Physical and mechanical properties of wood
The AD (0.62 to 0.68 g/cm3) and OD (0.58 to 0.65 g/cm3) values obtained this study were similar with respect to basic density33 in the same sample trees (Table 2), whereas our results for mean AD values were relatively higher than those for L. sibirica reported by Ishiguri et al.27 and lower than those reported by Koizumi et al.5. Radial variations of AD and OD showed similar patterns to those reported by other researchers of L. sibirica5 and L. kaempferi16. Meanwhile, Cáceres et al.3 reported an influence of extractives on density in L. kaempferi. They found that the hot-water extractive content of L. kaempferi varied between 2.9 to 6.9% among 20 provenances, suggesting that actual wood density might be about 5% lower than AD. As shown in Table 3, cold-water extractive content ranged from 7.3 to 16.1%, and the mean values of EOD (0.54 g/cm3, Table 2) were about 10% lower values compared to OD (0.61 g/cm3, Table 2). These results indicated that the effect of cold- or hot-water extractives on wood density might be greater in L. sibirica compared to other Larix species.
Ishiguri et al.27 reported that radial shrinkage at 1% moisture content change showed almost constant values from pith to bark, whereas tangential shrinkage increased up to 4 cm from pith and then became constant at around 0.3%. The mean values and radial variation patterns examined in this study for shrinkage in both the radial and tangential directions in L. sibirica were similar to those of L. sibirica examined by Ishiguri et al.27.
Although the tree ages varied, the mean values of MOE, MOR, and CS of the L. sibirica trees in the present study (Table 4) were similar to those found in a previous study for L. sibirica that grow naturally in Mongolia27 but lower than those for L. sibirica that grow naturally in Russia5 and higher than those for L. kaempferi planted in Japan22,24. The mean SS was higher than that of L. sibirica planted in Finland9 and L. kaempferi planted in Japan24. In the radial variation, similar radial trends were found in L. sibirica that grow naturally in Mongolia27 and in L. kaempferi planted in Japan22.
Based on the obtained results, the mean values of the physical and mechanical properties of L. sibirica collected from five different provenances in Mongolia are similar to those of L. sibirica and other Larix species found in other countries. Thus, wood resources from L. sibirica harvested in Mongolia can be used for similar purposes to other Larix species, such as construction materials.
Juvenile and mature wood
The boundary between juvenile and mature wood ranged from the 17th to 24th annual rings from the pith (Table 6). The results were similar to those reported for L. kaempferi trees17,22,42. However, Ishiguri et al.27 showed that juvenile wood might exist within 4 cm from the pith in L. sibirica. In the present study, the boundary was within 2 to 5 cm from the pith among the provenances, suggesting that juvenile wood formation in L. sibirica trees that grow naturally in Mongolia is not only affected by tree age but also by growing conditions.
We previously reported that mean values of annual ring width were 1.55, 2.47, 0.49, 1.86, and 1.74 for Khentii, Arkhangai, Zavkhan, Khuvsgul, and Selenge, respectively33. This result indicates that the radial growth rate was extremely slow in Zavkhan compared to other four provenances. Shiokura and Watanabe28 reported that suppressed radial growth in the initial stage of tree growth resulted in prolonging the juvenile wood formation period in Picea jezoensis and Abies sachalinensis. Although significant differences among provenances were also found in annual ring number from the pith in the boundary between juvenile and mature wood (Table 6), the difference in the earliest (17th) and the latest (24th) annual ring number from the pith in the boundary was only 7 years. Thus, the radial growth rate in L. sibirica does not have a strong effect on the cambial age at which the production of mature wood cells begins. However, further research is needed to clarify the relationship between the radial growth rate and annual ring number from the pith in the boundary between juvenile and mature wood in this species.
