Decay resistance and chemical properties of the heartwood in Larix sibirica naturally growing in Mongolia

1 Introduction

Larix species are widely distributed across the temperate and cold regions of the Northern Hemisphere – from northern Siberia and Europe to the mountainous regions of China and Japan (Bonan and Shugart 1989). With around 15 known species, larches are integral to forest ecosystems and are prized in the forestry industry for their superior strength properties, often outperforming other conifers (Osawa et al. 2010). In fact, their timber is mainly used for construction (Gupta and Ethington 1996; Nagao et al. 2003; Tumenjargal et al. 2020). In Mongolia, there are three Larix species: Larix sibirica Ledeb., Larix gmelinii var. gmelinii, and Larix czekanowskii Szafer. Of three Larix species, the most dominant species is L. sibirica, which naturally grows in northern and western parts of the country (Institute of Botany, Mongolian Academy of Sciences 2022). The forests of this species cover 7.8 million hectares, accounting for over 60 % of the forested area and about 80 % of the growing stock volume in the country (Forest Department, Government Implementing Agency, Mongolia 2022). Thus, L sibirica is the most important forestry species to produce construction timber in Mongolia.

When using wooden materials for construction, the durability and natural resistance of the wood are essential. If the wood has high natural durability, the lifetime of the construction materials may be prolonged without the use of artificial preservatives. An increase in the lifetime of the materials might result in advantages for carbon stocks in constructions for a long time without any harmful preservatives. To utilize wood as a construction material, the decay resistance of various forestry species, such as Larix species, has been evaluated (Curnel et al. 2008; Gambetta et al. 2004; Gierlinger et al. 2004a, b; Ishiguri et al. 2018; Jebrane et al. 2014; Morrell and Freitag 1995; Nezu et al. 2022; Sarkhad et al. 2022; Venäläinen et al. 2001, 2006; Windeisen et al. 2002). However, direct evaluation of the decay resistance of wood is time-consuming, as it takes several months or more (Curnel et al. 2008; Gambetta et al. 2004; Gierlinger et al. 2004a, b; Ishiguri et al. 2018; Nezu et al. 2022; Sarkhad et al. 2022; Venäläinen et al. 2001; Windeisen et al. 2002). Thus, in Larix species, relationships between decay resistance and some heartwood characteristics, such as wood-color parameters and extractive contents, have been investigated to facilitate the evaluation of decay resistance of wood (Curnel et al. 2008; Gambetta et al. 2004; Gierlinger and Wimmer 2004; Gierlinger et al. 2004a, b; Ishiguri et al. 2018; Jebrane et al. 2014; Luostarinen and Heräjärvi 2013; Morrell and Freitag 1995; Nezu et al. 2022; Pâques et al. 2013; Pâques and Charpentier 2015; Sarkhad et al. 2022; Venäläinen et al. 2001, 2006; Windeisen et al. 2002). For example, relationships between mass loss by fungi and polyphenol contents were investigated in Larix species. As a result, polyphenol contents were negatively correlated with mass loss by Poria placenta and Coniophora puteana in Larix decidua, Larix kaempferi, and their hybrids (Gierlinger et al. 2004b) and by Trametes versicolor and Fomitopsis palustris in L. kaempferi (Nezu et al. 2022). In addition, wood-color parameters, L* and a*, were correlated with mass loss by P. placenta and C. puteana in L. decidua (Gierlinger et al. 2004a), and by T. versicolor and F. palustris in L. kaempferi (Nezu et al. 2022). Unfortunately, information of the relationships between mass loss and related properties is still limited in L. sibirica grown in Mongolia. Thus, detailed research on decay resistance and related properties should be conducted in L. sibirica harvested in Mongolia to more effectively utilize the wood as construction material.

