Genetic correlations between spiral grain and growth and quality traits in Picea abies

Genetic correlations between spiral grain andgrowth and quality traits in Picea abies

Henrik R. Hallingback, Gunnar Jansson, and Bjorn Hannrup

Abstract: In Norway spruce (Picea abies (L.) Karst), spiral grain is a major cause of twist development in sawn timber;this problem could be addressed by breeding for reduced grain angles. This study presents estimates of genetic correlationsbetween grain angle under bark and height and diameter growth; branch number, angle, and thickness; stem straightness;ramicorn occurrence; and pilodyn penetration using data from three progeny trials. The genetic relationship between grainangle development and radial growth was also investigated by measuring multiple annual rings (3–15) in stem sectionsfrom two clonal trials. Grain angles under the bark exhibited substantial heritability but near-zero genetic correlations withall the other traits studied. The genetic correlations between multiple ring grain angle and radial growth were also close tozero among all rings. However, radial growth exhibited positive genetic correlations with grain angles at specific distancesfrom the pith and with radial grain angle trends, suggesting that the higher grain angles in juvenile wood extend furtherfrom the pith as a result of increased radial growth. Therefore, from a sawtimber perspective, the genetic relationship withradial growth may be unfavourable, despite the lack of genetic correlations between grain angle and radial growth at anyparticular annual ring.

Resume : Chez l’epinette de Norvege (Picea abies (L.) Karst), le fil spirale est une cause majeure de torsion du bois scie.On pourrait aborder ce probleme en selectionnant pour un angle du fil plus faible. Cette etude presente des estimations descorrelations genetiques entre l’angle du fil sous l’ecorce et la croissance en hauteur et en diametre, le nombre de branchesainsi que leur angle et leur epaisseur, la rectitude de la tige, la frequence des ramicornes et la penetration du Pilodyn, apartir de donnees provenant de trois tests de descendance. La relation genetique entre le developpement de l’angle du filet la croissance radiale a aussi ete etudiee en mesurant un groupe de cernes annuels (3–15) dans des sections de tiges pro-venant de deux tests clonaux. L’angle du fil sous l’ecorce avait une forte heritabilite mais les correlations genetiques avectoutes les autres caracteristiques etudiees etaient pratiquement nulles. Les correlations genetiques entre l’angle du fil dugroupe de cernes annuels et la croissance radiale s’approchaient aussi de zero dans tous les cas. Cependant, il y avait descorrelations genetiques positives entre la croissance radiale et l’angle du fil a des distances specifiques de la moelle d’unepart et les tendances radiales de l’angle du fil d’autre part. Ces resultats indiquent que l’angle du fil qui est plus elevedans le bois juvenile persiste de plus en plus loin au-dela de la moelle a mesure que la croissance radiale augmente. Dupoint de vue du bois de sciage, la relation genetique avec la croissance radiale pourrait alors etre defavorable malgre l’ab-sence de correlation genetique entre l’angle du fil et la croissance radiale d’un cerne en particulier.

[Traduit par la Redaction]

Introduction

In Norway spruce (Picea abies (L.) Karst), high grain an-gles have been shown to be the major cause of sawn timberdeveloping twist during drying, thus causing substantial eco-nomic losses for sawmills and the building industry (Jo-hansson et al. 2001). Spiral grain is the phenomenon wherethe tracheids become tangentially inclined relative to thestem axis at the time of cell division (designated the grainangle). High grain angles have also been shown to be asso-ciated with reduced strength and stiffness in sawn timber(Dinwoodie 2000).

Earlier studies have found significant genetic variation as-sociated with grain angles in a number of conifer species,suggesting that the trait is heritable (references in Harris

1989). For grain angles in Norway spruce, moderate to highheritabilities and very small genotype � environment inter-actions have been observed (e.g., Hallingback et al. 2008).The estimated additive genetic standard deviations of almostone degree may appear small, but a grain angle difference ofthat magnitude in sawn boards considerably affects the prob-ability of boards developing unacceptable twist during dry-ing (Backstrom and Johansson 2006). Thus, breeding forreduced grain angles would appear to be an attractive strat-egy to improve shape stability and mechanical strength.However, knowledge about genetic correlations betweengrain angle and other traits is of critical importance: if thegenetic correlations are unfavourable, breeding for reducedgrain angle may cause inadvertent genetic changes in impor-tant growth or quality traits.

Received 4 June 2009. Accepted 14 October 2009. Published on the NRC Research Press Web site at cjfr.nrc.ca on 29 January 2010.

H.R. Hallingback1 and G. Jansson. Uppsala Biocenter, Department of Plant Biology and Forest Genetics, Swedish University ofAgricultural Sciences, Box 7080, SE-750 07 Uppsala, Sweden.B. Hannrup. Skogforsk, The Forestry Research Institute of Sweden, Uppsala Science Park, SE-751 83 Uppsala, Sweden.

1Corresponding author (e-mail: [email protected]).

173

Can. J. For. Res. 40: 173–183 (2010) doi:10.1139/X09-173 Published by NRC Research Press

Only a few studies have been published describing ge-netic correlations between grain angles and height or diame-ter growth in Norway spruce, and the estimates have mostlybeen close to zero and nonsignificant, despite substantial ge-netic variation in both traits (e.g., Hannrup et al. 2003).Even fewer studies have investigated the genetic relationshipbetween grain angle and other wood properties in Norwayspruce. Apart from some indications that spiral grain in ju-venile wood is negatively correlated with mechanical stiff-ness and strength (Hannrup et al. 2004) and that it plays arole in the hydraulic conductivity of earlywood tracheids(Rosner et al. 2007), very few of the genetic correlationshave differed significantly from zero. Hitherto, no investiga-tions into the genetic relationship between grain angle andstem and branch characteristics in Norway spruce have beenpublished. Furthermore, most previous studies have takengrain angle measurements from a few annual rings underthe bark, because this is easily done in a relatively nondes-tructive way. A number of studies of Sitka spruce (Piceasitchensis (Bong.) Carr) have indicated that there may be noor only a slightly negative genetic correlation between grainangle and growth traits in the juvenile wood but that there isa positive correlation in young mature wood (Hansen 1999).Such dependence on the number of rings away from the pith(cambial age) is conceivable because grain angles in juve-nile wood are often left handed and increase rapidly in thefirst annual rings from pith, whereas grain angles in maturewood generally decrease linearly with increasing cambialage and can even become right handed (Gjerdrum et al.2002).

