Uniseriate Ray Characteristics for Wood Identification and Quality Indices of Six Korean Oak Species

Radial variation in uniseriate ray characteristics of six Korean oak species was observed to provide information for wood identification and quality evaluation. Radial variations in uniseriate ray characteristics, such as ray height, number, and spacing, were observed at five growth ring intervals from the pith to near the bark using optical microscopy. The transition point between the juvenile and mature wood was evaluated using a segmented regression model. All species showed a comparable trend in uniseriate ray number and spacing, gradually decreasing from the pith to near the bark. Transition zones for the six Korean oak species ranged from 21 to 39 years of growth. The highest uniseriate ray heights and spacings were observed in Quercus aliena . Quercus dentata exhibited the highest number of uniseriate rays. Across all species, uniseriate ray number and spacing were higher in juveniles than in mature wood. A negative correlation was observed between the uniseriate ray number and spacing and the uniseriate ray height. The strongest positive correlation was observed between uniseriate ray number and ray spacing. The most reliable parameters for estimating the demarcation point were uniseriate ray number and spacing. The ray characteristics may be used to identify the six Korean oak species. DOI: 10.15376/biores.19.4.7102-7119


INTRODUCTION
Six oak species, Quercus variabilis Blume (Qv), Quercus serrata Murray (Qs), Quercus mongolica Fisch.ex Ledeb (Qm), Quercus dentata Thunb.(Qd), Quercus aliena Blume (Qal), and Quercus acutissima Carruth (Qac) are widely distributed throughout the mountains of Korea and adapt to diverse soil conditions ranging from dry to wet (Son et al. 2004).They tended to form pure stands at high elevations.Oak trees demonstrate intermediate shade tolerance, which allows them to thrive beneath forest canopies.Over time, they ascend to the main canopy and establish dominant stands during the late successional stages (Lee et al. 2012).These oak species cover an extensive area of 1,037,650 hectares, with a combined growing stock of 159,261,862 m³ (Korea Forest Service 2022).Ecologically, oak trees play crucial roles in providing essential habitats for wildlife and understory vegetation.In addition, they contribute significantly to wood and charcoal production from an economic perspective (Kweon and Comeau 2021).
Oak species are popular for their high-quality wood, which is known for its beauty, strength, and durability (Santos et al. 2012).Oak trees are easily found and widely distributed worldwide (Nixon et al. 2006).Oak wood has diverse applications ranging from high-grade uses such as boat construction and building structures to low-grade applications such as packaging and fuelwood (Pasta et al. 2016;Bumgardner 2019).In Korea, oak wood has historically served as a common material in construction (Lee and Bae 2021).However, in recent years, its predominant use has shifted toward low-grade applications, notably for charcoal production (Korea Forest Service 2022).
Understanding radial variations in anatomical characteristics is crucial for ensuring wood quality and optimizing the utilization of different species (Jozsa and Middleton 1994;Butterfield 2003).The radial variation, spanning from the pith to the near bark of the wood, serves as a valuable parameter for identifying the transition point between juvenile and mature wood (Panshin and de Zeeuw 1964;Lee et al. 2009;Kim et al. 2009;Darmawan et al. 2015;Kim et al. 2021).Juvenile wood undergoes significant changes in cell properties, including shorter cells, thinner cell walls, narrower cell diameter, and a higher microfibril angle (Kozlowski 1971;Tsoumis 1991;Zobel and Sprague 1998;Moore and Cown 2017).Therefore, juvenile wood exhibits inferior physical-mechanical properties (Zobel and van Buijtenen 1989;Bao et al. 2001;Bhat et al. 2001;Passialis and Kiriazakos 2004;Alteyrac et al. 2006;Shmulsky and Jones 2019), durability (Dünisch et al. 2010), and drying quality (Rulliaty 2008;Moya et al. 2017) compared to mature wood.
Numerous studies have investigated radial variations in the anatomical characteristics of oak wood.Previous studies have primarily focused on the axial elements of oak wood, such as fiber properties (Helińska-Raczkowska and Fabisiak 1991;Lei et al. 1996;Tsuchiya and Furukawa 2009;Sousa et al. 2014;Savero et al. 2024), tracheid length (Helińska-Raczkowska and Fabisiak 1991), vessel length (Helińska-Raczkowska and Fabisiak 1991;Tsuchiya and Furukawa 2009), and vessel diameter (Lei et al. 1996;Tsuchiya and Furukawa 2009;Tulik 2014;Savero et al. 2024).In addition, Sousa et al. (2014) explored the radial variation in the radial elements of multiseriate ray features, including the height and width.Leal et al. (2006) conducted a study on the radial variation in the multi-and uniseriate ray properties of oak wood.
According to Reiterer et al. (2002) and Elaieb et al. (2019), ray characteristics in oak species play a crucial role as essential parameters influencing both the physical and mechanical properties of wood.In addition, De la Paz et al. (2005) reported that uniseriate ray characteristics significantly influence the mechanical properties of Quercus spp.from Mexico.Among the six Korean oak species (Qv, Qs, Qm, Qd, Qal, and Qac), their ray anatomy exhibited similarities, as reported by Eom (2015) and Savero et al. (2023).These species exhibit two distinct ray sizes: uniseriate and multiseriate.The multiseriate rays are composed entirely of procumbent cells and are sometimes associated with prismatic crystals.These rays commonly consist of more than 10 seriate cells and have heights exceeding 1 mm.Similarly, the uniseriate ray also consists of procumbent cells, but the difference is that the uniseriate ray does not contain prismatic crystals.
Detailed information on the anatomical characteristics is important to complete the identification keys and quality indices for these species.As mentioned above, numerous studies have focused on general anatomical features, such as the vessel and fiber properties of oak wood; however, there remains a gap in research regarding ray characteristics, particularly uniseriate rays.Therefore, this study aimed to examine the radial variation in uniseriate ray characteristics to obtain information on the identification keys and demarcation points between juvenile and mature wood, to provide wood quality indices for six Korean oak species.In addition, the relationships between ray characteristics were observed to provide additional information for evaluating wood quality.

