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Miscellaneous Reports Maine Agricultural and Forest Experiment Station

1999

MR412: Wood Properties of Red PineTakele Deresse

Robert K. Shepard

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Recommended CitationDeresse, T., and R.K. Shepard. 1999. Wood properties of red pine (Pinus resinosa Ait.). Maine Agricultural and Forest ExperimentStation Miscellaneous Report 412.

WOOD PROPERTIES OF RED PINE(Pinus resinosa Ait.)

Takele DeresseGraduate Research Assistant

and

Robert K. ShepardProfessor of Forest Resources

Department of Forest Management

College of Natural Sciences, Forestry, and AgricultureMaine Agricultural and Forest Experiment Station

University Of MaineOrono, Maine 04469

CFRU Information Report 42

ACKNOWLEDGMENTS

This report was reviewed by Dr. William Ostrofsky and Professor Alan Kimball of the Department ofForest Management, University of Maine. Funding for this work was provided by the Cooperative ForestryResearch Unit, the McIntire-Stennis Program, and a grant from the USDA.

3Maine Agricultural and Forest Experiment Station Miscellaneous Report 412

INTRODUCTION

This report describes important physical andmechanical properties of red pine (Pinus resinosaAit.) and the factors that influence the variation inthese properties. Some results from a recently com-pleted study on red pine in Maine (Deresse, 1998)are presented to help illustrate and explain some ofthe more important concepts and relationships. Inaddition to studies specific to red pine, importantfindings on other conifers are presented for com-parison with red pine and to provide a more compre-hensive review of conifer wood properties. The stud-ies on the other coniferous species help to highlightthe sources of wood property variation. With thecurrent emphasis on intensive management andshortened rotations, forest managers must giveincreased attention to the factors that affect woodproperties and ultimately the suitability of the woodfor intended uses.

WOOD PROPERTY VARIATION

Wood properties in conifers vary among species,among trees of the same species, and within indi-vidual trees. The main sources of variation in woodproperties are genotype, environment, and the in-teraction between the two. The genetic factors thatinfluence properties of wood can be categorized asthe heritability of the cambial characteristics, andthe provenance.

Of the hereditary factors, the largest percent-age of the total variation in tree growth character-istics and wood properties is explained by differ-ences that exist among trees of the same stand as aresult of inherited genes (Zobel and van Buijtenen,1989). The heritability of growth rate and tree form,and specific gravity and fiber length, is very high(McKimmy, 1966; McKimmy and Nicholas, 1971;Talbert et al., 1983, Yanchuk and Kiss, 1993). Inaddition to these well-documented, heritable treecharacteristics, the maturation of the vascular cam-bium is also believed to be controlled by geneticfactors (Lindström, 1996).

The other major genetically controlled source ofwood property variation is differences among prov-enances (Zobel and Van Buijtenen, 1989). Prov-enances are distinct populations of a species thathave differentiated as a result of an environment-driven evolutionary process. Provenances throughtime acquire features that best suit their specificgrowth requirements, and they adapt to the prevail-ing conditions. The adaptation process may alter ormodify the differentiation process of the xylary cells(woody cells) in their chemical composition or ultra-structure, hence modifying their properties. In agiven species this evolutionary differentiation be-

tween populations accounts for a relatively smallpercentage of the total variation that occurs in woodproperties (Zobel and van Buijtenen, 1989). Redpine has limited genetic diversity (Horton and Bedell,1960; Fowler, 1965; Fowler and Morris, 1977;Mosseler et al., 1991, 1992) and, therefore, differ-ences among trees and stands are smaller for thisspecies than for most other species.

Among the non-hereditary factors that influ-ence wood property variation, the environment isthe most important (Zobel and Van Buijtenen, 1989).The environmental factors can be divided into cli-mate (variation in climate), site quality, and standenvironment (competition among individual trees).Coniferous tree crowns continuously change andadapt, responding to these factors. The changes incrown morphology in turn initiate changes in theactivity of the vascular cambium and the derivationof xylary cells. The influence of the crown on thecambium is exerted through the distribution ofphotosynthates and the growth regulating hor-mones—auxins, gibberellins, abscisic acid, cytoki-nins and ethylene (Larson, 1969; Savidge andWareing, 1984). It is now well accepted that growthhormones play an important role in determiningcell size and the development of earlywood andlatewood, as well as the width of the juvenile woodalong the length of the bole.

Other non-hereditary factors that influence for-mation of wood and its variation in quality can becategorized as latent factors (Lindström, 1996).These include external factors such as gravity, wind,injuries, and growth-related stresses. The influenceof gravity or wind is believed to induce reactivechanges and to alter the differentiation process ofthe vascular cambium. In coniferous trees abnor-mal cells known as compression wood are formedunder these modifications. The tracheids that makeup compression wood are altered both chemicallyand ultrastructurally and become a source of varia-tion in wood properties. Compression wood containsa larger amount of lignin and a lower percentage ofcellulose than normal wood. The tracheids areshorter in length, larger in diameter, and the cross-sectional shape is rounded when compared to thetypical rectangular shapes of normal tracheids(Kollmann and Côté, 1968; Haygreen and Bowyer,1989).

Other latent factors, i.e., injuries or fungal at-tacks, are less significant since material with visibledefects from injuries is usually not processed. How-ever, effects of injuries and microbial attacks as acause of variation in wood properties should beconsidered due to possible changes in the develop-ment of wood that is in proximity to the injured oraffected tissues (Blanchette, 1992).

4 Maine Agricultural and Forest Experiment Station Miscellaneous Report 412

In addition to the sources of variation discussedabove, the normal process of tree-aging is a majorsource of wood property variation. Under normalgrowth, the cambium (in coniferous species) devel-ops from a tissue that produces juvenile wood cellsinto a tissue that differentiates into mature woodcells. As discussed earlier, the maturation process isclosely related to the inherited characteristics of theindividual tree and the relation between the cam-bium and the most active part of the crown. Eachindividual cambial derivative (whether it is a juve-nile wood or a mature wood), however, passesthrough successive developmental stages. In eachstage the xylary cell plays a different physiologicalrole and as such has variable properties.

