Radial Variation of Wood Anatomical Characteristics and Maturation Ages of Six Korean Oak Species

Abstract

The objective of this study was to examine and compare radial variation of the anatomical characteristics and the transition age from juvenile to mature wood of the six Korean oak wood species: Quercus variabilis (Qv), Quercus serrata (Qs), Quercus mongolica (Qm), Quercus dentata (Qd), Quercus aliena (Qal), and Quercus acutissima (Qac). Quantitative anatomical features were observed from the pith to the bark at five growth-ring intervals using optical microscopy. A segmented regression model was used to evaluate the transition from juvenile wood to mature wood. The clearest transition from juvenile to mature wood was observed in the radial variation of the earlywood vessel diameter and fiber length. The maturation age of the six Korean oak species ranged from 19 to 44 years. Qv exhibited the highest values for latewood vessel diameter, fiber length, and fiber diameter. Qac displayed the highest values for earlywood vessel diameter and fiber wall thickness. The highest fiber lumen diameter was observed for Qm. The differences in earlywood and latewood vessel diameters and fiber lengths could be utilized as identification keys for these species. Earlywood vessel diameter and fiber length have emerged as the most reliable indicators for estimating the transition from juvenile to mature wood.

1. Introduction

Oak (Quercus spp.) wood is used in various applications, including furniture, flooring, framing, railway sleepers, and boats [1,2]. This versatility can be attributed to its decorative figure, high strength, and durability [3]. The distribution of the oak wood species is found extensively across the United States, Europe, Africa, and Asia [4].

Korea is one of the countries in Asia that have oak species, covering 16.34% of the total forest area [5]. Oak is extensively distributed in the mountains of Korea, thriving in diverse soil conditions, ranging from dry to wet. It exhibits a tendency to form pure stands at high elevations. With intermediate shade tolerance, oak can thrive under canopies, gradually ascending to the main canopy and establishing dominant stands during late successional stages [6]. These oaks play a crucial role in ecology and economy, providing essential habitats for wildlife and understory vegetation [7]. There are six major oak species in Korea [8], such as 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).

From the 17th to the 20th centuries, Korean oak wood played a pivotal role in constructing palaces, shrines, and fortresses [9]. However, in the present day, its commercial value for high-value applications seems to have faded into obscurity. Instead, Korean oak finds itself predominantly employed in low-value applications such as bed logs for mushrooms, tool handles, wood chips, firewood, and charcoal [5,10].

Gaining fundamental insights into radial variations in anatomical characteristics is important for ensuring wood quality and optimizing utilization of the species [11,12]. Radial variation, extending from the pith to the near bark of wood, is a valuable parameter for determining the transition age from juvenile to mature wood [13,14,15,16]. As described by Kozlowski [17], Tsoumis [18], and Moore and Cown [19], juvenile wood forms near the pith and exhibits considerable changes in cell characteristics such as shorter cells, thinner cell walls, narrower cell diameter, and a higher microfibril angle. Therefore, juvenile wood possesses poorer physical [20,21] and mechanical properties than mature wood [22,23,24,25], as well as reduced natural durability [26] and diminished drying quality [27,28]. Therefore, more juvenile wood can significantly impact the overall wood quality [29].

Several studies have reported the radial variations in anatomical characteristics to determine the transition age from juvenile to mature wood in oak species. Helińska-Raczkowska and Fabisiak [30] reported radial variation in fiber, tracheid, and vessel lengths of Q. petraea from Poland. Lei et al. [31] reported radial variations in earlywood vessel diameter and fiber length in Q. garryana from the USA. Tsuchiya and Furukawa [32] investigated the axial element size radial variation, including the diameter and length of earlywood vessels and fiber length in Qs from Japan. Sousa et al. [33] examined the radial variations in fiber length, width, and wall thickness in Q. faqinea in Portugal. In addition, Tulik [34] reported the radial variation in earlywood vessel diameter in Q. robur from Poland, aiming to comprehend how alterations in the size of these vessels might affect the ability of oak trees to transport water.