The main aim of this study was to determine whetherthere are any genetic correlations in Norway spruce betweenspiral grain on one hand and growth, density, branch, orstem characteristics on the other. Because growth is consid-ered to be the most important objective trait, it is essential toknow how the age at measurement affects the correlation.Therefore, we also examined whether genetic correlationsbetween grain angle and radial growth are dependent oncambial age or on measurement distance from the pith.

The main aim was addressed by measuring grain angleunder the bark, along with other traits, in a series of three28-year-old Norway spruce progeny trials (the ‘‘old progenymaterial’’). In particular, the relationship between grain an-gles and growth traits was studied by estimating genetic cor-relations from several annual rings in stem sections fromtwo 19-year-old clone trials (the ‘‘young clonal material’’).Using this material, the second aspect of the study was ad-dressed. The genetic correlations estimated from both mate-rials are compared, and their implications for tree breedingpractice are discussed from a sawtimber perspective.

This study is unique in that it investigates stem andbranch quality traits in relation to grain angle in Norwayspruce. Furthermore, it also takes into account the annualring number and the distance from pith when assessing grainangles and when evaluating genetic correlations betweengrain angle and growth traits. Estimations of the genetic pa-rameters for grain angle and for a few of the growth traitshave already been published for both the old progeny mate-rial (Hallingback et al. 2008) and the young clonal material(Hannrup et al. 2002).

Materials and methods

Genetic material in the 28-year-old progeny trialsThree progeny trials (the ‘‘old progeny material’’) were

derived from controlled crosses between 36 parent plus-trees, which were phenotypically selected in southern Swed-ish stands for vigour, height, diameter, and straightness aswell as for branches with small diameters and horizontalgrowth. Each parent was represented in seven crosses ac-cording to a partial diallel mating design (Kempthorne andCurnow 1961). All three progeny trials, Vetlanda, Ton-nersjo, and Lonsboda, were simultaneously established using3-year-old seedlings at different locations in southern Swe-den. The progenies were randomized in single-tree plots,with each family represented by four progenies in each of10 blocks at each site at a spacing of 2 m � 2 m.

Genetic material in the 19-year-old clone trialsTwo 19-year-old clonal field trials (the ‘‘young clonal ma-

terial’’) situated at Hermanstorp and Knutstorp in southernSweden were used in this study. Bareroot seedlings werephenotypically selected for their superior nursery heightgrowth in 4-year-old commercial seedling stocks originatingfrom a narrow geographical range (48.77–49.458N, 19.25–20.258E, 650–880 m elevation) in Slovakia. Cuttings fromthe selected plants were rooted and grown for 2 years in thegreenhouse. The rooted cuttings were then randomly plantedat a spacing of 2 m � 2 m in single-tree plots within arandomized block design. Each clone was represented withone cutting in each of five blocks in each trial. The nurseryselection effects were assumed to be insignificant for thepurpose of this study because investigations of similar nurs-ery selections found the growth gains to be minor in thefield (e.g., Hogberg and Karlsson 1998).

Field assessment of the old progeny materialSeveral growth, stem quality, and branch quality traits

were measured in the 28-year-old full-sibling progeny trials(Table 1). An indirect assessment of wood density was alsoperformed based on pilodyn penetration. After 27 or 28 yearsin the field, the diameter at breast height (DBH27) wasmeasured. In addition, the grain angle under the bark(GAub28) was measured using a wedge grain angle gaugeas described by Hannrup et al. (2003). The wedge was gen-tly hammered into the stem at the internode closest to breastheight, carefully avoiding branch whorls. Measurementsfrom two opposite sides (north and south) of the stem weremade, and the mean used as an estimate to eliminate anymeasurement error associated with deviation of the stemaxis from the vertical (Harris 1989). Trees exhibiting left-and right-handed grains were given positive and negativevalues, respectively.

Some of the traits were very time consuming to assessand were only measured in the blocks with the lowest mor-tality, resulting in variable numbers of observations amongtraits (Table 2). The traits diameter at breast height at age18 (DBH18), stem straightness at age 10 (Str10), and pilo-dyn penetration at age 18 (Pil18) were also not assessed inall trials. The categorical traits Str10, abundance of interno-dal twigs at age 10 (Tw10), and number of ramicorns at age10 (Ram10) (Table 1) were transformed to produce normal

174 Can. J. For. Res. Vol. 40, 2010

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distributions within blocks prior to statistical analysis (Gia-nola and Norton 1981).

Sampling and measurement of the young clonal materialIn Hermanstorp, 172 ramets from 43 clones were meas-

ured, and 116 ramets from 30 clones were measured inKnutstorp. Twenty clones were common to the two trials.From each ramet, a 10 cm thick stem section was takenfrom the first internode above 80 cm. From the sampledstem sections, the width of each annual ring was measuredusing a dendrochronological high-precision measuring de-vice. Accumulated radial growth (RGy) at cambial ages y =1, 2, . . ., 17 was subsequently calculated by adding themeasured ring widths from pith to the relevant cambial age,y. The stem sections were split using a wedge-shaped bladeand a mallet to expose the pith and the grain angle on thesplit surface. The pith was then fixed to pins of a movablebar, which was part of a precisely manufactured protractordevice (Hannrup et al. 2002). Grain angles (GAy) for eachannual ring (y = 2, 3, . . ., 17) were recorded with this pro-tractor using the pith as a reference instead of the stemaxis. Thus, the need for repeated measurements on oppositesides of the stem section was eliminated.

