Biermann Ch. Handbook of Pulping and Papermaking

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534 25. WOOD AND FIBER-GROWTH AND ANATOMY

spondingly higher in cellulose than normal fibers.

Staining with safranin and fast green shows normal

fibers as red and gelatinous fibers as green. The

number of vessels is decreased in tension wood.

The microfibril angle is lower than in normal

wood, but the numerous incipient tension failures

in the fiber cell walls give a weak wood or pulp.

Elm is an example of a hardwood with much

branching (bifurcating trunk). It is bound to have

large areas of tension wood due to its normal

growth form. Tension wood from the genus

Eucalyptus is associated with permanent collapse

during drying.

Pith,

juvenile wood

The pith and the first five to ten growth

rings,

corresponding to a core of several inches of

wood, have properties that vary greatly from the

rest of the wood (Besley in Anon., 1962). The

juvenile wood is generally much weaker than

normal wood and often shows leaf scars. The

wood has lower specific gravity, shorter fibers,

and lower cellulose content than normal wood. In

loblolly pine the juvenile wood averages 7 growth

rings with a diameter of about 3.5 in. and has a

density of about 80% of that of normal wood.

Trees (such as southern pine) growing in the open

have more knot wood and juvenile wood of lower

quality than trees growing under some cover

(Paul, 1963).

False growth rings

False growth rings are an additional growth

region (visible in the cross section of wood) that is

produced by a pronounced decrease in the growth

rate followed by resumption of growth within a

single season. This may be a result of defoliation

followed by foliage growth or temporary adverse

conditions of temperature or moisture (too much

or too little). False growth rings are relatively

common in baldcypress.

Interlocking grain

Interlocking grain occurs in wood when the

fibers are inclined in one direction through several

growth rings, slowly return to perpendicular in

succeeding growth rings, and then gradually re-

verse to become inclined in the opposite direction

for several growtii rings, and reverse again to

repeat the process. Interlocked grain gives a

ribbon or stripe figure on radial surfaces.

Spiral grain

Spiral grain is a type of growth where the

fibers take a spiral course around the center of the

tree instead of being longitudinal. The spiral may

be clockwise or counterclockwise.

25.4 SILVICULTURE AND WOOD QUALITY

Wood is a variable material. Its properties

depend on

the

species, individual genetics, location

within a tree, and growing conditions. There are

many well—known relationships among the actual

properties such as fiber length and microfibril

angle with position within a growth ring or tree,

but other relationships vary with species. All of

these factors contribute to wood quality. There is

much talk about growing "super" trees that meet

specific purposes. For example, fast—growing

trees with long fibers and low lignin contents are

ideal for pulp and paper. However, genetic

selection of trees has been largely limited to

growth rates and specific gravities of wood.

Wood quality is in the domain of the forest scien-

tist, but the wood scientist must work with the

forest scientist to define wood quality.

Wood quality

Wood quality might be defined in a number

of ways from the practical to the esoteric. It may

be defined as the "suitability for its purpose" or

merely "the economic value of the wood." Like

wood chip quality, wood quality assumes someone

is actually monitoring it; i.e., there is no impetus

such as financial reward (or even recovery of

investment) for increased wood quality if

the

wood

user is not quantifying it.

Many properties contribute to wood quality,

and these depend on the end use of the wood.

Specific gravity, cellulose content, number of

knots,

uniformity, and microfibril angle of the

S—2 cell wall layer are several important factors.

Specific gravity is, in turn, a function of the

percentage of latewood and cell diameters and wall

thicknesses. These are functions of the species

and growing conditions (see pages 41 on fibril

angle and 153 on latewood versus early

wood).

is no coincidence that the two major U.S. structur-

al woods, Douglas—fir and southern yellow pine,

have thick, dense latewood.

SILVICULTURE AND WOOD QUALITY 535

There is some contradiction of the desired

properties of wood. For various paper types one

might want more or less latewood. There are

some common qualities that most wood users

agree on, however, including a higher level of

uniformity. To a degree, wood quality depends on

technology. New processes allow some woods to

be used in products where they could not be used

previously. Too, as technology changes, the ideal

requirements of the wood may change.

