Ecosystem Carbon Stocks of Restored Mangroves and Its Sequestration in Northern Sumatra Coast, Indonesia

The 2, 4, 6, 8, 10, 12 and 30-year restored mangroves were studied through non-destructive method by measuring their stem diameter and tree height. Two allometric references: (a) stem diameter (D 30 ) and (b) combined quadratic stem diameter and tree height (D 302 H) were used to estimate aboveground and belowground vegetation carbon stocks. The soil samples were collected from different depth and analysed in laboratory for soil carbon. The objective of this study was to estimate ecosystem carbon stocks of restored mangroves and its sequestration. The growth of restored mangroves induced an increase of tree biomass and a corresponding increase in vegetation carbon stocks from 3.7 MgC ha -1 at 2-year to 136.8 MgC ha -1 at 30-years. However, soil carbon stocks among restored mangrove plots spread randomly and didn’t follow the sequence of mangrove tree ages. Average (2-30 years) mangrove ecosystem carbon in Northern Sumatra estimated by D 302 H allometry (362.0 MgC ha -1 ) was higher than that estimated by D 30 allometry (344.1 MgC ha -1 ). This higher trend was also followed by its carbon sequestration with values of 41.1 MgCO 2 e ha -1 yr -1 estimated by D 302 H allometry and 31.5 MgCO 2 e ha -1 yr -1 estimated by D 30 allometry. It is concluded that the values of ecosystem carbon stock of restored mangroves and its sequestration estimated by combined quadratic stem diameter and tree height (D 302 H) are higher than if it is estimated by stem diameter (D 30 ). The D 302 H value is higher because this allometry calculates the volume of the tree stem, while D 30 allometry only calculates the surface of the stem. The range values of average ecosystem carbon sequestration of this study may be better used as a reference for Afforestation, Reforestation and Re-vegetation (ARR) verification than what has been used as default/conservative values.


Introduction
The concentration of CO 2 in atmosphere is predicted to rise by 40 ppm (from 370 ppm in 2000 to 410 ppm in 2025 (IPCC, 2001). [1] In his presentation at the World Blue Carbon Conference in Jakarta mentioned that the CO 2 concentration in atmosphere was detected around 400 ppm in 2015 and is predicted to continue increasing to 425 ppm by 2025. The concentration of CO 2 in atmosphere will continue to increase if there is no specific action on global climate change mitigation.
Mangrove ecosystem is the most effective bio-sequestration in absorbing atmospheric CO 2 [2], as well as accumulating its unsaturated vegetation carbon and storing it in sediment for long-term period [3] that has a significant role in reducing green-house gas effects [4]. Compared to the other beach plantations such as: saltmarses, seagrass, macroalgae, coral-reef algae, microphytobenthos dan phytoplankton [5] mangrove is the most productive plant in storing carbon. The high capacity of carbon absorption may be influenced by the characteristic of mangrove trees, tidal pattern and physical factors of estuarines [6] The reduction of CO 2 in the atmosphere through bio-sequestration is important to keep global warming from exceeding 2 o C. One effort to reduce the concentration of CO 2 in the air is to restore degraded mangrove ecosystem on a large scale and with sustainable efforts. The mangrove ecosystem restoration and protection program in Northern Sumatra has been carried out since 2005 and is still ongoing. This activity is not only able to restore ecological and economic functions for the community but is also able to increase the productivity of biomass for carbon reserves and sequestration. To date, 30.75 million seedlings have been planted in 12,300 ha of well-growing mangrove areas with a survival rate of 75 -92% as carbon storage. The restored mangroves have been validated and verified through the Voluntary Carbon Standard (VCS) -Afforestation, Reforestation and Re-vegetation (ARR) scheme. Thus the areas of restored mangroves that absorb excess CO 2 from the atmosphere can be maintained and will not be converted again into other land-uses.
The objective of this study was to estimate ecosystem carbon stocks of serial age restored mangroves and its sequestration that estimated by two different allometries. Identified challenges of this study whether innovation of carbon accounting methodology and accountability value generated by this study are in conformity with international standard; as well as whether the range of serial age carbon stocks and carbon sequestration can be used as a reference for carbon verification rather than the use of default/conservative values.

Study sites
Data for ecosystem carbon sequestration of 2 -30 year restored mangroves were collected from 5 villages in Northern Sumatra coast: Percut, Tanjung Rejo, Pangkalan Siata, Jaring Halus and Pangkalan Gading. Growth parameters (stem diameter and tree heigth) and soil samplings were collected for biomass and carbon estimation (Annex 5).