One of the most recognized age-related trans-formations of woody cells is their physiological tran-sition from sapwood to heartwood. Depending onthe property in question, heartwood (defined as thepart of the stem in which all cells including theparenchyma cells are dead) can be slightly or sig-nificantly different from sapwood. In coniferousheartwood, most tracheid pits are aspirated and insome species tylosids are deposited in the lumens ofthe cells (Hillis, 1987). In red pine and many coni-fers the tracheids become impregnated by extrac-tives, reducing the void volume. The deposition ofextractives, the formation of tylosids, and the aspi-ration of pits consequently affect many physicalproperties such as color, permeability, specific grav-ity, and fiber saturation point, hence becoming asource of wood variation.

The remainder of this report describes impor-tant physical and mechanical wood properties andthe factors that affect them. Included are specificgravity, longitudinal shrinkage, microfibril angle,modulus of rupture, and modulus of elasticity.

1. Specific Gravity1. Specific Gravity1. Specific Gravity1. Specific Gravity1. Specific GravityOf all indices that characterize wood properties,

specific gravity is used universally to define woodquality. Specific gravity is a dimensionless numberand is expressed as the oven dry weight of a sampleof wood divided by its green volume divided by thedensity of water:

specific gravity = [(oven dry wt)/(green vol)]/(density of water)

The strong relationship of specific gravity to me-chanical properties, fiber yield, and other proper-ties relevant to the end use of forest products, andthe relative ease of its determination, make it asimple and a good descriptor.

Identifying all of the factors that influence spe-cific gravity variation alone is impossible or at bestvery complex. The influence of crown and age onxylem cell differentiation rate, on tracheid size

(length, cell wall to cell lumen ratio), on the forma-tion of latewood and earlywood, and on specificgravity are a continuum of the same process. There-fore, specific gravity may be linked directly to allpreviously discussed factors of variation.

In a stand where growing conditions are equalor comparable, differences in mean whole-stem spe-cific gravity among trees mostly arise due to differ-ences in genotype. According to Zobel and Talbert(1984), 70% of the overall specific gravity variationin a species is due to these differences that occur ina given stand. The remaining 30% is accounted forby differences among provenances and sites. Com-pared to other conifers, provenance differences inred pine are less for the reasons that were statedearlier. Such red pine provenance uniformity inspecific gravity was clearly exhibited by Peterson(1968), where among 10 provenances studied onlyone deviated from the rest by more than 0.02.

Within a single tree, specific gravity varies bothvertically (along the length of the stem) and radially(from the pith to the bark) in distinct patterns thatare characteristic to a species. In both directions thevariation is influenced by age and distance from themost vigorous part of the crown. Due to better accessto growth-regulating hormones and photosynthates,the portions of the cambium that are directly influ-enced by the crown differentiate into juvenile woodtracheids. With age and crown recession up thestem, the cambium progressively evolves into atissue that ultimately differentiates into maturewood tracheids (Larson, 1969). One of the biggestsources of radial variation in specific gravity is dueto these differences between the juvenile and ma-ture wood.

Variation from pith to barkVariation from pith to barkVariation from pith to barkVariation from pith to barkVariation from pith to barkAt any given cross-section of a stem, specific

gravity varies with age from the pith outward to thebark. According to Panshin and de Zeeuw (1980),such age-related specific gravity variation in coni-fers can be categorized in three general patterns(types), of which two are most prevalent in temper-ate conifers. The Type-I pattern is exhibited mostcommonly in many Larix and Pinus and occasion-ally in Picea species. In this pattern, the mean ringspecific gravity increases from the pith to the barkin a linear or curvilinear trend, flattening in themature section. This trend may exhibit a slightdecrease in the outer rings of overmature trees. Inthe next most frequently encountered trend pat-tern, Type-II, the mean specific gravity of the juve-nile core decreases in its early formation, and thenincreases until the mature period is reached. Simi-lar to Type-I, in mature wood of Type-II species themean ring specific gravity fluctuates around an

5Maine Agricultural and Forest Experiment Station Miscellaneous Report 412

average maximum and may fall towards the outerrings of overmature trees.

Even though both types of patterns are encoun-tered in red pine, it usually exhibits the Type-IIpattern (Panshin and de Zeeuw, 1980). Examples ofType-II patterns were found by Peterson (1968),where 10 red pine provenances exhibited patternsin which specific gravity declined to the age of 13,after which the trend was characterized by a linearincrease and then a leveling off. Similar patternswere also evident in studies of red pine in Mainewhere Type-II patterns occurred in both naturaland plantation stands (Shepard and Shottafer, 1992;Deresse, 1998) as illustrated in Figure 1. Figure 1,based on the work of Deresse (1998), shows thechange in specific gravity at breast height with agefor dominant and large codominant trees from tworelatively fast- growing stands and two slower grow-ing, much older stands. The diameters at breastheight of the study trees from all stands rangedfrom approximately 12 to 15 in.

The graph illustrates two points. First, specificgravity in the two rings closest to the pith wasconsiderably greater than specific gravity in thethird and fourth growth rings. After the ninth andtenth growth rings it began to increase, eventuallyleveling off at about age 30, and in one of theyounger stands, at approximately age 40. This clearlyindicates that red pine must be grown to an age of atleast 30 years in order to begin to produce wood ofmaximum specific gravity. Specific gravity at theend of the juvenile period (about ring 30) was about30% higher than in rings five and six. Growingstands to a greater age will ensure a larger volumeof high specific gravity wood, which will yield agreater weight of pulp per unit volume of wood andbe stronger. Growing red pine under short rotations

means that the wood will have less desirable prop-erties for most uses than wood from older stands. Itis clear, based on the trends in Figure 1, that for twotrees of 12 in. DBH, one 40 years old and one 80years old, the older tree would have a considerablyhigher overall specific gravity.