Radial grain angle trends and grain angles at specifieddistances from pith

Wood science studies of Norway spruce (Ekevad 2005)and radiata pine (Pinus radiata D. Don; Booker 2005) haveshown that grain angles at specific distances from the pith aswell as radial grain angle trends based on distance from thepith are the most relevant for determining the shape stability

of sawn timber. Therefore, the evaluation of radial grain an-gles trends and grain angles at given distances from the pithwere deemed interesting. Prediction of these traits was pos-sible by assuming that the grain angle change rate was con-stant with respect to cambial age and radial growth asobserved by Gjerdrum et al. (2002).

The grain angle at the radial distance of 75 mm(GA75mm) from pith was predicted for the old progeny ma-terial by using the following model (proposed by Sall 2002):

½1� GA75mm ¼ GAub28

þ ðGAub28� 3:53Þ 2r

DBH27� 1

� �where r is the distance from the pith. Aside from the as-sumption of linear grain angle trends, one-half of DBH27was used as an approximation of the radial distance betweenGAub28 measurements and the pith. The grain angle inter-cept at the pith was assumed to be 3.53 (Sall 2002). In thisparticular case, the distance r = 75 mm was chosen, as itwas roughly one-half of the mean DBH27 in all three pro-geny trials (Table 2).

Radial grain angle trends were also estimated for each in-dividual tree in the young clonal material by using GAy andRGy. The following two alternative linear least squares re-gression models were used:

½2� GAyj ¼ GAageB0j þ GAageB1j � yþ eyj

½3� GAyj ¼ GAdistB0j þ GAdistB1j � RGyj þ eRGyj

Equation 2 estimated the grain angle slope (GAageB1) and

Table 1. Description of measured and calculated traits.

Abbreviation Description of the trait Unit

Traits measured or calculated in the old progeny materialHtx Tree height at field age x = 5, 10, and 20 years cmDI10 Diameter under whorl 5* at field age 10 mmDBHx Diameter at breast height at field age x = 18 and 27 mmBrD10 Diameter of branch{ 5 mm from whorl 5* at field age 10 mmStr10 Relative stem straightness{ at field age 10 Score (0–9)Br10 Number of branches in whorl 5* at field age 10 —Tw10 Abundance of internodal branches (twigs) at field age 10 on internode above whorl 5* Score (1–5)BrA10 Branch angle from stem internode above at field age 10 of branch{ in whorl 5* DegreesRam10 Number of whorls{ containing ramicorns at field age 10 —Pil18 Pilodyn penetration§ at field age 18 at internode closest to breast height mmGAub28 Grain angle under bark at field age 28 at internode closest to breast height DegreesGA75mm|| Predicted grain angle at the distance 75 mm from pith Degrees

Traits measured or calculated in the young clonal materialRGy Radial growth from pith at annual ring y mmGAy Grain angle at annual ring y DegreesGAageB1

|| Grain angle trend on annual ring number from pith 8�year–1

GAdistB1|| Grain angle trend on radial growth 8�mm–1

GArmm Grain angle in ring encompassing the distance r = 25, 38, and 50 mm from pith Degrees

Note: The field ages are the numbers of years the trials have grown in the field and should be regarded as approximate. See Table 2for further information.

*Whorl 5 refers to the fifth branch whorl counted from the top shoot at field age 10.{The branch growing from a predefined side of the stem was chosen for assessment.{The upper part of the stem from whorl 5 to the top was assessed.§A predefined side of the stem was chosen for the assessment.||Traits were calculated using eqs. 1–3.

Hallingback et al. 175


Table 2. Arithmetic means, genetic parameters and numbers of observations for growth and quality traits in the progenytrials at Vetlanda, Tonnersjo, and Lonsboda (old progeny material).

Trait Years in fieldNo. ofobservations Arithmetic mean CVA (%) bh2

Vetlanda: 57.418N, 15.158E, 230 m elevation; 88% survival at age 27Ht5 (cm) 5 2535 116 (1) 11 (4) 0.12 (0.04)Ht10 (cm) 10 2536 357 (2) 10 (3) 0.11 (0.04)Ht20 (cm) 21 983 963 (6) 9 (3) 0.19 (0.05)DI10 (mm) 10 1336 60 (1) 14 (4) 0.19 (0.05)DBH18 (mm) 18 990 103 (1) 14 (4) 0.22 (0.06)DBH27 (mm)* 27 3544 135 (1) 17 (5) 0.27 (0.06)BrD10 (mm) 10 1336 10.4 (0.1) 10 (4) 0.11 (0.04)Str10 — — — — —Br10 10 2536 6.12 (0.04) 10 (3) 0.10 (0.03)Tw10 10 1336 2.9 (0.1) 0.32 (0.13){ 0.10 (0.04)BrA10 (8) 10 2536 65.0 (0.2) 6 (2) 0.15 (0.04)Ram10 10 2536 0.31 (0.01) 0.14 (0.08){ 0.03 (0.02)Pil18 (mm) 17 986 19.8 (0.1) 6 (2) 0.24 (0.07)GAub28 (8)* 27 950 2.10 (0.05) 0.87 (0.26){ 0.37 (0.09)GAub75mm (8) — 950 1.86 (0.06) 1.13 (0.33){ 0.36 (0.09)

