3.2. Effect of Site, Clone and Site × Clone Interaction
The results of the analysis of variance are shown in
Table 4. As discussed in previous reports [
26,
27,
28], the clone effect was highly significant for anatomical, physical and mechanical properties. A clone effect is indicative of the genetic control of the studied properties, which were under weak to moderate genetic control [
15,
26,
27,
28]. Site effect was also highly significant for most of the examined traits. Several factors explain the significant site effect on the studied properties, including edaphic and climatic conditions [
20,
29].
The effect of site × clone interaction (
S ×
C) was significant for most of the anatomical (FP (fiber proportion), VP (vessel proportion), FWT (fiber wall thickness)) and shrinkage (VSH (volumetric shrinkage), RSH (radial shrinkage)) properties (
Table 4). For these properties, the ranking of the clones changes from one site to another [
30]. For the wood density and mechanical properties, the
S ×
C interaction was not significant and the ranking of clones does not change across sites. This result is expected for clonal trials and is in a good agreement with previous findings [
10,
11,
12,
16,
17,
18,
19,
20,
30]. Zobel and Jett [
30] reported that for wood properties, especially wood density, the G × E interaction (Genotype × Environment) is very small. However, considering the relatively low number of studied sites (three sites) and the number of replicates within each site (seven clones and five trees per clone), the results are indicative and a larger population should be sampled for more precise estimation of the various genetic and phenotypic parameters.
3.3. Phenotypic Correlations between Wood Properties
The results of the correlation analysis between the studied properties are presented in
Table 5. The correlations between fiber length and all other wood properties were not significant at both tree and clone levels. The non-significant correlation between fiber length and wood density is in good agreement with previous findings for hybrid poplar clones [
10] and for
P. trichocarpa [
31]. For both tree and clone levels, a close negative relationship was found between fiber and vessel proportions. Increasing the proportion of one element will lead to a decrease in the other. This result is in good agreement with previous reports on hybrid poplar [
15,
32] and other hardwoods [
29,
33,
34,
35,
36].
A positive relationship between fiber proportion and fiber wall thickness was also observed (
Table 5). Thus, clones with higher fiber proportion tend to develop thicker cell walls. Similarly, the negative relationship between fiber wall thickness and vessel proportion suggests that clones with higher vessel proportion have thinner cell walls. These findings explain the positive correlation between wood density and fiber wall thickness and the negative correlation between vessel proportion and wood density. Wood density was correlated to all anatomical features, except fiber length, at both the clone and tree levels. Indeed, the fiber morphological properties of wood largely determine its density [
37]. Higher fiber proportion and fiber wall thickness are associated with higher wood density [
38,
39]. On the other hand, a high percentage of vessel proportion will yield hydraulic conductivity, which could cause higher shrinkage, and a disruption in wood structure. Joon [
40] reported that the large number of vessel elements present in poplar wood is mainly responsible for the disruption of its structure.
Positive and significant correlations between tension wood proportion and fiber wall thickness were found (
Table 5). This means that higher tension wood proportion is associated with smaller fiber lumen area and thicker walls. On the other hand, vessel proportion was negatively correlated to tension wood proportion. These findings are in good agreement with previous findings for eastern cottonwood [
41].
The correlation between wood density and tension wood proportion was positive and significant at both the tree and clone levels. This result could be explained by the higher fiber proportion and greater fiber wall thickness of tension wood. In addition, the formation of tension wood was associated with the presence of a gelatinous layer that increases the amount of cellulosic material in the fiber. Okumura et al. [
42] suggested that increased wall thickness for tension wood fibers was mainly due to an increased thickness of the unlignified cellulosic G-layer of the secondary wood layer. The correlation between tension wood proportion and wood density at the clone level (0.78) was much higher than that at the tree level (0.35). Indeed, the clone is the only factor that showed a significant effect on the tension wood proportion (
Table 4). In addition, the high variance component for the error terms suggests a high dispersion of the data at the tree level.
The correlation between tension wood proportion and volumetric shrinkage was not significant (
Table 5). The volumetric shrinkage of wood is influenced by the tension wood content. The samples from the present study might have variable tension wood contents, which thereby explains the insignificant variation of volumetric shrinkage values among the tested clones, although this result is difficult to explain. However, Gorisek and Straze [
43] reported that chemical composition and cell wall organization such as high crystallinity of cellulose in the G-layer, small amounts of matrix substance, and smaller micro voids in cell walls, are probable reasons for the non-significant relationship between wood shrinkage and tension wood. On the other hand, the presence of tension wood was positively correlated to longitudinal, radial, and tangential shrinkages. Ollinmaa [
44] found significant positive correlations between longitudinal shrinkage and tension wood proportions for aspen and alder. Many authors also confirmed the existence of a positive correlation between tension wood and longitudinal shrinkage in poplar wood [
45] and other hardwoods [
46,
47]. Sassus [
48] described that axial shrinkage of tension wood is often more than five times higher than that of normal wood for beech and poplar.