As shown in Table 7, significant differences between juvenile and mature wood were found in the mean values of physical properties, tracheid length, and mechanical properties, except for SS: the values of the physical and mechanical properties of juvenile wood were lower than those of mature wood. These lower values can be explained by shorter tracheid length and lower wood density. Similar results were obtained by several researchers of softwood species17,22,24,28,29. For example, Koizumi et al.24 found that, in L. kaempferi, the mean MOE, MOR, CS, and SS values were 8.2 GPa, 93.3 MPa, 54.0 MPa, and 11.5 MPa in juvenile wood and 9.5 GPa, 97.2 MPa, 55.1 MPa, and 11.4 MPa in mature wood, respectively. Bao et al.25 reported that the mechanical properties of juvenile wood were significantly lower than those of mature wood in Larix olgenis and L. kaempferi. We also found lower mechanical properties, basic density, and shorter latewood tracheid length of juvenile wood in 67-year-old L. kaempferi22. Thus, the presence of juvenile wood should be considered when utilizing wood resources of this species as construction materials requiring higher strength properties.
Correlation among physical and mechanical properties of wood
Figure 4 shows the correlation coefficients of the physical and mechanical properties of three different wood types (all types of wood, juvenile wood, and mature wood). In general, wood density is positively related to shrinkage in the radial and tangential directions44. The results of this study showed significant correlations between radial shrinkage at 1% moisture content and EOD in mature wood and all wood, suggesting that EOD can predict shrinkage in the radial direction in this species. Wood density is also positively correlated with many types of mechanical properties of wood45,46. CS was positively correlated with all types of wood densities measured in this study. The MOE and MOR in mature wood and all wood only exhibited a significant positive correlation with EOD. These results indicate that MOE and MOR values were correlated with wood substances without extractives, and these values in juvenile wood might be related to other properties, such as microfibril angle. Luostarinen and Heräjärvi10 reported that water-soluble arabinogalactan contents were weakly correlated with SS in L. sibirica. SS was significantly correlated with AD, but not with EOD, suggesting that cold water-soluble extractives, such as arabinogalactan, might be affected on the SS in this species.
Based on these results, strength properties (e.g., bending properties and compressive strength) can be estimated with each other and predicted by EOD. In addition, SS might be influenced by the presence of cold water-soluble extractives, such as arabinogalactan.
Cáceres et al.3 reported that significant among-provenance differences were not found in basic and oven-dry densities, whereas hot-water extractive content was significantly affected by provenances in L. kaempferi. We also previously demonstrated that no significant differences among provenances were found in the basic density of L. sibirica naturally grown in Mongolia33. Although the cold-water extractive content significantly differed among provenances in this study, all examined densities, such as AD, OD, and EOD, showed no significant differences among the five provenances (Tables 2 and 3), indicating that wood density might not vary greatly among provenances. Thus, it can be concluded that genetic variations in relation to wood density might be small in L. sibirica trees naturally grown in Mongolia.
In half-sib families of P. jezoensis, F-values obtained by an ANOVA test for AD, MOE, and MOR among families gradually decreased from juvenile to mature wood47. In addition, Kumar et al.48 reported that estimates of narrow-sense heritability for MOE were generally higher in the corewood than in the outer wood in Pinus radiata. For Larix species, significant differences in wood density, CS, and SS but not in MOE and MOR were found in outer wood among 23 provenances for 31-year-old L. kaempferi24. Thus, genetic variations in the physical and mechanical properties of juvenile wood were higher than in mature wood in many softwood species. Significant differences were also found in most of the mechanical properties among provenances, except for CS (Table 4). In addition, significant differences were found in all examined physical and mechanical properties except for CS in mature wood among the five provenances, while no differences were found in juvenile wood for many properties (Table 7). Similar results were obtained in estimated MOE and MOR values at the 10th and 30th annual rings from the pith: no significant among-provenance variations were found in MOE and MOR at the 10th annual ring from the pith, but significant differences were found in the 30th annual ring from the pith (Table 5). Although the environmental conditions in the five provenances were not the same, the genetic variations in physical and mechanical properties among provenances were large in mature wood compared to juvenile wood for L. sibirica grown naturally in Mongolia. Further research is needed to clarify the genetic factors of the physical and mechanical properties of wood in L. sibirica.
Based on the results, there are significant among-provenance differences in the physical and mechanical properties of wood, especially in mature wood, in L. sibirica grown naturally in Mongolia. The physical and mechanical properties of wood in this species, especially in mature wood, can be improved by establishing tree breeding programs: families or clones with higher mechanical properties can be produced to achieve sustainable forestry in Mongolia.
Source: Ecology - nature.com