Variability in wood quality parameters in clones of Eucalyptus tereticornis

Variability in wood quality parameters in clones ofEucalyptus tereticornis Shakti S. Chauhan1* and Pankaj Aggarwal2

1School of Forestry, University of Canterbury,Christchurch, New Zealand2Institute of Wood Science and Technology, Bangalore,India

Postal AddressSchool of Forestry, University of Canterbury,Private Bag 4800, Christchurch, New ZealandPh. +64-3-3642987 ext 8521Fax: +64-3-3642124e-mail: [email protected]

Short Title: Wood quality in Eucalyptus tereticornis clones No. of Tables : threeNo. of Figures: Five

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Abstract:

Wood quality is becoming an important criterion inselection of superior genotypes in breeding.

Fourteen clones of Eucalyptus tereticornis were assessedfor wood quality namely basic density, acousticvelocity, longitudinal growth strain and volumetricshrinkage. The effectiveness of Pilodyn wood testerin screening trees according to wood density wasalso evaluated. The relationship between variouswood quality variables was also studied across theclones.

A significant variation was observed in the woodquality between clones. Within clones, thevariations in wood quality characters were notsignificant. Pilodyn penetration measurement instanding tree was found to be positively correlatedwith wood basic density. A significant positiverelationship was observed between acoustic velocityand basic density of wood.

The absence of any association of longitudinalgrowth strain with any of the measured wood qualityparameters suggests that superior clones having lowgrowth strains can be selected for solid woodproduction without compromising of wood density orstiffness.

Key words: acoustic-velocity/ clone/ eucalypts/growth-strain/ wood-quality

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Introduction:Eucalypts have gained significant importance inplantation forestry worldwide. Early propagationstrategies in most of the Eucalypts plantation focused onfast growth and yield mainly for pulp and paper industry.Many wood quality traits important for pulp productionslike basic density, fiber characteristics, lignin contentetc. are highly heritable (Raymond and Apiolza 2004,Raymond 2009) and therefore have gained a significantattention in selection and propagation of clones suitablefor pulp production. However, the wood qualityrequirements for solid wood products are substantiallydifferent from pulpwood production. The major qualityrequirements for solid wood products are wood density,stiffness, strength, stability, processing ease andaesthetic features. Most eucalypt species have moderateto high basic density, superior strength, stiffness andhardness. The inherent problems associated with thisgenus are large growth stresses, excessive shrinkage,difficulties in processing and drying etc. These inherentproblems have distracted both wood industry and consumersin utilizing this wood in high-valued sawn timberproducts. High growth stresses cause end-splitting inlogs, warping and deformation in boards during sawingleading to poor recovery of quality sawn timber. Manyeucalypts are prone to have high volumetric shrinkagethat can have significant commercial consequences interms of stability and recovery of timber (Greaves et al2004).

It is well recognized that many wood traits likebasic density, fibre length, stiffness and microfibrilangle that determine wood properties are under geneticcontrol. Also, growth stresses and shrinkage have beenreported to have a moderate to high heritability(Pelletier et al. 2008, Greaves et al. 2004, Murphy et al. 2005,Henson et al. 2004). Thus one expects substantialdifferences in these characters between differentfamilies, provenances, and clonal material. A largevariation has been observed in growth strains among

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families and provenances of various eucalypts (Yang etal. 2002, Raymod et al. 2004, Murphy et al. 2005), whileGerard et al. (1995) have shown influence of genotype onthe magnitude of inherent growth strains in severaleucalypts. Very high heritability have been reported inlog end splitting in logs of E. urophylla and E. grandis(Garcia 2001). The absence or low level of end splittingin log, an indirect indicator of growth stresses, hasbeen an important criterion in the tree-breedingprogramme in South Africa (Malan 1995). Santos et al.(2003) observed significant genetic variation in basicdensity, bowing in sawn wood (an other indicator ofgrowth strain), heartwood percentage, modulus ofelasticity in 41 open pollinated progenies of E. grandis

As clones can be selected for any heritable traits,clonal propagation can play a vital role in theproduction of quality sawn wood and reducing the largevariability in existing plantation resources. Selectionof appropriate clones with desirable wood quality traitsprovides an opportunity to grow trees with pre-definedwood characteristics for future timber production. Inthis paper, basic density, acoustic velocity,longitudinal growth strains and volumetric shrinkage wereassessed in 14 clones of Eucalyptus tereticornis, an importantcommercial species grown in India. The variability inthese wood quality traits between clones was analysed.The relationship between different wood quality traitswas also analysed in order to develop most appropriateselection strategies.

