from the soil up to the leaves through hollow cells (shaped like long drinking straws with very tiny openings) found in the sapwood (See Figure 1.1,). Leaves absorb carbon dioxide from the air, which they combine with chlorophyll (the green matter of leaves) and sunlight to manufacture food, in the form of various sugars, for the tree's use. This process is called photosynthesis. A by-product of this process is the release of oxygen. In fact, without the production of oxygen by trees and other green plants on our planet, humans and other animals could not survive.

Figure 1.1 Main parts of a tree and the process of photosynthesis.

Photosynthesis: CO₂+ H₂O = C₆H₁₂O₆+ O₂

Carbon dioxide (CO₂) from the atmosphere combines with water (H₂O) in the leaves during photosynthesis, a process catalyzed by chlorophyll and energized by sunlight, which produces the basic sugar, glucose (C₆H₁₂O₆), and releases oxygen (O₂) to the atmosphere.

The nutrients (sugar solutions) manufactured by the leaves are conducted through the inner bark (or phloem cells) to the areas of a tree where growth takes place--the tips of branches and roots and the cambium layer. (See Figure 1.1 and 1.2) The cambium is the layer of reproductive cells found between the inner bark (phloem) and sapwood portions of a tree. This very narrow layer of cells creates new sapwood cells toward the inside and new phloem cells toward the outside of the cambium. Thus the cambium layer is responsible for a tree's outward growth in diameter and circumference.

Figure 1.2 Principle features of a tree stem, cross-sectional (transverse) view.

As a tree gets bigger around, phloem cells get older; they are pushed farther away from the cambium (toward the outside) and gradually die. Their water transporting function is then taken over by younger phloem cells produced by the cambium. Dead phloem cells become part of the outer protective layer of trees that we call bark. Bark is important in protecting the tender cells in and near the cambium. Without bark, these cells would be under continual attack from insects, forest animals, fungi and birds and susceptible to physical damage from frost, wind and fire.

The woody portion of a tree is called xylem and it includes both the sapwood and heartwood. Heartwood is the darker-colored inner part of a trunk. This portion of a tree is composed of dead cells, which greatly contribute to the overall strength of the tree trunk. In many ways heartwood is similar to sapwood, but they differ in their chemical and physical properties.

Unlike animals, trees have no way to get rid of by-products or extractives produced by the chemical changes that take place in their living tissues. Some of these by-products could be harmful to the tree, so provision has been made to nullify such risk. The tree moves these substances toward its heartwood center; so heartwood, basically, is just sapwood in which waste substances have accumulated. This leads to two major differences in the properties of heartwood and sapwood. Heartwood, because of the presence of extractives and other substances, usually has:
(a) greater resistance to insect attack and decay by fungi, and
(b) reduced permeability, which can affect timber treatment because the natural cellular channels of heartwood can become clogged with extractive deposits (we will examine this in more detail in Lesson 5).

Cell Structure

A tree is a plant and all growing organisms, whether plant or animal, consist of cells. During its life, a plant cell is a very small individual unit with a cell wallcompletely enclosing the liquid inner-cell contents. It is these cells that accept preservatives during wood treatment. Plants grow by the formation of new cells. This occurs when individual cells divide in two, a process called cell division. By this process the plant increases in size and weight. Even a small piece of wood, such as a 1" x 1" x 1" cube, will contain many thousands of tiny cells produced by the continued process of cell division and expansion in the cambium.

As the circumference of the tree grows, the thin ring of cambium grows equivalently. Because of the climatic conditions in the tropics, the rate of growth (that is, the subdivision of cells) is almost constant throughout the year. However, in the United States there are very definite climatic seasons which affect the growth of wood cells. Figure 1.2 above, shows the cross-section of a typical tree. Each year the wood cells grow fast early in the growing season (spring), producing springwood orearlywood. Later in the season, as winter approaches, growth slows producing summerwood or latewood. In the depth of winter there may be no woody growth at all. This consistent pattern of fast growth followed by slow growth gives trees their distinctive annual rings. The earlywood cells have thin walls and large central openings or lumens. The latewood cells have thicker walls and smaller lumens. More wall material is produced in the latter part of the growing season.

Differences between softwood and hardwood cells
In softwoods, over 90 percent of the wood volume is made up of cells called longitudinal tracheids (pronounced tray-key-ids). See Figures 1.3 and 1.4, below. Tracheids are long (3-4 mm in length), thin cells oriented parallel to the vertical axis of the tree. Tracheids give softwood trees their structural support and those found in the inner sapwood area provide the conduits for the vertical movement of water and nutrients.

