Biomass and carbon storage in an age-sequence of Acacia mangium plantation forests in Southeastern region, Vietnam

Aim of study: The major objective of this study was to estimate the biomass increment and carbon (C) storage of the main ecosystem components in an age-sequence of three Acacia mangium plantation stands. Vietnam. Material and methods: In order to assess the biomass of different tree components, 36 trees with diameter at breast height ranging from 13.38 to 22.87 cm were harvested from the different aged stands. Biomasses of understory (shrubs and herbs), and litter were also determi ned. Carbon storage in the trees and understory biomass, litter, and mineral soil (0-50 cm) were determined by analyzing the C content of each compartment. Main results: The biomass in trees, understory vegetation, litter, and ecosystem increased with stand age. Soil C represented 61.99% of the total, aboveground tree biomass C made up 26.73%, belowground tree biomass C accounted for 7.01%, and litter comprised 2.96%, whereas only a small amount (1.30%) was associated with understory vegetation. The average C content of total tree (47.97%) was hi gher than those of understory and litter. Soil organic C stock in the top 50 cm depth in 4-, 7- and 11-year-old stands of A. mangium were 86.86, 126.88 and 140.94 Mg. C ha -1 respectively. Soil C concentration decreased continually with increasing soil depth. Total C storage of three planted forests ranged from 131.36 to 255.86 Mg. C ha-1, of which 56.09 - 67.61% of C storage was in the soil and 26.88 - 40.40% in the trees. Research highlights: These results suggest that A. mangium is a promising afforestation tree species with fast growing, high biomass accumulation and high C sequestration potential.


