Abstract
The anatomical properties of seven hybrid poplar clones grown in three sites in southern Quebec, Canada were investigated. Radial and longitudinal variations in selected anatomical properties of wood were measured by image analysis of transverse sections and by fiber quality analysis. Results indicate that all measured anatomical properties varied significantly across sites. Clonal variation was highly significant for all anatomical properties studied, and broad-sense heritability ranged from 0.10 (average vessel lumen area) to 0.76 (cell wall area percentage). Genetic gain was positive for all anatomical properties. The variation in radial pattern was characterized by a rapid increase in the first few years in fiber length, width, and proportion, wall thickness, and percent cell wall area. Ray proportion remained constant, whereas the vessel lumen area and proportion decreased with cambial age.
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ANATOMICAL PROPERTIES OF SELECTED HYBRID POLAR CLONES GROWN IN SOUTHERN QUEBEC
ASM Azmul Huda,a Ahmed Koubaa,a,* Alain Cloutier,b Roger E. Hernández,b and Pierre Périnet c
The anatomical properties of seven hybrid poplar clones grown in three sites in southern Quebec, Canada were investigated. Radial and longitudinal variations in selected anatomical properties of wood were measured by image analysis of transverse sections and by fiber quality analysis. Results indicate that all measured anatomical properties varied significantly across sites. Clonal variation was highly significant for all anatomical properties studied, and broad-sense heritability ranged from 0.10 (average vessel lumen area) to 0.76 (cell wall area percentage). Genetic gain was positive for all anatomical properties. The variation in radial pattern was characterized by a rapid increase in the first few years in fiber length, width, and proportion, wall thickness, and percent cell wall area. Ray proportion remained constant, whereas the vessel lumen area and proportion decreased with cambial age.
Keywords: Hybrid poplar; Wood anatomical properties; Site; Clonal variation; Age; Heritability; Genetic gain
Contact information: a: Canada Research Chair on Wood Development Characterization and Processing, Département des sciences appliquées, Université du Québec en Abitibi-Témiscamingue, 445 Boul. de l’Université, Rouyn-Noranda, Québec, Canada J9X 5E4; b: Université Laval, Département des sciences du bois et de la forêt, 2425 Rue de la Terrasse, Québec, Canada G1V 0A6; c: Ministère des Ressources naturelles et de la Faune du Québec, 2700 Einstein, Québec, G1P 3W8, Canada.
* Corresponding author: ahmed.koubaa@uqat.ca
INTRODUCTION
Forests are one of Canada’s most important natural resources. Poplar species and hybrids are among the most widespread and fastest growing tree species in North America, and along with aspen and mixed wood stands, they make up a substantial part of the natural forest. Poplars are increasingly harvested in Canadian forest industries, and they currently account for over 50% of all hardwoods and 11% of overall timber resources in Canada (Avramidis and Mansfield 2005).
Hybrid poplars are hybridizations of two or more species within the genus Populus, which, as one of the fastest growing temperate trees, has considerable commercial value (Zsuffa et al.1996). Hybrid poplars have been genetically modified through selection and crossbreeding to improve growth rate, trunk form, adaptability, and disease resistance (Hernández et al. 1998; Riemenschneider et al. 2001; Zhang et al. 2003; Pliura et al. 2007). Wood formation is a complex developmental process that includes the differentiation of vascular cambial initials into various xylem tissues, cell elongation, and secondary wall synthesis. As the secondary wall forms, the fiber and vessel cells undergo massive thickening, significantly influencing the wood quality. Wood properties are largely genetically determined (Zobel and Jett 1995) and can show growth variations in width and height depending on site conditions and age. For example, some fast growing tropical tree species consist of juvenile wood only when they are harvested at young age (Zobel and Sprague 1998). For optimum wood utilization, the age effects on wood properties must be determined, including the size and type distribution of cells (Peszlen 1994). In addition, genetic improvements designed to produce harvestable size trees at young age may also produce higher percentages of juvenile wood, which would have significant effects on wood properties and industrial processing applications (Burley and Palmer 1979; Mátyás and Peszlen 1997; Hernández et al. 1998). Accordingly, researchers have analyzed anatomical variations in wood elements within and among clones (Populus spp., Eucalyptus spp., Dalbergia spp.) in order to assess wood quality (Phelps et al. 1982; Koubaa et al. 1998; Rao et al. 2002; Pande and Singh 2005).