It is known that the heartwood of Larix species contains arabinogalactan, which is a water-soluble oligomer consisting of monosaccharides such as arabinose and galactose (Côté et al. 1966; Hashizume and Takahashi 1974; Hillis 1987; Mizumoto et al. 1994; Nakada 2018; Sasaya 1987; Taylor et al. 2002). Ohsawa et al. (1992) clarified that butt-rot basidiomycetes in Larix, such as Phaeolus schweinitzii, Sparassis crispa, and Oligoporus balsameus (belonging to brown-rot fungus), can utilize monosaccharides, arabinogalactan, cellulose, and hemicellulose. Takabatake et al. (1994) evaluated the effect of the addition of water extractives from Larix sp., which might include arabinogalactan, on the mycelial growth of six edible mushrooms. Mycelial dry weight was increased by the addition of water extracts of Larix sp. wood to the medium (Takabatake et al. 1994). Ishiguri et al. (2018) examined mass loss by T. versicolor and F. palustris, as well as the amounts of hot-water extracts, which may also include arabinogalactan, in L. sibirica grown in Mongolia. As a result, mass loss in the heartwood was higher than that in the sapwood, indicating that the larger mass loss values in the heartwood of L. sibirica might be related to the larger amount of arabinogalactan (Ishiguri et al. 2018). Based on the previous results, arabinogalactan might relate to the decay resistance of L. sibirica wood. However, further research is needed to clarify the effect of arabinogalactan on the decay resistance of L. sibirica.

The variations in wood-color parameters, chemical components, and mass loss among families and provenances have been investigated in Larix species to enhance the natural decay resistance of wood through tree breeding (Nezu et al. 2022; Pâques et al. 2013; Pâques and Charpentier 2015; Sarkhad et al. 2022). By understanding how phenolic content affects decay resistance and considering the wide variability in extractive content among species and genotypes, breeders can now find practical ways to improve the natural durability of larch, particularly in hybrid larch (Pâques et al. 2013). Therefore, the among-provenance variations of these wood properties related to decay resistance should also be clarified in other Larix species. In L. sibirica, geographic variations were found in the physical and mechanical properties of wood from trees naturally grown in Mongolia (Tumenjargal et al. 2018, 2020). However, information on the geographic variations of decay resistance, chemical properties, and their relationships in the heartwood of L. sibirica naturally grown in Mongolia remains limited. Therefore, further research is needed to clarify the relationships between decay resistance and heartwood characteristics, including chemical properties, in L. sibirica naturally grown in Mongolia, to improve natural decay resistance by establishing appropriate tree breeding programs.

The objectives of this study are to clarify the decay resistance of the heartwood of L. sibirica and the relationship between wood-color parameters or extractive contents and decay resistance. The geographic variations of these characteristics were also assessed. To achieve these objectives, the wood-color parameters, extractive contents (hot-water extracts, 1 % NaOH extracts, arabinogalactan contents, and polyphenol contents), and decay resistance in the heartwood of L. sibirica were investigated. The heartwood samples used in this study were the same as those used to elucidate the geographic variations in the physical and mechanical properties of L. sibirica wood in Mongolia (Tumenjargal et al. 2018, 2020). By using the same samples, it is possible to conduct a comprehensive evaluation of both physical and chemical traits relevant to wood durability. However, it should be noted that the sample trees are usual, naturally growing trees in Mongolia, but not selected trees with superior characteristics in decay resistance. Despite this limitation, the findings will provide valuable insights for enhancing decay resistance through tree breeding and for optimizing the utilization of this species. Thus, based on the results, the implementation of tree breeding to improve the natural decay resistance of this species was also discussed.

2 Materials and methods

2.1 Materials

Five natural forests of L. sibirica were selected from Mongolia: Khentii, Arkhangai, Zavkhan, Khuvsgul, and Selenge (Table 1, Figure 1). These provenances are famous for forestry sites producing L. sibirica wood in Mongolia. A total of 25 trees (five trees from each site) with straight, undamaged stems were selectively harvested. The mean number of annual rings, stem diameter at 1.3 m above ground level, and tree height ranged from 44 to 193, 22.5–26.6 cm, and 11.6–19.5 m, respectively (Table 1; Tumenjargal et al. 2018, 2020). In the present study, small samples (ca. 20 [R] by 20 [T] by 320 [L] mm) collected from heartwood (near the pith) were used.

Table 1:

Geographic and climatic information on the sampling locations (Tumenjargal et al. 2018).