Wood variability within a tree

Panshin and de Zeeuw (1980) is a useful,

general resource for this topic. Fig. 25-15 shows

the variation in fiber length for spruce pine as an

example of a softwood. It is well known that the

average fiber length in trees increases from the

pith outward to a nearly constant average fiber

length after about 30—50 years growth from the

pith. Also the fiber length in latewood is appre-

ciably higher than in the earlywood of softwoods.

Fig. 25-16 demonstrates the variability of

fiber length in a hardwood, southern red oak. The

same trend (as in softwoods) in average fiber

length is shown.

With other factors equal (like growth rate),

the specific gravity in a tree is variable, decreasing

from the center outward in oak (Paul, 1963);

other hardwoods have increasing specific gravity

from the pith outward. Softwoods generally have

^55

l:;5o

^ 40

EARLYWOOD

LATEWOOD

25}-

2.0 ••

4.0 r

3.0 [

9ni

.oi

,0^

5^ 4.0

i:{3.

2.0 ••

: 4.0 r

= 3.0 50.2

. 2.01

.^.PV^

^^'

4.5 r

3.5

4.5

3.5 f

4.5 r

3-5 58.50

2.5'-

5 ^-5 [

^3-5[ 50.25

-3.5 [ 4200

|4.5r

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2.5

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460

50^

302520 IS K)9 0 5 10 15 202530 30 25 20 15 10 9 0 5 K) 19 20 25 30

YEARS FROM PITH

Fig. 25-15. Fiber length analysis of macerated samples of spruce pine wood. Curves to the right

of the pith are tracheid length contour lines. From Manwiller (1972).

536 25. WOOD AND FTOER-GROWTH AND ANATOMY

100

.50^

>l.50 MM.

I.I0-I.50

MM.

< I.IO MM.

100 80

40 60 80 100

20 0 20

AGE (YEARS)

Fig. 25-16. Variation in fiber length of south-

ern red oak. Reprinted from Koch (1985).

Fig. 25-17. Fast (left, specific gravity of 0.41)

and slow (sp. gr. of 0.31) growing yellow-poplar

from a single tree. Reprinted from Paul (1963).

increasing specific gravity from the pith outward

to the bark.

Chamaecyparis

and

J?iuja

are excep-

tions where the opposite is true.

Silviculture and wood specific gravity

Latewood is much more dense than

earlywood in softwoods. Since slow—growing

softwoods have a higher proportion of latewood,

they also have a slightly higher density. However,

the percentage of latewood accounts for only about

45—75%

of the variation in wood specific gravity

since the specific gravity of both latewood and

earlywood is variable. The amount of moisture in

the soil and the size of the tree crown are key

variables that are inversely related to the percent-

age of latewood.

Other factors such as the age of the growth

ring and position within a tree also contribute to

the variation. In softwoods, the wood density

generally increases from the pith outward and

levels off (at age 20—50) until the tree is very old

(100 years old), when it may decrease.

Paul (1963) summarized his work of four

decades on the application of silviculture to control

specific gravity of wood and gave a good review

of the topic. Conifers have an ideal growth rate

with at least 10 growth rings/in., although 15 is

ideal. One worker suggested 12 ring/in. for

Douglas—fir in Germany.

With hardwoods, however, a slow growth

rate usually leads to a wood of low density. Fig.

25-17 demonstrates the effect of growth rate on

specific gravity of yellow—poplar (difftise—po-

rous) and Fig. 25-18 for white ash (ring—porous).

Contrasted this to Fig. 25-19, where slow growth

gives somewhat higher density in softwood.

Genetics plays a role in wood quality.

Selection of trees for fast growth, high—density

wood, and other desirable characteristics improves

wood quantity and quality. Tree selection, howev-

er, tends to reduce the genetic variability within

the species. Large tracts of land are planted with

a single species of the same age in monoculture

that is susceptible to massive damage by disease.

Many other factors contribute to wood quali-

ty. Do the logging operations involve clearcutting

of large tracts of land or is the logging limited to

thinning and cutting rows? The proper selection

of fungi from the Rhizobium genus in the growth

of leguminous plants under various conditions can

SILVICULTURE AND WOOD QUALITY 537

greatly improve their productivity. Should la-

bor—intensive pruning and tree planting be used?

How can one predict the economic return, which

may not occur for 40 years? Are economic

considerations important only for the relatively

short term when predictions can be made?