Materials
The wood samples were collected from three trees each of six Korean oak species grown at the Kangwon National University research forest in Chuncheon City, Gangwon Province, South Korea (37° 47′ 2.8932″ N, 127° 49′ 13.368″ E).The same trees were studied as those used in the authors' previous study (Savero et al. 2024), and the details of the sample trees for each species are presented in Table 1.

Sample preparation
Wood samples, in the form of disks with a thickness of 50 mm, were extracted 1.3 m above the ground (breast height).Subsequently, the wood disks were cut into woodblock specimens measuring 10 mm (longitudinal) × 10 mm (radial) × 10 mm (tangential).The specimens were prepared at five growth ring intervals from the pith to the nearby bark.

Optical microscopy
The wooden block specimens were softened by boiling them in a 1:1 mixture of water and glycerin for 12 h.Slices of the tangential surface, measuring 15 to 20 µm in thickness, were then obtained from the latewood of softened wood block using a sliding microtome (Lab Microtome, Swiss Federal Research Institute WSL, Birmensdorf, Switzerland).The slices were stained with a 1% safranin solution and light green, followed by dehydration using a series of alcohol concentrations (50%, 70%, 90%, 95%, and 99%).The stained slices were stored in xylene-filled vials.Before observation, permanent slides were prepared by attaching the stained slices to Canada balsam.Uniseriate ray characteristics were observed in the latewood in the tangential section without broad rays, as shown in Fig. 1, using an optical microscope (Eclipse E600, Nikon Corp., Tokyo, Japan) connected to an image analysis system (i-Solution Lite; IMT i-Solution Inc., Burnaby, BC, Canada).All the chemicals were the products of Daejung Chemical Co. Ltd., Siheung, South Korea.

Uniseriate ray characteristics measurement
The uniseriate ray height, number, and spacing were evaluated following the International Association of Wood Anatomists (IAWA) list of microscopic features for hardwood identification (IAWA Committee 1989).The characteristics were determined using tangential sections of segmented samples in each five growth ring intervals obtained from three trees of each species.The ray heights (in both µm and cells) were measured from 50 uniseriate rays.The number of uniseriate rays was examined in 25 areas measuring 1 mm².Furthermore, the uniseriate ray spacing in tangential direction was determined using 25 measurements from a 1 mm line.

