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ORIGINAL PAPER

Genetic variation in growth traits and stem–branch characteristics and their relationships to Eucalyptus clones

Shijun Wu1?Zhaohua Lu1?Jianmin Xu1?Guangchao Chen1?Yingan Zhu1?Guangyou Li1

?Northeast Forestry University and Springer-Verlag Berlin Heidelberg 2015

Eucalyptushas become an important genus in China because it displays adaptability to a wide range of site conditions and produces pulp wood and veneer on short rotations.The aim of this study was to estimate genetic parameters and relationships and consider implications for development of clonal forestry.We assessed growth traits,stem–branch characteristics,crown diameter and height of fresh branch for 20Eucalyptushybrid clones in China measured at the age of 44 months.Analysis of variance showed that there were signif i cant differences in growth traits,height of fresh branch and stem straightness among clones.Signif i cant differences in height,volume, crown diameter,height of fresh branch and branch size among replicates were also recorded.Coeff i cients of variation ranged from 9.84 to 28.54%for growth traits,12.03 to 17.25%for stem–branch characteristics,18.26%for crown diameter and 11.73%for height of fresh branch. Estimates of repeatabilities for height,diameter at breast height over bark,volume,crown diameter,height of fresh branch,stem straightness and branch size at clone mean level were 0.86,0.80,0.80,0.54,0.85,0.77 and 0.44 respectively.Diameter at breast height over bark and height had strongly positive phenotypic and genotypic correlations with volume,ranging from 0.96 to 1.00.The positive genotypic and phenotypic correlations between growth traits and other studied traits suggested that fast growing clones always had bigger crown diameter,higher height of fresh branch,straighter stems and relatively smaller branches.

Multiple comparison?Clonal variation?Genotypic and phenotypic correlations?Eucalyptusclones

Introduction

Tree plantations have become the basic source to meet the increasing demand for pulp and paper and wood products because of the steady increase in global population and the protection of natural forests by governments,particularly in China and other developing countries(Huang and Dell 2002;Bull et al.2006;Kien et al.2009).Eucalypts have been recognized as one of the three most quickly growing genera with strong adaptability and excellent wood properties(Xu and Dell 2002;Martin 2002;Warren et al.2009). The launching of large-scale commercial eucalypt plantation programs began in the 1990s in China(Qi 2007).A short-rotation species,eucalypt plantations now cover over 3.6 million hectares in China.The mean annual yield of eucalypt plantations is 20–30 m3ha-1a-1,and some reach 40 m3ha-1year-1(Qi 2007;Xu et al.2009).However, the mean annual increment in volume of eucalypt plantations in China is still less than the mean annual yield ofeucalypt plantations in some tropical and subtropical regions,and some commercial plantations in countries such as Brazil,South Africa,Congo and Australia can yield as much as 30–90 m3ha-1a-1(Mo et al.2002;Toit et al. 2010).Yang(2003)and Xu et al.(2003)reported that productivity of eucalypt plantations varies from 2 to 70 m3ha-1year-1,suggesting great potential for improving productivity of low-yield eucalypt plantations in southern China.Selection of new clones and increasing the productivity of existing plantations to supply the growing demand for wood products and to reduce the unit cost of wood production are urgent priorities in China(Wu et al. 2014).Therefore clonal selection has become an increasingly important practice for eucalypt plantation managers in China.

The objectives of this study were to:(1)compare growth traits,stem–branch characteristics,crown diameter and height of fresh branch of different clones,(2)estimate within-clonal variation and repeatability of traits,(3)look at the genotypic and phenotypic relationships between growth traits,stem–branch characteristics,crown diameter and height of fresh branch,and(4)explore the implications for tree improvement.

Materials and methods

Trial description

A trial was established at Jiuhe town of Heyuan City in Guangdong province of south China(23°82′N,115°09′E, 355 m a.s.l.),a region which is affected by the south subtropical monsoon with mean annual temperature of 20.9°C and mean annual rainfall of 1700 mm.Mean January temperature at this location is 11°C,mean for July 27°C and the minimum temperature was-4°C.The frost period was 56–65 days.Planting pits(50 cm× 50 cm×40 cm)were prepared and 0.4 kg of eucalypt fertilizer(N:P:K=16:16:16)was applied in the f i rst year for each tree.The details of assessed clones are presented in Table 1.Twenty clones were planted in April 2005.The widely planted U6 clone,bred in the early 1990s,was used as a check(CK)comparison.The f i eld design was a randomized complete block with 5-tree line plots at a spacing of 3×2 m.