Silviculture and compression wood

Proper forestry can minimize compression

wood formation in young, second—growth stands

under a relatively high degree of management

(Pillow and Luxford, 1937). In such even—aged

stands thinnings should be made to remove over-

topped trees if they are at an incline, twin trees

with two stems from a single stump, and defective

or crooked trees. The dominant trees tend to be

straighter than suppressed trees that may grow at

an angle trying to obtain light. Other trees should

be removed to obtain the proper stocking to give

the proper spacing between trees.

However, in partially cut stands, wind action

and increased vigor may contribute to increased

compression wood formation. In partial cuttings

made to improve a stand, the formation of com-

pression wood will be decreased if large, irregular

openings in the crown canopy are avoided and the

action of violent winds thereby reduced.

Wood

use

patterns and wood quality

In areas that use residual chips from sawmill

operations, one would expect very good wood

quality for packaging papers since most of the

wood comes from slabs containing wood of the

outer growth rings. On the other hand, wood that

comes from precommercial thinnings will have a

very high percentage of juvenile wood. Usually

it costs more to handle small wood than large

wood. Questions remain as to the long—term

effects of plantation grown wood of short rotation

compared to longer rotations on soil properties.

25.5 ANNOTATED BIBLIOGRAPHY

General wood anatomy

Brazier, J.D. and G.L. Franklin, Identifica-

tion of hardwoods. Forest

Prod,

Res, Bull

46,

London, 1961.

Core, H.A., W.A. Cote, and A.C. Day,

Wood Structure and Identification, 2nd ed.,

Syracuse University Press, 1979, 138 p. and

appendix. Numerous optical and scanning

Fig. 25-18. Slow Geft, specific gravity of 0.48)

and fast (sp. gr. of 0.65) growing white ash

from a single tree. Reprinted from Paul (1963).

Fig. 25-19. Old growth Oeft, 24 rings/in.) and

second growth,

rings/in.

redwood

(Paul,

1963).

538 25. WOOD AND FIBER-GROWTH AND ANATOMY

electron micrographs describe the important

features of wood; identification keys are

included.

Harrar, E.S. and J.E. Lodewick, Identifica-

tion and microscopy of woods and wood

fibers used in the manufacture of pulp, The

Paper Ind. February:630-637;

ibid.,

May:103-lll;

ibid.,

Aug.:327-335(1934). A

summary of wood and anatomy of fiber for

pulp and paper technologists. Coniferous

woods are considered in the first article,

hardwoods in the second, and fibers in the

third.

Hoadley, R.B., Identifying

Wood,

Taunton

Press,

Newtown, Connecticut, 1990, 223 p.

This book has a high circulation and is of

modest cost. It includes many color plates

and a long bibliography; it is a practical

book on wood identification with a hand lens

and includes sources of authentic wood

samples, prepared slides, equipment, etc.

Ilic, J.,

CSIRO

Atlas of Hardwoods, CSIRO,

Australia, 1991. Text is minimal and photo-

graphs are maximal: 69 pages show the

crosssections of 1284 species (1.6 by 1.7 in.,

in color) organized by family and genus at

6.5 X; 402 pages show microscopic images

of the three (A:, r, t) sections

(25

x), and

vessel—ray pitting (lOOx).

6. Jane, F.W., The Structure of

Wood,

A. and

C. Black, Ltd., London, 1956, 427 p. (2nd

ed., 1970). This resource has a more de-

tailed discussion of many aspects of wood

anatomy than other references. It is global in

perspective.

Koehler, A., Guidebook for the identification

of woods used for ties and timbers, USD A

Forest Service, Misc. RL-1, 1917, 79 p. and

31 plates showing 62 cross sections at

Numerous maps are included showing the

distribution of many species. The photo-

graphs were made by passing light through

thin specimens, which gives better detail than

photographing the cross sections of large

pieces.

8. Panshin, A.J., and C. de Zeeuw, Textbook of

Wood Technology, 4th ed., McGraw-Hill

Book Company, New York, N.Y., 1980, 722

"Structure, identification, uses, and prop-

erties for the commercial woods of the U. S.

and Canada" with many micrographs and

descriptions by wood species. Keys for

identification are included for hardwoods and

softwoods. The 4th ed. has minor changes

over the 1970 3rd ed. (705 p.) This work

goes back before the 1st ed. of 1949 to the

1934 work

Identification

of Commercial Tim-

bers of the

United

States of H.P. Brown and

A.J. Panshin. [Commercial timbers of the

United States was published in 1940 by the

same authors and has key references on

wood identification that are omitted in the

later editions, which is the origin of much of

the material; the quality of photographs in

the 1940 ed. with coated paper is higher than

the 1980 and 1970 eds. on uncoated paper.]