Provenance	Latitude	Longitude	Above sea level (m)	Mean annual temperature (°C)	Mean integrated value of annual precipitation (mm/year)	NAR		D (cm)		TH (m)
						Mean	SD	Mean	SD	Mean	SD
Khentii	48°51′N	110°05′E	1,214	−1.6	368	72	5	25.2	0.3	19.5	1.4
Arkhangai	47°22′N	101°43′E	1707	0.5	377	44	4	26.6	0.2	11.6	1.2
Zavkhan	48°41′N	98°17′E	1878	−5.0	242	193	6	23.6	0.2	15.9	1.0
Khuvsgul	48°31′N	99°15′E	1827	−1.8	226	49	5	22.5	0.3	15.1	0.7
Selenge	48°41′N	106°52′E	1,120	0.2	271	52	8	22.5	0.2	17.3	2.7

1. NAR, number of annual rings at 1.3 m above ground level; D, stem diameter at 1.3 m above ground level; TH, tree height; SD, standard deviation. Data on annual temperature and precipitation were provided from Information and Research Institute of Meteorology, Hydrology and Environment, Mongolia.

Figure 1:

Location of sampling sites. Triangles in the map indicate sampling sites.

2.3 Decay test

Figure 2 shows experimental procedure of decay test. The decay test was carried out according to the Japanese Industrial Standard (JIS K1571:2010) using two fungal species: T. versicolor (FFPRI 1030) as a white-rot fungus and F. palustris (FFPRI 0507) as a brown-rot fungus (Japan Industrial Standard 2010). Small-clear specimens (20 [R] by 20 [T] by 10 [L] mm) for the decay test were obtained from the small samples of heartwood. Six specimens were prepared for each tree – three for the white-rot fungus and three for the brown-rot fungus. The specimens were dried at 60 °C for 48 h and then sterilized with propylene oxide for two days. Subsequently, three specimens were placed on the fungal mat, spread on the medium (composed of 4 % glucose, 0.3 % peptone, 1.5 % malt extracts, and 2.0 % agar) in plastic bottles (9.5 cm in diameter, 850 mL in volume). The specimens were incubated at 26 ± 2 °C with 70 % relative humidity for 12 weeks. After the incubation, mycelium was removed from the specimens using a small brush and tweezers. The decayed specimens were air-dried for 24 h and oven-dried at 60 ± 2 °C for 48 h to determine their weight after the decay test. To verify fungal activity and compare the degree of decay, sapwood samples of Cryptomeria japonica D. Don were exposed to the same fungi and conditions. As the results, mass loss of sapwood of C. japonica was 15.6 % by T. versicolor, and 12.1 % by F. palustris (data not shown).

Figure 2:

Experimental procedures of decay test. Photographs (A and B) indicate wood block samples decayed by Trametes versicolor and Fomitopsis palustris after 12 weeks of incubation, respectively. The decay test was conducted according to the JIS K1571: 2010.

3 Results

3.1 Heartwood color

Mean values and standard deviations for L*, a*, and b* values of heartwood samples are presented in Table 2. Figure 3 also shows the heartwood color of the specimen’s surface. The mean L* values ranged from 71.1 to 75.6 across the different provenances, with the highest value observed in the Selenge provenance (75.6) and the lowest in the Zavkhan provenance (71.1). Regarding the a* parameter, values ranged from 6.6 to 8.2. The highest a* value was recorded in the Zavkhan provenance (8.2), while the lowest was in the Selenge provenance (6.6). The b* values ranged from 24.3 to 26.0. The highest b* value was found in the Zavkhan provenance (26.0), while the lowest was in the Selenge provenance (24.3). The overall mean values of L*, a*, and b* across all provenances were 72.9, 7.5, and 25.1, respectively.

Table 2:

Means and standard deviations of heartwood color.