Tonnersjo: 56.668N, 13.098E, 90 m elevation; 75% survival at age 27Ht5 (cm) 5 1496 124 (1) 10 (3) 0.13 (0.04)Ht10 (cm) 11 3765 396 (2) 12 (3) 0.17 (0.04)Ht20 (cm) 21 1314 840 (5) 11 (3) 0.23 (0.06)DI10 (mm) 11 1496 64 (1) 10 (3) 0.16 (0.04)DBH18 (mm) 18 1314 91 (1) 13 (4) 0.18 (0.05)DBH27 (mm)* 27 3464 139 (1) 15 (4) 0.24 (0.06)BrD10 (mm) 11 1496 12.3 (0.1) 9 (3) 0.11 (0.04)Str10 11 3534 7.0 (0.1) 0.46 (0.13){ 0.22 (0.05)Br10 11 1496 6.78 (0.06) 10 (5) 0.08 (0.04)Tw10 11 1496 3.5 (0.1) 0.29 (0.12){ 0.08 (0.03)BrA10 (8) 11 1496 69.8 (0.2) 7 (2) 0.32 (0.08)Ram10 11 3534 0.40 (0.01) 0.17 (0.06){ 0.04 (0.02)Pil18 (mm) 18 1301 19.2 (0.1) 6 (2) 0.32 (0.08)GAub28 (8)* 28 785 1.55 (0.04) 0.65 (0.22){ 0.32 (0.09)GAub75mm (8) — 785 1.27 (0.05) 0.80 (0.28){ 0.27 (0.08)

Lonsboda: 56.438N, 14.338E, 150 m elevation; 76% survival at age 27Ht5 (cm) 5 850 140 (1) 14 (5) 0.16 (0.05)Ht10 (cm) 11 2777 334 (3) 20 (5) 0.22 (0.05)Ht20 (cm) 21 948 837 (7) 13 (4) 0.23 (0.06)DI10 (mm) 11 850 60 (1) 14 (4) 0.21 (0.05)DBH18 (mm) — — — — —DBH27 (mm)* 27 2491 124 (1) 24 (6) 0.29 (0.07)BrD10 (mm) 11 850 11.7 (0.1) 13 (4) 0.16 (0.05)Str10 11 2782 5.4 (0.1) 0.47 (0.14){ 0.22 (0.06)Br10 11 850 7.27 (0.07) 9 (5) 0.09 (0.05)Tw10 11 850 2.6 (0.1) 0.41 (0.15){ 0.15 (0.05)BrA10 (8) 11 850 73.3 (0.3) 6 (2) 0.25 (0.08)Ram10 11 2782 0.77 (0.02) 0.34 (0.12){ 0.15 (0.05)Pil18 (mm) — — — — —GAub28 (8)* 28 567 1.47 (0.06) 0.85 (0.27){ 0.42 (0.11)GAub75mm (8) — 567 0.89 (0.09) 1.22 (0.39){ 0.34 (0.09)

Note: Standard errors are given in parentheses. See Table 1 for trait abbreviations.*Previous estimations of genetic variation and heritability have been published in Hallingback et al. (2008).{For these traits, CVA is inappropriate or impossible to determine; hence, the genetic standard deviation sA is given.

176 Can. J. For. Res. Vol. 40, 2010


intercept (GAageB0) for the individual tree j using annualring number from the pith (y) as the independent variable,thus describing the grain angle development depending oncambial age. All grain angle measurements from rings 4 to15 were included in accordance with Hannrup et al. (2002).Equation 3 estimated the grain angle slope (GAdistB1) andintercept (GAdistB0) using accumulated radial growth as theindependent variable instead, thus illustrating the radialgrain angle trend present in a sawlog. This model, proposedby Sall (2002), included all grain angle and radial growthmeasurements from the fifth ring all the way to the bark butexcluded measurements <15 mm from the pith.

By using both GAy and RGy data, it was possible to eval-uate the grain angle at any distance from pith within the fi-nal growth radius of the sampled trees of the young clonalmaterial. By selecting GAy values in each tree that occurredat RGy of 25, 38, or 50 mm, new traits (GA25mm,GA38mm, and GA50mm, respectively) were evaluated. To-gether with the grain angle prediction for 75 mm from thepith in the old progeny material (eq. 1), these traits illustratethe grain angle in the central and peripheral parts of hypo-thetical 50 mm � 100 mm boards sawn from the log(Fig. 1).

Statistical analysis of the old progeny materialTo estimate variance components for the traits measured

in the old progeny material, the following mixed linearmodel was used for each of the three trials separately:

½4� yijkl ¼ mþ bi þ pj þ pk þ fjk þ eijkl

where y is the trait measurement from the lth tree of the fullsib family of parents j and k located in the ith block; m isthe trait mean; b is the fixed block effect; and p, f, and eare the random parent effects, random family effects, and re-siduals, respectively.

Using a multivariate approach, each trait was consistentlyanalyzed together with Ht10, DBH27, and GAub28.GAub28 was chosen because it is of primary interest in thisstudy, and Ht10 and DBH27 were chosen because of thevery large number of measurements taken for these traits(totals of 9078 and 9499, respectively), thus improving pa-rameter estimates of the other traits by accounting for mor-tality and missing values (e.g., Wei and Borralho 1998). Inmatrix format for multitrait evaluation, eq. 4 can be ex-pressed as

½5� y ¼ Xbbþ Zppþ Zffþ e

using the same symbols for measurement and effect vectorsas in the univariate case. The design matrices Xb, Zp, and Zfwere used for the fixed block effects, parent effects, and fa-mily effects, respectively. All random effects were assumedto be independently and normally distributed with expectedmeans of zero and structured as

½6� Var

p

f

e

264375 ¼

P� Ip 0 0

0 F� If 0

0 0 R� Ie

264

375

where P, F, and R are the parental, family, and residualvariance–covariance matrices, respectively, and Ip, If, and Ie

are the respective identity matrices. Restricted maximumlikelihood values of variance and covariance componentswere estimated using ASReml software (Gilmour et al.2006).

Statistical analysis of the young clonal materialFor the young clonal material, variance components were

estimated for each of the trials separately, using the follow-ing mixed linear model:

½7� yij ¼ mþ bi þ cj þ eij

where yij is the trait measurement from the tree of clone j inthe ith block, m is the trait mean, b is the fixed block effect,c is the random clone effect, and e is the residual.