Demarcation point determination
A segmented regression model was used to evaluate the transition point between juvenile and mature wood, following the approach outlined by Rahayu et al. (2014).This model can be explained by using two functions within the curve.The first function, characterized by a steep slope, reflects the juvenile wood region, whereas the second function, represented by a flat line, corresponds to the mature wood region.The relationship between the uniseriate ray number and spacing, treated as dependent variables, and the number of growth rings from the pith to near the bark, considered as independent variables, was described using nonlinear regression analysis with a quadratic model and a plateau.The nonlinear least squares procedure (PROC NLIN) in SAS v.9.0 (SAS Institute Inc., Cary, NC, USA) was used to analyze the demarcation point.The proportion of mature wood was calculated using the method proposed by Savero et al. (2024).

Data analysis
Statistical analyses were conducted to explore the relationships between uniseriate ray characteristics among different species.One-Way Analysis of Variance (ANOVA) was used to compare means across species, followed by Duncan's test for post hoc verification at a 5% significance level.Additionally, Pearson's correlation was used to analyze the relationship between the uniseriate ray characteristics, with significance levels set at both 1% and 5%.All statistical analyses were performed using SPSS version 26 (IBM Corp., Armonk, NY, USA).

Radial Variation and Demarcation Point Uniseriate ray height
The radial variation in uniseriate ray height (µm) is depicted in Fig. 2. It demonstrated a fluctuating pattern, with an increase near the pith until 10 to 15 years of growth, followed by a decrease near the bark in all species.A similar pattern was observed in the radial variation of the uniseriate ray heights (cells), as shown in Fig. 3.However, there was a difference in trends between uniseriate ray height in µm and cells, the former exhibited an increasing trend, while the latter showed a decreasing trend.
The radial variation pattern of uniseriate ray height in µm and cells did not exhibit a juvenile-to-mature wood pattern.Additionally, segmented regression model analysis could not be conducted because the pattern did not fit well (R² < 0.2).This study is consistent with a previous study by Zobel and Sprague (1998), who reported no distinct juvenile-to-mature wood pattern in the ray height of Celtis occidentalis.bioresources.cnr.ncsu.eduSavero et al. (2024)."Uniserate rays of 6 oaks," BioResources 19(4), 7102-7119.7107 Compared with the present study, Leal et al. (2006) also observed a similar radial variation trend in Q. suber from Portugal.The height of uniseriate rays (µm) increased from the pith to near the bark, while the height of uniseriate ray (cells) decreased.In addition, Sousa et al. (2014) examined a similar trend in Q. faginea from Portugal, which was characterized by a decrease in the height of the uniseriate rays (cells).

Uniseriate ray number and ray spacing
Figure 4 illustrates the radial variation in the uniseriate ray numbers for the six Korean oak species.Initially, the uniseriate ray numbers were high near the pith but decreased until 20 to 40 years of the growth ring.Subsequently, they stabilized with a slight increase toward the bark for all species, except for Qs.Qs gradually decreased from near the pith to 30 years of growth, after which it remained constant until near the bark.The demarcation points between juvenile and mature wood were analyzed using a segmented regression model.The radial variation pattern of the uniseriate ray number fitted well (R² > 0.8).The transition zone of the uniseriate ray numbers ranged from 21 to 37 years of growth, with the lowest at Qm and the highest at Qd (Table 2).In the mature wood proportion, Qm had the highest proportion at 67%, whereas Qal had the lowest proportion (44%).The radial variation in the uniseriate ray spacing exhibited a similar pattern to that of the uniseriate ray numbers for the oak species (Fig. 5).Initially, it was high near the pith and gradually decreased until 20 to 30 years of growth, after which it slightly increased toward the bark.Similar to the uniseriate ray number, the radial variation pattern of the uniseriate ray spacing fit well in the segmented regression model.Table 2 presents the demarcation zones of uniseriate ray spacing, which ranged from 24 to 39 years for the growth rings.This range was slightly higher than the uniseriate ray number but was still comparable.Among the different wood types, the lowest demarcation point was found in Qac, whereas the oldest was found in Qs.In terms of mature wood proportion, Qd exhibited the highest portion at 66%, whereas Qal had the lowest at 42%.There were similar transition points between the radial variation of the uniseriate ray number and spacing to determine the juvenile and mature wood.The demarcation zones for the uniseriate ray number and spacing ranged from 21 to 39 years for growth rings.A comparable result was observed in a previous study by Savero et al. (2024), who reported the transition zone for six Korean oak species using fiber length and earlywood vessel diameter ranging from 19 to 44 years of growth.Tsuchiya and Furukawa (2009) identified a demarcation zone for earlywood vessel diameter in Qs from Japan, ranging from 11 to 38 years of growth rings.Therefore, we conclude that the uniseriate ray number and spacing in the six Korean oak species can be used to determine the demarcation point between juvenile and mature wood.