Second, there is a clear indication that betweenthe ages of about 10 and 30, the specific gravity oftrees from the younger, faster-growing stands wasless than specific gravity of trees from the olderstands, possibly because the influence of the livecrown on wood development in the lower portion ofthe bole of the rapidly growing trees was exerted toa greater degree and for a longer time than in theolder stands, where crown recession presumablybegan at an earlier age. This suggests the possibilityof an additional reduction in wood properties of fast-growing stands, over and above the reduction due tothe shorter rotations alone.

The juvenile period for specific gravity in redpine is considerably shorter than the juvenile periodfor red spruce, which may approach 70 years insome stands (Wolcott et al., 1987; Shepard, 1997). Itis somewhat longer than for balsam fir, which rangesfrom 30 to 35 years (Shepard - unpublished data). Incontrast, the juvenile period for loblolly pine isabout 10 to 12 years (Saucier, 1989).

Specific gravity variation across the stem frompith to bark is directly related to the variation in thepercentage of latewood in each growth ring. Formost experimental purposes, the latewood is cat-egorized by Mork’s definition, as a region of the ringwhere the radial cell lumens are equal to, or smallerthan, twice the thickness of radial double cell wallsof adjacent tracheids (Denne, 1989). In species suchas loblolly pine, an increase in specific gravity isstrongly associated with an increase in the propor-

Figure 1. Specific gravity variation with age at breast height for dominant and codominant trees sampled from old redpine stands (o) and (*), and young red pine stands (C) and (◆). Each observation is the average of samples from fivetrees and for two consecutive rings together (From Deresse, 1998).

0.280.300.320.340.360.380.400.420.440.460.48

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6 Maine Agricultural and Forest Experiment Station Miscellaneous Report 412

tion of latewood (Ifju and Labosky, 1972; Taylor andBurton, 1982). Kollmann and Côté (1968) illus-trated this close relation between latewood percent-age and specific gravity for several coniferous spe-cies, such as Scots pine, eastern white pine, andbalsam fir. However, the correlation between late-wood percentage and specific gravity appeared tovary among species.

Most studies agree that variation in the propor-tion of latewood accounts for a major proportion ofthe ring-to-ring variation in specific gravity. How-ever, the variation may also depend on the magni-tude of the differences between mean earlywoodand latewood specific gravity. Warren (1979), byisolating the weighted variance and covariance com-ponents of specific gravity (i.e., proportions of early-wood and latewood and their respective specificgravities) showed that the magnitude of earlywoodto latewood differences had significant importancein accounting for the specific gravity variation.Smith (1956) reached a similar conclusion where amultiple regression model that included the specificgravities of the earlywood and latewood, as well asthe proportion of the latewood, accounted for almostall of the specific gravity variation in second-growthDouglas-fir.

In red pine the influence of earlywood andlatewood variation on specific gravity generallyfollows patterns similar to those discussed above.Results from the 10 provenances studied by Peterson(1968) showed that the juvenile wood specific grav-ity was mostly influenced by the change in the cross-sectional cell size (cell wall thickness and lumenwidth), while the mature wood specific gravity ap-peared to be strongly associated with the percent-age of latewood. Comparable inferences were alsomade by Shottafer et al. (1972), who found a strongrelationship between specific gravity and latewoodpercentage for plantation-grown red pine.

In individual growth rings of temperate regionconifers, specific gravity increases from earlywoodto latewood. This increase follows different patternsand appears to be species specific. The difference inspecific gravity between the early- and the late-formed regions can be as much as two to four times(Kollmann and Côté, 1968). The width of earlywoodor latewood or the magnitude of the differencesbetween the two is influenced by age and growthrate. With the development of X-ray densitometry,image analysis, and other techniques for microden-sitometry, work on within-ring specific gravity varia-tion has been simplified and this variation can nowbe estimated in terms of its components. Estimatingintra-ring variation helps in understanding the ef-fects of growth rate or other management factorsthat influence growth (Liu and Tian, 1991; Walkerand Dodd, 1988; Zhang et al., 1996).

Variation along the boleVariation along the boleVariation along the boleVariation along the boleVariation along the boleSpecific gravity also varies along the length of

the bole within a ring (sheath) formed in one season.This variation from the base towards the apex of atree can be explained by differences in age of thecambium and the level of direct influence of thecrown. According to Simpson and Denne (1997), thespecific gravity of 52-year-old Sitka spruce treesvaried with tree height in patterns that varied withthe position of the test samples. In samples fromsheaths produced between ages 10 and 20, specificgravity decreased from the apex to the base. Incontrast, the specific gravity of samples from thesheaths produced between ages 40 and 50 followeda different pattern. Specific gravity decreased fromthe uppermost internode downward to the eighth totwelfth internode, and then increased until it reacheda maximum at about internode 20 from the top(Simpson and Denne, 1997). From a different ap-proach, where average specific gravity of stem cross-sections was analyzed, the relationship betweenspecific gravity and tree height was reported to benegative for jack pine (Spurr and Hsiung, 1954).Studies on native Maine conifers by Wahlgren et al.(1966) and Baker (1967) also indicated a specificgravity decline from the bottom up; however, thesepatterns appeared species specific and the magni-tude of the decline differed greatly among the spe-cies investigated.

2. Longitudinal Shrinkage2. Longitudinal Shrinkage2. Longitudinal Shrinkage2. Longitudinal Shrinkage2. Longitudinal ShrinkageWood is an anisotropic (unequal physical prop-

erties along different axes) and hygroscopic (ab-sorbs moisture from the air) material, and its physi-cal and mechanical properties vary with the changeof moisture content below the fiber saturation point(the point at which all liquid water has been re-moved from the cell lumen, but the cell wall is stillsaturated). This phenomenon of dimensional insta-bility is exceptionally important in the use of woodas a construction material. The dimensional changewith moisture is greatest in the tangential direc-tion, generally about two times more than the radialchanges. The smallest, but perhaps most important,dimensional changes occur in the longitudinal di-rection (along the length of the board). For mostsoftwoods longitudinal shrinkage from the green tothe oven-dry condition is estimated to be about 0.2%of the green length; however, juvenile wood mayshrink four to five times more than the averagevalue (Haygreen and Bowyer, 1989).