To date, there have been few comparative studies on the anatomical characteristics of the six Korean oak species. In a previous study, we reported a comparative analysis of macroscopic and microscopic anatomical features of six Korean oak wood species [35]. However, the information on the quantitative anatomical characteristics is crucial to completing the identification keys for these species. Furthermore, there is also limited study on the radial variation and maturation age of these species. Additional information on the maturation age is important to ensuring wood quality and increasing the economic value for high-value applications. Therefore, in this study, to offer the reader identification keys and quality indices for the effective utilization of these species, we examined and compared the radial variation of anatomical characteristics and the transition ages from juvenile to mature wood.

2. Materials and Methods

2.1. Materials

Three trees each of the six Korean oak species were harvested from the research forest of Kangwon National University in Chuncheon-si, Gangwon-do, Korea (37°47′2.8932″ N, 127°49′13.368″ E). Disks with a thickness of 5 cm were extracted at breast height (1.30 m above the ground). Detailed information on the wood disks of each species is presented in

Table 1. Sample tree information.

Trade Name	Scientific Name	Tree No.	Breast Height Diameter (cm)	Cambial Age (Years)
Oriental Cork Oak	Quercus variabilis Blume (Qv)	1	21.1	63
		2	23.8	64
		3	29.7	61
Jolcham Oak	Quercus serrata Murray (Qs)	1	22.2	69
		2	28.3	54
		3	29.5	93
Mongolian Oak	Quercus mongolica Fisch. ex Ledeb (Qm)	1	21.3	63
		2	23.7	65
		3	24.2	64
Korean Oak	Quercus dentata Thunb. (Qd)	1	21.3	82
		2	21.5	66
		3	23.7	70
Oriental White Oak	Quercus aliena Blume (Qal)	1	15.5	49
		2	20.6	44
		3	25.3	50
Sawtooth Oak	Quercus acutissima Carruth. (Qac)	1	15.7	48
		2	23.6	48
		3	25.8	48

.

2.2. Methods

2.2.1. Sample Preparation

Wood block specimens (10 mm × 10 mm × 10 mm) and small stick specimens (5 mm × 5 mm × 20 mm) were prepared from wood disks. Segmented samples were collected at intervals of five growth rings from the pith to near bark. Wood block specimens were used to observe the earlywood vessel diameter, latewood vessel diameter, fiber diameter, fiber lumen diameter, and fiber wall thickness. Simultaneously, stick specimens were used to measure fiber length.

The wooden block specimens were softened in a boiling mixture of glycerin and water (50:50) for 12 h. A softened wood block was used to obtain thin slices (15–20 μm) of the cross, tangential, and radial surfaces using a sliding microtome (Lab Microtome, Swiss Federal Research Institute WSL, Birmensdorf, Switzerland). Thin slices were stained with safranin and light green solutions (1% each), followed by dehydration using a graded series of alcohol (50%, 70%, 90%, 95%, and 99%). The stained slices were stored in a vial filled with xylene. Before observation, permanent slides were prepared from the stained slices and attached to Canada balsam. Additionally, the stick specimens were soaked in Schultz’s solution following the method outlined by Savero et al. [36] to obtain macerated fibers.

2.2.2. Observation of Anatomical Characteristics

Quantitative anatomical characteristics were examined 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), and evaluated according to the International Association of Wood Anatomists (IAWA) list of microscopic features for hardwood identification [37]. The earlywood and latewood vessel diameters were measured using cross-sections of segmented samples obtained from three trees, with each sample comprising 50 cells. Additionally, fiber diameter, fiber lumen diameter, and fiber wall thickness in tangential direction were measured on cross-sections of segmented samples from three trees, each containing 50 cells. Fiber length was determined from the macerated fibers of the segmented samples obtained from three trees, with each sample consisting of 50 fibers.

2.2.3. Determination of Mature Wood Transition Age

The transition from juvenile to mature wood was evaluated using a segmented regression model following Rahayu et al. [29]. This model can simultaneously estimate the parameters and the transition age of juvenile to mature wood. The radial variation patterns of earlywood vessel diameter, latewood vessel diameter, fiber length, fiber diameter, and fiber wall thickness from the pith to the bark can be explained by two segments or functions within the curve in this model. The first regression function represented juvenile wood with a steep slope, whereas the second function represented mature wood with a flat line.