Introduction
Emissions of CO 2 and other GHG have been increasing during the past century (IPCC, 2007); hence, posing environmental issues (Xu et al., 2007;Cheng et al., 2015), and leading to concerns of climate change and their mitigation strategies. Sequestering carbon (C) through forests is considered as a mitigation path for climate change (Dixon et al., 1994;Lal, 2005;Pan et al., 2011). Forests can maintain C balance via their substantial amount of C sequestration (Fan et al., 1998;Houghton, 2007;Pugh et al., 2019). Forests ecosystem have an irreplaceable role in minimizing accumulation of atmospheric CO 2 and maintaining global climate stability (Fang et al., 2001;Härkönen et al., 2011).
The global total amount of C in soil and vegetation is about 1912.2 Pg C, of which soil C content is 2.8 times higher than the amount of C stored in vegetation biomass (Scharlemann et al., 2014), showing the importance of soil in C sink. Tropical forests play a crucial role in driving the global C cycle (Zelazowski et al., 2011;Ngo et al., 2013). The accurate estimation of C storage in forest soils over time can help to understand the amount of C removed from the atmosphere through plants (Eglin et al., 2011).
According to FAO (2016), plantation forests cover an area of 291 million hectares and accounts for 7% of the global forest area. Planted forests are regarded as effective tools for countering climate change due to their ability to absorb C and they also play an increasingly important role in regulating climate in the future (Canadell et al., 2007). China has the largest forest plantation area in the world (~ 69 million hectares) (Xu et al., 2010). Most of these plantations are still immature and indicate a significant potential for C sequestration (Huang et al., 2012). Xu et al. (2007) revealed that the C sink function of vegetation in Chinese forests mainly came from the contribution of plantations. Thus, strengthening relevant studies on C stocks in plantations will have an extremely important guiding role in predicting and maintaining the long-term productivity of the system and minimizing atmospheric CO 2 concentration accumulation.
Biomass equations will be a key part of future C measurements and assessments in addressing the issues of climate change as biomass is a crucial component in determining forest C stocks and accumulation rates (Temesgen et al., 2015). The DBH is a commonly used variable in allometric equations because it has a close relationship with biomass and is easy and correctly identified (Brown, 1997;Brown et al., 1989;Snowdon et al., 2000;Phuong, 2011). Although some authors have shown that the inclusion of H in biomass allometries compared to the only use of DBH, substantially improves biomass assessment (Chave et al., 2005;Hunter et al., 2013). The accurate determination H in closed canopy forests is often challenging because it is difficult to decipher between adjacent trees and the tree to be measured (Chave et al., 2014). As a result, current methods to assess H result in estimates with high degrees of random and systematic error.
Acacia mangium Willd. is a fast-growing tree species native to parts of Australia, Papua New Guinea and Indonesia. It has been widely cultivated in Southeast Asia, Tropical America and many African countries in large-scales (Forss et al., 1996;Ren & Yu, 2008;Herdiyanti & Sulistyawati, 2009;Koutika & Richardson, 2019) mainly due to its fast growth, drought tolerance, adaptation to different soil types and pH, good wood quality for timber and energy production (NRC, 1983). These attributes make A. mangium a candidate tree species for afforestation or reforestation programs. A. mangium was introduced into Vietnam in 1980 (Turnbull et al., 1998) and it's now one of the most important afforestation tree species in some of the eight important forestry-ecological regions and Southeastern region in Vietnam (MAR-Da, 2014). It plays an important role in the national and regional ecosystem C cycles because its plantation area is over 600,000 hectares, accounting for approximately 16% of the total plantations in Vietnam (MARDb, 2014). These plantations are predominantly established on agricultural lands and degraded forest lands, and these are managed on a rotational length of 5-12 years for the production of both pulps and timbers (MARDb, 2014). A. mangium was an excellent afforestation tree species with fast growing, high yielding and high comprehensive utilization of whole trees. Besides producing woods for industries, A. mangium plantations also has role in providing environmental service such as felling negative effects on C cycle through the uptake and storage of C. Thus, there is need for accurate information regarding the C storage in A. mangium planted forests. Although numerous studies have been conducted in A. mangium plantations; growth and wood properties (Dhamodaran & Chacko, 1999), biomass and productivity (Ren & Yu, 2008), allometric biomass equations and C sequestration (Hai et al., 2009;Herdiyanti & Sulistyawati, 2009;Ilyas, 2013), microbial activity in litter and soil (Bini et al., 2013), soil nutrients (Lee et al., 2015), biological nitrogen fixation (Paula et al., 2015), soil C storage (Thanh & Cuong, 2017), benefits and its negative impacts on biodiversity (Koutika & Richardson, 2019). However, the biomass and C sequestration of A. mangium plantations in Southeastern region of Vietnam are largely unknown, which is critical to predict the responses of regional and global C balance to future climate change. Hence, the present study investigated the changes in biomass and C storage in A. mangium plantations across three different ages in Southeastern region, Vietnam. Specifically, the focuses of this study were: (i) to quantify the biomass of the ecosystem compartments across an age-sequence of A. mangium stands; (ii) to assess the changes in the C concentrations and C stocks of ecosystem compartments with increasing stand age; and (iii) to provide information for analyzing the ecological characteristics of A. mangium plantations in Southeastern region.

Study site
This study was carried out in Chang Riec Historical -Cultural Forest (11°00′30″ to 11°35′13″N and 106°00′00″ to 106°07′10″E), Tayninh Province, Vietnam (Fig. 1). The location has two distinct seasons: the dry season from December to April and the rainy season from May to November. The mean annual temperature is 26.9°C, with the maximum of 35.2°C between April and May and the lowest 21°C from December to February. The mean annual rainfall is 1967 mm with average rainy days of 155. Annual mean humidity is 78.3%, and monthly mean of sunshine is 181-277 hours (IBST, 2009). The terrain in the study area is relatively flat, elevation of 28-53 m a.s.l. with slopes of 3-5°. The soil type is grey brown, developed on ancient alluvium rocks, and soil depth over 100 cm. They are loam in texture with pH (H 2 O) of 5.10-5.23. The clay, silt, and sand contents are 12.76, 46.06, and 41.19%, respectively. The predominant planted forests of the area comprise A. mangium Willd., Acacia hybrid (Acacia auriculiformis A. Cunn. ex Benth. × A. mangium Willd.), Hopea sp., Dipterocarpus obtusifolius Teijsm. ex Miq., and Tectona grandis L.f. plantation forests. These plantations accounted for over 39.3% of the total forest area. A. mangium occupied approximately 20% of all plantations in this area and plays a crucial role in pulp and timber production. Our site includes a chronosequence A. mangium stands with ages of 4-, 7-, and 11-year-old. All the stands were the first rotation and no fertilization was applied. They were established in agricultural lands after the clear-harvesting of previous Cassava (Manihot esculenta Crantz). The initial planting density was 4 m × 2.5 m, and thinning operations were done once, twice, and three times for the 4-, 7-, and 11-year-old plantations, respectively. Stand density decreases as the stand age increases due to thinning operation carried out on the plantations. The DBH and H tend to increase with stand age (Table  1). Seed provenance from Dongnai Province was used in raising the seedlings of A. mangium used for the plantations. The diversity and abundance as well as growth of understory vegetation were well developed, especially for the 7-and 11-year-old plantations. The dominant species of shrub and herb layers in these plantations were