Using a multivariate approach, the four traits GAy, RGy,GA10, and RG10 were included in each separate analysis forthe annual rings y = 3, 4, . . ., 15. We included GA10 andRG10 because they were the traits with the most observa-tions, thus improving parameter estimates by accounting formortality and missing values in GAy and RGy. In matrix for-mat for multitrait evaluation, eq. 7 can be expressed as

½8� y ¼ Xbbþ Zccþ e

where Zc and c are the design matrix and vector of the clo-nal effects, respectively. The random effects were structuredin a similar way to eq. 6 and the same assumptions weremade. Bivariate analyses were performed for the two sitesseparately, using eq. 8 with the traits GAageB1 andGAdistB0 incorporated in pairs together with RG10 to esti-mate genetic parameters and the genetic correlations withradial growth. Equivalent multivariate analyses ofGA25mm, GA38mm, GA50mm, and RG10 were also per-formed.

Genetic interpretationIn the genetic interpretation of the old progeny material,

Fig. 1. Hypothetical sawing pattern of 50 mm � 100 mm boards inthe sawtimber produced in the investigated trials. Grain anglesevaluated at specific radial distances from pith are illustrated withopen circles. Pith (solid circle) is assumed to be located in the cen-tre of the log, and grain angles at specific radii are assumed to beequal in the tangential direction (Danborg 1994).



we assumed that there was no epistasis. Parental (bs2p), fam-

ily (bs2f ), and residual (bs2

e) variances and the parental cova-riance component (bsp1p2

) estimated using the multivariatemodels were translated into additive genetic variance (bs2

A),phenotypic variance (bs2

P), and additive genetic covariance(bsA1A2

) as follows:

bs2A ¼ 4bs2

p

bs2P ¼ 2bs2

p þ bs2f þ bs2

ebsA1A2¼ 4bsp1p2

For genetic interpretation of the young clonal material,clonal variance (bs2

c), residual variance (bs2e) and clonal cova-

riance (bsc1c2) estimated from the multivariate models were

translated into genotypic variance (bs2G), phenotypic variance

(bs2P), and genotypic covariance (bsG1G2

) as follows:

bs2G ¼ bs2

cbs2P ¼ bs2

c þ bs2ebsG1G2

¼ bsc1c2

Based on these genetic variances and covariances, addi-tional genetic parameters were calculated as follows:

bh2 ¼ bs2Abs2P

bH2 ¼ bs2Gbs2P

CVA ¼ 100bsA

�x

� �

CVG ¼ 100bsG

�x

� �

brA1A2¼ bsA1A2bsA1

bsA2

brG1G2¼ bsG1G2bsG1

bsG2

where bh2and bH2

are narrow- and broad-sense heritabilities,respectively; CVA and CVG are the percentage coefficientsof additive and genotypic variation, respectively; x is thetrait mean; and brA1A2

and brG1G2are the additive genetic and

genotypic correlation coefficients, respectively, estimatedbetween traits 1 and 2.

In both materials, log-likelihood ratio tests were used todetermine whether genetic trait correlations were signifi-cantly different from zero. Standard errors of genetic param-eters were estimated in ASReml (Gilmour et al. 2006) usingthe Taylor series expansion.

Results

Genetic variation and heritability in the old progenymaterial

The mean values of most traits from the old progeny mate-rial were similar among the investigated sites (Table 2).GAub28 means were in the range 1.478–2.108, and diametergrowth means at the same age (DBH27) were 124–139 mm.By interpolation of the height measurement trait, the meancambial age under the bark at breast height was estimated tobe approximately 23 years at Vetlanda and Lonsboda and24 years at Tonnersjo at the time of the GAub28 measure-ments (data not shown). The measured GAub28, as well aspredicted GA75mm, had heritability estimates of 0.27–0.42and bsA estimates in the range 0.658–1.228. Growth traits hadlower heritabilities, in the range 0.11–0.29, and genetic varia-tion estimates in the range 9%–24%. These estimates werefairly similar among the sites. The remaining traits exhibiteddiffering, but appreciable, genetic variation (6%–13%) andheritability (0.08–0.32) except for ramicorn occurrence(Ram10) heritability, which was very low in Vetlanda andTonnersjo (0.03–0.04).

Genetic correlations between GAub28 and other traitsGenetic correlations between GAub28 and other growth

and quality traits were low, were consistently nonsignificant(–0.16 to 0.38), and had large standard errors (Table 3). Themajority of the genetic correlations between GAub28 andgrowth traits were low and positive with a mean of 0.19.None of them were significant. In addition, these geneticcorrelations did not show any dependence on the time ofthe growth assessment. The genetic correlation betweenGAub28 and GA75mm was almost equal to one, butGA75mm was a trait predicted using GAub28 (eq. 1), so au-tomatic correlation was likely.

Genetic correlations between Ht10 and quality traitsIn Norway spruce tree breeding programmes in southern

Sweden, tree height after 10 years in the field (Ht10) is gen-erally regarded as a final selection trait (Karlsson and Ros-vall 1993). Therefore, genetic correlations between Ht10and the other traits are also presented to make comparisonswith the genetic correlations between the same traits andgrain angle (Table 3). Genetic correlations between Ht10and other growth traits were all strong and significant(0.72–0.99). In most cases, genetic correlations betweenHt10 and branch diameter at age 10 (BrD10), number ofbranches at age 10 (Br10), Tw10, and Pil18 were also sig-nificant and always positive (0.38–0.93). Furthermore,GA75mm was also positively correlated with growth (0.31–0.57). Most of the other quality traits exhibited weak (–0.09to 0.12 for branch angle at age 10) or inconsistent (–0.81 to0.62 for Str10 and Ram10) genetic correlations with Ht10.