Radial Variation and Demarcation Point Uniseriate ray height
The uniseriate ray height (µm) of six Korean oak species is shown in Table 3.In average values, the highest uniseriate ray height was found in Qal with 314.5 µm, while the lowest was in Qv with 294.2 µm.In juvenile wood, Qs exhibited the highest value at 320.4 µm, while in mature wood, Qal had the highest value at 325.3 µm.The lowest uniseriate ray height was observed in both juvenile and mature wood of Qv, measuring 289.9 and 296.9 µm, respectively.10.0 c (1.9) Note: Numbers in parentheses represent standard deviations.The same superscript letters and asterisks next to the mean values within columns indicate non-significant differences at the 5% significance level, as determined by Duncan's test for comparisons between species and ANOVA for comparisons between juvenile and mature wood, respectively.According to the IAWA lists, the ray per millimeter values are categorized as follows: ≤ 4 /mm (114), 4 to 12 /mm (115), and ≥ 12 /mm (116).
Consistent with uniseriate ray height in µm, the highest average of ray height in cells was 16.6 cells for Qal, while the lowest was Qv at 15.0 cells, as shown in Table 3.In both juvenile and mature wood, Qal exhibited the highest value of the uniseriate ray height (cells), measuring 16.6 cells and 16.5 cells, respectively.The lowest value was observed for Qv, at 15.2 cells for juvenile wood and 14.9 cells for mature wood.
The uniseriate ray height (µm) in mature wood exhibited higher values than that in juvenile wood for all species, except Qs.In contrast, uniseriate ray height (cells) in mature wood was lower than that in juvenile wood for all species.However, the uniseriate ray height (cell) values of Qm, Qal, and Qac were not significantly different between juvenile and mature wood.Furthermore, both the uniseriate ray height in µm and cells showed no significant difference between Qv and Qac, as well as between Qm and Qd.These findings are consistent with those of previous studies showing that mature wood has more cells than juvenile wood (Kozlowski 1971;Tsoumis 1991;Zobel and Sprague 1998;Barbour 2004;Moore and Cown 2017).Therefore, it seems that mature wood had lower uniseriate ray height (cells) values, but higher uniseriate ray height (µm) compared to juvenile wood.
The uniseriate ray height (both µm and cells) in this study was higher than that in previous studies.Leal et al. (2006) and Sousa et al. (2009) reported that Q. suber from Portugal showed uniseriate ray height (µm) averaging 226.5 and 216.3 µm, respectively.Leal et al. (2006) reported a uniseriate ray height (cells) with an average of 11.0 cells.Sousa et al. (2014) and Han et al. (2015) reported that the average heights of the uniseriate rays (cells) in Q. faginea from Portugal and Q. rubra from Canada were 9.3 cells and 12.7 cells, respectively.
The uniseriate ray height (both µm and cells) from this study differed significantly between species.According to Denk et al. (2017) and the World Flora Online (2024), the six Korean oak species are divided into two subgenera and sections.Qv and Qac belong to Quercus subgen.Cerris Oerst.sect.Cerris Dumort., whereas Qm, Qd, and Qal are categorized under Quercus subgen.Quercus sect.Quercus.In addition, Qs do not belong to any specific subgenus or section.Interestingly, Qal exhibits significantly different ray height compared to Qm and Qd, making it a potentially valuable additional identification feature within the Quercus subgen.Quercus sect.Quercus.
These species are grown in the same location, which makes it possible to eliminate the site factor.Therefore, it is suggested that uniseriate ray height (both µm and cells) could be utilized for identifying the six Korean oak species.