Provenance	n	L*		a*		b*
		Mean	SD	Mean	SD	Mean	SD
Khentii	5	73.9	3.3	7.6	1.6	25.5	1.5
Arkhangai	5	71.4	0.9	8.1	0.9	24.8	1.5
Zavkhan	5	71.1	4.7	8.2	1.1	26.0	1.1
Khuvsgul	5	73.0	1.1	7.2	1.0	25.1	0.9
Selenge	5	75.6	3.4	6.6	1.5	24.3	1.5
Total	25	72.9	2.7	7.5	1.2	25.1	1.3

1. n, number of trees; L*, lightness; a*, red/green value; b*, blue/yellow value; SD, standard deviation.

Figure 3:

Heartwood color of Larix sibirica collected from different provenances.

4 Discussion

Wood color, extractive contents, and decay resistance have been extensively examined in Larix species (Curnel et al. 2008; Gambetta et al. 2004; Gierlinger and Wimmer 2004; Gierlinger et al. 2004a, b; Ishiguri et al. 2018; Jebrane et al. 2014; Luostarinen and Heräjärvi 2013; Morrell and Freitag 1995; Nezu et al. 2022; Pâques et al. 2013; Pâques and Charpentier 2015; Sarkhad et al. 2022; Venäläinen et al. 2001, 2006; Windeisen et al. 2002). Table 7 lists comparable results of wood-color parameters, extractive contents, and mass loss of Larix species.

The heartwood color of L. sibirica tested in the present study was similar to those of L. kaempferi planted in Japan (Nezu et al. 2022, Table 7) and L. decidua planted in Europe (Gierlinger et al. 2004a, Table 7) but slightly differed from those of the L. kaempferi and L. decidua planted in Europe (Pâques et al. 2013, Table 7). The amounts of hot-water extracts and 1 % NaOH extracts obtained in the present study were almost the same values obtained in L. sibirica collected from Mongolia (Ishiguri et al. 2018; Sarkhad et al. 2022, Table 7). However, arabinogalactan contents showed about two times higher value than those of the same species collected from Finland (Venäläinen et al. 2006, Table 7). On the other hand, polyphenol contents (taxifolin equivalent content) showed a relatively low value compared to previous results obtained in L. sibirica, L. kaempferi, and L. decidua (Nezu et al. 2022; Pâques et al. 2013; Venäläinen et al. 2006, Table 7).

Arabinogalactan and taxifolin contents radially varied within heartwood in Larix species (Côté et al. 1966; Hashizume and Takahashi 1974; Hillis 1987; Sasaya 1987; Mizumoto et al. 1994; Nakada 2018; Taylor et al. 2002). These radial variations might result in differences in arabinogalactan and polyphenol contents between the previous and present studies. In contrast, mass loss after decay in the present study showed lower values than comparable data obtained in the Larix species (Ishiguri et al. 2018; Nezu et al. 2022; Sarkhad et al. 2022, Table 7). This is because of the environmental or genetic differences among provenances.

Arabinogalactan found in the heartwood of Larix species can be extracted quantitatively from the heartwood with water (Sjöström and Alén 1999). In the present study, arabinogalactan contents positively related to amounts of hot-water extracts and 1 % NaOH extracts (Table 5). Takabatake et al. (1994) examined the effects of cold-water extracts from sawdust of Larix species on the mycelial growth of edible mushrooms (white-rot fungi). With a few exceptions, mycelial weight increased on the medium containing the cold-water extracts. However, the results of the study also suggested that the increase in mycelial weight was related to the cold-water extracts, except for arabinogalactan, because the fungus quickly utilizes glucose as a nutrient source compared to arabinogalactan. On the other hand, Ohsawa et al. (1992) examined the ability of fungi inhabiting Larix trunks with butt-rot to utilize wood components in vitro. As a result, butt-rot basidiomycetes in Larix, such as P. schweinitzii, S. crispa, and O. balsameus (belonging to brown-rot fungi), can utilize monosaccharides, arabinogalactan, cellulose, and hemicelluloses (Ohsawa et al. 1992).

