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Abstract In order to explore the differences in the compressive strength parallel to grain （CSPG） between different clones and individual woods of Populus deltoides Marsh.， the CSPGs of 5 15yearold P. deltoides clones were determined and analyzed. The results showed that there were significant differences in CSPG of wood between different clones of P. deltoides. The CSPG of the tested clones reached more than 41 MPa， which was higher than the average compressive strength of P. deltoides （32.94 MPa）. Similar to the changes of stem height， the CSPG of the clones gradually increased with the increase of stem height. However， there were significant differences in CSPG of an individual plant at different tree heights. The CSPG of P. deltoides clones was related with their growth traits， but not significantly. Therefore， it is expected to achieve simultaneous and independent improvement on CSPG and the growth of P. deltoides， so as to get excellent individuals with fast growth and high CSPG.
Key words Populus deltoides Marsh.； Clone； Stem height； Compressive strength parallel to grain
Populus deltoides Marsh. is native to North America along the Mississippi River. It is widely introduced and cultivated in the Yangtze River basin in China. It is one of Chinas important fastgrowing tree species and is widely used in industries such as pulping and papermaking as well as woodbased panels[1-2]. In recent years， with the development of science and technology and the maturity of the manufacturing industry， Populus has also been gradually used as the structure materials in construction， furniture and so on. Therefore， the physical and mechanical properties of poplar wood have gradually attracted the attention of related researchers.
As for commercial tree species， wood traits are important selection indicators in the cultivation and breeding of the same species as their growth traits. At present， the genetic improvement of wood traits has been widely used in domestic and foreign commercial tree species， and the results are significant[3-6]. Populus is widely used in pulp materials， and the study on the wood properties of P. deltoides is relatively late at home and abroad. In 1989， Wang et al. found that there were vast genetic foundation for the wood density and fiber length of the F1 clones of P. deltoides × P. simonii Carr. and P. nigra L.）[5]. In the following years， Pan et al.， Alfas et al.， Yin et al. and Xu et al. further studied and analyzed the wood density and wood fiber of P. deltoides from macro and micro perspectives， respectively[7-12]. As for the study on the physical and mechanical properties of P. deltoides， Cao et al. measured the changes in wood physical and mechanical properties of P. deltoides at different stand densities； Tong et al. analyzed the differences in physicomechanical properties and the correlations with the growth traits of P. deltoides[3，13-15]. However， there are few studies on the compressive strength parallel to grain （CSPG） of P. deltoides， especially the study on the variation law of CSPG is still blank. In order to better explore the direction of development and utilization of the wood of P. deltoides， broaden the ways of wood utilization， and realize the diversification and efficient use of poplar wood resources， it is imperative to carry out research on the CSPG of P. deltoides. Therefore， using the clones of P. deltoides as the research object， this study explored the differences in CSPG between different clones and the variations rules of CSPG at different stem heights of P. deltoides， with the aim to provide referential basis for the diversified breeding of P. deltoides， and to provide technical parameters for the efficient use of P. deltoides and formulation of reasonable genetic improvement strategies.
Materials and Methods
Test time and place
The test place is located in the Populus Demonstration Forest Base in Junshan District of Yueyang City， which has a typical subtropical humid monsoon climate with an annual average temperature of 16.8℃， extreme high temperature of 39.3℃， extreme low temperature of -11.8℃， and annual sunshine duration of 1 726.7-1 792 h. The area is wet in spring and arid in autumn and winter. The average annual rainfall is 1 135.3-1 237.9 mm， and the annual average relative humidity is 80%. The terrain in the area is relatively flat， with an elevation of 28-35 m. The soil is mostly fluvoaquicture formed by rivers and lakes， and the soil is deep[16].
Test materials
The test samples were collected from the test forest of P. deltoides clones built in 2001. Using randomized block test， 5 typical P. deltoides clones （86， 80， 69， 58， 83） with straight stems and great increment were selected as test materials.
Test methods
For each clone， 3 model trees were taken and cut down， and a disc of 10 cm thick was taken respectively at the tree height of 1.3， 3.3， 5.3， 7.3 and 9.3 m. After drying， samples were made to measure the CSPG according to GB 192891 General Requirements for Physical and Mechanical Tests of Wood[15].