Genetic parameters for grain angle and radial growthtraits in the young clonal material

In the young clonal material, the maximum grain angleoccurred in the fourth annual ring, and there was a subse-quent steep decrease with increasing cambial age. Therefore,15 annual rings from the pith, the young clonal material hadgrain angles equal to or smaller than those recorded for most

178 Can. J. For. Res. Vol. 40, 2010


of the old progeny material at approximately 23–24 ringsfrom the pith (Fig. 2). Substantial genetic variation and highbroad-sense heritabilities were recorded for grain angles andfor accumulated radial growth in all the annual rings stu-died. As examples, the genetic parameters for GA10 andRG10 are shown in Table 4. The grain angle trend traits(GAageB1 and GAdistB1) also exhibited genetic variationand heritability. Furthermore, grain angles at specific dis-tances from the pith (GA25mm, GA38mm, and GA50mm)also all had bsG values >18 and large heritability estimates(0.31–0.47).

Genetic correlations between grain angle and radialgrowth traits in the young clonal material

Individual ring correlations between accumulated radialgrowth and grain angle were close to zero (–0.15 to 0.31),and all had large standard errors (Fig. 3). No obvious trendwas identified in association with increasing cambial age ateither of the sites. The genetic correlations between GAageB1and RG10 were also not significant at either site (Table 4). Onthe other hand, the genetic correlations between GAdistB1and RG10 were high and significant at both sites. In addition,the genetic correlations between the grain angle traits at spe-cific distances from the pith and RG10 were all positive andan increase from 0.17 to 0.41 with increasing distance fromthe pith was recorded at Hermanstorp.

Discussion

Means and genetic variation of the materialThe means and the genetic variation of most of the traits

were comparable with the observations of other authors,suggesting that both materials examined in this study werefairly representative of phenotypically selected breeding ma-terials of the same ages (Tables 2 and 4). Grain angle anddiameter means of the 28-year-old progeny trials corre-sponded to previous data pertaining to grain angle develop-ment relative to the distance from the pith (Fig. 2) as well as

Table 3. Additive genetic correlations between grain angle under the bark (GAub28), tree height at field age 10 years(Ht10), and other traits measured in progeny trials at Vetlanda, Tonnersjo, and Lonsboda (old progeny material).

GAub28 Ht10

Trait Vetlanda Tonnersjo Lonsboda Vetlanda Tonnersjo LonsbodaHt5 0.29 (0.21) 0.19 (0.24) 0.32 (0.22) 0.96 (0.02) 0.78 (0.09) 0.98 (0.03)Ht10 0.30 (0.21) 0.02 (0.22) 0.10 (0.21) — — —Ht20 0.35 (0.19) –0.02 (0.22) 0.04 (0.21) 0.93 (0.04) 0.90 (0.04) 0.99 (0.01)DI10 0.38 (0.19) 0.11 (0.22) 0.07 (0.21) 0.72 (0.10) 0.78 (0.08) 0.88 (0.05)DBH18 0.38 (0.19) 0.13 (0.22) — 0.83 (0.08) 0.84 (0.06) —DBH27 0.38*(0.18) 0.11*(0.22) 0.14*(0.20) 0.91 (0.05) 0.85 (0.06) 0.94 (0.02)BrD10 0.35 (0.21) 0.07 (0.24) 0.28 (0.21) 0.69 (0.13) 0.71 (0.11) 0.90 (0.06)Str10 — 0.02 (0.22) 0.15 (0.21) — –0.61 (0.13) 0.62 (0.13)Br10 0.34 (0.21) 0.09 (0.28) 0.38 (0.30) 0.51 (0.18) 0.38 (0.22) 0.61 (0.20)Tw10 0.34 (0.23) –0.05 (0.27) 0.15 (0.24) 0.64 (0.16) 0.65 (0.14) 0.93 (0.07)BrA10 0.00 (0.22) 0.00 (0.22) 0.10 (0.24) –0.09 (0.23) 0.12 (0.20) –0.06 (0.22)Ram10 –0.16 (0.29) 0.00 (0.26) –0.10 (0.24) –0.01 (0.31) 0.19 (0.22) –0.81 (0.09)Pil18 0.03 (0.22) 0.09 (0.22) — 0.63 (0.16) 0.56 (0.14) —GA75mm 0.97 (0.01) 0.92 (0.04) 0.86 (0.06) 0.48 (0.18) 0.31 (0.20) 0.57 (0.15)

Note: Standard errors are given in parentheses and estimates significantly different from 0 (p < 0.05) are given in boldface.See Table 1 for trait abbreviations.

*Previous estimations of these genetic correlations have been published in Hallingback et al. (2008).

Fig. 2. Developmental graph of mean grain angle on mean accu-mulated radial growth for annual rings 2–15 (*) at(a) Hermanstorp and (b) Knutstorp. For comparison, the means ofradial growth and grain angle under the bark of the three progenytrials are presented (~, Vetlanda; &, Tonnersjo; !, Lonsboda).The radial growth of the progeny trials are approximated by one-half the mean diameter at breast height after 27 years in the field.The general mean grain angle trend associated with radial growthof Norway spruce (Sall 2002) is also shown (line).



cambial age (Sall 2002). In addition, additive genetic varia-tion and heritabilities were similar to previously publishedestimates for grain angle under bark, growth traits, and den-sity assessment by pilodyn penetration (Costa e Silva et al.2000) as well as stem straightness and branch traits (Stef-fenrem 2008) in Norway spruce (Table 2). In contrast, thegrain angle means of the 19-year-old clone trials exhibitedboth higher maxima and more rapid declines by increasingdistance from pith than those previously recorded in Sweden(Fig. 2). A possible explanation for these differences may bethat the young clonal material was vegetatively propagatedand, therefore, experienced more rapid physiological agingthan the seedlings of the older progeny material (Harris1989). Grain angles measured in single annual rings as wellas averaged from radial pith to bark profiles have beenfound to exhibit high heritability in Norway spruce(Hannrup et al. 2002). The results of this study indicate thatthe radial grain angle pith-to-bark trend (GAdistB1) andgrain angles at specific distances from pith (e.g., GA50mm)also are heritable (Tables 2 and 4).