The correlation coefficients between tension wood and mechanical properties were also significant (
Table 5). This result is in good agreement with Pilate et al. [
49], who suggested that the presence of the G-layer contributes in a significant way to the specific mechanical properties of wood. The results of the present study indicate that tension wood will not negatively affect the mechanical performance of the wood. Similarly, Hernández et al. [
50] found that tension wood did not affect the machining properties of these same hybrid poplar clones. Clair et al. [
51] also found similar results for chestnut. For poplar, the secondary wall of the tension wood is replaced by a poorly lignified or purely cellulosic layer that is generally thick [
42]. Besides, tension wood is characterized by a higher proportion of fibers and a lower proportion of vessels [
52]. As a result, the increase in fiber proportion implies more walls by volume of wood tissue, thus, a higher density and higher mechanical properties [
15].
The phenotypic correlation between volumetric shrinkage and wood density was positive and significant at the clone and tree levels. For radial shrinkage, the correlation with wood density was significant but those of longitudinal and tangential shrinkage were not. A similar result was reported in
Populus ×
canadensis hybrid clones [
18]. However, Koubaa et al. [
18] recommended the direct measurement of shrinkage values for poplar since several anatomical features, such as growth ring angle, fibril angle or lumen diameter, might significantly influence the shrinkage of juvenile poplar wood. Volumetric shrinkage had no significant relationship with anatomical or mechanical properties in the present study. Volumetric shrinkage and swelling are affected by several wood properties, such as the heartwood to sapwood ratio and the microfibril angle in the S2 layer [
53]. Our results showed that, among the properties studied, wood density has the greatest effect on wood shrinkage but the correlations were only moderate or non-significant (
Table 5). Hence, the direct measurement of shrinkage values of tested poplar clones gives some degree of confidence on their dimensional stability.
A number of anatomical features are known to influence plant mechanical properties [
54]. This study showed that mechanical properties improved with increased fiber proportion, although no significant relationship with other anatomical properties was found, except for tension wood (
Table 5). Bendtsen et al. [
55] studied the mechanical properties of cottonwood and hybrid poplar NE-237 and found that anatomical properties and wood density had an effect on compression strength.
This study also found a positive relationship between wood mechanical properties and density, although this relationship was only moderately significant. Previous studies have found highly significant relationships between density and mechanical properties in hybrid poplar clones [
56,
57]. At the individual tree level, density showed a highly significant correlation with flexural MOR and ultimate crushing strength and a moderate but significant correlation with flexural MOE. On the other hand, density highly affected all mechanical properties at the clonal level. Similarly, all mechanical properties were moderately to highly correlated to wood density. Using the overall tree density instead of the density of the tested sample for the correlation analysis explains the moderate relationships. In addition, the tested poplar clones were only 15 years of age. Thus, the wood was mainly juvenile and could partially explain the weaker relationship between density and mechanical properties.
The correlation analysis showed that test sample density had a strong correlation with flexural MOE (
Figure 3) and MOR (
Figure 4) and the ultimate crushing strength (
Figure 5). This result is in good agreement with previous findings [
58,
59].
3.4. Genotypic Correlations between Wood Properties
Genotypic correlations among traits were moderate to strong, depending on traits (
Table 6). A significant negative genetic or genotypic correlation has been found between density and growth properties in many studies involving poplar and its hybrids [
11,
16,
17,
18,
19,
56]. However, no study has addressed the genotypic correlations among different wood properties in hybrid poplar clones.
Genotypic correlations between fiber length and other wood properties were negative (except for vessel proportion and flexural MOR) but weak and non-significant. The genotypic correlation between fiber length and density in the present study is in good agreement with a previous report [
31] on
Populus trichocarpa. In both phenotypic and genotypic correlations, we observed weak relationships among fiber length and other wood properties, which makes fiber length an independent trait for wood breeding strategies. However, weak correlations among these properties could be an indication that the properties are functionally or developmentally less related, and are therefore less integrated, phenotypically and genetically speaking. For example, this result suggests that it is difficult to improve both fiber length and basic density simultaneously. As a result, this weak genotypic correlation will have to be considered if density is used alone as a predictor for wood quality for hybrid poplar breeding programs.