Material and methods:Fourteen clones were selected from a vegetativemultiplication garden having 25 clones of Eucalyptustereticornis. The clonal trial was established in 1998 atNagroor, about 35 kms from Bangalore by the Institute ofWood Science and Technology, Bangalore with a singleclone row and 25 replications of each clone. The treeswere planted at spacing of 1.5 m 1.5 m. The trees were9-year old at the time of this study. Clones in this

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study were preselected based on their good growth andtree form in the orchard. Five trees from each clone wererandomly selected. Over-bark diameter, longitudinalgrowth strain and Pilodyn penetration at the breastheight were measured in each of the selected trees.Pilodyn measurements were taken on two opposite sides ofeach tree using a 6-Joule Pilodyn tester. A small windowwas created by removing the bark at the point of pilodynmeasurement and a single pin penetration was measured oneach side and the two readings averaged.

Three trees from each of the studied clones werefelled and a clear bole of about 3 m length wasextracted. Immediately on cutting, both ends were endcoated with the black paint to avoid any moisture loss andtransported to the laboratory. These logs were kept ingreen condition by continuous spraying water until allmeasurements were completed. First, a 25 mm thick discfrom both the ends of each log was cut and debarked. Thesediscs were immediately measured for weight and volume todetermine the green density. These discs were oven-driedat 103oC and again measured for weight and volume. Discvolume was measured by measuring weight of the disc inwater to an accuracy of 0.1 g. Wood basic density andvolumetric shrinkage were determined from thesemeasurements. Logs were measured for acoustic velocityusing a resonance based acoustic tool. Growth strain measurement: Longitudinal growth strain in standing trees was measuredusing the wire strain gauge method (Chauhan et al. 2007).A portion of the bark from vicinity of the measuring pointwas removed to expose the wood surface which wasthoroughly cleaned using cotton to wipe surface moisture.A 5-mm wire strain gauge of 350 ohms resistance was gluedto wood surface using a cynoacrylated based adhesive andleft to cure and bond strongly. Once the strain gauge waswell bonded to the wood surface, it was connected to apurpose built two channel strain meter in quarter-bridgeconfiguration and the bridge circuit was balanced tozero. When growth strains are measured in standing treesusing wire-strain gauge method, a potential complication

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is the additional strain induced by wind loading. It isnecessary to wait until the wind stops so that there is nointerference from this extraneous force.

Wood fibers were cut above and below the strain gaugeusing a hand drill machine to relieve the tensile stressin the fibers. The cut slots were about 15 mm wide andabout 20 mm deep (until a constant longitudinal growthstrain is recorded). As the slots are made, the fibres inthe isolated segment of wood contract longitudinally andexpand transversely. Change in strain in immediatevicinity of the gauge is indicated by the strain meter andrecorded. The configuration of holes and strain gauge isshown in Fig. 1.

Acoustic velocity measurement: Measurement of acoustic velocity is a well recognisedmethod for ranking trees or logs according to theirstiffness (Tsehaye et al. 2000, Dickson et al. 2003). Themodulus of elasticity (MoE) of wood is derived from theacoustic velocity in wood and wood density at the time ofvelocity measurement using the following equation

MoE = density V2

The acoustic velocity can be determined either by theresonance frequency of longitudinal vibration in the logor by measuring transit time of a stress wave from onepoint to another along the log length. A tool designed atthe Institute of Wood Science and Technology, Bangaloreand fabricated by Spektronics Ltd., Bangalore, was usedto measure wood stiffness in resonance mode. Forresonance frequency, the log was gently tapped at one endand a microphone was used to capture the vibrationsignals. A fast fourier transformation (FFT) of thecaptured signal was performed by the inbuilt software toidentify the resonance frequencies of the log. Acousticvelocity (V) was measured from the fundamental frequencyof longitudinal vibration (f1) and log length (l) usingfollowing relationship:

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V = 2 l f1

Results and discussion:Standing tree measurements: The descriptive statistics ofthe clone-wise data measured in standing trees is givenin Tab. 1. The table presents the variability in themeasured parameters within and between clones. Values inparenthesis are standard deviations within the clone.