Panshin and de Zeeuw (1980) conducted a literature review on longitudinal and radial variations in wood anatomical properties. They found three patterns of radial variation in tracheid and fiber length: 1) a rapid increase followed by constant length from pith to bark; 2) a smooth and continuous increase from pith to bark; and 3) an increase from pith to bark up to a maximum, followed by a smooth decrease. A similar trend was reported for vessel element length and for fiber and vessel diameter, although the increase was moderate. Fiber wall thickness increases from pith to bark in some species and remains constant in others. According to Mátyás and Peszlen (1997), wood properties vary greatly within and among poplar trees. However, the findings on variation in poplar wood properties are inconclusive and in some cases contradictory. More specifically, within-tree variation in anatomical properties in hybrid poplars has not been examined, except for a few studies on fiber length (Holt and Murphey 1978; Murphey et al. 1979; Yanchuk et al. 1984; Bendtsen and Senft 1986; Koubaa et al. 1998; DeBell et al. 1998). These studies found significant clone and longitudinal variation in fiber length. DeBell et al. (1998) found that the variation in fiber length was affected by age but not by growth rate, whereas Koubaa et al. (1998) found a slightly negative correlation between fiber length and growth rate.
Peszlen (1994) found a significant effect of age on poplar anatomical features. She reported an increase in fiber and vessel lumen area from pith to bark, with a rapid increase in vessel lumen diameter in the first few years followed by constant increase toward the bark. Lei et al. (1996) found a similar variation pattern in white oak. Mátyás and Peszlen (1997) found only slight changes in the radial distribution of vessel lumen, fiber lumen, and cell walls in poplar clones. Kern et al. (2005) noted that various fiber features, such as narrower fiber lumen area, greater cell wall thickness, change in the cellulose microfibril angle, or biochemical features of the lignin in cell walls, might mitigate potential mechanical weakening caused by greater vessel lumen area. Based on an understanding of the anatomical characteristics that determine wood quality in hybrid poplar clones, better selection criteria could be developed for specific final products.
Little information is available on the genetic parameters of the wood anatomical characteristics of poplar species, such as heritability and genetic gain, except for a few studies on fiber length (Farmer and Wilcox 1968; Koubaa et al. 1998; Zhang et al. 2012) and fiber width (Wang et al. 1991). Estimating heritability and genetic correlation with traits is particularly important for predicting the genetic gain from cloned material (Foster and Shaw 1988) and for better clonal selection.
Accordingly, knowledge of the variation in fiber anatomy is essential for obtaining improved wood quality, better clonal selection, optimum rotation cycles, and better end uses of hybrid poplar clones. We therefore investigated site, clonal, and within-tree variations in selected anatomical properties of hybrid poplar clones grown in field trials in the southern part of the province of Quebec, Canada. The results will contribute to the selection of superior clones in terms of wood anatomical properties.
MATERIALS AND METHODS
Study Area
Materials for this study were collected from three hybrid poplar clonal trials established by the Direction de la recherche forestière, Ministère des Ressources naturelles et de la Faune du Québec (Department of Forestry Research at Quebec’s Ministry of Natural Resources and Wildlife) between 1991 and 1995. Trees for hybrid clones trials were planted at the Saint-Ours and Windsor sites in 1993 and in 1991 at the Pointe-Platon site (Table 1). Trees for clone DNxM-915508 were obtained from a 1995 trial at the Pointe-Platon site (Table 2). The trial sites are located in Pointe-Platon (46040’N 71051’W), Saint-Ours (45054’N 73009’W), and Windsor (45042’N 71057’W) in southern Quebec, Canada (Fig. 1). The Saint-Ours site is located in the Champlain marine deposit, where the soil consists of a silty clay deposit (40% clay). The two other sites consist of sandy loam soil (Pliura et al. 2007). All sites were originally used for agriculture, but had been abandoned for several years before the hybrid poplar clones were planted. All tree plantation trial sites had a randomized block design with ten blocks each.