The genotypic correlations among fiber proportion and other wood properties were strong and positive, while the genotypic correlation with vessel proportion was negative. Hence, this result indicates the greater importance of fiber proportion for end-uses. As expected, the genotypic correlations were strong and negative between vessel proportion and other properties (
Table 6). Additionally, fiber proportion and vessel proportion always showed the opposite correlation with the other wood properties, especially at the genetic level. The relationship among fiber wall thickness and other wood properties showed strong genotypic correlation. At the genetic level, fiber wall thickness was associated with higher fiber proportion, indicating a tendency for higher mechanical properties. Indeed, the thicker fiber wall corresponds to a higher fiber proportion or smaller vessel diameter, which induces higher wood density and mechanical properties. All correlations with tension wood were positive and high, with the exception of vessel proportion, where a strong negative genotypic correlation was detected. The gelatinous fiber layer in tension wood has narrower vessels and a lower vessel area [
15]. In tension wood, the S3 layer of the secondary wall is replaced by the thick cellulosic layer known as the gelatinous fiber layer inside the lumen of the fiber. Kaeiser and Boyce [
41] reported that gravitational stimulus generally induces the formation of gelatinous fibers, which modify the anatomical characteristics of other elements of wood, such as modifications in the size of rays, vessels, and fibers in
Populus deltoides.
Strong genotypic correlations were observed between tension wood and shrinkage properties. Tension wood consists of a hydrophilic substance within the G-layers [
60]. As a result, when tension wood is dried and water removed rapidly, it causes a greater level of shrinkage and it impacts wood mechanical properties and, consequently, wood quality.
Density showed strong positive genotypic correlations with all anatomical and mechanical properties except fiber length and vessel proportion. Zhang et al. [
10] reported a similar result for fiber length and wood density in hybrid poplar. The strong genotypic correlations observed between density and these properties indicated that selection of any one of these properties would result in a highly correlated response to selection in the others. However, a breeding program based on density may lead to severe reductions in fiber length, as fiber length had a strong genotypic correlation with growth properties, whereas significant negative genetic correlations were found between density and growth properties [
11,
56]. The genotypic correlations among density and the various shrinkage properties were moderate. Moreover, the genotypic correlation between wood density and mechanical properties was positive and strong (
Table 6).
This study further found strong genotypic correlations between wood mechanical properties and anatomical properties, except for fiber length, which does not play an important role in mechanical properties. On the other hand, the strong positive relationships between mechanical properties and anatomical properties (fiber proportion and fiber wall thickness) at the genetic level present a possible strategy for wood quality improvement. Breeding strategies that aim to improve fiber proportion and fiber wall thickness, and thus increase mechanical wood properties, would have negligible influence on fiber length. The genotypic correlations among mechanical properties and density were very strong. As a result, the inclusion of wood density in tree breeding programs could lead to an improvement in mechanical strength properties. Moreover, these high genotypic correlations with MOE and MOR make density a strong candidate for the direct genetic improvement of general wood quality. The use of this property could ultimately benefit solid wood and fiber-based wood products. For example, selection for increased wood density for industrial implications would at the same time increase pulp yield and the value of solid wood products, and decrease production costs. However, the choice of the properties to be included in the improvement program often depends on their ease of assessment or determination. Therefore, wood properties such as fiber proportion, fiber wall thickness and easily measurable wood density can be used as a selection strategy for the improvement of mechanical wood properties.
Broad literature surveys suggested that genetic and phenotypic correlations for wood properties have the same sign and magnitude [
59,
60,
61]. Our findings also confirmed the relationships of phenotypic and genetic correlations reported in the literature. For example, phenotypic and genetic correlations of vessel proportion with all properties were all negative (
Table 5 and
Table 6). However, stronger genotypic correlations were found compared to phenotypic correlations for all wood properties. These results may be explained by the environmental influences that weaken the phenotypic correlation between wood properties in comparison to the genotypic correlation. This is consistent with findings from an earlier study showing environmental influence on the phenotypic and genotypic coefficients of variation for wood anatomical properties [
26].
Some of the genetic correlations, showed a relatively high standard error. This result could be explained by the relatively small sample investigated in this study. Thus, a larger sampling is needed for more precise estimation of the various genetic parameters.