Non-linear regression analysis of a quadratic model with a plateau was employed to describe the relationship between earlywood vessel diameter, latewood vessel diameter, fiber length, fiber diameter, and fiber wall thickness as dependent variables, and the number of growth rings from pith to bark as the independent variables. The transition age was obtained using the non-linear least squares procedure (PROC NLIN) in the SAS v.9.0 (SAS Institute Inc., Cary, NC, USA). Additionally, the mature wood proportion in the wood disks was determined using the following equation:

$$\mathrm{M}\mathrm{a}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e} \mathrm{w}\mathrm{o}\mathrm{o}\mathrm{d} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{p}\mathrm{o}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \left(\mathrm{\%}\right)=\left(\frac{\mathrm{C}\mathrm{a}\mathrm{m}\mathrm{b}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{A}\mathrm{g}\mathrm{e} \mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a}-\mathrm{J}\mathrm{u}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{i}\mathrm{l}\mathrm{e} \mathrm{A}\mathrm{g}\mathrm{e} \mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a}}{\mathrm{C}\mathrm{a}\mathrm{m}\mathrm{b}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{A}\mathrm{g}\mathrm{e} \mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a}}\times 100\right)$$

2.2.4. Data Analysis

The relationship of quantitative anatomical characteristics between species was performed using a one-way analysis of variance (ANOVA). Duncan’s test was employed as a post-hoc verification of the ANOVA test, with a 5% significance level. The statistical analyses were computed using SPSS v.26 (IBM Corp., Armonk, NY, USA).

3. Results and Discussion

3.1. Radial Variation

3.1.1. Earlywood Vessel Diameter

The radial variation in earlywood vessel diameter showed a similar pattern for all species, showing a rapid increase until 20 to 30 growth rings, and then a stable or slight increase (Figure 1). Tsuchiya and Furukawa [32] reported that earlywood vessel diameter of Qs grown in Japan rapidly increased from pith until 10 growth ring and then a slight increase or constancy until near bark. Lei et al. [32] also mentioned that vessel diameter in the earlywood of Q. garryana from the USA showed a rapid increase from pith until 15 growth ring and a little increase or constancy until near the bark.

The transition of earlywood vessel diameter from the pith to the bark, calculated using the segmented regression model, is shown in

Table 2. Transition age from juvenile to mature wood of six Korean oak species using the segmented regression model.

Parameters	Transition Age (Years)
	Qv	Qs	Qm	Qd	Qal	Qac
Earlywood vessel diameter	32–36	25–36	19–30	22–36	30–35	30–41
Latewood vessel diameter	54–60	34–41	48	33–41	47	43–48
Fiber length	33–44	31–36	26–35	34–43	40–44	36–44
Fiber diameter	17–40	67	41–44	24–54	33	13–29
Fiber lumen diameter	-	-	-	-	-	-
Fiber wall thickness	44–50	13	25–42	18–41	33–38	19–22

. All species had different transition ages or maturation ages, ranging from 19 to 41 years. Qac had the oldest maturation age range with 30–41 years, whereas Qm was the youngest with 19–30 years. According to Tsuchiya and Furukawa [32], the maturation ages of earlywood vessel diameters in Qs observed in this study fall within a comparable range to those of Qs in Japan, which range from 11 to 38 years. In addition, Lei et al. [31] reported that the maturation age of Q. garryana in the USA exceeds 30 years.

The maturation ages of the earlywood vessel diameters were used to calculate the proportion of mature wood, as listed in

Table 3. The mature wood portion of six Korean oak species based on the maturation ages.

Parameters	Mature Wood Proportion (%)
	Qv	Qs	Qm	Qd	Qal	Qac
Earlywood vessel diameter	44–48	54–61	54–70	56–67	30–32	15–38
Latewood vessel diameter	6–12	37–56	26	50–53	6	0–11
Fiber length	31–46	43–61	46–59	48–49	9–12	8–25
Fiber diameter	38–72	28	32–35	34–64	34	40–73
Fiber lumen diameter	-	-	-	-	-	-
Fiber wall thickness	22–28	76	35–60	50–73	24–25	54–60

. These findings indicate that Qm had the highest mature wood portion at 54%–70%, followed by Qd, Qs, Qv, Qal, and the lowest was in Qac with 15%–38%.