9. Phillips, E.W.J., Identification of softwoods

by their microscopic structure. For.

Prod.

Res.

Bull. 22:1-56(1948), London.

Staining of

wood

10.

Kutscha, N.P. and LB. Sachs, Color tests for

differentiating heartwood and sapwood in

certain softwood species, USD A FPL Rep.

No.

2246, 1962, 13 p. Includes 21 solutions

for 23 species.

11.

Parham, R.A., Tflppz

65(4):

122(1982).

It is

very useful to determine the hardwood frac-

tion of chips in chip mixtures using a reac-

tion which specifically stains hardwoods. A

very useful, well—known reaction is the

Maule reaction. Chips in a plastic mesh (for

ease of transfer) are treated at room tempera-

ture in a fume hood with

1 %

KMn04 for 10

min., brief rinse, 6 N HCl for 1 min, brief

rinse, 10 % NH3 for 1 min. The chips are

dried in the hood. The weight of the red

(hardwood) portion as a percentage of the

total is determined.

Plant

microtechnique

12.

Johansen, D.A., Plant Microtechnique,

McGraw-Hill, New York, 1940, 523 p.

ANNOTATED BIBLIOGRAPHY 539

13.

Sass, J.E., Elements of Botanical

Microtechnique, McGraw-Hill, New York,

1940,

222 p. The 3rd edition of this book

appeared in 1958. The book was rewritten

by G.P. Berlyn and J.P. Miksche as Botani-

cal Microtechnique and Cytochemistry and

published by Iowa State University Press

(Ames, Iowa) in 1976.

Fiber and wood

microscopy

14.

Carpenter, C.H., et al,

Papermaking

Fibers,

Tech. Pub, 74, State University College of

Forestry, Syracuse, New York, 1963. This

is a photomicrographic atlas of woody and

other fibers with 77 plates. Entire isolated

fibers of numerous species are featured. It is

an update of the 1952 work; the original

version of 1932 is the Atlas of

Papermaking

Fibers by C. H. Carpenter with 56 plates.

15.

Graff,

J.H., A

Color

Atlas for

Fiber

Identifi-

cation, Inst. Paper Chem., Appleton, Wise,

1940,

27 p. 4-

color plates. Three classes

of stains are described; the five color plates

are used to compare the results of the tests.

16.

Isenberg, I.H., Pulp and Paper Microscopy,

3rd ed., Inst. Paper Chem., Appleton, Wise,

1967,

395 p. This work is an expansion of

Graff (1949) above with much detail, but

without color plates. It includes a foundation

in optics and microscopy, fiber dimension

measurements by microscopy, descriptive

fiber morphologies of wood and nonwood

plant fibers, animal fibers, and mineral fibers

with numerous pictures of actual fibers, and

discussions on numerous color stains useful

for fiber characterizations. The color stains

can often be used directly on paper to charac-

terize its properties. This is a comb—bound

notebook.

17.

Parham, R.A. and R.L. Gray, The Practical

Identification of Wood Pulp Fibers, TAPPI

Press 1982, 1990, 212 p. This comb—bound

book has very high quality paper that helps

reproduce the photographs. This work is

very useful for wood fiber identification.

Optical microscopy and SEM photos are

included with each species. There are no

keys,

and the format is much like an atlas,

although there is moderate text included.

Numerous fibers from each species indicate

the variability of the fiber morphology.

18.

Strelis, I. and R.W. Kennedy, Identification

of North American Commercial Pulpwoods

and Pulp Fibres, University of Toronto

Press,

1967, 117 p. This work is especially

useful for fiber analysis; photomicrographs

of animal, mineral, and wood and nonwoody

vegetable fibers are included. Techniques for

fiber identification by solubility analysis and

staining are included. (Of course, FT—IR

analysis of individual fibers as a useful tool

for fiber analysis was not commonly avail-

able at the time of this work.)