Longitudinal shrinkage is very important incausing twisting, warping, and other types of boarddeformation that occur on drying. Longitudinalshrinkage resulting from large amounts of juvenilewood in lumber is responsible for the “rising truss”phenomenon (Gorman, 1984). This problem results

7Maine Agricultural and Forest Experiment Station Miscellaneous Report 412

Figure 2. Best-fit curves from a regression analysis showing the relationship between age and longitudinal shrinkage atbreast height in dominant and codominant trees from two red pine stands. From green to 12.4% MC -- young, fast-growing stand (o) and old, slow-growing stand ((); from green to 0% MC -- young, fast-growing stand (C) and old, slow-growing stand (◆) (from Deresse, 1998).

from the longitudinal movement of wood in thelower chord of house trusses as moisture contentchanges. This movement causes the truss to rise,separating the ceiling from the room partition.

Estimation of longitudinal shrinkage is an er-ror-sensitive procedure, even when shrinkage isrelatively large, as in juvenile wood. Therefore,measurements require special attention and preci-sion. Longitudinal shrinkage measurement can alsobe confounded by many of the natural and dryingdefects (i.e., grain deviation, warp in longer samples,and other stress-related defects). Constructing in-formative trends from longitudinal shrinkage datais also difficult because of large differences amongspecimens coming from different locations within atree. This large variation tends to obscure the influ-ence of other factors such as age, tree, site, andprovenance.

In softwoods longitudinal shrinkage when dry-ing is from green to oven dry follows a more or lessuniform trend. The trends reported by Foulger (1966)for white pine and by Ying et al. (1994) for loblollypine appear to be typical. In both studies longitudi-nal shrinkage rapidly decreased from the pith out-ward to rings 10 to 15 and then fluctuated around aminimum. Longitudinal shrinkage in red pine speci-mens from four stands also followed this generalpattern when dried from green to approximately12% MC and to 0% MC (Deresse, 1998).

Figure 2 illustrates the age-related variation inlongitudinal shrinkage of wood from two red pinestands. It is apparent that a greater proportion ofthe wood in trees from the younger stand than fromthe older stand would shrink longitudinally. This isbecause with trees of approximately the same DBH,the first 15 growth rings would constitute a much

greater proportion of the stem cross-section inyounger trees. Figure 2 also shows that longitudinalshrinkage when drying was from the green condi-tion to 0% moisture content was approximatelythree times greater than when drying was to 12%moisture content.

A variety of factors influence longitudinal shrink-age, and no single factor can consistently explainthe variation. Age, specific gravity, percentage ofearlywood, and microfibril angle have been cited byYing et al. (1994), and cell wall thickness and thenature of the middle lamella by Meylan (1968), assources of variation in longitudinal shrinkage.Among these sources, microfibril angle may be themost important.

The pattern of longitudinal shrinkage reportedby Ying et al. (1994) was similar to the pattern ofmicrofibril angle (MF-angle) decline in the samestudy. According to the authors, 21% of the varia-tion in longitudinal shrinkage of wood when goingfrom the green to the oven-dry condition was ex-plained by MF-angle, and MF-angle was a betterdescriptor of longitudinal shrinkage than specificgravity and proportion of earlywood. From a moretheoretical approach, Cave (1968) and Meylan (1968)concluded that the relations between MF-angle ofthe S2 layer and longitudinal shrinkage are similarin most conifers.

3. Microfibril Angle3. Microfibril Angle3. Microfibril Angle3. Microfibril Angle3. Microfibril AngleThe term microfibril angle (MF-angle) is used to

describe the helical angles that microfibrils make inrespect to the longitudinal axis of the xylary cells.Microfibrils are the smallest identifiable structuralunits of the cell walls and can readily be observedusing an electron microscope. There is no definite

00.05

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8 Maine Agricultural and Forest Experiment Station Miscellaneous Report 412

Figure 3. A digitized image showing the orientation of the cell walls (A), the orientation of the microfibrils (B), and theangle (ααααα) included between the two. The image from a field taken in the middle of ring 2 at breast height from a fast-growing red pine tree at X1200 magnification shows relatively large microfibril angles of approximately 35o (from Deresse,1998).

A

B

"

explanation on how microfibrils are formed, butirrespective of the phases and stages through whichthey may pass during formation, microfibrils can bedefined as aggregates of strongly bonded, cellulosicchains. In crystalline regions of microfibrils, cellulo-sic chains are arranged lengthwise and tend to beparallel to one other. Each crystalline region isencased in an amorphous paracrystalline sheath(Timell, 1965) and is separated by intermicrofibrillarspace from an adjacent unit (Stamm, 1964).

The formation and arrangement of microfibrilsdiffer between the cell wall layers. In the primarycell wall, the microfibrils are arranged more or lessin a random fashion and are loosely packed in thematrix material (hemicellulose and lignin). In con-trast, the three main layers of the secondary cell wallare made up of microfibrils that are closely packed andexhibit recognizable arrangements (Stamm, 1964;Wardrop, 1965; Kollman and Côté, 1968).

Of the three layers of the secondary cell wall, theS2 layer is the most important in determining theproperties of wood because of its thickness. Com-pared to the S1 and S3 layers, which combined havebetween 8 to 12 lamellae (higher aggregates ofmicrofibrils), the S2 layer is the thickest and hasbetween 30 and 150 lamellae. In the tracheids ofconifers the microfibrils of the S2 layer are highlyorganized and run parallel to one other, mostlyforming Z-helices around the cells. In normal wood

these helices make an angle with the vertical axis ofthe cells that is usually less than 30o (Kollman andCôté, 1968). Studies show that intermediate to strongcorrelation exists between the MF-angle of the S2layer and stiffness (Ifju and Kennedy, 1962;Tamolang et al., 1967), anisotropic elasticity (Cave,1968), and shrinkage (Meylan, 1968; 1972)

The MF-angles of the S2 layer are usually deter-mined indirectly. One widely used procedure is theuse of the interfibrillar spaces as reference direc-tions for the orientation of the MF-angles. It isbelieved that the interfibrillar spaces can be en-larged by drying small blocks of wood at 103±2oC.The enlarged spaces appear in the form of crack-likepropagations under a microscope (Stamm, 1964).The microscopic checks can further be enhanced bystaining for better visualization and measurementusing different techniques, as discussed in Baileyand Vestal (1937), Senft and Bendtsen (1985), andYing et al. (1994). Figures 3 and 4 present twodigitized images showing microfibril orientations inthe form of short dark lines that are more or lessparallel to each other and deviate from the orienta-tion of the tracheid cell walls. These dark lines areformed by iodine precipitation in the cracks of the S2layer that were formed during drying. Longitudinalshrinkage of the wood in Figure 3 would be muchgreater than longitudinal shrinkage of the wood inFigure 4.