DBH was greatest with clones 1, 4 and 10 whereasclones 27 and 116 had relatively poor growth. A largevariation was observed in longitudinal growth strainamong clones with clone 116 showing the smallest growthstrain (396 m/m) and clone 71 exhibiting the maximumstrain (1284 m/m). Of the 14 clones, four clones (Clones1, 7, 105 and 116) had mean growth strains of less than700 micro-strains and are expected to have fewer growthstress related problems in processing. Further, clones 1and 7 had superior radial growth as compared to that of105 and 1116. The pilodyn penetration in wood alsovaried from 8 mm to 12 mm. A two-way analysis of variance(ANOVA) suggested a significant difference between clonesin DBH, longitudinal growth strain and pilodynpenetration. There were no significant variations in thevariables within the clones. A Duncan’s multiple-rangetest was used to compare the significant differencesbetween the individual clones. The proximity of clonesfor their DBH, growth strain and pilodyn penetration areshown in Fig. 2. Clones are ranked from highest to lowest(from left to right) for individual variable. Within eachindividual cluster or group, clones have no significantdifferences. Longitudinal growth strain and pilodynpenetration exhibit much higher variability between theclones. It is evident that the DBH has fewer groupsindicating less variation in DBH between clones. Wood properties assessment: Variability in basic density, acoustic velocity, MoE andvolumetric shrinkage are given in Tab. 2. The modulus ofelasticity was determined from the acoustic velocitymeasured in log and the green density. Acoustic velocityis the average of three logs per clone whereas density

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variables and volumetric shrinkage are the average of sixvalues (two discs from each log). As expected, the greendensity had a small variation with the range of 987-1080kg/m3. A large range was observed in other wood propertiesamong the clones. Basic density ranged from 583 to 736kg/m3, acoustic velocity ranged from 3.13 to 4.30 km/s,MoE ranged from 10.36 to 17. 26 GPa and volumetricshrinkage ranged from 14.83 to 22.81%. A low standarddeviation for each wood quality variable within a clonesuggests uniformity in wood quality with in the clone.

Clone 71 was found to have the lowest acousticvelocity (3.14 km/s) and the highest volumetric shrinkage(22.81%) with a low basic density (609 kg/m3) among theclones. The clone also had the highest level of growthstrains (1284 m/m) which is large enough to causeserious growth stresses related defects during processingand a relatively poor radial growth (DBH – 12.30 cm).Therefore this clone would not be suitable for solid woodproduction. Amongst the low strain clones mainly clones1, 7, 105 and 116, clone 1 has the highest basic density(736 kg/m3) as well as the highest acoustic velocity(4.18 km/s) and correspondingly MoE of 17.26 GPa. Theclone also exhibited a moderate level of volumetricshrinkage (16.87%).This clone appears to have woodquality characteristics suitable for heavy structuralapplications where dense and stiffer timber is desirable.For many solid wood products like furniture and lightstructural applications, a high wood density is notalways desirable. Clones 7, 105 and 116 with moderatebasic density, acoustic velocity and volumetric shrinkageappear to be potential clones for solid wood production.However, among these three clones, clone 105 and 116 hadcomparatively low radial growth and therefore could yieldless wood volume so clone 7 would be a preferred clone.The significant differences in wood quality among clonesprovides an opportunity to select superior clones forsolid wood production with desired properties that canbe used in further breeding programmes.Relationship between traits

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Analyzing relationships between different wood qualityvariables is of significant importance in understandingthe possible trade-offs in selecting clones for aspecific wood quality trait. A Pearson’s correlationanalysis was performed to analyse the relationshipbetween various wood quality parameters (Tab.3). Thecorrelation was carried out with pooled data across theclones.

Over-bark DBH exhibited a moderate positiverelationship with both basic density and acousticvelocity but had no association with growth strains. Thissuggests that the tree growth rate does not influencegrowth strain. Longitudinal growth strain measured instanding trees did not exhibit any significantassociation with other measured wood quality variablesexcept a weak positive association with the volumetricshrinkage. Volumetric shrinkage was found to have amodest positive association with growth strains. A weakpositive relationship of volumetric shrinkage with growthstrain has been reported by Nicholson (1972; 1975). Asignificant positive relationship of mean tree growthstrains with volumetric shrinkage of the outer wood wasreported in wood from 9-year-old Eucalyptus nitens trees(Chauhan and Walker 2004). Clair et al. (2003) found asignificant positive correlation between growth stressand tangential shrinkage in normal wood, but this waswith only two trees of chestnut (Castanea sativa Mill.).However, the association between the two variables doesnot seem to be very strong.

The relationship of growth strain with basic densityand acoustic velocity is shown in Fig. 3. Both acousticvelocity and the basic density did not exhibit anysignificant relationship with the growth strain. Acousticvelocity has been associated with the microfibril anglein many softwoods where faster acoustic speed ischaracterized by low microfibril angle in the wood cellwalls. Since the high magnitude of longitudinal growthstrains is also linked with the low microfibril angle inwood (Okuyama et al. 1994), a positive relationshipbetween acoustic velocity and growth strain was

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anticipated. However, the lack of any significantrelationship observed between acoustic velocity andgrowth strains in the study nullifies the anticipatedhypothesis. Absence of any association between the twovariables was reported in Eucalyptus nitens (Chauhan et al.2007) and for E. globulus by Yang et al. (2002).