19.

Tappi Standard T 401 om-82, Fiber analysis

of paper and paperboard, 14 p., 32 referenc-

es.

This standard discusses fiber morphology

and many staining techniques for quantitative

analysis of fiber types.

20.

Tappi Standard T 263 om-82, Identification

of wood and fibers from conifers, 13 p., 3

references. It is a reprint of B.F,

Kukachka's article in Tappi 43(11):887-

896(1960) with some added information;

Araucaria—Agathis is

listed as having spirals

(should be pits 2—4 alternate).

Reaction wood

21.

Perem, E., Tension wood in Canadian hard-

woods. Can. Dept. of For. Pub. No. 1057,

1964,

38 p.

22.

Pillow, M.Y and R.F. Luxford, Structure,

occurrence, and properties of compression

wood, USD

Tech.

Bull. No. 546,1937,32

Silviculture

and wood

quality

23.

Anon., Wood Quality, Proceeding of three

symposia, Dept. of Lands and Forests Res.

Rep.

No. 46, Ontario Research Foundation

Rep.

No. 601, 1962. Published in Pulp

Paper

Mag.

Can, (2nd symp. in Aug., 1960

Woodland's Section, but in the nontechnical

540 25. WOOD AND FIBER-GROWTH AND ANATOMY

pages omitted in many libraries) and Timber

Mag. Can. (3rd symp., issue unknown).

24.

Kellison, R.C. and R.G. Hitchings, Harvest-

ing more southern pines will require pulp

mill changes, Pulp & Paper 59(7):53-56

(1985).

This article summarizes problems

associated with using young trees as an

increasing percentage of raw material.

25.

Paul, B.H., The application of silviculture in

controlling the specific gravity of wood,

USDA Tech. Bull. No. 1288, 1963, 97 p.

This is an superb reference on the topic; it

was first published in 1930 as No. 168.

Bark

anatomy

26.

Chang, Y.-P.,

Bark Structure

North

Amer-

ican Conifers, USDA Tech. Bull. No. 1095,

1954,

86 p. Keys are included to identify the

origin of bark samples.

27.

Chang, Y.-P., Anatomy of

Common

North

American

Pulpwood

Barks,

Tappi Monograph

Series 14, 1954, 249 p.

Pulpwoods

28.

Isenberg, I.H., Ed. (revised by Harder,

M.L., and L. Louden), Pulpwoods of the

United States

and

Canada,

3rd edition. Insti-

tute of Paper Chemistry, Appleton, Wiscon-

sin, 1981. This two—volume set (Volume 1,

conifers; Volume 2, hardwoods) lists tree

species, their silvics, wood properties, and

general pulping characteristics with referenc-

es.

It includes the commercial species of

North America.

Statistical

information

on wood

industry

29.

A.O.,

World Forest

Inventory

1963,

Rome,

1966.

31.

U.S. Bureau of the Census, Statistical Ab-

stract of

the

United States, 111th ed., 1991,

Washington, D.C., $28, pp 675-683 for

wood products industry.

Miscellaneous

32.

Bamber, R.K. and J. Burley,

The

properties

radiata

pine. Commonwealth Agric. Bu-

reaux, England, 1983, 84 p.; large bibliog-

raphy.

33.

Chattaway, M.M., The development of

tyloses and secretion of gum in the heart-

wood formation,

Aust.

J. Sci. Res. B. I'.lll-

240(1949).

34.

Howard, E.T. and E.G. Manwiller, Anatomi-

cal characteristics of southern pine

stemwood. Wood Sci. 2(2):77-86(1969).

35.

Koch, P.,

Utilization

of Hardwoods Growing

Southern

Pine Sites, USDA Agric. Hand-

book No. 605, 1985.

36.

Kukachka, B.F., Properties of imported

tropical woods, USDA Eor. Ser. Res. Paper

FPL 125, March, 1970, 67 p.

EXERCISES

Write a one—page essay discussing some of

the major differences between softwood

anatomy and hardwood anatomy.

Would you say compression wood generally

has advantages in pulping and papermaking

(compared to normal wood)?

Describe how softwood of the same species

could vary drastically in two different planta-

tions.

30.

FAO, Yearbook,

Rome, 1992.