9Maine Agricultural and Forest Experiment Station Miscellaneous Report 412

Figure 4. A digitized image showing the orientation of the cell walls (A), the orientation of the microfibrils (B), and theangle (ααααα) included between the two. The image from a field taken in the middle of ring 30 at breast height from a fast-growing red pine tree at X1200 magnification shows microfibril angles of approximately 9o (from Deresse, 1998).

A

B

"

In contrast to specific gravity and mechanicalproperties, less work has been done on MF-anglebecause of the tedious nature of the methods avail-able. To date all known procedures of MF-anglemeasurement require considerable time in mea-surement or specimen preparation, and the indirectX-ray diffraction method requires a more expensivetechnology.

Published results indicate that variation existsin MF-angle among species, among trees of thesame species, and within a single tree. Dependingon the methodology, techniques, and level of sam-pling, these variations can be very large. Havingthis in mind, Megraw (1985) cautions that infer-ences that can be made from MF-angle measure-ments are limited.

The mean MF-angle in the S2 layer varies withage and exhibits trends that probably indicate geno-typic characteristics of the species. The MF-angledeclines (the angle between the microfibril and thevertical axis gets smaller) with age from the pithoutward to the bark. Work by Erickson and Arima(1974) on 28-year-old Douglas-fir, Wang and Chiu(1988) on 34-year-old Japanese cedar, and Ying etal. (1994) on 25-year-old loblolly pine illustrate thistrend. In these studies larger angles were clearlyassociated with juvenile wood, and the mean ringMF-angle was found to be large in the first few ringsnear the pith and was followed by a steep decline inapproaching the ‘mature’ region. In all three studiesthe rate of MF-angle decline was significantly re-

duced after 10 years and leveled off in the outerrings. According to Wang and Chiu (1988), thispattern of MF-angle decrease with age was ob-served at all heights of the tree stems of Japanesecedar; however, the rate of decrease at the base ofthe stems was slower compared to higher positions.

MF-angle decreases from earlywood to late-wood. The mean earlywood MF-angle may be two tothree times larger than the mean latewood MF-angle. For mature Douglas-fir wood (age 36 to 45years), Ifju and Kennedy (1962) reported an aver-age ratio of 0.33 between the latewood and theearlywood MF-angles. Analyzing the trend of early-wood and latewood MF-angles in Japanese cedar,Wang and Chiu (1988) found that the latewood MF-angles leveled off at about 10o, and the earlywoodMF-angles at 15o to 25o.

In red pine mean ring MF-angles surveyed intwo young stands showed a decline with age (Deresse,1998). The results from this study also revealed thatwithin individual rings the largest and the smallestMF-angles were primarily associated with the tra-cheids formed at the beginning and at the end of thegrowing season, respectively. In one of the stands,for which every second ring from the pith wassurveyed to ring 40, the MF-angles in the earlyformed tracheids averaged 2.5 times larger than theMF-angles in the tracheids formed in the latter partof the growing season. Figure 5 illustrates the varia-tion of MF-angles (from one of the stands) with ageand for six equally spaced positions within each

10 Maine Agricultural and Forest Experiment Station Miscellaneous Report 412

Figure 5. MF-angle variation with age at breast height for dominant and codominant trees in a young red pine stand.Averages for ten samples at each of six within-ring positions (P) equally spaced from each other when going fromearlywood to latewood; P-1 (-o-), P-2 (—o—), P-3 (-*-), P-4 (—*—), P-5 (-(-) , P-6 (—(—) and ring mean MF-angle(—C—). Positions were 15%, 30%, 45%, 60%, 75%, and 90% of the distance across the ring (from Deresse, 1998).

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ring. The positions were at 15%, 30%, 45%, 60%,75%, and 90% of the distance across the ring. It isclear that MF-angle decreased until about the 15thgrowth ring, after which it changed very little.

MF-angle exerts some control over longitudinalshrinkage, with shrinkage generally increasing asMF-angle increases (Megraw et al. 1998). It is evi-dent from Figures 2 and 5, that the trend in longitu-dinal shrinkage closely follows the trend in MF-angle, with both decreasing until about the 15thgrowth ring from the pith and changing relativelylittle after that. A relationship exists between MF-angle and longitudinal shrinkage, because whenthe orientation of the microfibrils in the S2 layer ofthe cell is at a significant angle from the cell axis, thecell becomes shorter as the wood dries, and conse-quently, longitudinal shrinkage occurs (Haygreenand Bowyer, 1989).

In contrast to the trends summarized aboveand the trends illustrated in Ying et al. (1994),McMillin (1973), using the polarized light tech-nique, did not find differences in MF-angles be-tween the core, middle, or outerwood samples ofloblolly pine. Investigating differences in MF-angleof earlywood and latewood, the study also found nopattern that may indicate the effect of age on theMF-angle.

In most of the above studies the relation be-tween MF-angle and age is evident. The variation inMF-angle can also be correlated with tracheid length,tracheid cross-sectional size, and other physical andmechanical properties (Megraw, 1985). Strong cor-relations were found with tracheid length in Dou-glas-fir (r=-0.94; Erickson and Arima, 1974), with

tensile strength in Douglas-fir (r=-0.88; Ifju andKennedy, 1962), and with modulus of rupture (r=-0.54) and modulus of elasticity (r =-0.68) of the earlyjuvenile wood of red pine (Deresse, 1998).

