{'fraction_non_alphanumeric': 0.03309936248309614, 'fraction_numerical': 0.018352759353467706, 'mean_word_length': 4.659438775510204, 'pattern_counts': {'":': 0, '<': 13, '<?xml version=': 0, '>': 4, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
A Stand-Level Comparison of Carbon and Nitrogen Distribution in an Exotic Japanese Cedar Plantation and a Natural Oak Stand

: This study compared carbon (C) and nitrogen (N) distribution at a stand level in an exotic Japanese cedar ( Cryptomeria japonica D. Don) plantation and a natural Serrata oak ( Quercus serrata Murray) stand growing under similar site conditions in South Korea. The aboveground biomass (stems, branches, and leaves) of 20 trees (10 of each species), the forest ﬂoor, and the mineral soils to a depth of 30 cm were sampled to determine C and N concentrations. Except in branches, C concentrations were signiﬁcantly higher ( p < 0.05) in the Japanese cedar plantation than in the Serrata oak stand, whereas N concentrations, except in the stem bark, were signiﬁcantly lower in the Japanese cedar plantation. Reforestation with an exotic coniferous species signiﬁcantly increased the C stocks in the aboveground biomass and the N stocks in the forest ﬂoor and mineral soils compared with a natural oak stand. The N stocks in the aboveground biomass were dependent on either the N concentrations or the C stocks in the tree components, whereas soil C and N stocks were negatively related to soil fertility parameters such as C/N ratio. Although it is uncertain which factors are responsible for the difference in aboveground C and soil N stocks following the establishment of Japanese cedar plantations on former natural Serrata oak stands, tree replacement may have an impact on C and N allocation within different forest compartments. unaffected by Japanese cedar plantation development on a natural Serrata oak stand, indicating that reforestation did not have a signiﬁcant impact on soil C stocks over the 50-year period after the establishment of Japanese cedar plantations on former natural Serrata oak stands. Although it is uncertain how elevated N stocks in Japanese cedar plantations respond to forest C stocks over a long period of tree growth, the results of this study suggest that establishing Japanese cedar plantations on former natural Serrata oak stands may increase aboveground C and soil N stocks to levels higher than those found in natural forests.


Introduction
Quantitative evaluation of carbon (C) and nitrogen (N) stocks in forests is important because of the role of C sequestration in mitigating global climate change and sustainable forest productivity. Consequently, estimates of C and N stocks in forests have been made at global, national, regional, landscape, and stand scales [1][2][3][4][5]. The role of C and N stocks in forests is likely to vary on a stand scale because nutrient conversion rates and allocation mechanisms differ between tree species [1,6,7]. Several studies have reported the influence of tree species on the possible factors driving C and N stocks in trees, forest floor, and mineral soils [1,3,7]. The variability in C/N ratio, C, and N stocks at a stand scale can be closely linked with tree species composition as both above-and belowground litter inputs may differ by the tree species planted within the same site [3,6]. For instance, C in soil organic matter is the energy source of microbes involved in N transformation such as N mineralization and immobilization processes, which seem to be dependent on tree species [6]. Thus, the C cycle is tightly connected to the N cycle in forest stands [6]. However, it is uncertain which factors are responsible for the differences in soil C and N stocks on a stand scale. In addition, other factors such as tree biomass, site conditions, and forest management practices can result in variation of C and N stocks on a stand scale [2,4,8].
Carbon and N stocks in forests can change after plantation establishment on natural forest sites [9,10]. For example, the total ecosystem C stocks in plants and soils using a meta-