Biomass estimation and C concentration analysis
During 12th February to 30th March in 2019, four sample plots (40 m × 25 m) were established in each A. mangium stand. All the sampling plots were 500 -800 m apart (Fig. 1). The DBH (cm) of trees in each plot with DBH ≥5 cm were measured by using a diameter tape accuracy of 1 mm and their heights (H, m) were determined using a Blume-Leiss altimeter. Canopy closure was determined by using a specific smartphone application (Gap Light Analysis Mobile Application software), and 10 points were measured in each plot (Table 1). Three standard trees were chosen based on the mean DBH (Dm) and logged from each sampling plot for biomass determination, using the segmenting method (Hai et al., 2009;Phuong & Hai, 2011;Ngoan & Chung, 2018). A total of 36 trees were harvested ( Table 2). The sample trees were divided into four biomass components (stem, branches, leaves, and BGB). The fresh weights of all compartments were measured in situ, and samples of every components in each sample tree were collected for moisture content and C concentration analysis. The root biomass was collected through total excavation, extending radially out from the trunk and downwards to soil layer until no roots were visible. The ratio of different biomass components were computed to analyze changes of relative biomass partitioning with stand age.
In this study, the DBH was used as a predictive variable. The relationships between dry biomass and DBH were tested using linear and nonlinear functions. Accor-ding to Parresol (1999), biomass data normally exhibit heteroscedasticity. Consequently, biomass data was log-transformed and fitted as linear models to correct the heteroscedasticity of variance. The White's test (White, 1980) was also applied to check variance homogeneity of biomass models (Table 3). It is common that a correction factor (CF) would be used to correct for the systematic bias introduced by the anti-log transformation to obtain the expected biomass in original scale when a log-model was fitted to the biomass data (CF = exp (SEE 2 /2) (Baskerville, 1972). However, other studies found that anti-log transformation tended to overestimate biomass if the CF is applied, and suggested that the CF may be omitted if the bias (B) from anti-log was relatively small compared to the overall variation in the estimate of biomass (B = ((CF-1)/CF)×100) (Madgwick & Satoo, 1975;Zianis et al., 2011). In the present study, the CF values of all biomass equations were less than 1.009. The B values were also rather small ranging from 0.2 to 0.89% (Table  3). Therefore, the CF was not essential for the species in this study. The best biomass equations were selected based on various goodness-of-fit statistics namely the coefficient of determination (R 2 ), the Akaike information criterion (AIC), the root mean squared error (RMSE), and the standard error of estimation (SEE). We also accounted for the significance of the regression (p value) and the significance of parameters. Equations with higher R 2 , smaller AIC, RMSE, SEE, and significant parameters and p value (p<0.05) were selected as the best-fitted biomass allometric equations (Akaike, 1974;Adam & Ju- (1)
Biomass of shrubs and herbs was estimated by a destructive sampling of five 5 m × 5 m subplots. Shrubs (DBH<5.0 cm) were separated into three components: stem plus branches, leaves and roots, and herbs into aboveground and belowground components. In each plot, the litter contribution was obtained from a set of five quadrats (1 m × 1 m) were separately chosen in each of the four corners and the center of the plot. Litter was separated into two components: leaves plus sexual organs and twigs (<10 mm diameter). All materials were weighed freshly, and approximately 300-500 g of each compartment from each sample (shrubs, herbs, and litter) were taken for moisture and C analysis.
The abovementioned samples were taken to the laboratory of Centre of Forestry Research and Climate Change at the Vietnam National University of Forestry and ovendried at 70°C to a constant weight, then ratios of dry/fresh biomass were calculated and used to convert fresh biomass into dry biomass. The C contents of the components from the tree component samples, understory, and litter were determined by the dichromate oxidation method (Nelson & Sommers, 1982).

