Carbon storage of a tropical mangrove forest in Mui Ca Mau National Park, Vietnam

Catena 121 (2014) 119–126 Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena Carbon storage of a tropical mangrove forest in Mui Ca Mau National Park, Vietnam Nguyen Tai Tue a,⁎, Luu Viet Dung b, Mai Trong Nhuan c, Koji Omori a a b c Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Japan Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Japan Faculty of Geology, VNU University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam a r t i c l e i n f o Article history: Received August 2013 Received in revised form 29 April 2014 Accepted May 2014 Available online xxxx Keywords: Mangroves Carbon storage Sediment Carbon emissions Mekong Delta Vietnam a b s t r a c t Mangrove forests constitute the most important sink of carbon (C) in the tropics, the conservation of which is an essential mean in offsetting C emissions and climate change Mangrove forests are therefore suggested to be an important component of reducing emissions from deforestation and degradation (REDD+) schemes, which require scrupulous quantification of ecosystem C storage in order to monitor temporal C sequestration and emissions Despite this, proportionally less is known about ecosystem C storage of mangrove forests in Vietnam, where these systems constitute a large proportion of its coastline In this study, ecosystem C storage of a tropical mangrove forest in Mui Ca Mau National Park, Vietnam (CMNP) was quantified by measuring biomass of trees, roots, and downed woody debris, and sediment organic C and overall depth Results showed that aboveand below-ground C stock ranged from 90.2 ± 15.8 to 115.2 ± 19.3 and from 629.0 ± 32.5 to 687.0 ± 29.2 MgC ha−1, respectively The combination of the above- and below-ground C stocks resulted in a high ecosystem C storage, which ranged from 719.2 ± 38.0 to 802.1 ± 12.3 MgC ha−1, and slightly increased from fringe toward interior forest The 13,400 of mangrove forests in the CMNP were estimated to store 10.3 (±0.8) × 106 Mg of C, which is equivalent to 38.0 (±3.0) × 106 Mg of CO2e The present results suggest that the conservation of mangrove forest is needed to increase ecosystem C storage and to offset C emissions at the regional scale © 2014 Elsevier B.V All rights reserved Introduction Mangrove forests can store up to 1023 MgC ha−1, ranking among the most important carbon (C) sinks in the tropics (Donato et al., 2011) Recent studies have indicated that C storage by mangrove forests is two to three times higher than that of terrestrial forests (Adame et al., 2013; Donato et al., 2011; Kauffman et al., 2011) The C storage of mangrove forests therefore represents an important component in modeling regional and global C budgets Mangrove forests also provide a wide range of ecological services such as maintenance of biodiversity, nursery and breeding grounds for fish and invertebrates, and mitigation of disasters (e.g., typhoon, flood, and tsunami) (Alongi, 2011) As a result, mangrove forests have been recognized as a crucial component in climate change mitigation strategies (Alongi, 2011; Donato et al., 2011; Kauffman et al., 2011) and reducing emissions from deforestation and degradation (REDD +) schemes (Siikamäki et al., 2012) Ironically, mangrove forests are among the most threatened ecosystems in the tropics (Valiela et al., 2001), and rapidly deforested by land conversion and ⁎ Corresponding author at: 790-8577 Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Japan Tel.: +81 89 927 9643, +81 902 894 1610 (Cell Phone); fax: +81 89 927 9643 E-mail address: tuenguyentai@gmail.com (N.T Tue) http://dx.doi.org/10.1016/j.catena.2014.05.008 0341-8162/© 2014 Elsevier B.V All rights reserved natural disasters (Alongi, 2002) In which, aquaculture development is a major cause of mangrove degradation (Seto and Fragkias, 2007), and is a critical issue in many developing countries (Valiela et al., 2001) Mangrove forests have been reduced by at least 35% of their total area from 1980 to 2000 in Africa, Asia, and the Americas (Valiela et al., 2001), and the global mangrove deforestation rate is estimated to be between and 2% per year (FAO, 2007) The loss of mangrove forests will lead to critical consequences such as loss in biodiversity, ecosystem stability, ecosystem services, and the sequestration of C Moreover, the loss of above-ground biomass may exacerbate the decomposition rate of Crich sediments in mangrove forests, eventually emitting greenhouse gases (GHGs) to the atmosphere (Donato et al., 2011; Lovelock et al., 2011) Thus, conservation programs (e.g., REDD+) would benefit the protection of mangrove forests and biodiversity in developing countries (Donato et al., 2011; Siikamäki et al., 2012) The requirements of the REDD + schemes