Stand Characteristics
The study was conducted in the Namhae National Forest located in Namhae-gun, Gyeongsangnam-do, southern Korea. The mean annual precipitation in this area is 1839 mm year −1 , and the mean annual temperature is 14.1 • C. The soil is a well-drained slightly dry or wet brown forest soil (Inceptisol, USDA soil taxonomy) originating from granite, with a loamy and acidic property of pH 4.53-4.63 [16]. Study plots were located in a Japanese cedar plantation and an adjacent natural Serrata oak stand ( Figure 1). The plantation was established after clearcutting of natural Serrata oak stands 48 years ago prior to this study. The dominant understory species in the Japanese cedar plantation included Zanthoxylum schinifolium S. et Z., Smilax china L., Aralia elata Seem, Lindera glauca Blume, Morus mongolica C.K. Schied., Callicarpa japonica Thunb., Toxicodendron trichocarpum Kuntze, Lindera erythrocarpa Makino, and Cornus controversa Hemsl. Ex Prain. The dominant understory species in the Serrata oak stand included Prunus sargentii Rehder, Smilax china L., Cornus kousa F. Buerger ex Hance, Lindera glauca Blume, Smilax china L., Fraxinus sieboldiana Blume, and Quercus variabilis Blume.

Carbon and Nitrogen Stocks in the Aboveground Tree Biomass
The study design consisted of six 20 m × 10 m plots with two blocks located in each stand type. Each block has three replicated plots. To measure the C and N stocks in the aboveground tree biomass and develop nutrient content equations for each of the tree components, five trees per block of Japanese cedar (total: 10 trees) and Serrata oak (total: 10 trees), representing the diameter at breast height (DBH) range of the stand, were destructively sampled on 23 and 26 June 2013. Each sample tree was cut 20 cm above the ground and separated into its components (stem, branches, and leaves). Each tree component was weighed in the field using portable electronic balances. All sampling procedures were carried out in accordance with the technical standards formulated by the Korea Forest Research Institute [17].
The subsamples of each tree component were oven-dried at 85 °C for one week, then ground in a Wiley mill, and passed through a 40-mesh stainless steel sieve. Carbon and N concentrations in the ground materials were determined for each tree component using an elemental analyzer (Thermo Fisher Scientific Flash 2000, Milan, Italy). Although tree samples are usually oven-dried at 65 °C for the N concentration, drying wood samples at higher temperatures does not affect their N concentration [18]. The C and N concentrations were multiplied by the dry weights of the aboveground components to calculate the C and N content in each tree component.
Allometric equations were developed for each of the tree components (stem wood, stem bark, branches, and leaves) using the standard equation: log10y = a + b × log10(DBH), where y is the C and N content of the respective tree component, DBH is the diameter at breast height, and a and b are regression coefficients. DBH was used as the primary variable to the C and N contents because DBH can be measured accurately as a variable for the prediction of tree C and N stocks [4,19].

Carbon and Nitrogen Stocks in the Forest Floor and Mineral Soils
The forest floor samples were collected at the same time as the tree samples from three random points in each plot using a 900 cm 2 steel quadrat (30 cm × 30 cm). Forest floor samples consisted of needles with twigs (C. japonica), broadleaves (Q. serrata), a small branches piece (<2 cm), barks, and miscellaneous parts (e.g., reproduction fragments). Collected samples were placed in zipper paper, oven-dried at 65 °C, ground in a Wiley mill, and passed through a 40-mesh stainless sieve. Analysis of the C and N concentrations in the forest floor samples followed the same procedure used for tree C and N analysis. The