There have been contrary reports on the relationshipgrowth strains with basic density and MoE of wood and intrees/logs among eucalypts. A positive relationship ofgrowth strain with basic density was reported in normalvertical stems of E. grandis (Malan and Gerischer 1987), 36-y-old E. regnans (Chafe 1990) and 10-y-old E. globulus (Yang etal. 2002) but the relationship was absent in 8-y-old E. nitens(Chafe 1990), 10-y-old E. cloeziana (Muneri et al. 1999) and 4-y-old E. pilularis (Muneri and Leggate 2000). No significantassociation was reported between MoE and growth strain invertical stems of many eucalypts (Chafe 1990, Yang 2002)Although Clair et al. (2003) observed a modest positiveassociation between growth strain and modulus ofelasticity in the normal wood of chestnut. Since MoE isdetermined from acoustic velocity and green density, therelationship of MoE with other variables is very similarto that of acoustic velocity as the green density variedvery little in our trees.

Most relationships between longitudinal growthstrains and other wood quality variables observed in thisstudy and reported elsewhere appear to be specific tospecies, site and the sampling methodology. Inconsistentrelationships of growth strain and basic density and MoEemphasizes the need for caution in generalizing suchassociation across the species.

Pilodyn penetration as measured at breast height(1.4 m above ground) in standing trees had a strongnegative correlation with basic density. Pilodynmeasurement near breast height was reported to be theappropriate sampling height (Raymond and MacDonald,1998). The relationship between pilodyn penetration andbasic density is shown in Fig. 4. Cown (1978) suggestedusing the pilodyn for the purpose of ranking or groupingof genotypes according to density classes. The very high

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repeatability and heritability of pilodyn penetration ineucalypts makes it an efficient tool for high intensityselection. in screening or ranking clones according tobasic density, although its accuracy depends on the pindiameter and age. The utility of pilodyn in indirectselection for density in Eucalyptus nitens was demonstratedby Greaves et al. (1995).

Acoustic velocity was found to have a strongpositive relationship with basic density (Fig. 5). Theresults obtained here are in conformity with reports onother eucalypts. Dickson et al. (2003) reported a strongpositive association between acoustic velocity measuredin logs by Fakopp tool and basic density in Eucalyptusdunnii. They also found a significant positiveassociation of clear wood MoE with the basic density.The strong correlations between basic density withclearwood MOE have been reported for E. globulus and E. nitens(McKimm et al., 1988; Yang, 1997, Chauhan and Walker 2004).Since the green density was more or less uniform acrossthe clones, nearly 97% variation in MoE is explained bythe acoustic velocity measured in logs.

The study clearly indicates significant differencesin wood density, longitudinal growth strains, acousticvelocity and volumetric shrinkage in Eucalyptus tereticornis.The large variation in growth strain among clonesobserved in this study emphasizes the potential ofselection of clones for solid wood production. Selectionof clones with low growth strain could be an importantstep in quickly improving the quality of solid wood fromplantations as wood density and stiffness are of lessconcern.

ConclusionsLongitudinal growth stresses are one of the most

important wood quality traits in the majority ofeucalypts species affecting solid wood quality, productyield and product dimensions. A reduction in stresslevels in tree stems would be a most significantimprovement for the efficient utilization of eucalypts astimber. The absence of any significant relationship of

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longitudinal growth strains with either wood basicdensity or modulus of elasticity suggests that the cloneswith low strain can be selected without compromising woodbasic density and modulus of elasticity, two otherimportant wood quality parameters for solid woodproducts. The large variation in wood quality variablesamong clones provides an opportunity to select clones fortheir wood quality in future breeding programmes. Growthstrain measurement along with pilodyn and acousticvelocity measurement on standing trees provide a quickand reliable approach for selecting superior clones ofEucalyptus tereticornis.

Acknowledgements: Authors express their gratitude to the Director and GroupCoordinator of the Institute of Wood Science andTechnology, Bangalore for their support in carrying outthe study. Authors are also thankful to Prof. JohnWalker, School of Forestry, University of Canterbury,Christchurch for editing the manuscript.

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Cown, D.J., 1978. Comparison of Pilodyn and Torsiometermethods for the rapid assessment of wood density inliving trees. N.Z. J. For. Sci. 8(3): 384-91.

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4-year-old Eucalyptus pilularis. In: Opportunities forthe New Millennium, proceedings of Australian ForestGrowers 2000 Conference, September 2000, Cairns,Queensland, pp. 1–13.