Biomass and carbon measurement
All 2 -30 year mangrove trees were recorded for species composition. Each tree was measured for stem diameter and tree height through non-destructive method. The stem diameter 30 cm above the highest prop-roots (D 30 ) was recorded in which each multi-stemmed tree was treated individually. The height of each tree from land-base to the top of canopy was also measured. The aboveground and belowground biomass of each mangrove species were estimated by two allometric equations based on diameter (D 30 ) and quadratic diameter-tree height (D 30 2 H) variables (Annex 7). Then, the data on aboveground biomass (AGB, kg) representing each tree age were calibrated into carbon stocks (AGC, MgC ha -1 ).

Soil samplings
Five soil sampling plots were established at non-destructive plots, as seen in Annex 3. Soil samples were collected using soil auger at the 0 -15 cm; 15 -30 cm; 30 -50 cm; 50 -100 cm and 100 -200 cm soil depth. Each soil carbon sampling was accompanied by measurement of its parent material. Soil sample (30 -50 g) was taken at a thickness of 5 cm in the soil column at the mid-range estimate of each depth of soil (an example: 5-10 cm column range representing a depth range of 0-15 cm). Data on physical-chemical sediment factors (soil pH, soil temperature, sediment redox and soil texture) were collected from each plot.
Each soil sample was placed on an aluminum bowl then it was labeled with sampling location, plot code, soil depth, date and other related information. The collected sample was then dried in an oven at 600°C for 48 hours to keep the mass constant and then sealed in a vacuum plastic to slow down microbial activity. Each dried soil sample was analysed at the laboratory for the determination of soil density and carbon content analysis. The mangrove soil carbon sequestration was estimated by clustering method and the result was compared with other findings.
All data of soil carbon, vegetation carbon stocks, and ecosystem carbon sequestration were calculated using Excel software package. Linear and multiple regressions were established for further data analysis. Comparisons between mean data (p<0.05) were analysed using the Excel Statistical package. Ecosystem carbon sequestration of restored mangroves (MgCO 2 e ha -1 yr -1 ) was calculated by summing up biomass-and soil carbon sequestration.

Species composition
There were 9 species found in the study sites (Table 1). Those multi-species were able to perform a good vegetation composition as an ecosystem, as part of 23 species growing naturally in mangrove ecosystem of Northern Sumatra coast.
Rhizophora apiculata and R. mucronata were dominant because of used for restoration program. Both species are easy and adaptive to grow at local environment.

Growth parameters
The growth of restored mangroves increased significantly (P<0,001) with an increase of stem diameter, tree height and basal area ( Figure 1). Average stem diameter of 2-30 year restored mangroves was 6.0 cm with an increase of 0.5 cm yr -1 (y = 0.4083x + 2.3432). The growth of stem diameter in this study was higher than Rhizophora mangle stem (0.33 cm yr -1 ) growing at exposed sea tides in Braganca Peninsula, Northern Brazil [7] in which the stem diameter growth was only 0.12 cm yr -1 when a monthly rainfall <50 mm. This evidence shows that mangrove growth is not only influenced by abiotic factors and competition among individuals and species but it is also influenced by local climate factors.
In general, the tree high growth of restored mangroves increased along with an increase in plant life. The average tree height of 2-30 years was 8.32 m with an increase rate of 0.94 m yr -1 . Stated that the high growth of mangrove forest in Puerto Rico was inversely related to an increase of sediment salinity through allometric equation: y = -0.2x + 16.58 (R 2 = 0.72) and no mangrove can live in salinity 70-80 ppt [8]. It seems that vegetation structure and plant growth parameters (tree density, stem diameter and tree height) were strongly influenced by salinity level of mangrove sediment. The stem diameter and tree height increased as an increase of tree age. While basal area varied among individual tree and didn't follow rules of tree age. The average basal area of 2-30 year trees was 26.5 m 2 ha -1 with an increase of 2.6 m 2 ha -1 yr -1 (y = 4.0107x -14.707). The average increase of basal area in this study was higher than the increase of individual Sonneratia alba (7.0 to 79.6 m 2 ha -1 yr -1 ) and Bruguiera gymnorhiza (4.8 -27.4 m 2 ha -1 yr -1 ) growing at mangrove forests in Micronesia [9].
Basal area increased from 1.6 m 2 ha -1 at 2-year to 110.8 m 2 ha -1 at 30-year old. The basal area of 30 year R. apiculata (110.8 m 2 ha -1 ) in this study was much larger than the basal area of the same species living at natural mangrove forest in Maluku -Eastern Indonesia 23.97 m 2 ha -1 [10], in Ranong -Southern Thailand 23.97 m 2 ha -1 [11] and in Talidendang Riau -East Sumatra 29.5 m 2 ha -1 [12]. However, the increase trend of basal area of this study was in opposite with basal area increment of Rhizophora apiculata forests in the Mekong delta, Vietnam [13]. The differences in average basal area in specific sites may be significantly influenced by the hydro-geomorphological zone where the plant grows.
The shoot/root ratio of restored mangroves in this study was inversely proportional to the growth parametes. An average increase of tree age decreased 6% shoot/root ratio (S/R ratio) from 1.29 at 2 years; 1.16 at 4 years; 1.05 at 6 years; 0.96 at 8 years; 0.85 at 10 years; 0.78 at 12 years; and 0.74 at 30 years. This finding showed that mangrove rooting system was much more intensive and expansive when the trees grow up. This expansive rooting occured when the trees become older that makes easier taking up soil nutrients with its extensive rooting systems.