Table 1. Site Characteristics of Hybrid Poplar Clonal Trials
Fig. 1. Map of sampling sites located in the southern part of the province of Quebec, Canada
Sample Collection and Preparation
Seven hybrid clones (Table 2) were selected for this study. Five trees per site were randomly sampled for each clone, for a total of 105 trees. Trees were cut from the St-Ours and Windsor sites after 15 growing seasons. Trees were cut from the Pointe-Platon site after 17 growing seasons, except for clone DNxM-915508, which was felled after 13 growing seasons. Disks 10 cm thick were collected from each tree from breast height upward at 2.5 m intervals (Fig. 2) and used for anatomical analysis. Wax was applied to the disk edges to limit drying and prevent decay and other environmental alterations. Samples were then transported to the Wood Research Centre, Université Laval, Quebec, Canada and kept frozen until test sample preparation. A 2.5 cm thick slab was cut along a diameter of each disk (bark to bark passing through the pith) and then conditioned at 20 °C and 60% relative humidity for several weeks until reaching 12% moisture content.
A series of radially oriented sample blocks sized 1 (T) x 1 (R) x 2 (L) cm was systematically cut from annual growth rings (3, 6, 9, and 12) using a precision saw and a chisel. At the annual growth ring with a cambial age of 15, there was insufficient wood to prepare samples for properties measurement by WinCELL, an image analysis system specifically designed for wood cells analysis, from Regent Instruments, Québec, Canada. Cross sections of 20 μm were cut using a rotary microtome with a disposable blade positioned at approximately 15 degrees. Sections were then bleached with sodium hypochlorite solution (80 mL water + 5 drops of bleach) for 1 minute and washed in a distilled water bath for 1 minute. Sections were then double stained with 1% safranin stain for 5 minutes and 0.1% astra blue stain for 15 minutes. Excess stain was removed by washing sections successively in 50, 80, and 100% ethanol solution. Thin sections were further dehydrated using toluene and then permanently mounted on microscope slides with coverslips using permount. Samples were left for two weeks to ensure proper drying of the permount.
Fig. 2. Schematic diagram of the sampling procedure for the analysis of hybrid poplar wood anatomical properties
Table 2. Studied Hybrid Poplar Clones
Trees for the clone DNxM-915508 at Pointe-Platon were sampled from another trial PLA16495.
Samples were photographed at ×50 magnification using a Leica compound microscope (DM 1000) equipped with a PL-A686 high resolution microscopy camera to capture black and white images (tiff electronic file format) at 1200×1600 resolution using a green filter to maximize contrast (Fig. 3). WinCELL Pro 2004a (Régent Instruments Inc. 2004) was used to measure average fiber wall thickness, fiber lumen area, vessel lumen area, fiber diameter, and vessel diameter. Average fiber diameter accounts for average fiber lumen diameter and the respective two-sided fiber wall thickness. Cell wall area (%) was estimated by subtracting the percent areas of the vessel lumen, ray, and fiber lumen from the image area (Peszlen 1994).
The proportion of tissue in the different cell types was estimated from two cross-sections of each wood block. Vessel cells were distinguished from fiber and ray by analyzing 570 µm2fields (corresponding to four squares of the grid) and noting the tissue types within the field. Fiber proportion was measured using the same method. Ray proportion was obtained by subtracting from unity the vessel and fiber areas.
Fig. 3. Microscopic cross-section of hybrid poplar wood: V = vessel, R = ray, F = fiber, Scale bar = 50 µm
The same thick slabs used for wood anatomy analysis were used to analyze fiber properties from pith to bark at different tree heights using a Fiber Quality Analyzer (FQA) (OpTest Equipment Inc., LDA02, Ontario, Canada). A series of radially oriented thin tangential sections was systematically cut from annual growth rings 3, 6, 9, 12, and 15 for fiber length and fiber width measurement. Tangential sections were macerated using Franklin’s method. Sections were soaked in Franklin solution (1:1, glacial acetic acid: 30% hydrogen peroxide) and heated at 70 °C for approximately 48 hours. The solution was decanted and the remaining fibrous material was washed under vacuum with deionized water until reaching neutral pH. The distribution of fiber length and width was measured automatically using the FQA. A total of 5,000 fibers were measured for each sampled growth ring.