include a scrupulous monitoring of ecosystem C storage (MgC ha− 1), emissions of GHGs, and emission factors associated with land use change (Holly et al., 2007; Lydia et al., 2008) Although ecosystem C storage has been thoroughly quantified for mangrove forests in Asia-Pacific (Donato et al., 2011), oceanic islands (Kauffman et al., 2011), and Caribbean (Adame et al., 2013) regions, yet the assessment of C storage has not been conducted for mangrove forests of Vietnam where these systems constitute 120 N.T Tue et al / Catena 121 (2014) 119–126 a large proportion of its coastlines Thus, quantification of ecosystem C storage of mangrove forests is a prerequisite for future development of climate change mitigation strategies and the REDD + schemes in Vietnam, and will represent a crucial part of the puzzle when extrapolating C sinks at the regional and global scales (Donato et al., 2011) The mangrove forest area in Mui Ca Mau National Park, Vietnam (CMNP) is 13,400 ha, accounting for the largest proportion (~37%) of mangrove forests in the Mekong Delta, South Vietnam (Sam and Hong, 2003) Since the 1990s the mangrove forest has exhibited extensive levels of deforestation by massive construction of shrimp ponds (Clough et al., 2002), and this had an observable impact on marine production-based catch (de Graaf and Xuan, 1998), major changes in hydrological processes and soil acidification (Blasco et al., 2001), and may be an unforeseen driving force for the emissions of the GHGs (Lovelock et al., 2011) Despite these observations, the quantification of the C storage of mangrove forests from the region has not been performed Information and baseline data on C storage would benefit the development of more accurate models and mitigation strategies for climate change, and the development of effective REDD+ schemes In the present study, we hypothesized that the below-ground C stock accounts for the largest proportion of ecosystem C storage, and that ecosystem C storage of the CMNP's mangrove forest is comparable to that from the Asia-Pacific region We quantified the C storage of mangrove forest in the CMNP by measuring the biomass of trees, roots, downed woody debris, and sediment organic C and overall depth Materials and methods 2.1 Study area The present study was conducted in an estuarine mangrove forest of the CMNP in the Mekong Delta The CMNP is located between 08°32′– 08°41′N and 104°44′–104°55′E in the southernmost tip of the Mekong Delta (Fig 1) The mangrove forest is dominated by Avicennia alba, Avicennia officinalis, Excoecaria agallocha, Thespesia populnea, Xylocarpus moluccensis, Bruguiera parviflora, Bruguiera sexangula, Ceriops tagal, Rhizophora apiculata, and Sonneratia caseolaris with tree heights ranging from 6.3 to 12.1 m (Tinh et al., 2009) The mangrove forest provides many ecological functions and services, being an important habitat for critically endangered species of four-toed terrapin Batagur baska, hairy nosed otter Lutra sumatrana, and black-faced spoonbill Platalea minor, and it is an important stopover and wintering habitat for a large number of migratory birds (http://www.ramsar.org), and nursery grounds for many species of fish and invertebrates (Hong and San, 1993) Additionally, mangrove forest is thought to play a critical role in supporting the coastal fisheries of the Mekong Delta region, in which each hectare of mangrove forest has been estimated to support a marine catch of 450 kg year−1 (de Graaf and Xuan, 1998) The CMNP was declared as a Ramsar Site (http://www.ramsar.org) in 2006 and a UNESCO Biosphere Reserve (http://www.unesco.org) in 2009 The CMNP is located within a tropical monsoon climate zone with a rainy season from May to November and a dry season from December to April The rainfall averages 2400 mm per year with the highest precipitation level recorded in October (Tinh et al., 2009) The mean monthly temperature of the region ranges from 25.9 to 29 °C with an annual mean of 27.6 °C (Tinh et al., 2009) Tides are mixed diurnal and semidiurnal regimes with a range of tidal amplitude between 0.5 and 1.5 m The salinity of the Cua Lon River ranged from 22.9 to 26.9‰ during the sampling campaign (unpublished data) 2.2 Field sampling A total of three 135 m long transects was established parallel in fringe forest (near the river bank), through to transitional forest, and to interior forest during December 2012, with each transect having three m radius circular plots spaced 45 m from their respective center, and the distance between transects was approximately 70 m The fringe forest was dominated primarily by the A alba, A officinalis, and S caseolaris (Fig 1) The transitional and interior forests had a high density of R apiculata, A alba, A officinalis, and B parviflora (Fig 1) Within each