Data collection

Measurements of diameter at breast height over bark(DBH in cm),height(HGT in m),crown diameter(CD in m), stem straightness(STR),branch size(BRA)and height of fresh branch(HF in cm)were collected in December 2008, 44 months after the beginning of the experiments.Tree volume(VOL in m3ha-1)was calculated using the following formula as described by McKenney et al.(1991):

STR was assessed using a four-class relative scoring method,where class 4 trees had very straight stems without knots or scars,class 3 trees were straight or had a slight bend at one position with few knots and scars,class 2 trees had at least one slight bend or a larger bend with more knots and scars,and class 1 trees had very crooked stems. BRA was assessed using a three-class relative scoring method,where class 3 trees had small and uniform branches,class 2 trees had intermediate-sized branches,and class 1 trees had very large branches.Prior to analysis, class scores were transformed into asymptotic‘normal scores’to adjust for inadequate or variable spacing of classes and to improve the eff i ciency of data analyses(Hai et al.2008).

Statistical analysis

Variation among clones and replicates were analyzed by analysis of variance,using a linear model(Hansen and Roulund 1996):

where yijis the performance of the ramet of the ith clone within the jth replicate,and μ is the general mean,αiis the random effect of the ith clone,βjis the f i xed effect of the jth replicate,and εijis the random error.

The repeatability of the clone mean was calculated following Hai et al.(2008):

The genotypic coef fi cient of variation(CV)was calculated as(Pliura et al.2007;Hai et al.2008):

where x is the phenotypic mean.The equation expresses a standardized measure of the genetic variance relative to the mean of a trait.

The genotypic correlation rA(xy)of traits x and y was calculated as(Pliura et al.2007;Stackpole et al.2010):

Table 1 Clone number,clone identity,and parental combination of assessed clones

Results and discussion

Variance among clones and replicates

The analysis of variance of studied traits among clones and replicates is presented in Table 2.Clones differed signif icantly in growth traits HF and STR.BRA also differed signif i cantly between clones at the 0.05 level.Clones were similar in terms of CD.Replicates differed signif i cantly in HGT,VOL,CD,HF and BRA due at least in part to relatively uneven site conditions.DBH and STR were similar among replicates,suggesting less environmental effects on these traits.

Clonal multiple comparison

The clonal multiple comparison results(Table 3)showed that the mean values of VOL,HGT,DBH,HF,STR and BRA were 101.31 m3ha-1,14.41 m,11.02 cm,10.43 m, 3.54 and 2.43,respectively.The mean VOL of the most rapidly growing clone,the slowest growing clone,CK and all clones were 138.39,58.78,58.78,and 101.31 m3ha-1, respectively(Table 3;Fig.1).Clone 3(DH32-28)grew more rapidly than others and its VOL,HGT,DBH,HF,STR and BRA were 138.39 m3ha-1,16.32 m,12.26 cm, 11.56 m,3.54 and 2.42.As CK,Clone 17(U6)was the slowest growing and its VOL,HGT,DBH,HF,STR and BRA were 58.78 m3ha-1,11.75 m,9.42 cm,8.25 m,3.15 and 2.17.Selection gains predicted for VOL,HGT and DBH,based on comparison of the slowest growing to the fastest growing clone,were 235,138 and 130%.Based on comparison of the mean values for all clones with the means for the fastest growing clone,predicted gains were 34,25 and 13%.Due to the similarity of CD among clones,CD was not listed in Table 3.Stem–branch characteristics considered to be important traits for breeding have been widely used in tree improvement(Hai et al.2008;Kien et al. 2008).The mean value of STR and BRA were 3.54 and 2.43 implying most of the clones had straight stems and smaller branches.Clones 11 and 12 scored higher means for STR and BRA than the overall means.

Clonal variation and repeatability

Coeff i cients of variation and repeatability of all traits are listed in Table 4.Coeff i cients of variation ranged from9.84 to 28.54%for growth traits,12.03 to 17.25%for stem–branch characteristics,18.26%for CD and 11.73% for HF.These were in agreement with the variability observed in previous studies by Kien et al.(2010),in which coeff i cients of variation for growth traits ranged from 10 to 20%.These results also showed that HGT and DBH had least variation,stem–branch characteristics,and HF variation were moderate,and VOL had greatest variation, indicating that the scope for selection among clones would be considerable.These results agreed with previous studies ofAcacia auriculiformisA.Cunn.Ex Benth.(Hai et al. 2008)andEucalyptus(Kien et al.2008,2010).