The SAS® statistical package, version 9.2 (SAS 2008) was used for all statistical analyses. Normality and homogeneity of variance for residuals were tested using UNIVARIATE statistics. Data transformations were not considered necessary to satisfy the assumptions of the analysis of variance and other analyses. Analyses of variance were performed using PROC GLM with Type III (partial sums of squares) estimates to assess the relative magnitude of each source of variation. Analyses were performed among and within sites and clones. The variance in anatomical properties variables among sites was analyzed using the following mixed linear model (Eq. 1),
Xijlm = μ + Si + Cj + (S x C)ij + Al + Hm+ (A x H)lm + εijlm (1)
where Xijlm is an observation on the lth age and mth height of the jth clone from the ith site; μ is the overall mean; Al, Hm, and (A x H)lm are the fixed effects and their interactions, respectively; Si is the fixed effect due to the ith site; Cj is the fixed effect due to the jth clone; (S x C)ij is the interaction between site and clone; and εijlm is the random error. Some F ratios involved more than one mean square in the denominator and were tested with approximate degrees of freedom. Tree effects and the site-clone-age interaction were not considered in the analysis, as preliminary testing showed negligible contribution to the total variance. Furthermore, in many cases, the variance component for these terms could not be estimated or was not significant.
Tukey’s Studentized Range (HSD) was used to test the statistical significance (at p<0.05) of differences among means of clones for each site (PROC GLM, SAS). The variance components were estimated in the model using VARCOMP with the restricted maximum likelihood method (REML) and expressed as a percentage (VAR). The individual broad-sense heritability (H2) was calculated from the variance estimates, as follows (Eq. 2) (Becker 1984; Falconer and Mackay1996),
(2)
where 
and 
are the genotypic and phenotypic variance, respectively. Phenotypic variance ( )was calculated as shown in Eq. (3).
(3)
The genotypic coefficient of variation (CVG) and the phenotypic coefficient of variation (CVp) were calculated from Eqs. 4 and 5, respectively (Burton 1952; Henderson 1953).
(4)
(5)
The mathematical expression for the genetic gain (G) is expressed in Eq. 6. The potential genetic gain from individual tree selection is computed by selection differential (Eq. 7) and 10% selection intensity,
(6)
(7)
where h² is the heritability, S is the selection differential, i is the selection intensity (10%), and σP is the phenotypic standard deviation.
RESULTS AND DISCUSSION
Within- and Among-Site Variation
Growth site variable showed a highly significant effect on all studied wood anatomical properties (Table 3). Table 4 presents the mean and standard error of the anatomical properties of all studied hybrid poplar clones.
Trees from the Saint-Ours site showed the highest fiber length (0.99 ± 0.17 mm), and trees from Pointe-Platon showed the lowest (0.90 ± 0.19 mm), with a 10% difference between the lowest and highest length (Table 4). Highest fiber diameter, vessel lumen area, and vessel diameter were also found in trees from Saint-Ours, with the lowest from Pointe-Platon.
For fiber width, the Windsor site showed the highest average and Pointe-Platon the lowest average (Table 4). Pointe-Platon trees showed higher fiber wall thickness than trees from the two other sites, with a 12.4% difference between the highest and lowest average (Table 4).
Pointe-Platon trees showed the lowest average fiber lumen area, fiber diameter, vessel lumen area, and vessel diameter (Table 4). Pointe-Platon trees showed the highest fiber proportion and cell wall area, and the lowest average vessel lumen area and diameter and ray proportion (Table 4).
The significant site effect concurs with previous studies (Murphey et al. 1979; Phelps et al. 1982; Yanchuk et al. 1984; Bendtsen and Senft 1986; DeBell et al. 1998; Chauhan et al. 1999; Zhang et al. 2003; Pliura et al. 2007). Several factors may explain this significant site effect on the anatomical properties, including edaphic and climatic conditions (Peszlen 1994; Pliura et al. 2007). Trees from the Saint-Ours site showed higher fiber length, vessel proportion, and vessel dimensions. This could be explained by higher moisture availability and better drainage conditions as well as the soil surface deposition at the Saint-Ours site compared to the two other sites.