3.1.2. Latewood Vessel Diameter

All species exhibited different patterns of radial variation in the latewood vessel diameter, showing a rapid increase from the pith to the bark, except for Qal, which initially decreased (Figure 2). According to the analysis using a segmented regression model (Table 2), Qv showed the oldest maturation age at 54–60 years, followed by Qm, Qal, Qac, Qd, and the youngest was found in Qs ranging from 34 to 41 years. The maturation ages of latewood vessel diameters were higher than those of earlywood vessel diameters, resulting in a lower mature wood portion compared with earlywood vessel diameters (Table 3).

3.1.3. Fiber Length

Figure 3 shows the radial variations in fiber length among the six Korean oak species. All species exhibited a similar pattern, rapidly increasing from the pith and remaining constant with a slight increase or decrease after 30–40 years of growth rings. The radial variation pattern of fiber length in our study is consistent with those in previous studies on Quercus spp. [32,33,34,35]. The authors mentioned that the fiber length of Quercus spp. showed a rapid increase from pith until 10–20 growth rings and a slight increase or constancy until near bark.

The radial variation pattern of the fiber length fits the regression model well (R² > 0.8), and the transition ages from juvenile to mature wood are shown in Table 2. The fiber length maturation ages had a higher range than those of the earlywood vessel diameter, whereas they were lower than those of the latewood vessel diameter. In comparison with a previous study, the maturation ages of fiber length in this study aligned with those of Q. petraea from Poland [30] and Q. faginea from Portugal [33], both exceeding 30 years. However, our findings indicate higher maturation ages than those reported for Qs from Japan, ranging from 8 to 27 years [32], and Q. garryana from the USA, with an average of 15 years [31]. The mature wood portion in Table 3 shows that Qm had the highest portion, followed by Qs, Qd, Qv, Qac, and Qal.

3.1.4. Fiber Diameter

The radial variation patterns of the fiber diameters are shown in Figure 4. A similar pattern was observed in all species, with an increase from pith and a stable or slight increase from 20 to 40 years. A segmented regression model was applied, and the results are presented in Table 2. Qs had the oldest maturation age of 67 years, and Qac had the youngest of 13–29 years. Qac had the highest mature wood proportion, followed by Qv, Qd, Qal, Qm, and Qs (Table 3).

3.1.5. Fiber Lumen Diameter

All species displayed fluctuating patterns of radial variation in the fiber lumen, as shown in Figure 5. Therefore, the radial variation pattern did not fit the regression model (R² < 0.3), making it impossible to calculate the age of transition from juvenile to mature wood.

3.1.6. Fiber Wall Thickness

Figure 6 shows the radial variation in fiber wall thickness. A similar pattern was observed in all species, with an increase in the pith and a slight increase or decrease after 20–40 years of growth rings. The transition ages from juvenile to mature wood are presented in Table 2. Qs exhibited the youngest maturation age of 13 years and the highest proportion of mature wood, with 76% (Table 3).

3.2. Anatomical Characteristics to Determine Transition Age

The transition ages for the earlywood vessel diameter, latewood vessel diameter, fiber length, fiber diameter, and fiber wall thickness ranged from 19 to 41 years, 33 to 60 years, 26 to 44 years, 13 to 67 years, and 13 to 50 years, respectively. Earlywood vessel diameter exhibited a similar range to fiber length, and these characteristics demonstrated consistent results across species (see Table 3). The radial variation patterns of these traits also indicated a clear transition from juvenile to mature wood (Figure 1 and Figure 2). According to Helińska-Raczkowska and Fabisiak [30], Lei et al. [31], Tsuchiya and Furukawa [32], and Sousa et al. [33], determination of the transition age from juvenile to mature oak wood relies on the earlywood vessel diameter and fiber length. Consequently, among these anatomical features, we identified earlywood vessel diameter and fiber length as the most reliable parameters for determining the maturation ages of the six Korean oak wood species.