Genetic correlations between grain angle and other traitsGrain angle assessed at single annual rings showed

weakly positive but nonsignificant genetic correlations withgrowth traits in the old progeny material as well as theyoung clonal material (Table 3, Fig. 3). Thus, the geneticcorrelation estimates were fairly similar despite differencesin age and the inclusion of both dominance and epistatic ef-fects in the young clonal material. Therefore, these resultsagree with other studies that have reported low genetic cor-relations between grain angle and growth (e.g., Hannrup etal. 2003). On the other hand, both negative and greater pos-itive genetic correlations than those presented here havebeen recorded in Norway spruce (Costa e Silva et al. 2000)

as well as in other species (e.g., Jayawickrama 2001). How-ever, the methods for assessing grain angle and the extent towhich cambial age was taken into account varied substan-tially among studies. The estimates in this study acquiredfrom seedling trees as well as cuttings indicate that breedingfor higher growth rate will have limited, if any, effects onthe grain angle at any specified cambial age in the range of3–24 years.

The fact that the genetic correlations between GAub28and branch growth traits (BrD10, Br10, and Tw10) weresimilar to the genetic correlations between GAub28 andgrowth traits is probably only a reflection of the strong ge-netic relationship between these latter traits and growth traits(e.g., Ht10 in Table 3). The other wood, stem, and branchtraits all appear to be genetically uncorrelated withGAub28, although the estimates must be interpreted cau-tiously because the standard errors are large. Until now,studies of the genetic relationship between grain angle andbranch and stem characteristics have been available only forother conifer species, including Scots pine (Pinus sylvestrisL.; Hannrup et al. 2003), Sitka spruce (Hansen and Roulund1998), and radiata pine (Jayawickrama 2001). None of thesefindings contradict the results of this study.

The near-zero additive genetic correlation between pilo-dyn penetration and grain angle concurs with the estimationsof Costa e Silva et al. (2000). On the other hand, Gaspar etal. (2008) observed high and adverse genetic correlations be-tween grain angle under bark and mean density of stem sec-tions in maritime pine (Pinus pinaster Ait.). Steffenrem etal. (2009) presented similar observations for Norway spruce,but the genetic material in that study was restricted to 13half-sib families. Furthermore, the near-zero genetic correla-tions between density and grain angle from stem sectionspresented by Hannrup et al. (2004) agree with the results of

Table 4. Arithmetic means, genetic parameters, and genetic correlation estimates with radial growth at the 10thannual ring for grain angle trend traits in the clone trials at Hermanstorp and Knutstorp (young clonal material).

TraitNo. ofobservations Mean bsG

bH2 brG (RG10)

Hermanstorp: 56.758N, 15.038E, 180 m elevationRG10 (mm) 172 46 (1) 5 (2)* 0.53 (0.08) —GA10 (8){ 167 2.11 (0.12) 1.04 (0.32) 0.43 (0.09) 0.00 (0.21)GAageB1 (8�year–1){ 168 –0.27 (0.01) 0.09 (0.03) 0.40 (0.09) –0.06 (0.21)GAdistB1 (8�mm–1) 168 –0.086 (0.005) 0.040 (0.012) 0.43 (0.09) 0.53 (0.16)GA25mm (8) 165 3.44 (0.13) 1.08 (0.32) 0.47 (0.08) 0.17 (0.20)GA38mm (8) 160 2.65 (0.12) 1.03 (0.31) 0.43 (0.09) 0.36 (0.18)GA50mm (8) 121 1.91 (0.16) 1.25 (0.39) 0.47 (0.09) 0.41 (0.18)

Knutstorp: 55.978N, 13.308E, 75 m elevationRG10 (mm) 116 45 (1) 5 (2)* 0.38 (0.10) —GA10 (8){ 115 2.10 (0.18) 1.24 (0.46) 0.46 (0.10) 0.16 (0.26)GAageB1 (8�year–1){ 115 –0.25 (0.02) 0.11 (0.05) 0.26 (0.10) 0.38 (0.27)GAdistB1 (8�mm–1) 115 –0.072 (0.007) 0.038 (0.019) 0.25 (0.10) 0.70 (0.20)GA25mm (8) 115 3.03 (0.18) 1.34 (0.47) 0.47 (0.10) 0.11 (0.25)GA38mm (8) 110 2.46 (0.20) 1.26 (0.50) 0.35 (0.10) 0.27 (0.26)GA50mm (8) 85 2.23 (0.23) 1.16 (0.54) 0.31 (0.11) 0.15 (0.29)

Note: Standard errors are given in parentheses. Correlation estimates significantly different from 0 (p < 0.05) are given inboldface.

*CVG values for RG10 were 12% and 11% at Hermanstorp and Knutstorp, respectively.{Previous estimations of genetic variation and heritability have been published in Hannrup et al. (2002).

180 Can. J. For. Res. Vol. 40, 2010


the current study. In all, this suggests that grain angle can beincorporated into breeding programmes without any adversegenetic responses in the other stem and wood quality traitsconsidered herein, although further investigations of the ge-netic relationship between grain angle and density might beadvisable.

In contrast to the low and nonsignificant genetic correla-tions with grain angle, stem, wood, and branch traits exhib-ited much stronger genetic relationships with height growth.BrD10, Br10, Tw10, and Pil18 all exhibited substantial un-favourable genetic correlations with Ht10 (Table 3). Similarrelationships have been shown repeatedly before for branchdiameter (e.g., Steffenrem 2008) as well as pilodyn penetra-tion (e.g., Costa et al. 2000), and those could well be re-garded as typical for Norway spruce. In this context, theweak adverse genetic relationship between GAub28 andgrowth traits should not necessarily be regarded as a majorconcern even if it had been significant.