sampling plot, the above-ground C stock was obtained by measuring the biomass of living and standing dead trees, and downed woody debris The below-ground C stock was obtained by measuring mangrove roots and sedimentary organic C (Donato et al., 2011; Kauffman et al., 2011) For tree biomass measurement, all trees rooted within the circular plot with a stem diameter N5 cm or a tree height N1.3 m were measured for the stem diameter at breast height (DBH) at 1.3 m or above the highest prop root of R apiculata In all sampling Fig Sampling locations in mangrove forest of Mui Ca Mau National Park, Vietnam Mangrove photos show an example of the typical floristic composition in the fringe forest (dominated by A alba, A officinalis and S caseolaris), transitional forest (dominated by R apiculata, and A alba), and interior forest (dominated by R apiculata) N.T Tue et al / Catena 121 (2014) 119–126 plots, the standing dead trees were measured for total height, diameter at base, and the DBH of the stem due to only the stem remaining Wood samples of mangrove species A alba, B parviflora, R apiculata, and S caseolaris were collected for determining their specific gravity and C content The pneumatophores of the A alba, A officinalis, and S caseolaris were counted and harvested for determining the average weight within 30 × 30 cm quadrants The planar intercept method was used to measure downed woody debris mass, consisting of the fallen twigs, branches, prop roots, stems of trees lying down on the forest floor (Kauffman and Donato, 2012) Briefly, four subtransects were perpendicularly settled at the center of the sampling plot, and had an azimuth of 45° from the main transect The diameter of any downed woody debris intersecting the subtransect was tallied using a wood gauge, according to four diameter classes: 0–0.64 cm, 0.65–2.54 cm, 2.55–7.5 cm, and ≥ 7.6 cm In which, the downed woody debris ≥7.5 cm from all plots was examined to be rotten and further measured for the diameter The length of each subtransect was 2, 5, 10, and 12 m for tallying the downed woody debris classes of 0–0.64 cm, 0.65–2.54 cm, 2.55–7.5 cm, and ≥7.6 cm, respectively Samples of each downed woody debris class were collected for determining their specific gravity and C content To measure the sediment C pool, the depth of sedimentary organic C was measured by gradually forcing a metal probe into the sediment stratum The sediment depths from all plots were determined to be N4 m In each sampling plot, a sediment core (250 cm in depth) was drilled using a Russian peat borer (52 mm in diameter) for minimizing the sediment disturbance during the drilling process Immediately following collection, sediment cores were placed in PVC tubes, sealed in the aluminum foils in order to minimize the gas exchange, then placed in a cooler, and processed within 24 h after collection Laboratory processing started with first removing the outer layer (~ 0.5 cm in thickness) of the sediment cores, then slicing into 5, 10, and 15 cm intervals for the core depths of 0–50, 50–100, and 100–250 cm, respectively The sediment samples were packed in labeled polyethylene bags for further analysis Subsamples of all sediment slices were also placed in plastic cubes (1 cm3) for determining bulk sediment density All sediment samples were maintained on ice in coolers and transported to the laboratory where they were frozen at −20 °C until further processing and analysis 2.3 Sample preparation and analysis In the laboratory, the plastic cubes containing sediments, the tree wood samples of a known volume, and the downed woody debris were weighed and dried in a drying oven at 60 °C until constant weight The bulk sediment density was obtained via weight of the dried sediments in plastic cubes (Donato et al., 2011) Volume of downed woody debris sample was measured by a waterdisplacement method outlined by Kauffman and Donato (2012) The specific gravities of tree wood and downed woody debris samples were calculated by the ratio of the oven-dried weights and their volumes Sample preparation and analysis for C content of wood and sediment samples followed methodologies outlined by Tue et al (2011) Briefly, the wood and sediment samples were completely dried at 60 °C in a drying oven, and subsequently pulverized using an agate mortar and pestle For sediment organic C analysis, the pulverized sediments were first treated with acid HCl 1N for 24 h to remove carbonates After acid treatment, the samples were thoroughly rinsed by Milli-Q filtered distilled-deionized water and re-dried at 60 °C in a drying oven for 24 h The C content of the sediment and wood samples was analyzed by a combustion method using an element analyzer connected to an isotope ratio mass spectrometer (ANCA-GSL; Sercon, UK) The sediment organic