Figure 1.3
Schematic drawing of typical southern pine wood. Adapted from Koch, Peter. 1972. Utilization of the Southern Pines. USDA Forest Service Ag Handbook No 420 . Based on Howard, E.T., and Manwiller, F.G. 1969. WoodScience 2: 77-86.

Transverse view. 1-1a, ray; B, dentate ray tracheid; 2, resin canal; C, thin-walled longitudinal parenchyma; D, thick-walled longitudinal parenchyma; E, epithelial cells; 3-3a, earlywood longitudinal tracheids; F, radial bordered pit pair cut through torus and pit apertures; G, pit pair cut below pit apertures; H, tangential pit pair; 4-4a, latewood longitudinal tracheids.

Radial view. 5-5a, sectioned fusiform ray; J, dentate ray tracheid; K, thin-walled parechyma; L, epithelial cells; M, unsectioned ray tracheid; N, thick-walled parenchyma; O, latewood radial pit; O1, earlywood radial pit; P, tangential bordered pit; Q, callitroid-like thickenings; R, spiral thickening; S, radial bordered pits; 6-6a, sectioned uniseriate heterogenous ray.

Tangential view. 7-7a, strand tracheids; 8-8a, longitudinal parenchyma (thin-walled); T, thick-walled parenchyma; 9-9a, longitudinal resin canal; 10, fusiform ray; U, ray tracheids; V, ray parenchyma; W, horizontal epithelial cells; X, horizontal resin canal; Y, opening between horizontal and vertical resin canals; 11, uniseriate heterogenous rays; 12, uniseriate homogenous ray; Z, small tangential pits in latewood; Z1, large tangential pits in earlywood.

Figure 1.4
Earlywood (left) and latewood (right) tracheids:

a, intertracheid bordered pits; b, bordered pits to ray tracheids; c, pinoid pits to ray parenchyma. To simplify the drawing, tangential intertracheid pits have not been depicted. These pits are distributed along the length but are most frequent near the tracheid ends.

Adapted from Koch, Peter. 1972. Utilization of the Southern Pines, USDA Forest Service Ag Handbook No 420. Based on Howard, E.T., and Manwiller, F.G. 1969 WoodScience 2: 77-86.

Other cells in softwoods lie in narrow bundles across the tracheids. These cells are oriented in a radial direction from the outside of the tree trunk towards its center and are referred to as ray cells or rays. They transport waste materials (extractives) toward the heartwood and may be used for storage of various food substances.Rays are bundles of cells usually only one cell wide and seldom more than three. Because softwood rays are so narrow, they are usually invisible to the naked eye. Horizontal transport of liquids across the annual rings is accomplished by the ray cells.

Hardwood trees are more highly developed than the softwoods and their cell structure is more complex and variable. See Figures 1.5 and 1.6 below. They have evolved a special way of conducting water from the roots to the leaves. Large, hollow cells (called vessels) lie within a mass of fibertracheids. In hardwoods all vertical water conduction is done through these vessels. Each vessel is made up of short segments joined end-to-end (like drain pipes). The vessels are much larger in diameter than thefiber tracheids and can often be seen as tiny holes on the ends of wood in tree species like ash, oak or elm. In contrast to the longitudinal tracheids found in softwoods, which provide support and conduct liquids, the fiber tracheids in hardwoods primarily provide support.

Figure 1.5
Schematic drawing of a typical hardwood-sweetgum. (magnified 330X)

Transverse surface: 1-1^a, boundary between two annual rings (growth proceeding from right to left); 2-2^a, wood ray consisting of procumbent cells; 2^b2^c, wood ray consisting of upright cells; a-a6 inclusive, pores (vessels in transverse section); b-b⁴ inclusive, fiber tracheids; c-c³ inclusive, cells of longitudinal parenchyma; e, procumbent ray cell.

Radial surface: f,f¹, portions of vessel elements; g¹, portions of fiber tracheids in lateral surface aspect; 3-3^a, upper portion of a heterocellular wood ray in lateral sectional aspect; i, a marginal row of upright ray cells; j, two rows of procumbent ray cells.

Tangential surface: k, portion of a vessel element in tangential surface aspect; k¹k², overlapping vessel elements in tangential surface aspect; 1, fiber tracheids in tangential surface aspect; 4-4^a, portion of a wood ray in tangential sectional view; m, an upright cell in the lower margin; n, procumbent cells in the body of the ray.

Adapted from Koch, Peter. 1985. Utilization of Hardwoods Growing on Southern Pine Sites. USDA Forest Service. Ag Handbook No. 605. From Panshin, A.J. and de Zeeuw, C. 1980. Textbook of Wood Technology. Used with the permission of McGraw-Hill Book Company.

Figure 1.6
Hardwood cell types are extremely varied.