Clonal Variation in Fiber Anatomical Properties
The clone effect on the examined wood anatomical properties was highly significant (Table 3). The variance component analysis indicated that the clone effect varied among the studied properties, ranging from 0.2% to 47.9%. The clone effect on fiber width, although significant at the 0.05 probability level, was low (1.8%). Similarly, the clonal variance component of the vessel lumen area was also negligible (0.2%). In contrast, the variance component for the cell wall area percentage was the highest (47.8%) among all the anatomical properties (Table 3).
Differences among clones were also significant for wood element proportions (fiber, vessel, and ray), as shown in Table 3. The highest interclonal variation was observed for vessel proportion, whereas ray proportion showed a significant clone effect but a low variance component (Table 3).
The clone effect was reflected in the differences in means among clones for all studied properties. For example, clone DxN-4813 showed the highest fiber wall thickness, whereas clone DxN-3565 showed the highest fiber lumen area and average fiber diameter. Clone DxN-131 showed the highest vessel lumen area, and clone DxN-3570 showed the highest average vessel diameter (Table 4).
Table 3. Results of the Analysis of Variance of Wood Anatomical Characteristics of Hybrid Poplar Clones (F values and variance components of 11 wood anatomical properties: fiber length (FL), fiber width (FW), fiber wall thickness (FWT), average fiber lumen area (AFLA), average fiber diameter (AFD), average vessel lumen area (AVLA), average vessel diameter (AVD), fiber proportion (FP), vessel proportion (VP), ray proportion (RP), and cell wall area (CWA)).
*Significant at P<0.05 probability level.
**Significant at P<0.01 probability level.
ns Non-significant at P<0.05 probability level.
a Variance component as a percentage of the total variance.
Table 4. Least Squares Means of Clones at Different Sites and Multiple Comparison Tests of Hybrid Poplar Clones (fiber length (FL), fiber width (FW), fiber wall thickness (FWT), average fiber lumen area (AFLA), average fiber diameter (AFD), average vessel lumen area (AVLA), average vessel diameter (AVD), fiber proportion (FP), vessel proportion (VP), ray proportion (RP), and cell wall area (CWA)).
Means within a column followed by the same letter are not statistically different at p=0.05 for each site separately.
The highest fiber proportion was found for clone DxN-3565 and the lowest for clone DNxM-915508, for a 17.4% difference (Table 4). The highest vessel proportion percentage was found for clone DxN-3570 and the lowest for clone DxN-3565, for a 27.4% difference. The highest and lowest ray proportions were found for clone DNxM-915508 and DxN-3565, respectively.
The effect of site x clonal interaction was also highly significant (Table 3). This indicates that clonal variation varied across the sites. The variance components of the site x clone interaction for fiber lumen area and fiber proportion were higher than for the other anatomical properties (Table 3). This variation was probably caused by the low number of clones involved (7) and the fact that the trees were sampled from small clonal test plots rather than a commercial plantation with large monoclonal blocks. Clonal tests are subject to effects from neighboring plots due to competition among clones. However, these environmental effects, although significant, are negligible compared to the clonal and within-tree variation.
According to Monteoliva et al. (2005), fiber anatomical properties have a major influence on the quality of pulp and paper products as well as solid wood products. Pliura et al. (2007) and Huda et al. (2011) found that fiber wall thickness was positively correlated to wood density in poplar hybrids. At the cellular level, increased cavitation resistance and stem mechanical strength were associated with thicker cell walls (Jacobsen et al. 2005). Greater fiber length is preferred for pulp and paper production because a better fiber network is achieved, resulting in higher paper strength. Karlsson (2006) also argued that narrow fiber width is desirable for pulp and paper applications because it results in smoother paper and more uniform formation.