matter content was obtained via loss on ignition measurement Two grams of the pulverized sediments was first dried at 60 °C in a drying oven for h, and then heated at 550 °C in a temperature-monitored muffle furnace for h Organic matter content 121 was obtained by subtracting the weight from before and after combustion at 550 °C 2.4 Ecosystem C storage estimation procedure The above-ground C stock was estimated from mangrove tree and downed woody debris biomass Tree biomass was calculated using allometric equations (Table 1) The tree C pool was calculated by multiplying tree biomass with wood C content Volume and mass of the downed woody debris were calculated using the equations reported by Kauffman and Donato (2012) The downed woody debris C pool was calculated as the product of the downed woody debris mass multiplied by the C content The below-ground C stock was calculated by the sum of the sediment and the root C pools Mangrove root mass was estimated by the common allometric equations for the species A alba, A officinalis, B parviflora, and S caseolaris (Komiyama et al., 2005), and the specific equation for R apiculata (Ong et al., 2004) (Table 1) The root C pool was estimated by multiplying the root mass with C content The sediment C pool was estimated up to m in depth Because the total organic C (TOC) content showed a lower coefficient of variation between 150 and 250 cm in depth, the TOC content over that depth range was averaged and applied to deeper sediment layers The sediment C pool was calculated following Eq (1)     −1 −3 ¼ Bulk sediment density g cm Sediment C pool MgC Â TOCð%Þ Â depth intervalðcmÞ ð1Þ Ecosystem C storage (MgC ha−1) was calculated by summing up the above- and below-ground C stocks The total C storage of mangrove forest in the CMNP was scaled up by multiplying the mean ecosystem C storage (MgC ha− 1) with the total area of mangrove forest (13,400 ha), and converted to carbon dioxide equivalents (CO2e) using a factor of 3.67 (Kauffman and Donato, 2012) 2.5 Statistical analysis A one-factor ANOVA followed by a Tukey post-hoc was used to test the differences in the above- and below-ground C stocks, and the ecosystem C storage of the fringe, transitional, and interior forests In addition, ANOVA test was used to assess whether the TOC content significantly decreased with sediment depth For statistical testing, the sediment layers were grouped into six depth intervals, consisting of 0–15, 15–30, 30–50, 50–100, 100–150, and 150–250 cm Prior to statistical analysis, the data was examined for normality and homogeneity of the variance using the Shapiro–Wilk's test and Levene's test, respectively For all Table Allometric equations for calculating above- and ground-biomass (in kg) of mangroves based on stem diameter at breast height (DBH, cm) and specific gravity (g cm−3) Species Allometric equation A Above-ground biomass A alba Stem: Ws = 0.079211DBH2.470895 Branch: WB = 0.481575(1.246280)DBH Leaves: WL = 0.171711(1.96367)DBH A officinalis Biomass: WTop = 0.251ρDBH2.46 B parviflora Biomass: WTop = 0.168DBH2.31 R apiculata ¼ 0:235DBH2:42 W Biomass: Top W stilt ¼ 0:0209DBH 2:55 S caseolaris Biomass: WTop = 0.251ρDBH2.46 A Below-ground biomass A alba Biomass: WR A officinalis Biomass: WR B parviflora Biomass: WR R apiculata Biomass: WR S caseolaris Biomass: WR = = = = = 0.199ρ0.899DBH2.22 0.199ρ0.899DBH2.22 0.199ρ0.899DBH2.22 0.00698DBH2.61 0.199ρ0.899DBH2.22 References Chukwamdee and Anunsiriwat (1997) Komiyama et al (2005) Clough and Scott (1989) Ong et al (2004) Komiyama et al (2005) Komiyama et al (2005) Komiyama et al (2005) Komiyama et al (2005) Ong et al (2004) Komiyama et al (2005) 122 N.T Tue et al / Catena 121 (2014) 119–126 Table Floristic composition (mean ± SE) of mangrove forest in Mui Ca Mau National Park, Vietnam Density (tree ha−1) DBH (cm) Total basal area Total tree biomass Each species Total species Percentage (%) Range (mean) (m2 ha−1) (Mg ha−1) Fringe forest A alba A officinalis B parviflora R apiculata S caseolaris 1668 ± 885 758 ± 418 65 477 ± 38 163 3033 ± 695 55 25 16 4.3–28.5 (10.4) 148.3 ± 34.6 197 ± 31.2 Transitional forest A alba B parviflora R apiculata 195 ± 225 228 1343 ± 228 1690 ± 283 12 79 4.2–27.9 (12.9) 108.3 ± 22.2 237.9 ± 53.8 Interior forest A alba A officinalis B parviflora R apiculata 260 ± 172 260 163 1517 ± 586 2058 ± 614 13 74 4.4–26.1 (12.3) 126 ± 24.6 250 ± 42.7 Species statistical tests, the significance level was accepted at a p value ≤ 0.05 All statistical tests were performed using SPSS 17.0 for windows (SPSS Inc., 2007) Results 3.1 Floristic composition of mangrove forest, specific gravity and C content of mangrove woods Mangrove species composition, density, DBH, total basal area, and tree biomass from the CMNP are shown in Table The mangrove forest