Carbon and Nitrogen Stocks in the Aboveground Tree Biomass
The study design consisted of six 20 m × 10 m plots with two blocks located in each stand type. Each block has three replicated plots. To measure the C and N stocks in the aboveground tree biomass and develop nutrient content equations for each of the tree components, five trees per block of Japanese cedar (total: 10 trees) and Serrata oak (total: 10 trees), representing the diameter at breast height (DBH) range of the stand, were destructively sampled on 23 and 26 June 2013. Each sample tree was cut 20 cm above the ground and separated into its components (stem, branches, and leaves). Each tree component was weighed in the field using portable electronic balances. All sampling procedures were carried out in accordance with the technical standards formulated by the Korea Forest Research Institute [17].
The subsamples of each tree component were oven-dried at 85 • C for one week, then ground in a Wiley mill, and passed through a 40-mesh stainless steel sieve. Carbon and N concentrations in the ground materials were determined for each tree component using an elemental analyzer (Thermo Fisher Scientific Flash 2000, Milan, Italy). Although tree samples are usually oven-dried at 65 • C for the N concentration, drying wood samples at higher temperatures does not affect their N concentration [18]. The C and N concentrations were multiplied by the dry weights of the aboveground components to calculate the C and N content in each tree component.
Allometric equations were developed for each of the tree components (stem wood, stem bark, branches, and leaves) using the standard equation: log 10 y = a + b × log 10 (DBH), where y is the C and N content of the respective tree component, DBH is the diameter at breast height, and a and b are regression coefficients. DBH was used as the primary variable to the C and N contents because DBH can be measured accurately as a variable for the prediction of tree C and N stocks [4,19].

Carbon and Nitrogen Stocks in the Forest Floor and Mineral Soils
The forest floor samples were collected at the same time as the tree samples from three random points in each plot using a 900 cm 2 steel quadrat (30 cm × 30 cm). Forest floor samples consisted of needles with twigs (C. japonica), broadleaves (Q. serrata), a small branches piece (<2 cm), barks, and miscellaneous parts (e.g., reproduction fragments). Collected samples were placed in zipper paper, oven-dried at 65 • C, ground in a Wiley mill, and passed through a 40-mesh stainless sieve. Analysis of the C and N concentrations in the forest floor samples followed the same procedure used for tree C and N analysis. The C and N stocks in the forest floor were calculated by multiplying the C and N concentrations by the dry weight of the forest floor samples.
Soil samples underlying the forest floor sampling point were collected from three randomly selected points in each plot. At each point, a soil pit of 50 cm × 50 cm was excavated to collect soil samples at three depths (0-10 cm, 10-20 cm, and 20-30 cm). Two soil samples were collected at each soil depth using 400 cm 3 stainless steel cans. One of the two soil samples was used to determine bulk density after drying at 105 • C for 24 h. The other one was air-dried and put pass through a 2 mm sieve to measure the amount of coarse fragments larger than 2 mm. The sieved soil samples were ground, passed through a 40-mesh (0.425 mm) stainless steel sieve, and an elemental analyzer (vario MACRO cube, Elementar Analysensysteme GmbH, Germany) was used to determine the C and N concentrations in the soil.
Soil C and N stocks at each soil depth were calculated using the following formula (Equation (1)): where NS represents the C and N stocks (kg ha −1 ) at each soil depth ( i ), NU i is the concentration (mg kg −1 ) of soil C and N at each soil depth, BD i is the bulk density (g cm −3 ) at each soil depth, D i is the soil depth (cm), and F i is the volumetric coarse fragment content (% by volume) at each soil depth.
In order to analyze soil chemical properties, soil P concentration, extracted by NH 4 F and HCl solutions [20], was determined using a UV spectrophotometer (Jenway 6505, Staffordshire, UK). Exchangeable K, Ca, and Mg concentrations, extracted by an NH 4 Cl solution [20] with a mechanical vacuum extractor (Model 24VE, SampleTek, Science Hill, KY, USA) were determined using ICP-OES (Perkin Elmer Optima 5300DV, Shelton, CT, USA).

Data Analysis
The accuracy of the allometric equations was evaluated using the coefficient of determination (R 2 ) and root mean square error (RMSE). Bias correction factors (CF) in the logarithmic transformation were calculated using the standard error of the estimate [21]. All data were checked for normality using the Shapiro-Wilk test. Means with checking the assumption for equal variances were analyzed with Student's t-test using the PROC T-test procedure of SAS [22] for significant differences at p < 0.05. Principle components analysis and correlation were carried out using Canoco 5.1 [23] to analyze the associations of variables in the soil C and N stocks.