Soil sampling and analysis
Soil samples were taken at fixed depth (0-10, 10-20, 20-30 and 30-50 cm) using a soil corer with four replicates for each age stand. The SBD of the different soil layers was determined by using a soil bulk sampler with a volume of 100 cm3 stainless-steel cutting ring. The sample was dried to 105°C to constant weigh and weighed to determine SBD (Blake & Hartge, 1986). The SOC was analyzed by using the dichromate oxidation method (Nelson & Sommers, 1982).

Carbon stock estimation
Plant biomass C stocks (Cvs) was calculated as following equation: where: C vs , plant C stocks (Mg . C ha -1 ); B, plant dry biomass (Mg . ha -1 ); P c , plant biomass C concentration (%).
It should be noted that organic C in soil is usually only calculated as organic C that exists in organic materials with a diameter (<2 mm) (IPCC, 2003;Deng et al., 2017;Thanh & Cuong, 2017), so Cst was calculated following the equation: where: Cst soil C stocks (Mg . C ha -1 ); SOC, soil organic C concentration (g . kg -1 ); SBD, soil bulk density (g . cm -3 ); and S d , soil depth (cm).

Statistical analysis
One-way analysis of variance (ANOVA) followed by the least significant difference and then the (LSD) test was used to detect the mean difference in biomass and C stocks among different stand ages. In ANOVA, assumptions of normality and homogeneity of variances were tested with significance level at p<0.05 confidence interval. Due to the focus of this study on analyzing allometric relationships between tree component biomass and DBH across stand ages, additivity of the biomass equation was not considered (Zianis & Mencuccini, 2004;Yang et al., 2019). Statistical analysis of data and regression analysis for developing allometric equations were conducted using SPSS 25.0 software package (IBM Corp, 2017).

Allometric biomass models for biomass of A. magium plantations
The allometric equations using power functions (stem, branches, BGB, and TB) and exponential function (leaves) correlating biomass were established for the various components with DBH (Table 3 and Fig. 2). The allometric equations that were developed interpreted over 92% of the variability for the tree compartments (p<0.001). The equations were most fitted for the AGB and TB among the various tree components (p<0.001). The parameters of the equations varied with tree biomass compartments.

Biomass distribution in different components of A. mangium plantations
With increasing stand age, the biomass of most A. mangium tree components increased, including AGB, There was a significant difference in TB between 11-, 7-, and 4-year-old stands (p<0.05), with the highest values in the 11-year-old stand (210.57 Mg . ha -1 ), and the lowest in the 4-year-old stand (72.72 Mg . ha -1 ). Fo-liage biomass increased gradually with age, with values of 3.98, 5.11, and 8.03 Mg . ha -1 in the 4-, 7-, and 11-year-old stands. The overall biomass of the ecosystem increased significantly with stand age (p<0.05). The litter biomass ranged from 11.43 Mg . ha -1 in the 4-year-old stand to 13.29 Mg . ha -1 in the 11-year-old stand, and did not differ between the different stands (p>0.05). There were significant differences in the biomass of each tree component among the stands with the exception of the foliage biomass between 4-and 7-year-old stands (p<0.05). For the 4-and 7-year-old stands, the biomass decreased as stem > BGB > branches > leaves. However, for the 11-year-old stand, biomass decline was in the order as stem > branches > BGB > leaves (Fig. 3a). In the understory vegetation, the VAGB, VBGB, and TVB were significantly different in different ages (p<0.05), indicating similar trends of increasing with stand age (Table 4). The TVB was the highest (5.99 Mg . ha -1 ) in 11-year-old stand and the lowest (4.86 Mg . ha -1 ) in 4-year-old stand. The stem amounted to the largest contribution to TB regardless of stand age, accounting for 54.66%, 54.96%, and 53.57% of TB for the 4-, 7-, and 11-year-old stands, respectively. The ratio of BGB and foliage biomass to TB decreased with increasing stand age. Conversely, the contribution of branch biomass to TB increased with stand age (Fig. 3a). The BGB/AGB ratio was 0.32, 0.31 and 0.2 for 4-, 7-, and 11-year-old stands, respectively (Table 4). The proportions of TB of A. mangium   accounted for total ecosystem biomass were 81.70%, 89.34% and 91.61%, respectively, for the three stands (Fig. 3b).