Uniseriate ray number and ray spacing
Uniseriate ray numbers varied among species, as presented in Table 3.The average value of the uniseriate ray number showed that Qd had the highest value at 55.5/mm², while Qs had the lowest at 43.0/mm².In both juvenile and mature wood, Qd showed the highest number of uniseriate rays among the species, at 60.9/mm² and 52.3/mm², respectively.In contrast, Qs exhibited the lowest values for both juvenile and mature wood, averaging 45.8/mm² and 40.8/mm², respectively.
For the uniseriate ray spacing of the six Korean oak species, Qal had the highest average value of 10.6/mm, whereas the lowest was observed in Qs and Qd (9.2/mm).In both juvenile and mature wood, Qal exhibited significantly greater ray spacing, averaging 10.9/mm and 10.2/mm, respectively.In contrast, the lowest value in juvenile wood was found for Qm at 9.2/mm, whereas in mature wood, Qs was 8.9/mm.According to the IAWA Committee (1989), the uniseriate ray spacing of six Korean oak species is categorized as anatomical feature number 115, with 4 to 12 rays per mm.
Both the uniseriate ray number and spacing in juvenile wood were significantly higher than those in mature wood, except for Qm.
In the present study, the uniseriate ray number was higher compared to Q. brantii from Iran, with an average of 30.0/mm² (Soheili et al. 2023).In contrast, the present findings regarding uniseriate ray spacing aligned with that of Q. robur from Turkey, which was 10.3/mm (Gülsoy et al. 2005).However, it was slightly lower than the uniseriate ray spacing observed in Q. pontica from Turkey (Yilmaz et al. 2008) and Q. rubra from Canada (Han et al. 2015), which were 11.3/mm and 11.7/mm, respectively.
As mentioned in the previous section, the differences in ray number and spacing between Qv and Qac from Quercus subgen.Cerris Oerst.sect.Cerris Dumort., as well as between Qm, Qd, and Qal from Quercus subgen.Quercus sect.Quercus might serve as additional information for identifying these species.Thus, a significant difference in the uniseriate ray number and spacing among all species suggests that these parameters can serve as identification keys for the six Korean oak species.

Relationship Between Uniseriate Ray Characteristics
Figure 6 illustrates the relationship between the uniseriate ray number (Fig. 6A) and spacing (Fig. 6B) to the uniseriate ray height (µm) in six Korean oak species.The correlation between the number of uniseriate rays and their height (µm) significantly differed at 5%, with a Pearson correlation coefficient of -0.28 as shown in Table 4.However, the correlation between uniseriate ray spacing and height (µm) was weak and not significantly different, with a Pearson correlation coefficient of -0.75.

A B
Fig. 6.Relationship between uniseriate ray number (A) and ray spacing (B) to uniseriate ray height (µm) of six Korean oak species.In contrast, a positive correlation was observed among uniseriate ray number (Fig. 7A), spacing (Fig. 7B), and uniseriate ray height (cells).However, the correlation was not statistically significant.Table 4 shows that the uniseriate ray number and height (cells) A positive correlation was also found between uniseriate ray height in µm and cells (Fig. 8A), as well as between uniseriate ray number and spacing (Fig. 8B).Both relationships were statistically significant the 1% level, as indicated in Table 4.A strong correlation was observed between the uniseriate ray number and the spacing, with a Pearson's correlation coefficient of 0.83.In contrast, the relationship between uniseriate ray height in µm and cells exhibited a weaker correlation, with a Pearson correlation coefficient of 0.41.Compared to the previous study reported by Leal et al. (2006), the relationship between uniseriate ray height in µm and cells exhibited a positive correlation.