Test instruments
The CSPG of the wood of P. deltoides was measured using WDW20E microcomputer controlled electronic universal testing machine produced by Jinan Times Shijin Test Machine Co.， Ltd. according to GB 19352009.
Statistical analysis
The data was processed with Excel 2007. SPSS 20.0 was used to analyze the mean， standard deviation， variance， coefficient of variation， multiple comparisons， and correlations.
Results and Analysis Analysis on CSPGs of different P. deltoides clones
As an important strength indicator for engineering structural materials， CSPG is an indispensable breeding standard for the development and utilization of timber forest. In order to better develop the resources of Populus trees in Dongting Lake area， the CSPGs of the selected P. deltoides clones were measured （Table 1）.
As shown in Table 1， the average values of CSPGs of the 5 P. deltoides clones were all higher than 41 Mpa， and clone 83 had the largest CSPG of 48.44 MPa， followed by clones 86， 58 and 80 with the values of 45.45， 44.00 and 42.25 MPa， respectively， while clone 69 had the smallest value of GSPG with the average of 41.06 MPa. Compared with the average CSPG of 32.94 MPa recorded by Cheng et al.， the GSPGs of the 5 P. deltoides clones were all higher than the average， and the highest was 15.5 MPa higher while the least was still 8.12 MPa higher[14].
Combined with analysis of variance， there was a significant difference in CSPG between the P. deltoides clones. The results of multiple comparisons （Table 1） showed that the CSPG of clone 83 presented extremely significant difference from the other 4 clones， clone 86 showed extremely significant difference from clones 80 and 69， and significant difference from clone 58； clones 58 showed extremely significant difference from clones 80 and 69； the difference was significant between clones 80 and 69. Therefore， P. deltoides clones showed great genetic variation in CSPG， which had broad genetic improvement space， thereby laying the foundation for the breeding of P. deltoides wood resources.
Variations of CSPG at different stem heights
Based on the determination of CSPGs of P. deltoides clones at different stem heights， the variation of CSPGs at different stem heights were analyzed. As shown in Table 2， the CSPGs of the 5 P. deltoides clones changed with the increase of stem heights. The CSPGs of the clones varied from 29.14 to 52.69 MPa among the clones， and the variation coefficients of the stem heights among the clones ranged from 6.79 to 18%. Clone 80 had the largest coefficient of variation of 18%， followed by clone 58， which also had the coefficient of variation reached 16.11%， indicating that there was a large variation in the CSPGs of the two clones with the stem heights. The results of variance analysis （Table 2） showed that there were significant differences in CSPG of the P. deltoides clones at different stem heights， suggesting that the pressure intensities were different at different stem heights of the P. deltoides clones. Therefore， it should make selective use of the wood from different stem heights when used as the engineering structural materials， so as to make the best of the materials. Correlation analysis of CSPG and growth traits of P. deltoides wood
The correlation between various traits of woods directly relates to the formulation of genetic improvement strategies. If the correlation is positive， the improvement of one of the traits can make the related traits get corresponding improvement， thereby improving the comprehensive traits of the woods because of the accumulative effect. If the correlation is negative， the improvement of one of the traits can affect the positive gain performances of other correlated traits， so it has certain selectivity in the genetic improvement of woods. If there is no correlation between the traits of woods， the traits are independent of each other， and the improvement of any one of the traits will not affect the normal performances of other traits， and therefore it can make random improvement according to the breeding goals. Similarly， it is also essential to make the correlation analysis to the traits for the breeding and genetic improvement of P. deltoides clones.
Based on the measurement of tree height， diameter at breast height （DBH）， wood volume per plant， and CSPG of the tested P. deltoides clones， correlation analysis was further made between CSPG and the growth traits of tree height， DBH， and wood volume per plant （Table 3）. The results showed that the CSPG of P. deltoides clones was negatively correlated with DBH with the correlation coefficient of -0.538， but the correlation was not significant； the CSPG showed positive correlation with tree height with a correlation coefficient of 0.542， and the correlation with wood volume per plant was very small with the correlation coefficient of only 0.106. The above results showed that there was certain correlation between the CSPG and growth traits of P. deltoides clones， but the correlation was not significant. Therefore， the selection of CSPG would have no effect on the genetic improvement of the growth traits of P. deltoides clones.