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Table1: Variability in parameters measured in standingtrees (five trees per clone) Value in the bracket isstandard deviation.

CloneNo.

DBH(cm)

Growthstrainsm/m)

Pilodynpenetration

(mm)1 15.06 (1.04) 631 (244) 9.65 (0.4)4 14.67 (1.10) 800 (151) 8.55 (1.10)6 13.05 (0.64) 842 (108) 9.95 (0.41)7 14.2 (1.12) 669 (92) 9.75 (0.95)8 12.55 (1.59) 778 (166) 11.05 (1.22)10 14.65 (1.39) 1010 (148) 10.05 (1.08)27 11.85 (1.55) 746 (236) 9.48 (0.04)71 12.26 (1.90) 1284 (340) 12.95 (0.48)83 12.61 (1.81) 833 (160) 10.6 (0.89)84 13.12 (2.11) 775 (276) 11.5 (0.50)99 13.47 (1.10) 1061 (47) 11.1 (0.89)105 12.74 (1.79) 677 (89) 10.75 (0.35)115 12.64 (0.97) 943 (97) 11 (0.82)116 12.1 (1.08) 396 (70) 11.05 (0.94)

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Table 2: Wood quality variables in clones of Eucalyptus.Value in the bracket is standard deviation.

CloneNo

Greendensit

y(kg/m3

)

Basicdensity(kg/m3)

Acousticvelocity(km/s)

MoE(GPa)

Volumetricshrinkage

(%)

1 987(37) 736 (30) 4.18

(0.05)17.26(0.8)

16.87(0.69)

4 1087(14) 708 (02) 3.73

(0.10)15.10(0.6)

17.68(0.16)

6 1030(17) 640 (08) 3.84

(0.04)15.19(0.1)

19.39(0.92)

7 1039(42) 645 (07) 3.73

(0.06)14 49(0.7)

17.32(0.40)

8 1000(72) 658 (08) 3.94

(0.21)15.43(0.6)

18.77(0.37)

10 1049(24) 611 (03) 3.57

(0.04)13.35(0.2)

16.03(1.20)

27 1033(50) 675 (12) 3.78

(0.11)14.76(0.1)

16.09(0.04)

71 1051(34) 609 (16) 3.14

(0.01)10.36(0.3)

22.81(1.02)

83 1067(49) 592 (08) 3.65

(0.02)14.25(0.5)

19.87(0.29)

84 1020(08) 583 (14) 3.51

(0.03)12.56(0.3)

17.24(1.80)

99 1062(23) 641 (22) 3.56

(0.09)13.44(0.5)

17.94(1.12)

105 1022(54) 637 (10) 3.37

(0.07)11.56(0.5)

15.28(1.47)

115 1044(33) 629 (18) 3.30

(0.14)11.39(0.7)

16.38(1.06)

116 1038(29) 629 (10) 3.41

(0.10)12.13(1.0)

18.02(2.35)

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Table 3: Correlation between wood quality variables (n=42)

VariablesStrain

Pilodyn

Basicdensity

Acousticvelocity

MoE Vol.shrinkage

DBH 0.00 -0.47** 0.40** 0.37* 0.43* -0.06

Strain 0.28 -0.13 -0.30 -0.30 0.31*

Pilodyn -0.74*** -0.58*** -0.63*** 0.35 *

Basic density 0.61*** 0.64*** -0.20

Acoustic velocity 0.97*** -0.08

MoE -0.03*** P<0.001; ** P<0.01; * P<0.05

Figure Captions

Fig.1: Schematic presentation of strain measurement intrees.

Fig.2: Proximity of the clones for various variablesusing Duncan’s test (a) DBH (b) Growth strains (c)Pilodyn penetration

Fig. 3: Relationship of growth strains with basic densityand acoustic velocity in wood

Fig.4: Relationship of wood basic density with pilodynpenetration in trees.

Fig.5: Relationship between wood basic density andacoustic velocity measured in logs

Fig.1

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Strain meter

15mm

15 mm Slots

Strain gauge

Tree

586587588589590591592593594595596597598599600601

Fig.2

1 4 10 7 99 84 6 105

115

83 8 71 116

27

(a) DBH

71 99 10 115

6 83 4 8 84 27 105

7 1 116

(b) Growth strains

71 84 99 8 116

115

105

83 10 6 7 1 27 4

(c ) Pilodyn penetration

602603604605606607608609610611

612613614615616617618619620

621622623624625626627628629

630631632633634635

Fig.3

Fig. 4

636637638639640641642643644645646647

648649650

651

Fig. 5 652653654655656

657