Forest Products 1990, 4. Which are more prevalent globally:

woods or softwoods?

hard-

SOFTWOOD ANATOMY

26.1 GROSS ANATOMY OF SOFTWOODS

Introduction

This chapter presents the detailed anatomy of

the wood of temperate softwoods. The anatomy of

various woods is important for several reasons.

The characteristics of papermaking fibers depend

much upon their anatomy. The pulping character-

istics of various fiber sources are dependent upon

their species and growing conditions. Some

species are lumped together as an indistinguishable

lumber commodity, but may have greatly differing

pulping and papermaking characteristics. For

example, southern pine is really four different

species of pine. For these reasons it is important

to be able to verify the species types of wood

chips or even pulp mixtures. One may wish to

examine the alterations of individual fibers by

processes such as refining. Identification of trees

in forests allows the use of flora, bark, twigs, and

other features; this topic is not discussed here.

The gross description and uses of many of

the wood species contains information from the

USDA Wood Handbook (Ag. Handbook No. 52,

1935,

1955, 1974, 1987) with minor alterations.

Some of the hardwoods contain anatomy informa-

tion from Koch (1985) with minor alterations.

Ag.

Handbook No. 101 (1956) was of some use,

and Koehler (1917) was used extensively. These

resources are in the public domain. Some details

have been added from the references listed in

Chapter 25. The wood uses are somewhat dated

as plastics and other materials have replaced some

the

uses; nevertheless, the listed uses say much

about the wood properties in manufacturing and

use.

Other photographs and information are from

OSU forest products courses courtesy of

Drs.

R.L.

Krahmer and A.T. Van Vliet.

Softwoods are distinguished from hardwoods

by the absence of vessels. The characteristic

features of softwoods that are visible with a hand

lens will be demonstrated. In the next section

microscopic features of softwood will be consid-

ered. Then, in the section after that, the anatomy

of many individual softwood species is given.

Resin canals

The most pronounced feature of softwoods is

resin canals (shown by microscope in Fig. 26-1) in

those species which have them. Resin canals are

not formed by cells, but are voids surroimded by

epithelium cells. (In contrast, hardwood vessels

are made up of individual cells.) Tyloses may

occasionally form in the resin canals of heartwood.

The pines, spruces, larches, and Douglas—fir

genera of Pinaceae contain normal resin canals in

the longitudinal and radial directions. The radial

canals are part of fusiform rays and are smaller

than longitudinal resin

canals.

The resin canals in

pines are particularly large and numerous and

occur throughout the growth ring. In the other

three genera, they are small, less numerous,

appear to be missing from some growth rings, and

may be grouped in small, tangential rows. Brown

and Panshin (1940) give northern white pine

longitudinal resin canals as 135—150 /xm average

diameter but as high as 200 /xm; radial canals

average 80 /xm in diameter. They may be difficult

to find in eastern spruce samples (Hoadley, 1990).

The large resin canals of pines appear as

short dark lines on the longitudinal surfaces (r, t)

due to the pitch or dirt trapped by the pitch. Epi-

thelium cells, which secrete resin, surround both

the longitudinal and ray resin canals. In pines

they are thin—walled and easily torn away when

free—hand sectioning (or even often when micro-

toming an embedded sample as in Fig. 26-1). In

the other three genera they are thick—walled and

usually remain intact with free—hand sectioning.

True firs, hemlocks, and cedars may form

traumatic resin canals in response to injury in the

tree.

These small resin canals occur in relatively

long rows in the tangential direction when the

wood cross section is viewed, but do not occur

enough to cause difficulty in wood identification.

These genera only have traumatic longitudinal

resin canals; radial resin canals (fusiform rays)

are always absent. North American hardwoods

normally lack resin canals but can form traumatic

resin canals as well (Fig. 27-3).

541

542 26. SOFTWOOD ANATOMY

III

Fig. 26-1. Longitudinal (top left) and radial

(top right) resin canals from Picea sitchensis

with thick-walled epithelium and from Pinus

strobus (bottom) with thin-walled epithelium.

Earlywood latewood transition

Another easily observed trait is the transition

between the earlywood and latewood. It is charac-

terized as abrupt or gradual (shown through a

microscope in Fig. 26-2). For example, in the

soft pines it is gradual but in the hard pines it is

abrupt. Engelmann spruce has a more abrupt

transition than the other domestic spruces.