Stand and Soil Characteristics
Mean stand densities of 467 trees ha −1 and 1067 trees ha -1 were measured for the Japanese cedar plantations and Serrata oak stands, respectively (Table 1). Mean DBH and stand basal area were higher for the Japanese cedar plantation (DBH: 28.9 cm; basal area: 29.5 m 2 ha −1 ) than for the Serrata oak stand (DBH: 12.0 cm; basal area: 13.5 m 2 ha −1 ).
Although soil nutrient data prior to plantation establishment were not available, soil nutrient concentrations (N, P, K, Ca, and Mg) after plantation development showed a significant difference (p < 0.05) between the Japanese cedar plantation and the Serrata oak stand. Concentrations of N, K, Ca, and Mg in the surface soil (0-10 cm) were significantly higher in the Japanese cedar plantation than in the Serrata oak stand (p < 0.05), but the soil C concentration was not significantly different between the Japanese cedar plantation and the Serrata oak stand at all three soil depths ( Table 2).

Carbon and Nitrogen Concentration in Aboveground Tree Biomass and the Forest Floor
Mean C concentrations in the stem and leaf components ( Figure 2) were significantly higher (p < 0.05) in Japanese cedars (stem: 48.7%; leaf: 47.5%) than in Serrata oaks (stem: 46.2%; leaf: 46.6%), whereas the concentration of C in stem bark tended to be higher in Serrata oaks (47.6%) than in Japanese cedars (46.4%). The N concentration in leaves was significantly higher (p < 0.05) in Serrata oaks (1.97%) than in Japanese cedars (0.81%). Consequently, C/N ratio was significantly different between Japanese cedars and Serrata oaks except for stem bark. Nitrogen concentration in the forest floor was significantly lower (p < 0.05) for the Japanese cedar plantation than for the Serrata oak stand, whereas C/N ratio showed an opposite trend (Table 3). However, the dry weight and C concentration were not significantly different (p > 0.05) between the Japanese cedar plantation and the Serrata oak stand.

Allometric Equation for Estimation of Carbon and Nitrogen Content
The allometric equations used to estimate C and N content of the tree biomass components were significant (p < 0.05). The allometric equations provided better fits of above- Nitrogen concentration in the forest floor was significantly lower (p < 0.05) for the Japanese cedar plantation than for the Serrata oak stand, whereas C/N ratio showed an opposite trend (Table 3). However, the dry weight and C concentration were not significantly different (p > 0.05) between the Japanese cedar plantation and the Serrata oak stand.

Allometric Equation for Estimation of Carbon and Nitrogen Content
The allometric equations used to estimate C and N content of the tree biomass components were significant (p < 0.05). The allometric equations provided better fits of aboveground C content, ranging from 52-98% of the variation compared with aboveground N content (Table A1 in Appendix A). The slope values (b) of the allometric equations for total C and N content of aboveground components were generally higher for Serrata oak than for Japanese cedar (Table A1).

Carbon and Nitrogen Stocks at the Stand Level
Carbon stocks in the aboveground tree biomass were significantly higher in the Japanese cedar plantation, which had a greater basal area, than in the Serrata oak stand. However, N stocks in stem wood and stem bark were significantly lower in the Japanese cedar plantation than in the Serrata oak stand. In contrast to the woody components, N stocks in the leaves were significantly higher in the Japanese cedar plantation than in the Serrata oak stand (Figure 3). Carbon stocks in the forest floor and the soil were not significantly different (p > 0.05) between the Japanese cedar plantation and the Serrata oak stand; however, N stocks in the soil were significantly higher in the Japanese cedar plantation than in the Serrata oak stand (Figure 4).