Relationship Between Uniseriate Ray Characteristics
In the introduction it was discussed that ray characteristics impact wood quality, including physical and mechanical properties (Reiterer et al. 2002;Elaieb et al. 2019).Barbe and Keller (1996) attributed this impact to the higher proportion of rays in the anatomical elements of wood, which typically ranges from 3% to 30% of the total wood volume.Elaieb et al. (2019) observed that ray numbers correlate positively with tangential and radial shrinkage of wood.Additionally, De la Paz et al. (2005) reported that uniseriate ray characteristics, particularly uniseriate ray spacing, are positively associated with compression perpendicular to the grain.Rahman et al. (2005) also noted a positive correlation between ray volume and specific gravity, as well as radial compression strength.Furthermore, it is essential to recognize that juvenile and mature wood also play a role in overall wood quality (Zobel and van Buijtenen 1989;Bao et al. 2001;Bhat et al. 2001;Passialis and 2004;Alteyrac et al. 2006;Rulliaty 2008;Dünisch et al. 2010;Moya et al. 2017;Shmulsky and Jones 2019).
The present results indicate that Qal exhibits higher uniseriate ray characteristics than other species.Ozden and Ennos (2014) reported that the hexagonal shape of ray cells contributed to giving greater strength, stiffness, and toughness to the wood.Thus, wood with higher values of ray height, density, and spacing resulted in higher mechanical properties.Additionally, Qal showed a younger transition age compared to Qs and Qd, but an older transition age compared to Qv and Qac.Wood with younger transition age produced a higher mature wood portion in the same age, the mature wood had a better overall quality than juvenile wood (Zobel and van Buijtenen 1989;Bao et al. 2001;Bhat et al. 2001;Passialis and Kiriazakos 2004;Alteyrac et al. 2006;Rulliaty 2008;Dünisch et al. 2010;Moya et al. 2017;Shmulsky and Jones 2019).According to Oh (1999), Qal has higher bending strength than Qs, Qm, and Qac, although it is lower than Qv.Furthermore, a previous study by Savero et al. (2024) suggests that Qv may have the best quality among the species based on vessel and fiber properties.Therefore, the present findings indicate that Qal may have better quality than Qs, Qm, Qd, and Qac.

CONCLUSIONS
1.The radial variation patterns of uniseriate ray height in µm and numbers of cells exhibited a similar fluctuating pattern for all species.All species followed a similar trend in uniseriate ray number and spacing, gradually decreasing from the pith to near the bark.The segmented regression model fitted well with the uniseriate ray number and spacing, resulting in a transition zone for the six Korean oak species, ranging from the 21 st to 39 th growth rings.
2. The highest average uniseriate ray height (µm and cells) and spacing were observed in Quercus aliena Blume (Qal) and Quercus dentata Thunb.(Qd) exhibited the highest average uniseriate ray number.Quercus variabilis Blume (Qv) showed the lowest average uniseriate ray height (µm and cells), whereas Quercus serrata Murray (Qs) had the lowest average in both uniseriate ray number and spacing.Across all species, uniseriate ray height (cells), number, and spacing were higher in juveniles than in mature wood.In contrast, mature wood of all species displayed higher uniseriate ray height (µm) than juvenile wood, except for Qs.
3. A positive correlation was observed between uniseriate ray characteristics.However, a negative correlation was observed between the uniseriate ray number and spacing to the uniseriate ray height (µm).The strongest correlation was observed between the uniseriate ray number and ray spacing.
4. The most reliable parameters for estimating the demarcation point between juvenile and mature wood were the radial variation in the uniseriate ray number and spacing.Uniseriate ray characteristics might also be used to identify the six Korean oak species, along with other qualitative and quantitative anatomical characteristics, for more accurate wood identification.Additionally, the results could be useful for silvicultural management and determining the optimal harvesting times to produce the best quality wood from the six Korean oak species.To fully understand the wood quality of these species, future research should explore their physical, mechanical, and chemical properties natural durability.

Table 2 .
Demarcation Points and Mature Wood Proportion of Six Korean Oak Species using Segmented Regression Model Numbers in parentheses represent standard deviations.The same superscript letters next to the mean values within columns indicate non-significant differences at the 5% significance level, as determined by Duncan's test for comparisons between species.

Table 3 .
Uniseriate Ray Characteristics of Six Korean Oak Species

Table 4 .
Pearson Correlation Coefficient of Uniseriate Ray Characteristics of Six Korean Oak Species