can be divided into three vegetation zones based on the species composition, being dominated by A alba, A officinalis, R apiculata, and S caseolaris in the fringe forest, by R apiculata, B parviflora and A alba in the transitional forest, and by R apiculata, A alba, A officinalis, and B parviflora in the interior forest The highest mangrove tree density was expressed in the fringe forest, followed by the interior and transitional forests (Table 2) Overall, mean tree biomass markedly increased from the fringe toward the interior forest (Table 2), and the total basal area was the highest in the fringe forest and was the lowest in the transitional forest The specific gravity of mangrove woods decreased in the order of R apiculata, B parviflora, A alba, and S caseolaris, and was higher in the living tree than in dead tree (Table 3) The C content of mangrove wood varied slightly among mangrove species, and was higher in the living tree than in dead tree For the downed woody debris, the specific gravity and C content varied slightly among the size classes, and was highest in the size class ≥7.5 cm (Table 4) Table Specific gravity and C content of mangrove tree woods from Mui Ca Mau National Park, Vietnam Mean, and mean ± SE values are given where n = 2, and n ≥ 3, respectively Species Condition/type Specific gravity (g cm−3) TOC (%) n Avicennia alba Avicennia alba Bruguiera parviflora Bruguiera parviflora Bruguiera parviflora Rhizophora apiculata Rhizophora apiculata Rhizophora apiculata Sonneratia caseolaris Living tree Dead wood Living tree Dead wood Root Living tree Dead wood Root Living tree 0.77 0.42 0.76 0.56 0.58 0.84 0.6 0.89 0.53 45.2 43.7 44.7 44.1 41.9 45.3 45.3 44.6 45.1 1 3 3 ± 0.05 ± 0.05 ± 0.03 ± 0.03 ± 0.04 ± 0.02 ± 1.0 ± 0.4 ± 0.3 ± 0.2 ± 0.2 3.2 TOC and organic matter contents of mangrove sediments The TOC content obviously decreased from surface to bottom of the sediment cores (ANOVA, p b 0.0001, Table 5, Fig 2) The TOC content of sediments in the fringe forest was significantly lower than that of sediments in the transitional (p = 0.025) and the interior forests (p = 0.002) The TOC content of sediments in the transitional forest was similar to that of sediments in the interior forest (p = 0.679) The organic matter content (OM) ranged from 2.9 to 20.2%, with an average of 7.8 ± 2.9% (Fig 3) The TOC content was positively correlated with organic matter content by the following linear regression model (Eq (2)) (Fig 3):   TOC%ị ẳ 0:227 OM%ị ỵ 0:244 R ¼ 0:56; p b 0:0001; n ¼ 225: ð2Þ The TOC content and the bulk sediment density had an inverse relationship, which was described by the following non-linear least-squares regression model (Eq (3)) (Fig 4): 0:289TOCị Bulk sediment density ẳ 1:539e   R ¼ 0:62; p b 0:001; n ẳ 225: 3ị 3.3 Above-ground C stock estimates The mean living tree C pool increased from the fringe toward the interior forest (ANOVA, p b 0.05), and accounted for a significant proportion of the above-ground C stock (Fig 5) The remaining contribution from dead tree and downed woody debris C pool was a small proportion (b4%), and tended to increase from the fringe toward the interior forest (ANOVA, p b 0.05; Fig 5) The mean above-ground C stock overall had Table Diameter, quadratic mean diameter, specific gravity, and carbon content (mean ± SE) of downed woody debris from mangrove forest of Mui Ca Mau National Park, Vietnam Size classes Diameter (cm) Quadratic mean diameter (cm) Specific gravity (g cm−3) n Carbon content (%) n ≤0.64 0.65–2.54 2.55–7.5 ≥7.6 0.50 1.20 4.50 9.8 0.51 1.28 4.76 − 0.56 0.51 0.53 0.64 40 68 56 11 42.6 40.6 41.2 40.7 5 ± ± ± ± 0.09 0.44 1.57 2.9 ± ± ± ± 0.19 0.12 0.11 0.07 ± ± ± ± 2.0 4.4 2.8 0.7 N.T Tue et al / Catena 121 (2014) 119–126 123 Table Bulk sediment density, total organic C (TOC) content, and sediment C pool (mean ± SE) of mangrove forest in Mui Ca Mau National Park, Vietnam Bulk sediment density (g cm−3) TOC (%) Sediment C pool (MgC ha−1) Fringe forest 0–15 15–30 30–50 50–100 100–250 N250 Total 0.75 0.77 0.78 0.85 0.99 1.02 ± ± ± ± ± ± 0.06 0.04 0.06 0.07 0.08 0.01 2.2 2.0 2.1 1.7 1.4 1.4 ± ± ± ± ± ± 0.3 0.3 0.4 0.4 0.2 0.1 24.8 23.2 32.1 70.1 212.5 221.6 584.2 ± ± ± ± ± ± ± 3.6 2.8 6.0 13.4 28.8 12.4 28.0 Transitional forest 0–15 15–30 30–50 50–100 100–250 N250 Total 0.61 0.66 0.62 0.77 1.08 1.17 ± ± ± ± ± ± 0.06 0.07 0.12 0.11 0.14 0.08 3.2 2.9 2.8 1.9 1.4 1.3 ± ± ± ± ± ± 1.0 1.0 1.0 0.3 0.3 0.3 28.7 28.0 33.1 70.1 236.2 232.9 629.0 ± ± ± ± ± ± ± 7.0 7.6 9.5 11.6 44.7 30.3 47.2 Interior forest 0–15 15–30 30–50 50–100 100–250 N250 Total 0.66 0.68 0.73 0.79 1.20 1.20 ± ± ± ± ± ± 0.04 0.05 0.12 0.13 0.13 0.05 3.3 3.3 2.8 2.3 1.3 1.3 ± ± ± ± ± ± 0.6 1.1 1.1 0.4 0.3 0.0 31.7 33.5 39.9 88.3 234.4 227.2 655.0 ± ± ± ± ± ± ± 4.6 8.8 11.5 10.6 48.0 11.2 30.8 Depth (cm) an