Carbon and Nitrogen Stocks at the Stand Level
Carbon stocks in the aboveground tree biomass were significantly higher in the Japanese cedar plantation, which had a greater basal area, than in the Serrata oak stand. However, N stocks in stem wood and stem bark were significantly lower in the Japanese cedar plantation than in the Serrata oak stand. In contrast to the woody components, N stocks in the leaves were significantly higher in the Japanese cedar plantation than in the Serrata oak stand (Figure 3). Carbon stocks in the forest floor and the soil were not significantly different (p > 0.05) between the Japanese cedar plantation and the Serrata oak stand; however, N stocks in the soil were significantly higher in the Japanese cedar plantation than in the Serrata oak stand (Figure 4).

Relationship between Soil C or N Stocks and Soil Property
Principal component analysis was performed to determine the key information from the multivariate data set by plotting all the soil data, along with the first two principal components. The first and second principal components accounted for 92.1% of the total variation in the Japanese cedar plantation and 88.4% of the total variation in the Serrata oak stand, respectively ( Figure 5). There were positive correlations between C and N stocks and C and N concentrations, whereas the soil bulk density and C/N ratio were negatively correlated with C and N stocks.

Relationship between Soil C or N Stocks and Soil Property
Principal component analysis was performed to determine the key information from the multivariate data set by plotting all the soil data, along with the first two principal components. The first and second principal components accounted for 92.1% of the total variation in the Japanese cedar plantation and 88.4% of the total variation in the Serrata oak stand, respectively ( Figure 5). There were positive correlations between C and N stocks and C and N concentrations, whereas the soil bulk density and C/N ratio were negatively correlated with C and N stocks.

Carbon and Nitrogen Status of the Aboveground Tree Biomass, the Forest Floor, and the Mineral Soils
It is important to understand the distribution of C and N in forests following the conversion from natural forests to forest plantations because C and N interaction following forest replacement is one of the main processes responsible for sustainable forest productivity and C sequestration [2,24]. However, the differences in C and N distribution of natural and planted forests may have been affected by many factors, such as tree species, stand age, forest management, and site conditions [11,18,24]. For example, the difference in C and N distribution by stand density may be affected by thinning activities, which are the most commonly applied practices for manipulating the growth of plantations [25]. In addition, forest tending works, such as thinning, have become one of the most im-

Carbon and Nitrogen Status of the Aboveground Tree Biomass, the Forest Floor, and the Mineral Soils
It is important to understand the distribution of C and N in forests following the conversion from natural forests to forest plantations because C and N interaction following forest replacement is one of the main processes responsible for sustainable forest productivity and C sequestration [2,24]. However, the differences in C and N distribution of natural and planted forests may have been affected by many factors, such as tree species, stand age, forest management, and site conditions [11,18,24]. For example, the difference in C and N distribution by stand density may be affected by thinning activities, which are the most commonly applied practices for manipulating the growth of plantations [25]. In addition, forest tending works, such as thinning, have become one of the most important forest management activities organized by the Korean government [26]. However, the difference in DBH, tree height and basal area between stand types may be due to the rapid growth patterns of exotic coniferous trees compared with slow-growing oaks. For example, the mean stand volume throughout the country examined in 2015 was higher for Japanese cedar plantations (346 m 3 ha −1 ) than for Serrata oak (126 m 3 ha −1 ) stands [5].
The results of this study showed that the C and N concentrations in the aboveground tree components were related to tree species, although some soil conditions differed between the Japanese cedar plantation and the Serrata oak stand. The high C concentrations in stem wood and new leaves of Japanese cedar could be associated with differences in nutrient concentration, polyphenolic compounds, and lignin concentrations. In addition, other studies reported that C concentrations in tree tissues were negatively correlated with nutrient concentration [4,27], whereas C concentrations in wood are known to be positively related to the proportion of lignified wood [28]. The lignin concentration in stems, reported by Chong and Park [29], is higher for Japanese cedar (32.0%) than for Serrata oak (22.6%). A previous study observed similar results in that C concentrations in tree components were found to be significantly higher for a coniferous tree species (Larix kaempferi Carriere) than a deciduous hardwood species (Quercus variabilis Blume), as a consequence of high lignin and low nutrient concentrations [24].
In this study, species-specific differences in mean N concentrations in the aboveground tree components were observed. These differences between Japanese cedar and Serrata oak are in agreement with the results of studies by Paré et al. [13] and Kim [24], which indicate that the N concentrations in tree components are generally lower in coniferous species than in deciduous hardwood species. Meerts [30] suggests that the low N concentration in stems of coniferous trees is related to the low proportion of living parenchyma cells in the stem tissues.
The N concentration and C/N ratio of the forest floor are a good proxy for predicting organic matter decomposition rates [31]. The low N concentration and high C/N ratio of the forest floor under Japanese cedar plantations were markedly higher than those under Serrata oak stands, which could subsequently increase the C stocks in the N-poor forest floor. In addition, no significant effect on C stocks of the forest floor between either tree species could be associated with site-specific effects of the two species or differences in aboveground litterfall inputs. The nitrogen status of the forest floor did not reflect the N concentration or C/N ratio in the mineral soil. This opposite trend indicates that the rooting systems play an important role in the C and N inputs incorporated into the mineral soil. For example, the annual fine root production observed in Japan was greater in Japanese cedar (72-320 g m −2 yr −1 ) [32] than in Serrata oak (144 g m -2 yr −1 ) [33]. Furthermore, the N concentration of fine roots was slightly higher in Japanese cedar (7.7 to 17.9 mg g −1 ) [34] than in Serrata oak (8.0 to 13.0 mg g −1 ) [35].