Color

Sapwood always has a relatively light color

from near white to tan or yellow. The heartwood

of some species may be light or dark or of

distinctive color. Redwood heartwood is a distinc-

tive brown—red color. Junipers have a deep pur-

ple—red heartwood. Douglas—fir usually has a

bright reddish hue. A reference collection for

color comparison is very usefiil.

Fig. 26-2. Abrupttransitionbetweenearlywood

and latewood of Larix occidentalis (left) and

gradual transition of Abies concolor.

Odor

Odor is particularly useful in green wood of

freshly cut surfaces. One must learn odors on

known samples of the wood; do not try to recog-

nize them by their descriptions. Kiln—dried wood

will not exhibit as strong an odor as green wood.

The pine genera all share a characteristic odor.

Cedars,

Thuja

plicata and Libocedrus decurrens

have aromatic smells which may be accented by

moisture and/or mild heat. Douglas—fir also has

a unique odor.

Dimpled surfaces

Some species have a dimpled grain

[lodgepole (pronounced) and ponderosa pines and

sitka spruce] on the tangential surface (t) that is

GROSS ANATOMY OF SOFTWOODS 543

distinctive; this is caused by crystals in the bark

that press against the cambium, which deforms it.

The growth rings reflect this by being uneven

when viewed with magnification.

Greasy feel etc. (Hoadley, 1990)

Taxodium species have a greasy or tacky feel

while redwood feels dry. Tamarack feels waxy

(less greasy than baldcypress). Figure in wood,

the appearance of the tangential surface (such as

flat—sawn

wood) is not overly relevant to pulp

and paper, though it can be used in wood identifi-

cation. Figure is most pronounced on woods with

an abrupt latewood transition such as with Doug-

las—fir and southern pines or ring porous hard-

woods such as oak and hickory. Special figures

also occur by grain deviations such as at the crotch

of trunk splits and at burls (outgrowths of the

trunk) especially with hardwoods. Spruces are

separated from white pines or firs by their glossy

(having luster) longitudinal surfaces.

The volatile material of Jeffrey pine contains

a high percentage of /z—heptane while those of

ponderosa pine are high in pinenes. For some

mixtures of species machines are being developed

to sort logs or boards by, more or less, smelling

each one. This technique is similar to that used at

airports to detect plastic explosives in luggage.

There is some literature on chemotaxonomy,

taxonomy based on the chemical components of

wood and other tissues.

Hand lens

key

for softwoods

Table 26-1 is a dichotomous hand lens key

designed for major U.S. commercial softwoods of

the western U.S. It is presented courtesy of R.L.

Krahmer and A.C. Van Vliet. Each number is a

step where one must decide between a pair of

mutually exclusive choices. The format is typical

of many keys used to identify species of wood.

Microscopic examination of wood is often needed

for conclusive identification of most woods.

26.2 MICROSCOPIC ANATOMY OF

SOFTWOODS

Rays

Most softwood rays are uniseriate (one cell

wide) unless they contain resin canals; fiisiform

rays,

when present, constitute about 5% of the

rays.

A few species (redwood, incense cedar) are

biseriate for at least a portion of the ray, but many

species can be sporadically biseriate. The height

of the ray (number of cells) is a useful tool for

softwood identification. Cedars tend to have low

rays (Port—Orford—cedar and eastern redcedar

are generally 6 or less high) while the true firs and

baldcypress tend to have high rays (25 to over 40

high and occasionally almost 1 mm high).

Ray tracheids

Ray tracheids (Fig. 26-3) are easily found in

many species, especially the hard pines. In other

species they are found only with great difficulty

and should be counted as absent. Ray tracheids

are differentiated from ray parenchyma by their

small bordered pits in the cross—field. In larches

and hemlocks they occur at the upper and lower

margins and are called marginal ray tracheids. In

pines they occur marginally and throughout the

height of the ray. In coniferous species rays

containing both tracheids and parenchyma are

termed heterocellular while rays with only one

type of cell are termed homocellular. Alaska

Fig. 26-3. Sketch of Pinus resinosa

(r)

showing

two rows of dentate ray tracheids on the upper

margin of a wood ray and one on the lower

margin. The three layers of ray parenchyma

show 1-2 simple cross-field pitting. From

Kukachka (1960).