Agricultural Biotechnology2018
Conclusion and Discussion
First， the average CSPGs of the tested P. deltoides clones all reach above 41 MPa， which is almost 10 MPa higher than the average CSPG （32.94 MPa） reported by Cheng et al.[17]， and there are extremely significant differences in the CSPGs of different clones. The results suggest that P. deltoides clones have excellent genetic resources of CSPG and broad genetic basis， which provides reference for the genetic improvement of P. deltoides resources as well as the development and utilization of wood. Second， the CSPGs of different P. deltoides clones all increase with the increase of stem heights. However， there are differences in the variation coefficients of CSPGs at different stem heights of different clones， in which clone 80 has the largest variation coefficient of 18%， followed by clone 58 of 16.11%. On the other hand， the other 3 clones show stable variation in CSPGs at different stem heights， and the coefficients of variation of the 3 are all less than 10%. Combined with the growth traits of the clones， the results show that there is no specific rule for the variation of CSPG with the changes of stem heights， which has certain relationship with the physiological characteristics of P. deltoides clones[18-22]. However， further studies are needed to confirm it.
Third， the correlations between CSPG and growth traits of P. deltoids clones show that the CSPG of P. deltoides clones is negatively correlated with DHB， and positively correlated with tree heights but not significant， while the correlation degree with the wood volume per plant is too small to count. Xu et al.[23]also reached the same conclusion in the study of the variation in CSPG of wood for provenances of exotic loblolly pine. Therefore， the CSPG of wood show a certain degree of independence to the growth traits， which is conductive to the improvement of CSPG and breeding of P. deltoides clones， making it possible to get the excellent clones and plants with good wood growth traits and high CSPG.
In view of this， our further study is intended to make comprehensive measurement to the wood properties of the fastgrowing clones obtained through a combination of macroscopic and microscopic methods. At the same time， regional testing is planned to provide bases for the popularization and application of fastgrowing excellent clones of Populus engineering structural materials， with the aim to increase the utilization rate of P. deltoides wood and broaden the development and utilization of P. deltoides resources.
References
[1]LU WH. Study on the variation of wood properties of polar clones in Populus deltoides Marsh[D]. Nanjing： Nanjing Forestry University， 2008： 1-6.
[2]QU Y. Breeding of fine poplar clones[D]. Taian： Shandong Agricultural University， 2014： 1-10.
[3]TONG ZK， YU YM， ZHENG YP. A study on timber physical and mechanical properties of new Aigeiros clones[J]. Forest Research， 2002， 15（4）： 450-456.
[4]BENETKA V， BARTAKOVA I， MOTTL J. Productivity of Populus nigra L. ssp. nigra under shortrotation culture in marginal areas[J]. Biomass and Bioenergy， 2002， 23：327-336. [5]WANG MX， HUANG MR， RUAN XG， et al. Genetic improvement of wood characters of new clones in the Aigeiros section[J]. Journal of Nanjing Forestry University （Natural Science Edition）， 1989， 13（3）：9-16.
[6]XIE XM， XIE HF， ZHANG YH. Correlation analysis of wood properties and growth traits of clones in Populus tomentosa[J]. Shandong Forestry Science and Technology， 2008（2）： 34-35.
[7]PAN HX， HUANG MR， RUAN XG， et al. Research on wood properties improvement VII study on early selection of wood density in new polar clones of Populus deltoids × P. simonii[J]. Scientia Silvae Sinicae， 1998， 34（1）： 73-81.
[8]PLIURA A， YU QB， ZHANG SY， et al. Variation in wood density and shrinkage and their relationship to growth of selected young poplar hybrid crosses[J]. Forest Science， 2005， 51（5）： 472-482.
[9]CHAVEY N， BARLOW P， SUNDBERG B. Understanding the role of the cytoskeleton in wood formation in angiosperm trees： hybrid aspen （Populus tremula × P. tremuloides） as the model species[J]. Tree Physiology， 2002， 22：239-249.
[10]COUTAND C， JERONIMIDIS G， CHANSON B， et al. Comparison of mechanical properties of tension and opposite wood in Populus[J]. Wood Science and Technology， 2004， 38（1）：11-24.
[11]YIN ZF， FAN RW. Differentiation in a secondary xylem vessel element of Populus deltoids using ultrastructural analysis[J]. Journal of Zhejiang Forestry College， 2008， 25（4）： 431-436.