Carbon and Nitrogen Stocks in Aboveground Tree Biomass, the Forest Floor, and the Mineral Soils
Findings of this study indicate that replacement of natural oak forests with exotic Japanese cedar plantation had a significant effect (p < 0.05) on the parameters related to aboveground tree biomass and to N stocks in the forest floor and mineral soils. In comparison with the Serrata oak stand, the higher C stocks in aboveground tree components of the Japanese cedar plantation, which had a large stand basal area, could be related to rapid growth characteristics and accumulation of C in the form of living aboveground biomass. Other studies have reported that differences in the aboveground C stocks in forests were attributed to differences in stand basal area [24,26] and wood density [36]. However, basic wood density could not be associated with the difference of aboveground C stocks in two different species because basic wood density in Serrata oak (0.66 Mg m -3 ) was greater than that (0.35 Mg m -3 ) of Japanese cedar [5]. In addition, the higher C stocks noted in the aboveground biomass of the Japanese cedar plantation could be related to the high C concentrations in the tree components of Japanese cedars and to stand age, given that trees in the 48-year-old Japanese cedar plantation were older than in the 37-year-old Serrata oak stand. Omoro et al. [9] suggest that changes in biomass C stocks after the replacement of indigenous forests with exotic tree species could depend on plantation age. The mean value of aboveground C stocks was 52,008 kg C ha −1 for the Japanese cedar plantation and 25,832 kg C ha −1 for the Serrata oak stand. The aboveground C stocks in the Japanese cedar plantation were similar to exotic 36-year-old L. kaempferi plantations (62,970 kg C ha −1 ) in Korean forests [24].
The N stocks in the aboveground biomass appeared to be dependent on either the N concentrations or the C stocks in the tree components. For example, the significant difference in N stocks in stems was mainly attributed to N concentrations rather than to stem C stocks, whereas the differences in N stocks in stem bark and leaves were due to the C stocks rather than the N concentrations. Thus, the total N stocks were similar in the Japanese cedar plantation (203 kg N ha −1 ) and the Serrata oak stand (201 kg N ha −1 ) because of the differences in N stocks in the tree components. However, the N stock values obtained in this study were slightly lower than the values of 253 kg N ha −1 for 36-year-old L. kaempferi plantations and 280 kg N ha −1 for 40-year-old Q. variabilis stands [24].
The C stocks in the forest floor were not significantly attributed to the different tree species in the Japanese cedar plantation (6536 kg C ha −1 ) and the Serrata oak stand (8473 kg C ha −1 ). The difference in the N stocks, however, while not related to the amount of forest floor, could possibly be due to the difference in N concentration between the Japanese cedar plantation and the Serrata oak stand. A previous study has reported that N concentration in the forest floor is generally higher in deciduous broad-leaf litter than in needle leaf litter [24].