increasing trend from the fringe toward the interior forest (ANOVA, p b 0.05), being 90.2 ± 15.8, 109.2 ± 24.2, and 115.2 ± 19.3 MgC ha−1 for the fringe, transitional, and interior forests, respectively (Fig 5) The above-ground C stock had an overall mean of 104.9 ± 20.7 MgC ha−1 for mangrove forest in the CMNP Fig Correlation between sediment organic C content (TOC, %) and sediment organic matter content (OM, %) of 225 sediment samples from mangrove forest in Mui Ca Mau National Park, Vietnam 30.8 MgC ha−1 for the fringe, transitional, and interior forests, respectively The sediment C pool had an overall mean of 622.7 ± 44.2 MgC ha−1 for mangrove forest in the CMNP, and contributed N90% proportion to the below-ground C stock The mean below-ground C stock increased from the fringe toward the interior forest (ANOVA, p b 0.05; Fig 5), with respective means of 629.0 ± 32.5, 656.2 ± 56.2, 687.0 ± 29.2 MgC ha−1 for the fringe, transitional, and interior forests The overall mean of the below-ground C stock was 657.4 ± 43.6 MgC ha−1 for mangrove forest in the CMNP 3.4 Below-ground C stock estimates 3.5 Ecosystem C storage in mangrove forest The root C pool contributed b 8% of the below-ground C stock, with mean values of 44.9 ± 4.7, 27.1 ± 9.0, and 32.0 ± 1.8 MgC ha−1 for the fringe, transitional, and interior forests, respectively The sediment C pool had an increasing trend with the distance from the fringe forest (ANOVA, p b 0.05), being 584.2 ± 28.0, 629.0 ± 47.2, and 655.0 ± Fig Variation of total organic C content (TOC, %) by depth of mangrove sediment cores Points denote means and bars denote standard errors The combination of the above- and below-ground C stocks resulted in a large capacity for ecosystem C storage in the CMNP mangrove forest (Fig 5) Mean ecosystem C storage increased from the fringe toward the interior forest (ANOVA, p b 0.05), being 719.2 ± 38.0, 765.4 ± 79.4, and 802.1 ± 12.3 MgC ha−1 for the fringe, transitional, and interior Fig Non-linear correlation between sediment organic carbon (TOC, %) and bulk sediment density (g cm−3) of 225 sediment samples from mangrove forest in Mui Ca Mau National Park, Vietnam 124 N.T Tue et al / Catena 121 (2014) 119–126 In the present study, the specific gravity of mangrove woods exhibited interspecific variation, with the highest and lowest specific gravities found in R apiculata and S caseolaris, respectively The pattern was similar to that of the same species in mangrove forests from Thailand, Indonesia (Komiyama et al., 2005) and Australia (Clough and Scott, 1989) The C content of mangrove woods was generally high with an overall mean of 45.0%, 43.3%, and 42.7% for living trees, roots, and dead trees, respectively The slight decrease in C content of the dead trees may be due to biodegradation of wood lignocelluloses during decomposition (Benner and Hodson, 1985) Although the C content of mangrove wood is an important factor in converting tree biomass to the aboveground C stock, yet the measurement of C content is seldom available for mangrove woods The C content of the mangrove woods has been estimated to be 42% (Kathiresan et al., 2013), 46.4% (Donato et al., 2011), and 48% (Adame et al., 2013) The C content based on empirical measurements in the present study will therefore provide useful data for calculating above-ground C stock of mangrove forests 4.2 Variation in TOC content of sediment cores Fig Above-and below-ground C stock of mangrove forest in Mui Ca Mau National Park, Vietnam Top panel: above-ground C stock; bottom panel: below-ground C stock; and vertical bars denote standard errors forests, respectively The overall mean ecosystem C storage was 762.2 ± 57.2 MgC ha−1 for mangrove forest in the CMNP Discussion 4.1 Floristic composition of mangrove forest In the present study, mangrove community changed in species composition from a dominance of A alba, A officinalis, and S caseolaris in the fringe forest, to R apiculata, B parviflora, and A alba in the transitional forest, and to R apiculata, A alba, and A officinalis in the interior forest (Table 2) The floristic characteristics were highly consistent with earlier observations of species composition (Hong and San, 1993) and with respect to tree density (Tinh et al., 2009) in the CMNP The tree biomass found in the present study, ranging from 197 ± 31.2 to 250 ± 42.7 Mg ha−1, was two-fold higher than that of medium-sized mangrove forests (3–5 m in height) in Mexico (Adame et al., 2013) and Sundarbans (Ray et al., 2011), but similar to that reported from mangrove forests in Palau, Micronesia (Kauffman et al., 2011), and tall mangrove forests in Mexico (Adame et al., 2013), Borneo and Sulawesi (Donato et al., 2011) The mangrove forest in the present study showed exceptionally high levels in tree biomass from