Carbon concentration and accumulation in biomass
The C contents of different components (tree, understory, and litter) were summarized in Table 5. There were no significant differences in C concentration in the tree (p>0.05), understory (p>0.05), and litter (p>0.05) layer with the exception of shrubs (stem plus branches), aboveground herbs, and twigs of litter. Generally, trees had the highest C content while the understory had the lowest. The C concentration varied from 46.72% to 48.63% among components within individual trees. The highest and lowest C content in each stand was recorded for the stem and BGB, respectively. The mean C concentration of total tree was 47.97%. The C contents of aboveground components of the shrub and herb layers in all three stands were   higher than those of their belowground components. The mean C contents of shrubs, herbs, and litter were 44.80, 43.17%, and 44.45%, respectively.
The AGBC averaged was 26.29 Mg . C ha -1 , 52.81 Mg . C ha -1 , and 84.98 Mg . C ha -1 in the 4-, 7-, and 11-year stands, respectively. The BGBC increased with stand age, from 8.24 Mg . C ha -1 at the 4-year-old stand to 16.40 and 16.54 Mg . C ha -1 at the 7-and 11-year-old stands, representing 23.86%, 23.70%, 16.29% of the TBC in the three aged stand (Table 6). Except for the foliar biomass C between 4-and 7-year-old plantations, significant differences were observed in the biomass C of each tree component at different stand ages (p<0.05). Stem biomass C contributed more to TBC than the other tree components. The ratio of each component to TBC indicated a similar pattern to the ratio of each component to TB (Fig. 4a).
Most of the C in the biomass was stored in the TB, and litter, while only a small amount was stored in understory vegetation (Fig. 4b). The total biomass C in trees, understory, and litter increased with stand age. The TBC was 34.53, 69.21, and 101.52 Mg . C ha-1 for the 4-, 7-, and 11-year-old plantations, corresponding to an annual rate of biomass C accumulation of 8.63, 11.56, and 8.08 Mg . C ha -1 year −1 (Table 6).

Carbon concentration and storage in soil
The SOC concentration was reduced significantly with increasing soil depth in all three stands (p<0.05; Fig. 5a). The SOC content of the upper 0-10 cm soil layer was the highest in all three stands, and the values for each stand were 12.70, 20.08, and 21.90 g . kg −1 , respectively. The SOC content of the four sampled soil layers (0-10 cm, 10-20 cm, 20-30 cm, and 30-50 cm) increased significantly with increasing stand age (p<0.05). Figure 5b describes trends in the soil layer C stock across the A. mangium stand age-sequence. The Cst in the 30-50 cm soil layer followed an significant increasing trend with stand age (p<0.05, Fig. 5b). Although Cst in the three soil layers (0-10 cm, 10-20 cm, and 20-30 cm) increased with age, the relationship was not significant. The Cst found in the 0-50 cm soil layer was 86.86, 126.88, and 140.94 Mg . C ha -1 in the 4-, 7-, and 11-year-old stands, respectively. More than 60% of soil C was stored within 0-30 cm soil layer of each stand (Fig. 5b).

Carbon storage in the plantation ecosystem
Mean total ecosystem C stock of 128.47, 203.77, and 251.27 Mg . C ha -1 were observed in the 4-, 7-, and 11-year-old plantations ( Table 6). The total ecosystem C stock increased with stand age, and there were significant differences in the ecosystem C storage (p<0.05). The C stock fractions across the three stands was in the following order: soil layer (56.09-67.61%) > tree layer (26.88-40.40%) > litter layer (2.42-3.84%) > understory layer (1.09-1.67%) (Fig. 6). Averaging across stands, 61.99% of the total C was in the soil, 33.75% in the trees, 2.96% in the litter, and 1.30% in the understory layer. The proportion of C accumulated in soil decreased with increasing age and the proportion accumulated in biomass increased.