4. Modulus of Rupture and Modulus of Elasticity4. Modulus of Rupture and Modulus of Elasticity4. Modulus of Rupture and Modulus of Elasticity4. Modulus of Rupture and Modulus of Elasticity4. Modulus of Rupture and Modulus of ElasticityModulus of rupture (MOR) and modulus of elas-

ticity (MOE) are important properties for the use ofwood as a structural material. MOR is an indicationof the bending strength of a board or structuralmember, and MOE is an indication of the stiffness.The correlation of MOR and MOE with specificgravity is typically very strong, as reported byShottafer et al. (1972) and Shepard and Shottafer(1992) for red pine, Wolcott (1985) for red spruce,and Han (1995) for red maple. However, in someconiferous species, such as Abies fabri, Piceaasperata, and Pinus massoniana, the relationshipof MOE to specific gravity is weaker than the rela-tionship between MOR and specific gravity (Zhang,1997), and this was also found to be true in fast-growing red pine (Deresse, 1998). It has been re-ported that wood samples having similar specificgravity can also exhibit significantly differentstrength values due to factors that may be associ-ated to other factors to which specific gravity is lesssensitive (Perem, 1958; Zhang, 1995; Deresse, 1998).The determination of MOR and MOE together withspecific gravity, therefore, is important to betterunderstand these relationships.

When analyzed among trees and within a tree,mechanical property variation tends to follow simi-lar patterns to those observed in specific gravity.The largest variation in mechanical properties of

11Maine Agricultural and Forest Experiment Station Miscellaneous Report 412

Figure 6. Modulus of rupture (MOR) variation with age at breast height for dominant and codominant trees sampled fromold red pine stands (o) and (*) and young red pine stands (C) and (◆). Each observation is the average of samples fromfive trees (from Deresse, 1998).

100020003000400050006000700080009000

10000

0 10 20 30 40 50 60 70 80 90 100 110

Ring number from the pith

MO

R (

psi)

wood is found between trees of the same stand, andthe remaining variation can be explained in a simi-lar way to the variation in specific gravity. Theradial variation in a single tree of a coniferousspecies can be characterized as strongly and posi-tively dependent on cambial age (number of ringsfrom the pith). Work by Wolcott (1985) on red spruceand Shepard and Shottafer (1992) on red pine showeda rapid increase in MOR and MOE from the pith tothe boundary of the juvenile core and a tendency toplateau in the mature wood, as shown in Figure 6 forMOR of red pine (Deresse, 1998). These variationtrends in many instances are distinct to each spe-cies, and they may also reflect the environmentunder which the trees develop.

Figure 6 shows the same general relationshipbetween MOR and age as between specific gravityand age in Figure 1. (The relationship betweenMOE and age was much like the relationship be-tween MOR and age.) In three of the four standsMOR leveled off at about age 30 (or continued toincrease very slightly after that age). However, inone of the young stands, MOR continued to increaseuntil at least age 40. MOR in the other young standwas considerably lower. The difference between thetwo stands is attributed largely to measured differ-ences in MF-angle, with angles in the stand havingthe lower MOR being greater than those in thestand having the higher MOR (Deresse, 1998). Thesame was true of MOE. A major difference in therelationship between MOR (and MOE) and age andthe relationship between specific gravity and age isin the relative increase in the two properties thatoccurred during the juvenile period. Specific gravityincreased by about one-third during the juvenileperiod. In contrast, MOR approximately doubled

during the juvenile period, and MOE was approxi-mately three times greater at the end of the juvenileperiod (Deresse, 1998). Thus, strength and stiffnessincrease relatively much more with age than spe-cific gravity does. The same is true for other species.

The juvenile periods for MOR and MOE in redpine are shorter than in red spruce, where theyrange from 40 to 60 years (Wolcott et al., 1987;Shepard, 1997). They are about equal to those forbalsam fir (Shepard, unpublished data). By con-trast, the juvenile period for both MOR and MOE inloblolly pine is reported to be about 13 years(Bendtsen and Senft, 1986).

Compared to the physical properties such asspecific gravity and shrinkage, there are more con-cerns about the decline in the quality of wood thatcomes from intensively managed stands in terms ofmechanical properties. Many studies support theopinion of Bendtsen (1978) and Senft et al. (1985),who feel that the decline in mechanical properties isattributable to the accelerated growth that leads toan early harvest of trees containing a larger propor-tion of juvenile wood. Therefore, the differences inquality are attributable largely to the differences injuvenile and mature wood.

A study by Pearson and Ross (1984) that cov-ered three sources of loblolly pine (a 41-year-oldnaturally regenerated stand, a 25-year-old planta-tion, and a 15-year-old plantation of geneticallyselected stock, with all trees of comparable dbh)supports the above discussion. The results fromthat study showed that in all stands MOR and MOEincreased as sample distance from the pith in-creased. The magnitude of this increase, however,differed markedly among the three sources. Thesedifferences were particularly large in wood from

12 Maine Agricultural and Forest Experiment Station Miscellaneous Report 412

near the pith. Overall, the samples from the naturalstand exhibited the highest values and the geneti-cally selected trees the lowest. According to theauthors these differences were attributable to theage difference of the three sources.

In contrast to the approach taken by Pearsonand Ross (1984), where comparisons were madeprimarily on the basis of the physical position of thespecimens, the use of microbending test specimens(Bendtsen and Senft, 1986) enables the separationof the age effect in quantifying differences that existbetween materials of different sources. The applica-tion of microbending tests has been demonstratedin Wolcott (1985), Shepard and Shottafer (1992),and Han (1995), where the methodology was used todetermine the transition periods from juvenile tomature wood. In Deresse (1998), results frommicrobending tests were used to statistically sepa-rate the effects of age, stand, and ring width on thevariation of MOR and MOE in two red pine stands.Results from this study indicated that the variationobserved in the data, and the mechanical propertydifferences between the two stands, could not befully explained by age only, as had been reported byPearson and Ross (1984) for loblolly pine. Multipleregression analysis showed that ring width (aver-age ring width in samples containing more than onering) was a highly significant source of variation,and a large proportion of the differences betweenthe two stands could be explained by the differencesin growth rate. Ring width was negatively corre-lated to both MOR and MOE; however, the impact ofchanges in ring width was stronger on MOE thanMOR (Deresse, 1998).