Table 2 Analysis of variance of growth traits and stem–branch characteristics

Table 3 Multiple comparisons of assessed clones in growth traits and stem–branch characteristics(α=0.01)

Fig.1 Mean values(±SE)of VOL(tree volume in m3ha-1)of assessed clones

Table 4 Coeff i cient of variation and clonal repeatability of growth traits and stem–branch characteristics

Estimates of repeatabilities for growth traits,CD,HF, STR and BRA at clone mean level were 0.86,0.80,0.80, 0.54,0.85,0.77 and 0.44 respectively.The repeatabilities of HGT and HF were always higher than for other traits. Meanwhile,CD and BRA had lowest repeatability.This conclusion is consistent with previous studies ofEucalyptusclones(Varghese et al.2008),Poplar clones(Pliura et al.2007)andAcaciaclones(Hai et al.2008).Varghese et al.(2008)reported that repeatabilities of growth traitsranged from 0.24 to 0.62 inE.camaldulensisandE. tereticornis.Lu et al.(2005)found that repeatabilities of growth traits and stem–branch characteristics ranged from 0.61 to 0.93 in 20 eucalypt clones.Hai et al.(2008)found that repeatabilities at tree ages of 3 and 4 years ranged from 0.28 to 0.47 for HGT,0.21–0.56 for DBH,0.21–0.54 for VOL,0.21–0.32 for STR,and 0.21–0.28 for BRA inAcacia auriculiformis.

Genotypic and phenotypic correlations between traits

Genotypic and phenotypic correlations between traits are listed in Table 5.In this study,DBHOB and HGT had strongly positive phenotypic and genotypic correlations with VOL,ranging from 0.96 to 1.00,while the correlations between DBHOB and VOL were generally higher than for those between HGT and VOL,indicating the importance of DBHOB in selecting for improved stem volume.VOL was calculated from DBH and HGT of each tree,therefore positive correlations to some degree always exited among DBHOB,HGT and volume whether inEucalyptus,Populusor other species(Hansen and Roulund 1996;Greaves et al.1997;Kumar and Singh 2001;Hai et al.2008;Kien et al.2010).STR and BRA generally had positive genotypic and phenotypic correlations with growth traits,agreeing with Kien et al.(2009)who found that genetic correlations between growth and stem traits were low to moderate in two open-pollinated progeny trials ofEucalyptus urophyllaat two sites in northern Vietnam.The phenotypic and genotypic correlations between STR and BRA were 0.79 and 1.03,respectively.The genotypic correlation between STR and BRA was above 1.00,probably due to imprecise estimation and may therefore be unreliable.The phenotypic and genotypic correlations between HF and CD were 0.72 and 0.87,suggesting that indirect selection of HF would yield a strong favorable response in CD.The positive genotypic and phenotypic correlations we recorded between growth traits and other studied traits imply fast growing clones always had larger CD,higher HF,straighter stems and relatively smaller branches.These correlations indicate that traits are likely controlled by the same set of genes.

Table 5 Phenotypic(above diagonal)and genotypic(below diagonal)correlations among growth traits and stem–branch characteristics

Major conclusions and implications for breeding programs

Joint analysis of clonal trials showed four implications for tree improvementinEucalyptusin China.Primarily,Clone 3 (DH32-28),Clone 1(DH32-26),Clone 6(DH4-26),Clone 5 (DH32-25)and Clone 4(DH33-9)displayed more rapid growth than others,and Clone 17(U6)was the slowest growing,implying more attention should be paid to selection of parent stock for breeding in order to improve genetic diversity.Secondly,tree heightand diameteratbreastheight had least variation,stem–branch characteristics and height offresh branch variation were moderate,and volume had the greatest variation,indicating that the scope for selection among clones is considerable.Thirdly,the differences in diameter at breast height and stem straightness between replicates were not signif i cant,implying less environmental effects on these traits.Fourthly,the repeatabilities of tree height and height of fresh branch were always higher than thatofothertraits,while crown diameterand branch size had lowest repeatabilities.These results should be studied for tree breeding,especially for improvements in pulp wood production.Furthermore,the positive genotypic and phenotypic correlations between growth traits and other studied traits suggest that fast growing clones always had larger crown diameters,higher height of fresh branches,straighter stems and relatively smaller branches.

AcknowledgmentsThe authors thank Prof.K.Harding and RE. Pegg for suggestions and corrections to improve early drafts of this manuscript.We thank Dr.Siming Gan and Dr.Jie Zeng for their suggestions and help.Comments from anonymous reviewers are also appreciated.

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22 November 2013/Accepted:5 January 2014/Published online:15 July 2015

Project funding:This study was f i nically supported by the National Twelfth Five-Year Science and Technology Plan‘Breeding of High yield and High Resistance New Species of Eucalyptus’’

(2012BAD01B0401),‘Genetic Research of Pulp Yield and Veneer of Eucalyptus urophylla Hybrid Clones’(RITFYWZX201304).

The online version is available at http://www.springerlink.com

Corresponding editor:Hu Yanbo

Shijun Wu and Zhaohua Lu have equally contributed to this work.

?Jianmin Xu

jianmxu@163.com

1Research Institute of Tropical Forestry,Chinese Academy of Forestry,Guangzhou 510520,Guangdong, People’s Republic of China