Biomass C of A. mangium stands at different ages
Stand age plays an important role in the distribution of biomass and C (Xie et al., 2016;Justine et al., 2017;Zhang et al., 2019). Results of this study show that the biomass and C of A. mangium in both aboveground and belowground parts increased significantly with increasing stand age. Similar results were reported in previous studies (e.g. Hai et al., 2009;He et al., 2009;Cao et al., 2012;Justine et al., 2015;. Furthermore, the accumulation rate of TBC was much higher in 4-and 7-year-old plantations than  in 11-year-old plantation. The highest accumulation rate of TBC was found in the 4-7-year-old (11.56 Mg . C ha -1 ), in line with previous studies for A. mangium forests (Hai et al., 2009;He et al., 2009), demonstrating a fast biomass C accumulation of trees in this stage. The mean TBC of 68.4 Mg . C ha -1 in this study was much greater than those reported for 4-12-year-old A. mangium plantations (39.8 Mg . C ha -1 ) and 2-12-year-old A. auriculiformis forests (21.5 Mg . C ha -1 ) in Vietnam conducted by Hai et al. (2009), and Acacia plantations of less than 6 to over 16 years (57.3 Mg . C ha -1 ) in the Pearl River Delta region of Southern China , slightly lower than that the 71.5 Mg . C ha -1 in 4-11-year-old A. mangium forests in Guangxi Province of China (He et al., 2009), but within the range of 34.4-85.6 and 44.8-118.2 Mg . C ha -1 reported for Asian and Global forests (FAO, 2010). The different rate of accumulated biomass C stock could be attributive to the effect of climatic conditions, plant species, stand age, stand density, biomass, soil nutrient, and water availability (Herdiyanti & Sulistyawati, 2009;Lee et al., 2015;Zribi et al., 2016). Our data demonstrated that the A. mangium plantation in Southeastern region can accumulate large amounts of biomass C in both aboveground and belowground parts. Moreover, fertilization was not done in these plantations. This suggests that A. mangium can grow well in soils without fertilization, has great biomass C accumulation potentials in this region. The present study illustrated changes in biomass and C partitioning with plantation age. We observed an increase in C accumulation in branches along with a decrease in BGB and foliage while relatively stable in ratio of stem biomass C to TBC. This result was in accordance with findings for Acacia auriculiformis (Hai et al., 2009) and Cunninghamia lanceolata (Xie et al., 2016). This allocation pattern may be interpreted by the strategies that trees use to survive during stand development. The fractions of BGB and leaves in early stages of growth are crucial for the survival of young seedlings and the likelihood that they survive to the next developmental stage. With the development, tree foliar biomass productivity no longer increases, resulting in declined demand for water and nutrient supply from BGB ( Vanninen et al., 1996). On the contrary, Hai et al. (2009) reported that the fraction of biomass C stored in stem of A. mangium increased during 4-12 years, while the proportion of biomass C stored in branches, leaves, and BGB decreased. He et al. (2009) found an increase in the stem biomass C partitioning along with a decrease in branch and foliar biomass C partitioning for A. mangium stands. The biomass C partitioning into different components can be affected by both biological and environmental conditions. Particularly, in the tree crown, which is characterized spread and lengths of branches, is partly associated with aerial growing space, the branch biomass C proportion might be expected to reflect the stand density. For instance, differences of stand density in the 11-year-old plantation (610 tree . ha -1 ) used in the current study and year-old plantation (825 tree . ha -1 ), and He et al.'s 11-year-old plantation (775 tree . ha -1 ) might interpret the different the allocation patterns with age.
In this study, decline in the BGB/AGB ratio with age might be related to root rot and death in the older trees resulting to alleviated BGB and therefore greater BGB/AGB ratio in the younger than in the other stands. The BGB/ AGB ratio average value of 0.28 observed here is within the range of 0.28-0.31 and 0.20-0.33 reported in the Asian and Global forests (FAO, 2010) but higher than the values of 0.23, 0.24, and 0.17 reported for A. mangium plantations by Qin et al. (2007), Ren & Yu (2008), and Hai et al. (2009), respectively. These different values might arise due to the BGB/AGB ratio dependent on forest age (Peichl & Arain, 2006;Xie et al., 2016), tree species (Hai et al., 2009;Ruiz-Peinado et al., 2012), and vegetation category (Mokany et al., 2006). Nevertheless, the BGB accounted for approximately one-quarter of the TB in this chronosequence study, which emphasizes the importance of BGB in the accurate estimation of the biomass and C of A. mangium planted forests.
Understory vegetation plays a crucial role in stabilizing the ecosystem and sequestering CO 2 (Xu et al., 2000), but their biomass C stock account only approximately 1.09-1.67% (median 1.30%) of the total ecosystem C stock in the A. mangium plantations (Fig. 6), which represents a relatively small proportion in the ecosystem. Gao et al. (2014) demonstrated that absence of the computation for understory vegetation could lead to underestimation of C sequestration capacity. Previous studies found that the biomass C storage in understory vegetation did not show a clear changing trend with stand age Deng et al., 2017;Yue et al., 2018;Zhang et al., 2019). In this study, however, the C stock of the understory vegetation increased with stand age, which was in accordance with the results of He et al. (2009) andZhang et al. (2018). The distinction in species ecological characteristics, light, and nutrients probably contributed to this difference (Abdallah & Chaieb, 2012). Perhaps in the other previous studies that understory biomass C storage did not clearly increase with stand age was due to the more intense competition for light and nutrients. Our results indicated that the C stock in understory were dependent on stand age.
We found that the biomass C storage of the litter increased with stand age, which was consistent with results from previous studies (Deng et al., 2017;Zhang et al., 2018). In contrast, other investigations found that the litter C stock did not correspond clearly to stand age (Ming et al., 2014;Yue et al., 2018). In general, litter C stock is determined by the balance between litter input and de-composition, hence any elements impacting the amount of input and decomposition rate would influence the litter C storage. The lower quantity of C stored in litter in the 4-year-old stand development was due to decreased leaves, branches, and understory biomass standing in the period. In the present study, it was also found that the C stock of the litter in each stand is greater than that of the total understory vegetation, with the largest proportion of 3.8% in the 4-year-old stand, demonstrating that forest litter should be regarded as an important component in plantation ecosystems.