The consequences of plantation development on soil C stocks in a Japanese cedar plantation could be minimal. Other studies have shown that total soil C stock did not change following afforestation with coniferous forests [2] and that soil organic C stocks in indigenous forests are greater than those in plantations [9]. Total soil C stocks (Japanese cedar plantation: 59,107 kg C ha −1 ; Serrata oak stand: 51,079 kg C ha −1 ) at 0-30 cm soil depth were similar to the mean C stocks of Korean coniferous forest soils (59,100 kg C ha −1 ) at 0-30 cm soil depth [5]. In contrast to soil organic C stocks, soil N stocks in the Japanese cedar plantations could be attributed to a high N concentration in mineral soil layers, as there was a trend of increasing N in soils 0-20 cm deep after the regeneration of exotic coniferous tree. In addition, higher biomass production and N concentrations in fine root biomass were observed in a pine (4.08 Mg ha −1 ; 5.6 mg g −1 ) plantation than in a natural sawtooth oak (3.17 Mg ha −1 ; 4.3 mg g −1 ) stand under similar site conditions in Korea [37]. If soil N stocks are regulated by species differences between above-and belowground litter, N concentration in below-ground litter could be an important determinant of soil N stocks after the replacement of natural forests by plantations of exotic species.
Soil properties, such as C and N concentrations, explained the tree species-related variability of C and N stocks ( Figure 5). The positive relationship between soil C or N stocks and soil C or N concentration may be attributed to soil organic matter, which is closely connected to C and N cycling at the forest ecosystem levels. However, soil C and N stocks were negatively related to soil fertility parameters, such as the C/N ratio. This property tends to be associated with the rates of C and N mineralization, which in turn results in high soil N stocks in Japanese cedar plantations.
Although higher C and N stocks were observed under plantation forests than under natural forests (Table 4), the opposite patterns were also observed in natural forests [10,11], with no difference in the C stocks in soil between plantations and natural forests [38]. Thus, it is difficult to determine the effects of forest replacement on C and N stocks of tree biomass, forest floor, and mineral soils, because of the differences in the quantity and quality of above-and below-ground litter, litter decomposition, and tree C allocation between plantations and natural forests.

Conclusions
At a stand scale, Japanese cedar plantations that replace natural Serrata oak stands sequester more aboveground tree C and soil N than natural stands. The influence of tree replacement was pronounced in the C and N stocks of tree components, as well as the N stocks of the forest floor and mineral soil. These differences could be attributed to different C and N cycle mechanisms, influenced by above-and below-ground litter inputs, and differences in stand characteristics between plantations and natural stands. Soil C stocks, however, are relatively unaffected by Japanese cedar plantation development on a natural Serrata oak stand, indicating that reforestation did not have a significant impact on soil C stocks over the 50-year period after the establishment of Japanese cedar plantations on former natural Serrata oak stands. Although it is uncertain how elevated N stocks in Japanese cedar plantations respond to forest C stocks over a long period of tree growth, the results of this study suggest that establishing Japanese cedar plantations on former natural Serrata oak stands may increase aboveground C and soil N stocks to levels higher than those found in natural forests.

Conflicts of Interest:
The authors declare no conflict of interest. Allometric equation form is log 10 y = a + b × log 10 (DBH). The R 2 is the coefficient of determination. p-values represent the significance of the equations. RMSE: root means squared error. CF: correction factor.