[12]XU YM， WENG WY， ZHANG YH. Difference in timber fiber vessel and basic density of Populus deltoides with different diameter classes[J]. Anhui Forestry Science and Technology， 2011， 37（1）：2-9.
[13]CAO FL. Effects of planting density on wood propertie[J]. Journal of Nanjing Forestry University， 1994， 18（2）： 41-47.
[14]CHENG JQ. Xylology[M]. Beijing： China Forestry Publishing House， 1985.
[15]CHEN JB， XIANG DY， ZHANG ZY， et al. Studies on longitudinal compressive strength variance of Eucalyptus cloeziana[J]. China Forestry Science and Technology， 2009， 23（4）： 63-66.
[16]LI YJ， TANG YX， TANG J， et al. Distribution and ecological risk assessment of heavy metal elements in soils of the beaches in Dongting Lake[J]. Journal of Central South University of Forestry & Technology （Natural Science Edition）， 2011， 31（2）：55-60.
[17]CHENG JQ， YANG JJ， LIU P. Chinese timber log[M]. Beijing： China Forestry Publishing House， 1992.
[18]CHANG SS， HU JB， LIU Y. The principal component analysis on the physicmechanical properties of Eucalyptus urophylla × E. grandis[J]. China Forestry Science and Technology， 2006， 20（2）： 39-41. [19]HERNANDEZ RE. Selected mechanical properties of fastgrowing poplar hybrid clones[J]. Wood & Fiber Science Journal of the Society of Wood Science & Technology， 1998， 30（2）： 138-147.
[20]MIRANDA I， ALMEIDA MH， PEREIRA H. Provenance and site variation of wood density in Eucalyptus globules Labill， At harvest age and its relation to a nondestructive early assessment[J]. Forest Ecology and Management， 2001， 149： 235-240.
[21]SHEN ZM， FANG XG， HAN LX. Analysis on the relationship between increment and density of 6year Pinus taeda[J]. Journal of Zhejiang Forestry Science and Technology， 2005， 25（2）：1-3.
[22]LEI H， GARTNER BL， MILOTA MR， et al. Effect of growth rate on the anatomy， specific gravity， and bending properties of wood from 7 yearold red alder （Alnus rubra）[J]. Canadian Journal of Forest Research， 1997， 27（1）：80-85.
[23]XU YM， LIN H， ZHANG ZW， et al. Variation in crushing strength parallel to the grain of wood for provenances of exotic loblolly pine and its relationship with tree age， wood density and latewood percentage[J]. Journal of Northeast Forestry University， 2005， 33（4） ）： 19-22.
Editor： Na LI Proofreader： Xinxiu ZHU
Key words Populus deltoides Marsh.； Clone； Stem height； Compressive strength parallel to grain
Populus deltoides Marsh. is native to North America along the Mississippi River. It is widely introduced and cultivated in the Yangtze River basin in China. It is one of Chinas important fastgrowing tree species and is widely used in industries such as pulping and papermaking as well as woodbased panels[1-2]. In recent years， with the development of science and technology and the maturity of the manufacturing industry， Populus has also been gradually used as the structure materials in construction， furniture and so on. Therefore， the physical and mechanical properties of poplar wood have gradually attracted the attention of related researchers.
As for commercial tree species， wood traits are important selection indicators in the cultivation and breeding of the same species as their growth traits. At present， the genetic improvement of wood traits has been widely used in domestic and foreign commercial tree species， and the results are significant[3-6]. Populus is widely used in pulp materials， and the study on the wood properties of P. deltoides is relatively late at home and abroad. In 1989， Wang et al. found that there were vast genetic foundation for the wood density and fiber length of the F1 clones of P. deltoides × P. simonii Carr. and P. nigra L.）[5]. In the following years， Pan et al.， Alfas et al.， Yin et al. and Xu et al. further studied and analyzed the wood density and wood fiber of P. deltoides from macro and micro perspectives， respectively[7-12]. As for the study on the physical and mechanical properties of P. deltoides， Cao et al. measured the changes in wood physical and mechanical properties of P. deltoides at different stand densities； Tong et al. analyzed the differences in physicomechanical properties and the correlations with the growth traits of P. deltoides[3，13-15]. However， there are few studies on the compressive strength parallel to grain （CSPG） of P. deltoides， especially the study on the variation law of CSPG is still blank. In order to better explore the direction of development and utilization of the wood of P. deltoides， broaden the ways of wood utilization， and realize the diversification and efficient use of poplar wood resources， it is imperative to carry out research on the CSPG of P. deltoides. Therefore， using the clones of P. deltoides as the research object， this study explored the differences in CSPG between different clones and the variations rules of CSPG at different stem heights of P. deltoides， with the aim to provide referential basis for the diversified breeding of P. deltoides， and to provide technical parameters for the efficient use of P. deltoides and formulation of reasonable genetic improvement strategies.