Carbon concentration
A factor of 50% is commonly used for biomass -C conversion (Brown, 1997;IPCC, 2003). However, it was showed that the C content of different plant compartments might be either above or below 50% (e.g. Thomas & Martin, 2012;Cheng et al., 2014;Zhang et al., 2019). The mean C concentration of total tree of 47.97% in this study is within the range of 40.15-55.87% and 45.54-49.45% reported for the same tree species (Hai et al., 2009;He et al., 2009) but lower than that for A. auriculiformis (49.32%), A. hybrid (51.22%), and Eucalyptus urophylla (53.11%) (Hai et al., 2009). Previous investigations indicated that the C concentrations of planted forests are closely associated with their sample chemical compositions, tree components, wood type, geographical location, climate, and soil conditions (Bert & Danjon, 2006;He et al., 2013). Hence, all of these factors must be taken into account when assessing C contents under different edaphic and climatic conditions. The mean C concentrations of shrubs, herbs, and litter were lower than 45%. Thus, using a coefficient of 50% may result to large errors in up-scaling of C pool. For example, assuming 50% C content for understory in the 7-year-old plantation and for litter in the 11-year-old plantation would result in a difference of 2.62 Mg . ha -1 (11.75%) and 6.65 Mg . ha -1 (9.29%), respectively. Therefore, we suggest that a component-specific C content value for individual compartments is suitable for assessing forest C pool among all stands.