Based on the results of the present study, clone DxN-3570, which showed high fiber length and narrow fiber width, would be promising for pulp and paper manufacturing. Similarly, clones DxN-4813 and DxN-3565, which showed the highest cell wall thickness, would have good density and mechanical properties, resulting in higher yield for fiber-based products (Huda et al.2011). In fact, of the seven studied hybrid poplar clones, these two presented the highest wood density (Huda et al. 2011; Hernández et al. 2011).
Although only seven hybrid poplar clones were included in this study, the significant variation in morphological properties within sites and clones indicates good opportunities for selecting the most performing clones in terms of anatomical properties, both for breeding and for processing for specific end uses.
Except for fiber length, few data on anatomical properties of poplar species have been reported in the literature (Table 5). The results on anatomical properties obtained in this study are generally in good agreement with previously reported data except for fiber width, average fiber diameter and ray proportion (Table 5) which showed higher values than previously reported data.
Moreover, the literature contains little data on genetic parameters such as heritability and genetic gain for the wood anatomical characteristics of poplar species, aside from a few studies on fiber length, which can be compared with the present study. Genetic variations such as heritability and genetic gain for particular properties are important tools for predicting genetic gains (Foster and Shaw 1988).
Table 5. Anatomical Properties of Various Populus Clones and Hybrids (fiber length (FL), fiber width (FW), fiber wall thickness (FWT), average fiber lumen area (AFLA), average fiber diameter (AFD), average vessel lumen area (AVLA), average vessel diameter (AVD), fiber proportion (FP), vessel proportion (VP), ray proportion (RP), and cell wall area (CWA)).
Based on these variations, hybrid poplar clones could be better selected for higher heritability and genetic gain, even with simple selection procedures. The high heritability and genetic gain for fiber length, fiber wall thickness, fiber proportion, vessel proportion, and cell wall area percentage indicate that these properties are primarily genetically determined in hybrid poplars, and can be selected through phenotypic performance.
Table 6. Estimates of Genetic Parameters of 7 Hybrid Poplar Clones (fiber length (FL), fiber width (FW), fiber wall thickness (FWT), average fiber lumen area (AFLA), average fiber diameter (AFD), average vessel lumen area (AVLA), average vessel diameter (AVD), fiber proportion (FP), vessel proportion (VP), ray proportion (RP), and cell wall area (CWA)).
The clonal and environmental variations (Table 3) were used to estimate various genetic parameters, including heritability and genetic gain for anatomical properties (Table 6). The phenotypic coefficient of variation was higher than the genotypic coefficient of variation for all studied properties (Table 6). This indicates that the apparent variation in clones was not only genotypic, it was also due to environmental factors. The extent of the environmental influence on a given property is indicated by the difference between the phenotypic and genotypic coefficient of variation. A small difference indicates a low environmental effect (Saifullah and Rabbani 2009).
Broad-sense heritability for anatomical properties ranged from 0.10 to 0.76, with high heritability for fiber wall thickness and cell wall area (Table 6). The heritability for fiber length was 0.59, which is comparable to that reported for hybrid poplar (H2=0.41) by Koubaa et al. (1998). However, heritability for fiber length in different species of Populus tremuloides was reported at 0.43 by Yanchuk et al. (1984), and at 0.61 by Klasnja et al. (2003) and at 0.36 by Farmer and Wilcox (1968) for Populus deltoides. These results confirm that fiber length in hybrid poplar is only moderately genetically determined, as suggested by Koubaa et al. (1998).
Heritability for fiber width was 0.28, in good agreement with the heredity for hybrid poplar (H2=0.33) found by Wang et al. (1991). The highest heritability was obtained for cell wall area (0.76), followed by fiber wall thickness (0.73), fiber proportion (0.62), and vessel proportion (0.58). Genetic gain for anatomical properties of hybrid poplar clones ranged from 0.83 to 6.02. The highest genetic gain was obtained for fiber wall thickness (6.02), followed by fiber length (5.11), fiber proportion (5.11), and cell wall area (4.67). The genetic gain for vessel proportion was 3.82.
Within-Tree Variation
The effects of tree height and cambial age were highly significant on most of the examined properties, except for vessel area and ray proportion (Table 3). The patterns for radial and longitudinal variation in fiber properties, vessel properties, and element proportions are shown in Figures 4 to 10, respectively.