the R apiculata dominated zone, which was nearly two times higher than that observed by Hutchison et al (2013) estimated for mangrove forests of Vietnam, and at the higher end of reported range for R apiculata forest in Malaysia (Putz and Chan, 1986) The high above-ground biomass of mangrove forests in the CMNP indicated the benefit of mangrove forest in C sequestration and offsetting C emission (Siikamäki et al., 2012), and production of seeds for future mangrove planting (Clough et al., 2000) The TOC content ranged from 0.82 to 5.65% with an overall mean of 2.02 ± 0.87%, which was at the lower end of reported range for C-rich sediments in mangrove forests of Asia-Pacific region (Donato et al., 2011), and 5-fold lower than that of tall mangrove forest in Mexico (Adame et al., 2013), but higher than that from North Vietnam (Tue et al., 2011, 2012) The TOC content significantly increased from the fringe toward the interior forest (Fig 2; Table 5), indicating that geomorphological setting and mangrove zonation were main factors affecting C accumulation processes It has been reported that the accumulation of C in mangrove sediments highly varies with tidal amplitudes and proportional contributions of autochthonous (mangrove litter and root) and allochthonous (phytoplankton, riverine seston) C sources (Tue et al., 2012) The increase in TOC content from the fringe toward the interior forest is due to a less tidal flushing, and/or higher in situ mangrove litter and fine grain size accumulations in the interior forest (Saintilan et al., 2013; Tue et al., 2012) The progressive decrease in TOC content from the surface to the bottom of sediment cores was similar to earlier studies in mangrove forests of North Vietnam (Tue et al., 2011), Asia-Pacific region (Donato et al., 2011), Australia (Saintilan et al., 2013), Gulf of Mexico (Bianchi et al., 2013), and Mexico (Adame et al., 2013) The decreasing trend of TOC content with depth may reflect a change in the proportional contribution of autochthonous and allochthonous sources, and/or the loss of organic matter during decomposition (Tue et al., 2011, 2012) The present study found a significant positive correlation between TOC and sedimentary organic matter content (Fig 3, Eq (2)), indicating that TOC content of mangrove sediments may be estimated from the organic matter content using Eq (2) The linear regression model (Eq (2)) suggested that the loss on ignition method for the determination of TOC content using a Bemmelen factor of 0.58 (Schumacher, 2002) should be used with caution for mangrove sediments, because it can considerably overestimate the TOC content by approximately 20% The TOC content had an inverse, non-linear relationship with bulk sediment density (Fig 4, Eq (3)), which is similar to earlier reports for terrestrial forest soils (Perie and Ouimet, 2008) and mangrove sediments (Donato et al., 2011) The present results suggested that the bulk sediment density of mangrove sediments is possible to be calculated from TOC content using Eq (3) Although the bulk sediment density is an important factor for estimating the C density and the sediment C pool (Eq (1)), few studies have simultaneously reported bulk sediment density with TOC content for mangrove sediments The non-linear relationship between TOC content and bulk sediment density (Fig 4, Eq (3)) therefore contributes a useful method for estimating the C density and sedimentary C stock in the mangrove forests (Donato et al., 2011) N.T Tue et al / Catena 121 (2014) 119–126 Fig Comparison of ecosystem-level C storage of the mangrove forest in Mui Ca Mau National Park (CMNP) to different mangrove and terrestrial forests The mean ecosystem C storage was taken from Donato et al (2011) for mangrove forests from Ganges– Brahmaputra Delta, Bangladesh (Gb); Java, Indonesia (Ja); Palau, Micronesian Island (Pa); Sulawesi, Indonesia (Su); Yap, Micronesian Island (Ya); Kosrae, Micronesia (Ko); Borneo, and Indonesia (Bo); from Adame et al (2013) for mangrove forests from Mexican Caribbean (Km); and from Dixon et al (1994) for terrestrial forests from a high latitude forest (HLF), middle latitude forest (MLF), and low latitude forest (LLF) 125 underscoring their importance in offsetting GHGs emissions and climate change The 13,400 of mangrove forest in the CMNP is estimated to store upwards of 10.3 (±0.8) × 106 Mg of C, which can be converted to 38.0 (±3.0) × 106 Mg of carbon dioxide equivalent (CO2e) The CO2e storage capacity of mangrove forest in the CMNP is equivalent to one-third of total C emissions of Vietnam for the year 2011 (112.67 × 106 Mg of CO2e, (IEA, 2013)) The results indicated that mangrove forest in the CMNP can rank among the largest C sinks in the tropics (Donato et al., 2011) Thus, the removal of mangroves by