Soil organic C in A. mangium plantations
Among the three plantation development stages of this study, the soil C concentration was the highest in the surface soil layer (0-10 cm) and indicated a decreasing trend with depth (Fig. 5a). Soil organic matter is the dominant source of soil C and is greater in topsoil (Du et al., 2015). The C concentrations of the top soil layers (0-10 cm, 10-20 cm, and 20-30 cm), and the deep layer (30-50 cm) significantly increased with stand age, probably due to the increasing litter productivity in older stands (Ming Forest Systems August 2020 • Volume 29 • Issue 2 • e009 et al., Thanh & Cuong, 2017). This finding contributes to interpret the increase C stocks in belowground as stands age. Although changes in Cst following afforestation and stand age have been widely reported in previous studies, there still exist great deals of controversies concerning the effects of stand age on Cst level. Some studies reported no significant increase in Cst with stand age (Uri et al., 2012;Yue et al., 2018). Some studies have shown an increasing Cst in the early period after afforestation followed by a gradual decrease with age increment (Noh et al., 2010;Du et al., 2015;Zhang et al., 2019). Some studies indicated a decrease in Cst at the early age and then gradually increased with age (Chen et al., 2013;Wang et al., 2013). These discrepancies may be due in part to how much other influencing factors, such as climate, soil characteristics, forest type, forest management, litter input, root metabolism, and previous land use, which could overshadow the effect of stand age (Jandl et al., 2007;Zeng et al., 2013;Thanh & Cuong, 2017). We found Cst in the mineral soil layer increased with plantation age. Previous studies about Cst with stand age reported same results as our study (Hai et al., 2009;He et al., 2009;Cheng et al., 2015;Deng et al., 2017;Thanh & Cuong, 2017). The increase in Cst with stand age might be due to a larger accumulation of organic matter in older stands (BGB and litter). Additionally, 0-50 cm soil layer Cst accounted for the largest proportion of ecosystem C reservoirs, particularly the Cst in the upper 0-30 cm layer. Approximately 60.37% to 63.33% of Cst in the 0-50 cm range of soil was stored in the 0-30 cm range, where soils can be disturbed by human disturbances. Therefore, this study suggests that protection of organic C in the topsoil from disturbances of planted forests is important for improving C sequestration. The soil C values in the upper 0-30 were much greater than those for the Cst stored in A. mangium and A. auriculiformis planted forests across all age classes described by Hai et al. (2009) for the 0-30 cm upper mineral soil horizon profile.

Ecosystem C storage in A. mangium plantations
The results suggested that age plays a crucial role in affecting the overall ecosystem C stock and can provide insight into C sequestration Justine et al., 2015). Total ecosystem C stock in the 11-year-old plantation was twice the amount of the 4-year-old plantation, illustrating a large C sequestration potential during A. mangium development. Fractions of ecosystem C stocks in the three stands were in the order of soil layer > tree layer > litter layer > understory layer. It was observed that soil and tree biomass were the main C pools across all stand ages, accounting for 95.74% of C storage in the ecosystem, which is congruent with pre-vious studies (He et al., 2009;Li et al., 2013;Ming et al., 2014;Du et al., 2015). This highlights the importance of including accurate assess of Cst in ecosystem C accounting.

Conclusions
The results of the present study indicated the first complete and thorough assessments of changes in biomass compartments and their C content and C stock of ecosystem compartments for A. mangium forests in an age-sequence (from 4-to 11-year-old plantations) in Southeastern region, Vietnam. The biomass of each component of the tree could be predicted from an allometric equation using DBH as the independent variable. Tree, understory, litter, and ecosystem biomass increased with stand age. The highest C concentration was observed in tree stem while the lowest was recorded in the tree BGB. The use of component-specific C content values other than a fixed factor of 0.5 to convert biomass to C storage is required in order to more precisely evaluate the forest C storage. Soil C content increased significantly with increasing stand age, and the SOC concentration decreased continually with increasing soil depth. Carbon stocks across the 0-50 cm soil depth increased with stand age. Trees and mineral soil were two dominant C pools in the age-sequence. Most increases in C stocks in belowground and total ecosystems were attributable to increases in SOC and tree biomass. Our findings suggest that A. mangium species was a fast-growing afforestation tree species with high potential biomass C sequestration. Therefore, it can facilitate the management of the forest as a C sink.