Materials and Methods
Test time and place
The test place is located in the Populus Demonstration Forest Base in Junshan District of Yueyang City， which has a typical subtropical humid monsoon climate with an annual average temperature of 16.8℃， extreme high temperature of 39.3℃， extreme low temperature of -11.8℃， and annual sunshine duration of 1 726.7-1 792 h. The area is wet in spring and arid in autumn and winter. The average annual rainfall is 1 135.3-1 237.9 mm， and the annual average relative humidity is 80%. The terrain in the area is relatively flat， with an elevation of 28-35 m. The soil is mostly fluvoaquicture formed by rivers and lakes， and the soil is deep[16].
Test materials
The test samples were collected from the test forest of P. deltoides clones built in 2001. Using randomized block test， 5 typical P. deltoides clones （86， 80， 69， 58， 83） with straight stems and great increment were selected as test materials.
Test methods
For each clone， 3 model trees were taken and cut down， and a disc of 10 cm thick was taken respectively at the tree height of 1.3， 3.3， 5.3， 7.3 and 9.3 m. After drying， samples were made to measure the CSPG according to GB 192891 General Requirements for Physical and Mechanical Tests of Wood[15].
Test instruments
The CSPG of the wood of P. deltoides was measured using WDW20E microcomputer controlled electronic universal testing machine produced by Jinan Times Shijin Test Machine Co.， Ltd. according to GB 19352009.
Statistical analysis
The data was processed with Excel 2007. SPSS 20.0 was used to analyze the mean， standard deviation， variance， coefficient of variation， multiple comparisons， and correlations.
Results and Analysis Analysis on CSPGs of different P. deltoides clones
As an important strength indicator for engineering structural materials， CSPG is an indispensable breeding standard for the development and utilization of timber forest. In order to better develop the resources of Populus trees in Dongting Lake area， the CSPGs of the selected P. deltoides clones were measured （Table 1）.
As shown in Table 1， the average values of CSPGs of the 5 P. deltoides clones were all higher than 41 Mpa， and clone 83 had the largest CSPG of 48.44 MPa， followed by clones 86， 58 and 80 with the values of 45.45， 44.00 and 42.25 MPa， respectively， while clone 69 had the smallest value of GSPG with the average of 41.06 MPa. Compared with the average CSPG of 32.94 MPa recorded by Cheng et al.， the GSPGs of the 5 P. deltoides clones were all higher than the average， and the highest was 15.5 MPa higher while the least was still 8.12 MPa higher[14].
Combined with analysis of variance， there was a significant difference in CSPG between the P. deltoides clones. The results of multiple comparisons （Table 1） showed that the CSPG of clone 83 presented extremely significant difference from the other 4 clones， clone 86 showed extremely significant difference from clones 80 and 69， and significant difference from clone 58； clones 58 showed extremely significant difference from clones 80 and 69； the difference was significant between clones 80 and 69. Therefore， P. deltoides clones showed great genetic variation in CSPG， which had broad genetic improvement space， thereby laying the foundation for the breeding of P. deltoides wood resources.