The ray cells of hardwoods are not unlike those in softwoods, but hardwood ray cells often form much wider bands or ribbons. They can be so wide as to be visible to the naked eye. In fact, the rays are responsible for much of the distinctive grain pattern or figure of our common hardwood species. Were it not for the different colors and structural features of exposed vessels and rays, most species of hardwood would look similar.

Cell wall structure
The wall of a typical wood cell is composed of several layers, which are formed as new cells are created at the cambium layer. (See Figure 1.7 below). The middle lamella, composed mainly of lignin, serves as the glue bonding adjacent cells together. The wall itself is made up of a primary wall and a three-layered secondary wall, each of which has distinct alignments of microfibrils. Microfibrils areropelike bundles of cellulose molecules, interspersed with and surrounded by hemicellulose molecules and lignin.

Figure 1.7
Cell wall organization.

Idealized model of typical wall structure of a fiber or tracheid. The cell wall consists of: P-primary wall; S1, S2, S3-layers of the secondary wall; W-warty layer (not always evident); ML-middle lamella, the amorphous, high-lignin-content material that binds cells together. Adapted from Koch, Peter. 1985. Utilization of Hardwoods Growing on Southern Pine Sites. USDA Forest Service. Ag Handbook No. 605. From Wood Ultrastructure - An Atlas of Electron Micrographs, by Cote, W.A. 1967. By permission of University of Washington Press, Seattle.

In the primary wall the microfibrils form a loose, irregular net-like orientation. In the outer (S1) layer of the secondary wall, the microfibrils are more precisely oriented, but are nearly perpendicular to the long axis of the cell. In the S2 layer, the microfibrils run almost parallel to each other in a tight spiral around the cell. This layer is the thickest and has the greatest effect on how the cell, and therefore the wood, behaves.

The smaller the angle the microfibrils make with the long direction of the cell, the stronger the cell is. In the innermost (S3) layer of the cell wall the microfibrils are once again oriented almost at right angles to the cell's long axis.

As the cell wall is forming, small openings called pits are created. (See Figure 1.8 below). Pits are thin spots where the secondary wall has not formed. Pits are normally matched in pairs between adjacent cells and allow liquids to pass freely from one cell to the next. Obviously the function of pits is very important, especially to the wood treater. However, because they are very small in some species they can be easily plugged by deposits in the heartwood, making the cell wall almost impermeable to liquids and therefore difficult to treat.

Figure 1.8
Pits provide tiny passageways for flow of water and liquids

Chemical Composition of Wood

Earlier in this lesson we learned that photosynthesis, which occurs in the leaves (or needles), produces glucose (C₆ H₁₂ O₆ ), a solution of sugar in water. Glucose is carried via the phloem tissue (or inner bark) to the growing tissues in the tree, that is, the cambium layer and the tips of branches and roots, where a very important chemical process occurs.

Glucose molecules (as many as 30,000) link end to end with each other in long straight chains to form cellulose molecules. Because so many glucose molecules will link together, cellulose is said to have a high degree of polymerization. However, even the longest cellulose molecules, which are about 10 microns long,(1 micron = .001 mm) are too small to be seen even with an electron microscope.

Cellulose, the main building material of all plant cells including trees, makes up about 50 percent of the dry weight of wood. Because bonding between and within glucose molecules is so strong, cellulose molecules are very strong and they are the reason wood is so strong. Lateral bonding between cellulose molecules is also quite strong, causing them to group together to form strands that, in turn, form the thicker,ropelike structures called microfibrils.Microfibrils can be seen with an electron microscope.

Hemicellulose, the second chemical component of wood, makes up 15 to 25 percent of the dry weight of wood. Unlike cellulose, which is made only from glucose, hemicellulose consists of glucose and several other water-soluble sugars produced during photosynthesis. The degree of polymerization (that is, the number of sugar molecules connected together) is lower for hemicellulose and they form branched chains rather than straight chains. Hemicellulose surrounds strands of cellulose and helps in the formation of microfibrils.

The third chemical component of wood is lignin, a complex chemical, completely different from cellulose. Lignin makes up about 15 to 30 percent of the dry weight of wood. It occurs in the wood throughout the cell wall, helping to cement microfibrils together. However, it's also concentrated toward the outside of cells and between cells. Lignin is a three-dimensional polymer, though its exact structure is not fully understood. Lignified plants differ from those which do not have lignin, (for example, grasses). Wood would be similar to cotton (which is almost 100 percent cellulose) if it wasn't for lignin. Lignified plants such as trees and shrubs are stiff and are able to grow tall. Lignin is thermoplastic, which means it becomes pliable at high temperatures and hard again when it cools.