The variation in fiber length with cambial age at each of the three sampled heights is illustrated in Fig. 4a. At all heights, fiber length was short near the pith, increased rapidly during the early years, and then continued to increase at a lower rate towards the bark (Fig. 4a). This variation pattern concurs with previous studies (Peszlen 1994; Mátyás and Peszlen 1997; DeBell et al. 1998; Koubaa et al. 1998). The increase in fiber length from pith to bark could be explained by the increase in length of cambial initials with increasing cambial age, as discussed in previous studies (Peszlen 1994; Koubaa et al. 1998, DeBell et al. 1998; Jorge et al. 2000; Fang and Yang 2003; Fang et al. 2004). The fiber length of hybrid poplar is shorter near the pith due to a high proportion of juvenile wood. The general pattern of fiber length found in this study did not allow a clear differentiation between juvenile and mature wood (Fig. 4a).
Fig. 4. Variation and standard error for (a) fiber length and (b) fiber width with cambial age and tree height in hybrid poplar clones
Fig. 5. Variation and standard error for fiber wall thickness with cambial age and tree height in hybrid poplar clones
Maturation age is defined as the age at which the size of wood elements stabilizes. The radial variation pattern for the studied anatomical properties showed no evidence of a transition from juvenile to mature wood at any cambial age. Thus, the radial variation in anatomical properties indicated that the wood in the studied hybrid poplar clones was still juvenile.
The radial variation in fiber length was the main source of variation, accounting for 71.1% of the total variation (Table 3). This result concurs with a previous study on P. x euramericanaclones (Koubaa et al. 1998), where cambial age accounted for 80.5% of the total variation.
The radial variation in fiber width accounted for 10.7% of the total variation and was characterized by an increase from pith to bark to reach a maximum at cambial age 9, followed by a slight decrease in the outer rings (Fig. 4b). The same variation pattern was obtained for the three sampled heights.
Fiber wall thickness tended to increase from the pith outwards. This trend is clearly shown at heights 1.3 m and 6.2 m (Fig. 5). However, at height 3.75 m, a slight decrease from pith to bark is shown. The radial variation in fiber wall thickness accounted for 1.2% of the total variation (Table 3).
Average fiber lumen area (Fig. 6a), average fiber diameter (Fig. 6b), and cell wall area (Fig. 7) increased from the pith outwards. Although the age effect on these properties was significant, it contributed little to the total variation. It accounted for only 2.4%, 1.0%, and 4.0% for the lumen area, fiber diameter, and cell wall area, respectively (Table 3). The average vessel lumen area (Fig. 8a) remained relatively constant. This result is supported by the non-significant effect of age on vessel area (Table 3). The average vessel diameter showed a decreasing trend with increasing cambial age (Fig. 8b). However, despite the significant effect of age on vessel diameter, it accounted for only 3.5% of the total variation (Table 3). The radial variation pattern from pith to bark for the fiber lumen and vessel lumen area confirms the results of other studies in poplar clones. For example, although the fiber lumen area increased slightly with age, the vessel lumen area remained relatively constant in poplar clones (Mátyás and Peszlen 1997).
Figures 9 and 10 present the radial variation in wood element proportions. Fiber proportion showed a significant increase from pith to bark for the three sampled heights (Fig. 9a), whereas vessel proportion showed the inverse trend (Fig. 9b). Cambial age showed significant variation in both properties, accounting for 2.6% and 4.3% of the total variation in fiber and vessel proportion, respectively (Table 3). However, no particular radial pattern of ray proportion was observed. In fact, the age effect on ray proportion was non-significant (Table 3). These results contradict those of Isebrands (1972), who found that wood formed in the juvenile crown of eastern cottonwood usually had a lower percentage of vessels than wood in more mature crowns. He also reported a noticeable decrease in the percentage of fibers with increasing distance from the pith and tree height.