Genetic correlations between growth and grain angledevelopment by cambial age

The genetic correlations between GAy and RGy in theyoung clonal material showed no dependence on cambialage (Fig. 3). Such a dependency would also have been re-flected by substantial genetic correlations between GAageB1and radial growth. Only material from Knutstorp exhibited aweakly positive GAageB1–RG10 genotypic correlation, butthis was not significantly different from zero (Table 4). Inaddition, the growth measurements made at different treeages in the old progeny material did not result in differentgenetic correlations with GAub28 (Table 3). Therefore,these observation do not provide evidence of any depend-

ence of genetic grain angle – growth correlations on cambialage within the 25 innermost annual rings in Norway sprucebreeding materials.

Genetic correlations between growth and grain angledevelopment by distance from the pith

Even if grain angle development does not show any sub-stantial genetic relationship with radial growth at any givenannual ring, the performance of a sawn board is dependenton the properties of the wood formed within that specificpart of the sawlog. Assuming that 50 mm � 100 mm boardswere flat sawn from logs that were completely straight andthat the pith never deviates from the centre of the log, it isclear that the centre of such boards would be located at25 and 75 mm from the pith, sharing an edge 50 mm frompith (Fig. 1). From this perspective, the traits describing thegrain angle development by distance from pith (GA25mm,GA50mm, GA75mm, and GAdistB1) are the most relevant(e.g., Ekevad 2005).

Therefore, it is interesting that the genetic correlation esti-mates between GAdistB1 and RG10 in the young clonal ma-terial were strongly positive and significant in both of thetrials (Table 4). In addition, at Hermanstorp, the genetic cor-relations with RG10 were also higher for grain angles atgreater distances from the pith and lower for grain anglescloser to the pith. At Knutstorp, these correlations had largestandard errors, and therefore, no such trend was apparent. Itis also noteworthy that the genetic correlations betweenGA75mm and Ht10 in the old progeny material were fairlyhigh and significantly different from zero in two of the threetrials (Table 3).

These genetic correlations suggest that rapid growth stillresults in greater grain angles at specific distances from thepith, even though no genetic relationships appear to exist be-tween growth and grain angle at specific cambial ages. Thisdiscrepancy in genetic correlations could be explained as aneffect of increased stem juvenility. Thus, the higher grainangle associated with juvenile wood and low cambial agewould be spread further out from the pith as a consequenceof increased radial growth. Similar relationships have beeninvestigated and discussed elsewhere using other wood traitsas indicators of wood juvenility (e.g., Cameron et al. 2005).

Implications for tree breedingAccording to the results of this study, grain angles meas-

ured at a standard number of annual rings from the pith arenot strongly correlated with growth traits, stem straightness,branch quality traits, or density. If the grain angle is meas-ured at a young age and the cambial age at the time ofmeasurement is taken into account, breeding for a lowergrain angle level in the juvenile core is possible and wouldnot have a great effect on other important traits.

However, despite the lack of genetic correlations betweengrain angles and growth traits at a specific cambial age,faster growth appears to cause the high grain angles associ-ated with juvenile wood to be spread further out from thepith, thus affecting larger volumes of wood. Consequently,breeding for faster growth with the aim of reducing the rota-tion length and keeping the stem diameter constant at the fi-nal cut would increase the proportion of wood with highgrain angles. In that sense, the results of this study still sug-

Fig. 3. Genetic correlations between grain angle of individual an-nual rings and accumulated annual ring widths at (a) Hermanstorpand (b) Knutstorp. Error bars are SEs.



gest that growth traits and grain angle development may beregarded as genetically adversely correlated.

Moreover, the efficiency of using single annual ring as-sessments with the aim of breeding for a reduction in shapestability problems in sawn timber is questionable to someextent, because these generally do not take the measurementdistance from the pith into account. In addition, the geneticrelationship between grain angles in individual rings and theoverall grain angle trend from juvenile to mature wood isstill poorly understood in Norway spruce.

Therefore, grain angle age–age correlations using meas-urements from multiple annual rings in older material andgenetic correlations between grain angles in specific annualrings and at specific distances from pith will be an importantand interesting topic for future studies. One should not betempted to use the strong genetic correlation betweenGAub28 and GA75mm (Table 3) as an indicator of this rela-tionship since the GA75mm for the old progeny materialwas calculated solely from GAub28 and DBH27 and wasbased on certain assumptions (eq. 1). Therefore, theGAub28 – GA75mm genetic correlation may be biased up-wards because of automatic correlation.

Conclusions

No substantial correlations between grain angle under thebark at a cambial age of 22–25 years and pilodyn penetra-tion, stem straightness, branchiness, branch diameter, rami-corn occurrence, and branch angle were observed. Thesetraits can probably be bred for independently of grain angle.The genetic correlations between grain angle measured insingle annual rings (3–15 and 22–25 years) were generallyadverse but weak (always <0.4) and consistently nonsignifi-cant. The genetic correlations between grain angle and radialgrowth were not significantly dependent on cambial age, buta positive dependence on distance from pith and a positivegenetic correlation between radial growth and grain angletrend with distance from the pith was observed. Therefore,in a sawtimber context, the results suggest that breeding forfaster growth alone will still result in unfavourable radialgrain angle trends and that the evaluation of the impact ofdifferent breeding strategies on shape stability or strengthshould make use of grain angles at specific distances fromthe pith rather than grain angles at specific cambial ages.

AcknowledgementsHeartfelt thanks go to Gudmund Ahlberg and Bo Karlsson

at Skogforsk in Ekebo for technical assistance and for theirhelp in searching the archives for historical measurementdata. Further grateful thanks go to Tim Mullin and AndersFries for useful comments on the manuscript. The measure-ments of grain angle for the samples from the two clonaltrials were made at the Institute of Botany, Universitat furBodenkultur, Austria, which also is acknowledged. JaniceMartin improved the manuscript with respect to the Englishlanguage. This study was conducted within the frameworkof the Research School of Forest Genetics and Tree Breed-ing and was also supported by the host company of the firstauthor, Bergvik Skog.

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Genetic correlations between spiral grain and growth and quality traits in Picea abies