changes in the land use (e.g., conversion to shrimp ponds) and forest degradation can have the potential to significantly increase the release CO2e to the atmosphere (Lovelock et al., 2011) Although mangrove forest in the CMNP has been admitted as a Full Protection Zone since the mid-1990s (Tuan, 2013), yet the mangrove forest has continually faced illegal practices of charcoal production, and rapid expansion of shrimp aquaculture (Clough et al., 2002), coastal erosion, sea level rise, and coastal water pollution by shrimp farms (Tho et al., 2006), and tropical typhoon (Clough et al., 2000) The present results suggest that effective incentives to conserve the mangroves are needed to increase ecosystem C storage (Donato et al., 2011) and coastal fisheries productivity (de Graaf and Xuan, 1998), and to offset GHGs emissions in the Mekong Delta region The conservation of mangrove forest should be a necessary component in sustainable coastal management plans, climate change mitigation strategies, and in REDD+ schemes 4.3 Ecosystem C storage of the mangrove forest Acknowledgments The mean above-ground C stock ranged from 90.2 ± 15.8 to 115.2 ± 19.3 MgC ha−1 and increased from the fringe toward the interior forest (Fig 5) The higher above-ground C stock (Fig 5) in the interior forest is a result of the dominance of R apiculata with the highest tree biomass (Table 2) and wood specific gravity (Table 3) The above-ground C stock was similar to that of mangrove forests in Asia-Pacific region (Donato et al., 2011) and Mexico (Adame et al., 2013), and of terrestrial forests (Dixon et al., 1994) (Fig 6), and greater than that from Sundarbans mangrove forest (Ray et al., 2011) The high aboveground C stock demonstrated that mangrove forest in the CMNP may play an important role for the C sequestration (Ray et al., 2011) The mean below-ground C stock ranged from 629.0 ± 32.5 to 687.0 ± 29.2 MgC ha−1, and increased slightly from the fringe toward the interior forest (Fig 5) The below-ground C stock accounted for approximately 90% of the ecosystem C storage of mangrove forest in the CMNP This result was similar to reported values of estuarine mangrove forests from Asia-Pacific region (Donato et al., 2011) The pattern can be explained by a combination of the high alluvial accumulation rates and inputs of autochthonous C sources in mangrove forests (Saintilan et al., 2013; Tue et al., 2012) The below-ground C stock had a statistically significant but weak, positive correlation with the above-ground C stock (R2 = 0.27, p b 0.05), indicating that the autochthonous source (e.g., mangrove litters) was a considerable C contributor to the belowground C stock Overall, the present results support conclusions that the below-ground C stock is the largest C pool of the ecosystem C storage from mangrove forests (Adame et al., 2013; Donato et al., 2011; Kauffman et al., 2011) The combination of relatively high above- and below-ground C stocks showed a high level of C storage of mangrove forest in the CMNP (Fig 5) The ecosystem C storage ranged from 719.4 ± 38.0 to 802.1 ± 12.3 MgC ha−1 with an overall mean of 762.0 ± 57.2 MgC ha−1, which was comparable to reported ranges of mangrove forests from Ganges– Brahmaputra Delta (Bangladesh); Java (Indonesia); Palau (Micronesian Island) (Donato et al., 2011); and Mexico (Adame et al., 2013) (Fig 6) Particularly, ecosystem C storage of the mangrove forest in the CMNP was two to three times higher than that of terrestrial forests (Dixon et al., 1994) (Fig 6) These results indicated the capacity of the CMNP mangrove forest in sequestering and storing C at the ecosystem-scale, The authors are grateful to the staff of VNU Hanoi University of Science for their help with sampling This work was supported by the Grant-in-Aid for Scientific Research (No 24-02386) of the Japan Society for the Promotion of Science We express our sincere thanks to anonymous reviewers and Dr Todd W Miller for their critical reviews and comments which significantly improved the manuscript Appendix A Supplementary data Supplementary data associated with this article can be found in the online version, at 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A of cinalis, R apiculata, and S caseolaris in the fringe forest, by R apiculata, B parviflora and A alba in the transitional forest, and by R apiculata, A alba, A of cinalis, and B parviflora in. .. changed in species composition from a dominance of A alba, A of cinalis, and S caseolaris in the fringe forest, to R apiculata, B parviflora, and A alba in the transitional forest, and to R apiculata,... diameter at breast height (DBH) at 1.3 m or above the highest prop root of R apiculata In all sampling Fig Sampling locations in mangrove forest of Mui Ca Mau National Park, Vietnam Mangrove photos