Fiber length at all cambial ages increased with increasing tree height (Fig. 4a). Although significant (= 5%), the variation in fiber length with tree height was marginal, accounting for less than 0.5% of the total variation (Table 3). Fiber width tended to increase with increasing height near the pith but in the outer rings, the opposite trend was observed (Fig. 4b). The longitudinal variation in fiber width was also marginal. It accounted for less than 2% of the total variation (Table 3).
Fig. 6. Variation and standard error for (a) average fiber lumen area (AFLA) and (b) average fiber diameter (AFD) with cambial age and tree height in hybrid poplar clones
Fig. 7. Variation in cell wall area percentage with cambial age and tree height in hybrid poplar clones
Fig. 8. Variation and standard error for (a) average vessel lumen area (AVLA) and (b) average vessel diameter (AVD) with cambial age and tree height for hybrid poplar clones
Fig. 9. Variation and standard error for (a) fiber and (b) vessel proportions with cambial age and tree height in hybrid poplar clones
Fig. 10. Variation and standard error for ray proportion with cambial age and tree height in hybrid poplar clones
Fiber wall thickness (Fig. 5), average fiber lumen area (Fig. 6a), average fiber diameter (Fig. 6b), and cell wall area (Fig. 7) increased with increasing tree height. Despite the significant effect of tree height on these properties, the longitudinal variation was marginal, accounting for less than 1.6% of the total variation (Table 3). Panshin and de Zeeuw (1980) found that fiber diameter and wall thickness in hardwoods decreased from the base of the stem to the crown. In the present study, three heights were sampled at 1.3 m, 3.75 m, and 6.2 m, well above the base of the tree crown (data not shown).
The effect of tree height was non-significant on vessel lumen area, and no partic-ular longitudinal variation pattern was found (Fig. 8a). However, the average vessel diameter decreased with increasing tree height (Fig. 8b). Although significant, the variation in height accounted for only 1.9% of the variation (Table 3).
The longitudinal variation in the fiber, vessel, and ray proportion are shown in Figs. 9a, 9b, and 10, respectively. Ray proportion did not vary with tree height (Fig. 9). However, fiber proportion increased with increasing tree height, whereas vessel proportion showed the opposite trend. Although statistically significant (= 5%), these variations accounted for less than 2.5% (Table 3) of the total variation, and were also considered marginal. The longitudinal pattern of fiber proportion runs contrary to the study by Isebrands (1972), who found a noticeable decrease in fiber proportion with increasing tree height.
The results on the longitudinal variation in the anatomical properties of hybrid poplar concur with previous studies in poplar clones (Fang and Yang 2003), red alder (Gartner et al. 1997), and several other hardwoods (Panshin and de Zeuuw 1980; Kellison 1981; Zobel and van Buijtenen 1989). Although the variations in some wood anatomical properties with tree height were statistically significant, they were relatively small and considered marginal.
CONCLUSIONS
This study examined site, clone, and within-tree variations in selected anatomical properties of hybrid poplar clones grown in southern Quebec, Canada. The effect of site on the studied properties was highly significant, and was explained by environmental conditions and site quality. The highly significant clone effect indicated genetic variation in the examined properties. Thus, broad-sense heritability ranged from 0.10 (for average vessel area) to 0.76 (for cell wall area). It would therefore be possible to select superior hybrid poplar clones in terms of desired anatomical properties. Similarly, up to 6.02% genetic gain could be obtained for fiber wall thickness.
The within-tree variation indicated highly significant radial variation in most studied trends. However, the longitudinal variation, although significant in most cases, was marginal and had no practical implications.
ACKNOWLEDGMENTS
The authors are grateful to the Canada Research Chair Program, the Fonds québécois de la recherche sur la nature et les technologies (FQRNT), Quebec’s Ministère du Développement économique, de l’Innovation et de l’Exportation (MDEIE), and the Réseau Ligniculture Québec (RLQ) for financial support, and to the Centre de recherche sur le bois (CRB), Université Laval for laboratory facilities and to Quebec’s Ministère des Ressources naturelles et de la Faune (MRNF) for providing access to the experimental plantations and supplying tree material.
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Article submitted: May 16, 2012; Peer review completed: June 26, 2012; Revised version received and accepted: June 29, 2012; Published: July 3, 2012.