Vegetation carbon stocks
In principle, mangrove ecosystem is an effective carbon accumulation machine [14]. Measuring an ability of restored mangrove ecosystem in absorbing and storing carbon is the core of this study. The carbon is stored in plant components and sediments. However, part of vegetation carbon will be gone due to decomposed deadwoods and litters as a food chain of fauna or flowing to the sea through tidal pattern.
Average (2-30 years) vegetation carbon stock estimated by D 30 allometry of this study (57.5 MgC ha -1 ) was smaller than [19] findings at several natural mangrove forest in Indonesia: Bunaken National Park (69.2 MgC ha -1 ), Kubu Raya (134.8 MgC ha -1 ), Tanjung Puting National Park (140.9 MgC ha -1 ), Teminabuan (196.3 MgC ha -1 ), Timika (255.1 MgC ha -1 ), Sembilang National Park (300.5 MgC ha -1 ) and Bintuni Bay (323.6 MgC ha -1 ), but it was higher than the mangrove carbon stock in Cilacap (6.9 MgC ha -1 ). The significant different may be due to the referenced carbon stocks collected from older age of natural mangrove forests, while the average stock in this study was to include the young planted mangroves starting from 2-year old. The differences of carbon stocks in different locations may be influenced by different mangrove age, soil fertility, individual tree growth, species, biotic and abiotic factors, and local climate between restored mangroves and natural mangrove forests.
The increase trend of mangrove carbon biomass until 30-year old in this study was consistent with [6] modified from [20] that the biomass of Rhizophora apiculata in the Mekong delta -Vietnam still increases until 40-year old. Moreover [6] modified from [21] also mentioned the development of mangrove ecosystem along the French Guiana can effectively accumulate carbon until 70-year old mangroves in which their growth is divided into 3 stages: early development (0-15 years), maturity (15-70 years) and senescence (after 70 years).
Difference in mean value of vegetation carbon stocks between D 30 2 H allometry and D 30 allometry at 2-30 years was 17.7 MgC ha -1 (22.9%). It means that carbon estimation using D 30 allometry can be used as a conservative value and the stcok of D 30 2 H allometry will be as a maximum estimation. This conservative value will be a strenght of D 30 allometry that will not be over-prediction in estimating carbon stocks on specific site. However, the vegetation carbon stocks estimated by D 30 and D 30 2 H allometries need to be compared with the stocks calculated by the destructive study. The comparison values between both calculation methods will figure the accuracy level to the real condition in the field.

Vegetation carbon lost
Mangrove ecosystem as a CO 2 reservoir is able to accumulate and store carbon, but it will be released into atmosphere if the vegetation is cut or cleared. The vegetation carbon stocks in this study are divided into ideal-and actual conditions ( Table 2). Ideal condition means that the vegetation carbon stocks from undisturbed tree biomass are considered intact. The actual condition is disrupted, indicating that carbon stocks from tree biomass are impaired. The actual tree conditions being sampled in the plots were in disturbed condition. The disturbance was indicated by the presence of standing trees but some parts of tree components such as branches, twigs, leaves/ fruits/flowers or proof-roots were lost. Average vegetation carbon loss of restored mangroves estimated by D 30 allometry (1.7 MgC ha -1 ) was higher than carbon lost estimated by D 30 2 H allometry (1.4 MgC ha -1 ). Loss of carbon stocks of restored mangroves increased with age. Field observation showed that older trees tended to be more vulnerable for community use. Access to restored mangroves is one of important factors of losing carbon where the mangroves within walking distance are fragile from illegal cutting. Loss of carbon stocks due to natural factors and community actions on mangroves cannot be avoided, but it can be minimized through awareness program and law-enforcement.
The extent of mangrove forest ecosystem in some countries is reduced by almost 50% due to human influences, in particular due to deforestation and land conversion [22]. The impact of mangrove forest loss is declining ability of ecosystem to sequester carbon. When the mangrove ecosystem is gone or degraded, the organic carbon materials stored for decades or even thousand years will be released back on the form of CO 2 emissions into atmosphere.
Mangrove ecosystem can contribute to climate change in the case of increasing atmospheric CO 2 concentration if the ecosystem is converted to other land-use. Estimated that loss of 35% of the world's mangrove ecosystems will result in carbon emissions of 3.8 x 1014 gC of vegetation carbon stored in above-ground biomass. This figure was considered to be below the forecast because it has not included total sub-surface biomass and tree canopy [22].
Globally deforestation mangrove forests contributed to emissions of 0.02 -0.12 PgC yr -1 or about 10% of global deforestation emissions [18]. Conversion of mangrove ecosystem into farmland in Malaysia produced 75 MgC ha -1 yr -1 soil carbon emissions from hundred years of sediment accumulation into the atmosphere, which is equivalent to 50 times average soil carbon sequestration [18]. CO 2 emissions from mangrove ecosystem also occur in Indonesia because the deforestation rate is very fast and ranks on the top in the world. Mangrove degradation in Indonesia was caused by conversion and land-use change for intensive fish-and shrimp ponds; tree cutting for charcoal, construction and settlement as well as other commercial activities. Over 2 million hectares of mangrove ecosystem in Indonesia have been damaged in the last 50 years.
In addition, preliminary study of [23] showed average aboveground carbon sequestration at Percut natural mangrove forest was 9.98 MgC ha -1 yr -1 . If the data were converted to the total loss of 69,304 ha mangroves in North Sumatera coast, the mangrove deforestation in this province contributed 691,340 MgC yr -1 or equivalent to 2,537,219 MgCO 2 e yr -1 carbon emissions to atmosphere. The carbon stocks in study sites will decrease drastically if there are no mangrove restoration and protection efforts. Assuming mangrove degradation of 4% per year, the ecosystem carbon stocks will decrease by 30% for 30 years. The response to carbon losses is to increase mitigation efforts that involve local communities in managing mangrove ecosystem.