EFFECT OF GROWTH STIMULATINGFACTORS AND VIGOR ON PROPERTIES

OF WOOD

Growth rate, defined as the number of woodycells that are derived from the vascular cambiumper unit time, is primarily a product of the influenceof genetics and environment. Therefore, one canconsider growth as a permanent trace of the effect ofall factors. It is also understood that the rate ofgrowth is variable, and that at any given height ina tree stem it declines with age as a result of adecline in the influence of the crown. Because thereare differences in growth between parts of the bolestrongly influenced by the crown and those lessinfluenced by the crown, it is important to recognizethis when growth rate is discussed in relation towood properties.

Growth rate can be partially controlled throughthe manipulation of the stand environment. Standmanipulation in the form of initial spacing, thin-

ning, or fertilization intrinsically affects wood prop-erties through changes in crown morphology. How-ever, the magnitude of the changes on properties ofwood differ from species to species.

In discussing the influence of fast growth onwood properties, it is most appropriate to clearlyidentify the periods of fast growth in question or thestage in tree development at which this stimulatedgrowth has occurred. The effect of fast growth atearly stages of tree development should not beexpected to have the same effect as increased growthin a later stage of development. For example, in-creased initial growth rate favors formation of alarge juvenile core. This means that DBH alone isnot necessarily a good indicator of wood properties;age must also be considered, as was illustratedpreviously. It is also important to understand thatdifferences exist in the way different species re-spond to factors that stimulate or retard growth. Insome species (i.e., red pine) the early growth envi-ronment (competition) (White and Elliot, 1992;Puettmann and Reich, 1995) was reported to inducelasting crown adaptation, and by inference a lastingeffect on wood properties.

Spacing is one of the best tools to control standdevelopment, and increased spacing has a positiveeffect on growth rate. Comparing initial spacing togrowth and specific gravity, Baker and Shottafer(1970) and Larocque and Marshall (1995) found thatincreased spacing favored radial growth in red pineand that specific gravity appeared to decline. In thelatter study the mean earlywood and latewood den-sities also declined with an increase in spacing.Similar observations on the relationship betweenspacing and specific gravity were also made forNorway spruce by Lindström (1996) and for Sitkaspruce by Petty et al. (1990).

In contrast to the findings discussed above, anearlier report by Baker (1969) for 16-year-old redpine trees planted at three spacings found no cleardecline in average tree specific gravity as spacingincreased. Similarly, Jayne (1958) did not find anysignificant differences for red pine that was plantedat three different spacings and on two differentsites.

The duration of the initial-spacing effect ongrowth and wood properties depends upon the over-all development of the stand. Discussing this short-term and long-term influence on growth and spe-cific gravity of unthinned Sitka spruce stands,Simpson and Denne (1997) reported that initialspacing had the expected effects on radial growth inthe first 15–20 years of growth, where wider ringswere strongly and positively correlated with thewider spacings. However, in later years (25–40years of growth) this correlation was reversed andbecame negative. In contrast to ring width, the

13Maine Agricultural and Forest Experiment Station Miscellaneous Report 412

correlation between specific gravity and initial spac-ing became increasingly positive with age.

The effect of thinning and fertilization on spe-cific gravity is well reviewed in Zobel and vanBuijtenen (1989). In contrast to initial spacing, theinfluence of these practices appears to depend uponconditions under which specific applications aremade. The direct influence of thinning may beshort-lived and may cause a radial growth increasewhile specific gravity and fiber length may decline(Erickson and Harrison, 1974; Megraw, 1985).

The effect of fertilization also depends on theobjectives of its application. If fertilizers are appliedto remedy a soil deficiency that is a source of ex-treme growth retardation, the results could be animprovement in certain aspects of wood quality.Otherwise, as Larson (1969) explained, fertilizationmay have an adverse effect by reducing the rate ofcrown recession and prolonging the juvenile period.The effect of fertilization on red pine properties,discussed in Gray and Kyanka (1974), illustratesthe variable effects, with radial growth and MOEimproving and all other physical and mechanicalproperties showing a slight or significant decline.

The effect of fast growth (radial growth rate) onwood properties has been much debated. Wood prop-erty variation that appears to be related to growthrate may be the result of the differences betweenjuvenile and mature wood. Illustrative results ofsuch differences are discussed in Pearson andGilmore (1971, 1980) and Pearson and Ross (1984).The three studies were based on loblolly pine logs ofcomparable size that were sawn and tested undersimilar techniques. Samples that originated fromstands of different age groups exhibited propertyvariations that were largely explained by the agedifferences. Pearson and Gilmore (1971), by analyz-ing the results from MOR and MOE tests, alsoconcluded that if age was statistically removed byusing specific gravity as its descriptor all sampleswould belong to the same population, irrespective ofthe ring width.

In many conifers, and especially the hard pines,growth rate seems to have little effect on mostproperties of wood (Taylor and Burton, 1982; Zhang,1995). Zobel and van Buijtenen (1989) list studiesthat comprehensively cover different aspects of thistopic. However, it is also important to point outfindings on loblolly pine that indicate specific grav-ity decreased as growth rate increased (Yao, 1970).This relation between specific gravity and growthrate for samples taken at breast height of loblollypine trees was also found by Pearson and Gilmore(1971).