Variations of CSPG at different stem heights
Based on the determination of CSPGs of P. deltoides clones at different stem heights， the variation of CSPGs at different stem heights were analyzed. As shown in Table 2， the CSPGs of the 5 P. deltoides clones changed with the increase of stem heights. The CSPGs of the clones varied from 29.14 to 52.69 MPa among the clones， and the variation coefficients of the stem heights among the clones ranged from 6.79 to 18%. Clone 80 had the largest coefficient of variation of 18%， followed by clone 58， which also had the coefficient of variation reached 16.11%， indicating that there was a large variation in the CSPGs of the two clones with the stem heights. The results of variance analysis （Table 2） showed that there were significant differences in CSPG of the P. deltoides clones at different stem heights， suggesting that the pressure intensities were different at different stem heights of the P. deltoides clones. Therefore， it should make selective use of the wood from different stem heights when used as the engineering structural materials， so as to make the best of the materials. Correlation analysis of CSPG and growth traits of P. deltoides wood
The correlation between various traits of woods directly relates to the formulation of genetic improvement strategies. If the correlation is positive， the improvement of one of the traits can make the related traits get corresponding improvement， thereby improving the comprehensive traits of the woods because of the accumulative effect. If the correlation is negative， the improvement of one of the traits can affect the positive gain performances of other correlated traits， so it has certain selectivity in the genetic improvement of woods. If there is no correlation between the traits of woods， the traits are independent of each other， and the improvement of any one of the traits will not affect the normal performances of other traits， and therefore it can make random improvement according to the breeding goals. Similarly， it is also essential to make the correlation analysis to the traits for the breeding and genetic improvement of P. deltoides clones.
Based on the measurement of tree height， diameter at breast height （DBH）， wood volume per plant， and CSPG of the tested P. deltoides clones， correlation analysis was further made between CSPG and the growth traits of tree height， DBH， and wood volume per plant （Table 3）. The results showed that the CSPG of P. deltoides clones was negatively correlated with DBH with the correlation coefficient of -0.538， but the correlation was not significant； the CSPG showed positive correlation with tree height with a correlation coefficient of 0.542， and the correlation with wood volume per plant was very small with the correlation coefficient of only 0.106. The above results showed that there was certain correlation between the CSPG and growth traits of P. deltoides clones， but the correlation was not significant. Therefore， the selection of CSPG would have no effect on the genetic improvement of the growth traits of P. deltoides clones.
Agricultural Biotechnology2018
Conclusion and Discussion
First， the average CSPGs of the tested P. deltoides clones all reach above 41 MPa， which is almost 10 MPa higher than the average CSPG （32.94 MPa） reported by Cheng et al.[17]， and there are extremely significant differences in the CSPGs of different clones. The results suggest that P. deltoides clones have excellent genetic resources of CSPG and broad genetic basis， which provides reference for the genetic improvement of P. deltoides resources as well as the development and utilization of wood. Second， the CSPGs of different P. deltoides clones all increase with the increase of stem heights. However， there are differences in the variation coefficients of CSPGs at different stem heights of different clones， in which clone 80 has the largest variation coefficient of 18%， followed by clone 58 of 16.11%. On the other hand， the other 3 clones show stable variation in CSPGs at different stem heights， and the coefficients of variation of the 3 are all less than 10%. Combined with the growth traits of the clones， the results show that there is no specific rule for the variation of CSPG with the changes of stem heights， which has certain relationship with the physiological characteristics of P. deltoides clones[18-22]. However， further studies are needed to confirm it.
Third， the correlations between CSPG and growth traits of P. deltoids clones show that the CSPG of P. deltoides clones is negatively correlated with DHB， and positively correlated with tree heights but not significant， while the correlation degree with the wood volume per plant is too small to count. Xu et al.[23]also reached the same conclusion in the study of the variation in CSPG of wood for provenances of exotic loblolly pine. Therefore， the CSPG of wood show a certain degree of independence to the growth traits， which is conductive to the improvement of CSPG and breeding of P. deltoides clones， making it possible to get the excellent clones and plants with good wood growth traits and high CSPG.
In view of this， our further study is intended to make comprehensive measurement to the wood properties of the fastgrowing clones obtained through a combination of macroscopic and microscopic methods. At the same time， regional testing is planned to provide bases for the popularization and application of fastgrowing excellent clones of Populus engineering structural materials， with the aim to increase the utilization rate of P. deltoides wood and broaden the development and utilization of P. deltoides resources.
References
[1]LU WH. Study on the variation of wood properties of polar clones in Populus deltoides Marsh[D]. Nanjing： Nanjing Forestry University， 2008： 1-6.
[2]QU Y. Breeding of fine poplar clones[D]. Taian： Shandong Agricultural University， 2014： 1-10.
[3]TONG ZK， YU YM， ZHENG YP. A study on timber physical and mechanical properties of new Aigeiros clones[J]. Forest Research， 2002， 15（4）： 450-456.