3.3. Quantitative Analysis

3.3.1. Vessel Diameter

Table 4. Quantitative anatomical characteristics of six Korean oak species.

Parameters	Wood	Korean Oak Species						IAWA List
		Qv	Qs	Qm	Qd	Qal	Qac
Earlywood vessel diameter (μm)	Juvenile	252.8 b (50.7)	255.4 b (44.6)	217.2 e (36.4)	235.2 c (44.9)	229.5 d (50.1)	271.9 a (54.5)	43
	Mature	302.5 b (35.4)	304.7 b (32.9)	257.2 e (30.6)	283.1 d (30.3)	287.5 c (33.5)	329.4 a (35.8)
	Average	279.0 b (49.9)	283.6 a (45.4)	240.3 e (38.6)	265.3 c (43.2)	248.1 d (52.9)	285.8 a (56.3)
Latewood vessel diameter (μm)	Juvenile	51.1 a (14.2)	22.3 d (4.9)	25.1 c (5.2)	19.5 f (3.5)	20.7 e (4.3)	46.4 b (10.9)	40–41
	Mature	73.9 a (13.2)	26.4 d (4.8)	27.9 c (5.2)	23.4 f (4.4)	25.0 e (4.8)	60.2 b (11.1)
	Average	63.1 a (17.8)	24.7 d (5.2)	26.7 c (5.4)	22.0 e (4.5)	22.1 e (5.0)	49.7 b (12.5)
Fiber length (μm)	Juvenile	1343.8 a (218.1)	1196.1 c (178.7)	1077.5 f (171.6)	1161.4 d (181.0)	1141.8 e (176.6)	1234.8 b (240.2)	72
	Mature	1447.0 a (204.4)	1280.2 c (181.3)	1175.1 d (183.9)	1284.2 c (175.6)	1281.8 c (201.5)	1402.6 b (225.4)
	Average	1387.3 a (218.4)	1242.2 c (192.1)	1128.9 f (183.8)	1224.3 d (195.2)	1171.8 e (191.5)	1269.5 b (246.7)
Fiber diameter (μm)	Juvenile	15.8 a (2.3)	15.3 c (2.3)	15.7 a (2.4)	14.9 d (2.0)	15.5 b (2.3)	15.6 ab (2.4)	-
	Mature	17.2 a (2.5)	15.9 d (2.6)	16.8 b (2.4)	16.3 c (2.4)	16.1 cd (2.0)	17.1 a (2.3)
	Average	16.4 a (2.5)	15.6 c (2.5)	16.3 a (2.5)	15.6 c (2.3)	15.6 c (2.3)	15.9 b (2.4)
Fiber lumen diameter (μm)	Juvenile	4.2 c (1.1)	5.0 b (1.5)	6.0 a (1.8)	4.1 c (1.3)	5.1 b* (1.6)	3.5 d (1.1)	-
	Mature	4.4 c (1.7)	5.1 b (1.6)	5.4 a (1.9)	4.6 c (1.7)	5.2 b* (1.8)	3.9 d (1.7)
	Average	4.3 c (1.4)	5.1 b (1.5)	5.7 a (1.9)	4.3 c (1.5)	5.1 b (1.7)	3.6 d (1.2)
Fiber wall thickness (μm)	Juvenile	5.8 b (1.1)	5.1 c (0.9)	4.8 e (0.9)	5.4 d (0.8)	5.2 c (0.9)	6.0 a (1.1)	-
	Mature	6.4 b (1.1)	5.4 e (0.9)	5.7 d (1.0)	5.9 c (0.8)	5.4 e (0.8)	6.6 a (1.1)
	Average	6.1 b (1.1)	5.3 d (1.0)	5.3 d (1.0)	5.6 c (0.9)	5.2 d (0.9)	6.2 a (1.1)