In contrast to species that are characterized byan abrupt transition from earlywood to latewood,there is some question as to the relationship be-

tween wood properties and growth rate for thosespecies that have gradual transition, such as whitepine, and red, black, and white spruce. Brazier(1977), reviewing wood property variation in spe-cies characterized by a gradual earlywood to late-wood transition and a narrow latewood band, pointedout the negative influence of growth rate on specificgravity. In several coniferous species (Abies fabri,Abies nephrolepis, Picea asperata, and Piceakoraiensis) characterized by gradual transition fromearlywood to latewood, similar observations werealso made by Zhang (1995), and the effect of growthrate was more pronounced on mechanical proper-ties than on specific gravity. The influence of growthrate on the properties of red pine wood was alsofound to vary. The results in Deresse (1998) showthat no specific gravity difference was exhibitedbetween two young, fast-growing stands as a resultof their difference in growth rate. In contrast, sig-nificant differences existed between the two standsin MOR and MOE, as discussed earlier.

In other coniferous species, such as Sitka spruce,a statistically significant negative relationship be-tween ring width and specific gravity was found byBrazier (1970). Simpson and Denne (1997) foundthe same relationship in Sitka spruce after takingthe effect of age into consideration. In the latterstudy the relation between ring width and specificgravity, however, appeared to differ when analyzedby age and tree height.

Compared to the findings discussed in the twostudies above, a stronger negative correlation be-tween ring width and specific gravity was reportedfor Norway spruce. Investigating trees from a fer-tilization experiment, Lindström (1996) found astrong relation between the logarithmic value ofring width and ring specific gravity. A non-linearregression was also found to best model the relationbetween the two variables, indicating the varyingeffect of ring width on the measured specific gravityvalues.

In Norway spruce the ring width effect on spe-cific gravity appeared to vary depending upon thewidth of the rings. For example, ring width changesin narrow rings (ranging between 0.04 in. and 0.12in.) had a larger relative impact on specific gravitythan changes in wider rings. With an increase inring width, the magnitude of the specific gravitychange as a result of a change in ring width becamemore variable, and the relation between the twodiminished. This type of variability was also ob-served for samples taken from the inner core of thestem in Sitka spruce (Simpson and Denne, 1997).

Deresse (1998) found a similar relationship inred pine, where specific gravity was negativelyrelated to ring width (Figure 7) and positively re-lated to age. Over the range in ring width from

14 Maine Agricultural and Forest Experiment Station Miscellaneous Report 412

approximately 0.18 in. to 0.35 in., there was norelationship between specific gravity and ring widthin red pine. Specific gravity did not increase as ringwidth decreased until ring width decreased below0.18 in. It should be emphasized that the higherspecific gravities at the narrow ring widths arepartly related to the fact that specific gravity in-creases with age, and as age increases, ring widthdecreases. The source of variability in the widerrings could be a result of other factors, such asreaction wood that can be present in the juvenilewood (Simpson and Denne, 1997).

Work by Petty et al. (1990) supports the resultsfrom the above studies. The results for the outer-most five rings at breast height from 48-year-oldtrees of Norway and Sitka spruce exhibited aninverse linear relation between ring width and spe-cific gravity. The relations were strong in Sitkaspruce (r = -0.85), which had predominantly nar-rower rings. In contrast, in the Norway sprucespecimens that contained rings 0.12 in. and morewide, the correlation between the two variables waslower (r = -0.44). A similar relationship was alsoevident in western hemlock (DeBell et al., 1994), butthe “average drop” in specific gravity was extremelylarge. Samples containing rings 20-24 from the pithshowed an average decrease in specific gravity from0.47 to 0.37 for an average ring width increase from0.08 in. to 0.32 in.

These growth-rate-related property variations(in specific gravity and some mechanical proper-ties), encountered in species like the spruces withgradual transition from earlywood to latewood orother species that lack a clear abrupt transition (i.e.,red pine), are mostly attributed to the nature of thetransition wood that develops under a variablegrowth rate. It has been discussed that specific

gravity of those species, and by inference manymechanical properties, is closely related to the vari-ability and width of the earlywood and latewood. Itis believed that in these species the relative width ofthe latewood, meaning its proportion, is more af-fected by ring width than in species that have anabrupt transition from earlywood to latewood. Ac-cording to Taylor and Burton (1982), there is anegative correlation between ring width and late-wood proportion in loblolly pine, especially in ringsformed in the early stages of tree development. InBrazier (1977) the same phenomenon was also re-viewed based on the results of Larson (1969). Theseresearchers attributed the variation to the chang-ing nature of the “intermediate-wood”. The point isthat the intermediate-wood, found between the trueearly-wood and true latewood, in slow growth quali-fies more as latewood while in vigorous growth ittends to be more like earlywood.

In recent reports, DeBell et al. (1994) on west-ern hemlock and Zhang et al. (1996) on black spruce,documented a strong positive correlation betweenearlywood width and overall ring width, meaning astrong negative relation between ring width and thepercentage of latewood. The increase in ring widthand consequently the increase in the earlywoodproportion also had an adverse effect on meanearlywood specific gravity, and a slight decline inthe earlywood specific gravity was evident in bothspruces.

Compared to its effect on specific gravity andmechanical properties, the effect of growth rate onfiber length appears to be similar in most coniferousspecies. This influence on softwood tracheid lengthis particularly visible in the early stages of treedevelopment. The relation between growth rate andtracheid length in most studies was found to be

0.26

0.3

0.34

0.38

0.42

0.46

0.02 0.06 0.10 0.14 0.18 0.22 0.26 0.30 0.34

Ring width (in.)

Spe

cific

gra

vity

Figure 7. Specific gravity variation with ring width at breast height through age 30 in dominant and codominant treesfrom two young, fast-growing red pine stands (from Deresse, 1998).

15Maine Agricultural and Forest Experiment Station Miscellaneous Report 412

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Wood Properties Of Red Pine(Pinus resinosa Ait.)

Takele DeresseGraduate Research Assistant

andRobert K. Shepard

Professor of Forest Resources

Department of Forest ManagementCollege of Natural Sciences, Forestry, and Agriculture

University of MaineOrono, Maine 04469

CFRU Information Report 42

ACKNOWLEDGMENTS

This report was reviewed by Dr. William Ostrofsky and Professor Alan Kimball of the Department ofForest Management, University of Maine. Funding for this work was provided by the Cooperative ForestryResearch Unit, the McIntire-Stennis Program, and a grant from the USDA.