[4]BENETKA V， BARTAKOVA I， MOTTL J. Productivity of Populus nigra L. ssp. nigra under shortrotation culture in marginal areas[J]. Biomass and Bioenergy， 2002， 23：327-336. [5]WANG MX， HUANG MR， RUAN XG， et al. Genetic improvement of wood characters of new clones in the Aigeiros section[J]. Journal of Nanjing Forestry University （Natural Science Edition）， 1989， 13（3）：9-16.
[6]XIE XM， XIE HF， ZHANG YH. Correlation analysis of wood properties and growth traits of clones in Populus tomentosa[J]. Shandong Forestry Science and Technology， 2008（2）： 34-35.
[7]PAN HX， HUANG MR， RUAN XG， et al. Research on wood properties improvement VII study on early selection of wood density in new polar clones of Populus deltoids × P. simonii[J]. Scientia Silvae Sinicae， 1998， 34（1）： 73-81.
[8]PLIURA A， YU QB， ZHANG SY， et al. Variation in wood density and shrinkage and their relationship to growth of selected young poplar hybrid crosses[J]. Forest Science， 2005， 51（5）： 472-482.
[9]CHAVEY N， BARLOW P， SUNDBERG B. Understanding the role of the cytoskeleton in wood formation in angiosperm trees： hybrid aspen （Populus tremula × P. tremuloides） as the model species[J]. Tree Physiology， 2002， 22：239-249.
[10]COUTAND C， JERONIMIDIS G， CHANSON B， et al. Comparison of mechanical properties of tension and opposite wood in Populus[J]. Wood Science and Technology， 2004， 38（1）：11-24.
[11]YIN ZF， FAN RW. Differentiation in a secondary xylem vessel element of Populus deltoids using ultrastructural analysis[J]. Journal of Zhejiang Forestry College， 2008， 25（4）： 431-436.
[12]XU YM， WENG WY， ZHANG YH. Difference in timber fiber vessel and basic density of Populus deltoides with different diameter classes[J]. Anhui Forestry Science and Technology， 2011， 37（1）：2-9.
[13]CAO FL. Effects of planting density on wood propertie[J]. Journal of Nanjing Forestry University， 1994， 18（2）： 41-47.
[14]CHENG JQ. Xylology[M]. Beijing： China Forestry Publishing House， 1985.
[15]CHEN JB， XIANG DY， ZHANG ZY， et al. Studies on longitudinal compressive strength variance of Eucalyptus cloeziana[J]. China Forestry Science and Technology， 2009， 23（4）： 63-66.
[16]LI YJ， TANG YX， TANG J， et al. Distribution and ecological risk assessment of heavy metal elements in soils of the beaches in Dongting Lake[J]. Journal of Central South University of Forestry & Technology （Natural Science Edition）， 2011， 31（2）：55-60.
[17]CHENG JQ， YANG JJ， LIU P. Chinese timber log[M]. Beijing： China Forestry Publishing House， 1992.
[18]CHANG SS， HU JB， LIU Y. The principal component analysis on the physicmechanical properties of Eucalyptus urophylla × E. grandis[J]. China Forestry Science and Technology， 2006， 20（2）： 39-41. [19]HERNANDEZ RE. Selected mechanical properties of fastgrowing poplar hybrid clones[J]. Wood & Fiber Science Journal of the Society of Wood Science & Technology， 1998， 30（2）： 138-147.
[20]MIRANDA I， ALMEIDA MH， PEREIRA H. Provenance and site variation of wood density in Eucalyptus globules Labill， At harvest age and its relation to a nondestructive early assessment[J]. Forest Ecology and Management， 2001， 149： 235-240.
[21]SHEN ZM， FANG XG， HAN LX. Analysis on the relationship between increment and density of 6year Pinus taeda[J]. Journal of Zhejiang Forestry Science and Technology， 2005， 25（2）：1-3.
[22]LEI H， GARTNER BL， MILOTA MR， et al. Effect of growth rate on the anatomy， specific gravity， and bending properties of wood from 7 yearold red alder （Alnus rubra）[J]. Canadian Journal of Forest Research， 1997， 27（1）：80-85.
[23]XU YM， LIN H， ZHANG ZW， et al. Variation in crushing strength parallel to the grain of wood for provenances of exotic loblolly pine and its relationship with tree age， wood density and latewood percentage[J]. Journal of Northeast Forestry University， 2005， 33（4） ）： 19-22.
Editor： Na LI Proofreader： Xinxiu ZHU