In the present study, amounts of hot-water extracts, 1 % NaOH extracts, and arabinogalactan significantly correlated with mass loss by F. palustris but not by T. versicolor (Table 5). In addition, correlation coefficients were determined between the mass loss and the subtracted value of arabinogalactan contents from hot-water contents. As a result, the correlation coefficient was 0.688 (p < 0.001) in F. palustris and 0.129 (p = 0.540) in T. versicolor (data not shown). Based on the results, it is considered that arabinogalactan might be positively correlated with mass loss in brown-rot fungus but not in white-rot fungus. In addition, as mentioned above, many butt-rot fungi in Larix species belong to brown-rot fungi. Thus, resistant trees against butt-rot fungi can be selected by using a criterion of arabinogalactan content in future tree breeding programs of Larix species.

Relationships between wood-color parameters, extractive contents, and mass loss have been investigated for Larix species (Gierlinger et al. 2004a, b; Nezu et al. 2022; Pâques et al. 2013; Windeisen et al. 2002). Windeisen et al. (2002) reported that a negative correlation (r = −0.80; r² = 0.64) was found between cyclohexane/ethanol extract contents and mass loss of inner heartwood by a brown-rot fungus (C. puteana) in 20 cultivated larch trees. A color parameter, a* positively correlated with polyphenol contents in L. decidua, L. kaempferi, and their hybrids grown in Europe (Gierlinger et al. 2004a; Pâques et al. 2013), and L. kaempferi grown in Japan (Nezu et al. 2022). In addition, negative correlations were found between polyphenol contents and mass loss by P. placenta and C. puteana in L. decidua, L. kaempferi, and their hybrids (Gierlinger et al. 2004b), and by T. versicolor and F. palustris in L. kaempferi (Nezu et al. 2022). In the present study, as shown in Table 5, the contents of hot-water extracts, 1 % NaOH extracts, and arabinogalactan correlated with L* (negatively) and a* (positively). In addition, a positive correlation was found between mass loss by F. palustris and arabinogalactan content. Based on the results, it is considered that selecting trees with higher value of L* and lower value of a* may result in the selection of trees with higher resistance against brown-rot fungus. Similar relationships have been observed in other durable hardwood species, such as teak (Tectona grandis) wood. Campos et al. (2025) found that heartwood extractive content in teak wood was positively correlated with decay resistance and inversely related to L* parameter. This parallel with Larix supports the idea that heartwood color and extractives are consistent indicators of decay resistance across species. This kind of evidence from other species supports using these traits to assess wood durability and help in tree breeding.

In the previous study, Tumenjargal et al. (2020) found significant geographic variations in the physical and mechanical properties of wood in the same samples as the present study. Venäläinen et al. (2001) examined growth characteristics, wood properties, and decay resistance against C. puteana (brown-rot fungus) in 25-year-old grafted L. sibirica in Finland, indicating that the genetic determination appeared stronger for decay resistance than for growth characteristics or heartwood formation but weaker than for wood density or latewood formation (Venäläinen et al. 2001). In the present study, relatively higher variance components of provenance were found in 1 % NaOH extracts, wood-color parameters and polyphenol content (Table 6). Trees from Selenge province showed the highest lightness (L*), while those from Khentii province had higher polyphenol content, possibly reflecting genetic and environmental differences. In contrast, arabinogalactan content varied only slightly among provenances, suggesting that genetic effects might be limited on arabinogalactan contents. These findings indicate that geographic variation exists not only in wood color parameters and chemical composition but also corresponds to the results previously reported in physical and mechanical properties of wood. Although no significant correlation was found between polyphenol content and mass loss in this study (Table 5), the selection of trees with higher polyphenol content will increase the natural decay resistance of this species.

5 Conclusions

In the present study, decay resistance, wood-color parameters, and extractive contents were investigated for heartwood of L. sibirica collected from five forestry sites in Mongolia. Arabinogalactan content was positively correlated with both hot-water and 1 % NaOH extracts. Extractive contents were also positively correlated with mass loss by F. palustris but not with T. versicolor. Extractive contents correlated with L* (negatively) and a* (positively), indicating a relationship between heartwood color and chemical composition. Significant geographic differences were observed in wood-color parameters and polyphenol content, suggesting local environmental or genetic influences. Based on the results, it is concluded that selecting trees with higher L*values, lower a* values, and lower arabinogalactan content from different provenances may improve decay resistance against brown-rot fungi, including butt-rot fungi, in the tree breeding program of this species.