Note: Numbers in parentheses are standard deviations. The same superscript letters and asterisks beside the mean values within columns denote non-significant outcomes at the 5% significance level using Duncan’s test for comparisons between species and non-significant outcomes at the 5% significance level using ANOVA test for comparisons between juvenile and mature wood, respectively. IAWA lists numbers for mean tangential vessel diameter as follows: ≤50 μm (40), 50–100 μm (41), 100–200 μm (42), ≥200 μm (43). The numbers for mean fiber length are ≤900 μm (71), 900–1600 μm (72), and ≥1600 μm (73).

shows the quantitative anatomical characteristics of the six Korean oak species. Qac exhibited a significantly wider diameter in both juvenile and mature earlywood vessels with 271.9 μm and 329.4 μm, respectively, whereas Qm displayed the narrowest values of juvenile and mature earlywood vessels with 217.2 μm and 257.2 μm, respectively. Consistent with the juvenile and mature wood analyses, the widest average earlywood vessel diameters were exhibited in Qac with 285.8 μm and Qs with 283.6 μm, while Qm showed the narrowest with 240.3 μm.

Latewood vessel diameter varied among the different oak species, as shown in Table 4. The widest latewood vessel diameters in both the juvenile and mature wood were observed for Qv with 51.1 μm and 73.9 μm, respectively, whereas Qd exhibited the narrowest values in juvenile and mature wood with 19.5 μm and 23.4 μm, respectively. Consistent with the juvenile and mature wood results, the average latewood vessel diameter also indicated that Qv had the widest diameter compared to the other species with 63.1 μm, whereas Qd and Qal exhibited the narrowest with 22.0 μm and 22.1 μm, respectively.

The earlywood and latewood vessel diameters of mature wood were significantly larger than those of juvenile wood across all species. Similarly, Barbour [38] reported that mature wood has a wider vessel diameter than juvenile wood.

The earlywood and latewood vessel diameters of Qv and Qac were consistent with those reported by Wang et al. [39]. The study noted that Qv from China exhibited a narrower earlywood vessel diameter and a wider latewood vessel diameter, averaging 242.8 μm and 87.9 μm, respectively, in contrast to Qac, which was 296.2 μm and 84.6 μm, respectively. Kim et al. [40] and Savero et al. [36] emphasized the significance of vessel diameter in species identification. Therefore, the results of earlywood vessel diameter and latewood vessel diameter can serve as valuable identification keys for differentiation among these six Korean oak species.

3.3.2. Fiber Length

Table 4 illustrates the juvenile and mature wood fiber length of six Korean species. In both juvenile and mature wood, Qv exhibited the longest fibers compared to the other species with 1343.8 μm and 1447.0 μm, while Qm had the shortest fibers with 1077.5 μm and 1175.1 μm, respectively. In all species, mature wood exhibited significantly longer fibers than juvenile wood. These findings align with those of Kozlowski [17], Zobel and van Buijtenen [20], Tsoumis [18], and Moore and Cown [19], who noted that mature wood tends to have longer cells than juvenile wood.

Consistent with the juvenile and mature wood results, the longest fibers were observed in Qv with 1387.3 μm, whereas the shortest fibers were observed in Qm with 1128.9 μm. A significant difference was observed in fiber length between the six Korean oak species. According to Savero et al. [36], fiber length is a key parameter for species identification. Therefore, the findings related to fiber length in our study may be valuable as an identification key, facilitating distinction between the six Korean oak species.

3.3.3. Fiber and Lumen Diameters

The widest fiber diameter in both juvenile and mature wood was observed in Qv with 15.8 μm and 17.2 μm, respectively, whereas the narrowest values were observed in Qd for juvenile wood with 14.9 μm and Qs for mature wood with 15.9 μm. In line with the juvenile and mature wood results, Qv exhibited the widest fibers at 16.4 μm, whereas Qd displayed the narrowest fibers at 15.6 μm. There was a significant difference in fiber diameter between the species, except between Qv and Qm, as well as between Qd, Qs, and Qal.

The fiber lumen diameter varied among the different oak species (Table 4). Qm had the widest fiber lumen diameter in both juvenile and mature wood with 6.0 μm and 5.4 μm, whereas the narrowest values were exhibited by Qac with 3.5 μm and 3.9 μm, respectively. Consistent with the results for juvenile and mature wood, the widest fiber lumen diameter was observed in Qm with 5.7 μm, whereas the narrowest fiber lumen diameter was observed in Qac with 3.6 μm.

The fiber and lumen diameters in mature wood were wider than those in juvenile wood, except for the lumen diameter of Qm. There were no significant differences in the lumen diameters of juvenile and mature wood between the Qal. Our findings on fiber and lumen diameters align with those of previous studies reported by Zobel and Sprague [41] and Barbour [38], which indicated that mature wood tends to have wider lumens and cells than juvenile wood.

3.3.4. Fiber Wall Thickness

Table 4 shows the fiber wall thickness in the juvenile and mature wood of six Korean species. In both juvenile and mature wood, Qac exhibited the thickest fibers compared to the other species with 6.0 μm and 6.6 μm, whereas Qm had the thinnest fibers in juvenile wood with 4.8 μm, and Qs and Qal showed the thinnest fibers in mature wood with a similar value of 5.4 μm. In each species, mature wood displayed notably thicker fibers than juvenile wood. These results are consistent with the observations of Kozlowski [17], Tsoumis [18], Moore and Cown [19], and Shmulsky and Jones [21], indicating that mature wood typically has thicker cells than juvenile wood.

Among the six oak species, there was variation in the average fiber wall thickness, Qac had the thickest fiber walls at 6.2 μm, whereas Qal had the thinnest walls at 5.2 μm. There were no significant differences in the fiber wall thickness among Qs, Qm, and Qal.

3.4. Anatomical Characteristics Affecting Wood Quality

The anatomical characteristics can be attributed to the wood quality including physical and mechanical properties [20]. The wood with longer cells, thicker cell walls, and a wider cell diameter generally exhibits higher quality [20,21,22,23,24,25,26]. Recent studies by Hamdan et al. [42] and Chen et al. [43] have observed that vessel diameter, fiber length, and fiber wall thickness significantly affected the density and mechanical properties of wood. In addition, juvenile and mature wood played a crucial role in determining the anatomical characteristics that will impact the wood quality [17,18,19,20,21,22,23,24,25,26,27,28,29].

In this study, among the six Korean oak species examined, Qv exhibited longer cells, thicker cell walls, and a wider cell diameter compared to the other species. Qv displayed a younger transition age from juvenile to mature wood in comparison to Qal and Qac, whereas Qv indicated an older age compared to Qs and Qm. Despite this, the overall indications suggest that Qv may have a better wood quality than the other species. According to Pyo et al. [44], Qv exhibited a higher basic density than Qs, Qm, and Qac. Chang et al. [45] showed that Qv had a lower shrinkage rate than Qm. Furthermore, Oh [46,47] reported that Qv exhibited higher compression and bending strength than Qs, Qm, Qal, and Qac.

4. Conclusions

All species exhibited a similar pattern of radial variation in earlywood vessel diameter, fiber length, and fiber wall thickness. The clearest transition from juvenile to mature wood was observed in the radial variation of the earlywood vessel diameter and fiber length. The maturation age of the six Korean oak species ranged from 19 to 44 years.

Qv exhibited the highest average values for latewood vessel diameter of 63.1 µm, fiber length of 1387.3 µm, and fiber diameter of 16.4 µm. In contrast, Qac displayed the highest values for earlywood vessel diameter and fiber wall thickness, showing 285.8 µm and 6.2 µm, respectively. Furthermore, Qm exhibited the highest value for the fiber lumen diameter of 5.7 µm. Mature wood exhibited higher vessel diameters and fiber properties than juvenile wood in all species.

In conclusion, the differences in earlywood and latewood vessel diameters and fiber length between species could be utilized as identification keys for six Korean oak species. To achieve more accurate wood identification, it is essential to analyze both quantitative and qualitative anatomical characteristics. Radial variations in earlywood vessel diameter and fiber length emerged as the most reliable indicators for estimating the transition from juvenile to mature wood.

The results of this study will provide valuable insights into silvicultural treatments and optimal harvesting times for producing higher-quality wood from the six Korean oak species. Further research on natural durability, physical, and mechanical properties is needed to enhance our understanding of the quality of the six Korean oak woods.