Soil carbon stocks
Mangroves have stilt-or prop-roots system that can stick into sediment and allow oxygen entering into the submerged roots. Such complex rooting structures can slow down the flow of water at high tides that cause water-borne materials to settle on the sediment surface [22] Therefore the soil carbon stocks are not only derived from litters and deadwood components of mangrove plant itself but also come from outside mangrove ecosystem in the form of particles trapped by root system.
The maximum soil depth varied and significantly different among the plots, such as 267 cm at 4-year plot and 300 cm at 2-and 10-year plots (Annex 8). The soil density of 30-year plot (0.96; 0.92; 0.97; 0.93 dan 0.99 g cm -3 ) didn't follow the level of soil depth. The pattern of SD variation in this study was similar to [24] finding but its range (0.92 -0.97 g cm -3 ) was much higher than soil density of natural mangrove forests in Palau (0.20 -0.25 g cm -3 ) and in Micronesia (0.37 -0.51 g cm -3 ).
The range of average soil density in this study (0.71 -1.17 g cm -3 ) was higher than [19] findings at several natural mangrove forests in Indonesia (0.28 -0.76 g cm -3 ). Average C-content in this study (5.32%) was also smaller than the mean C-content (9.60%) at several natural mangrove forests in Sembilang, Cilacap, Kubu Raya, Tanjung Putting, Bunaken, Teminabuan, Bintuni and Timika [19]. Average mangrove soil carbon stocks increased according to the soil depth (Table 3). This indicated that the soil carbon stocks didn't increase according to the growth of decomposed litters and deadwoods from sediment surface. Litters and accumulated necromasses were degraded by micro-organisms through aerobic and anaerobic respiration processes [25]. The magnitude of soil carbon stocks is more influenced by the ability of sediments to store carbon through soil evolution of allochthonous sedimentation process rather than by biomass decomposition. It was proven that the soil carbon stocks at a depth of >100 cm were much larger than the stocks at soil surface due to their stable condition without being influenced by tidal patterns.
Soil carbon stocks at all depths differed significantly (P<0.01) across all plots of different age groups. The average of soil carbon stocks of 6-year plots (618.0 MgC ha -1 ) was significantly much higher than the 30-year plots (231.5 MgC ha -1 ). The smallest soil carbon stock was located at 2-year plot (80.1 MgC ha -1 ) and the largest was at 6-year plot (618.0 MgC ha -1 ). This indicated that soil carbon stocks were more influenced by historical factors of soil carbon accumulation and sedimentation ability of mangrove ecosystem rather than by decomposition process of tree biomass. Mangrove root production significantly affects soil composition rather than litter fall decomposition [26]. Average sediment accretion rates of mangrove sediments worldwide were 5.0 mm yr -1 [13].
The average (2-30 years) soil carbon stock in this study (285.5 MgC ha -1 ) was smaller than Murdiyarso et al.

Ecosystem carbon stocks
Mangrove ecosystem storing CO 2 from biomass of plant components is called vegetation carbon stock. While, the decomposed biomass and deposited soil in sediment is called organic carbon soil. The carbon stocks of mangrove ecosystem represent total amount of vegetation carbon that includes dead woods and litters, and organic soil carbon ( Figure 2).
Mangrove ecosystem is able to store larger soil carbon than those stored in vegetation. The estimated carbon stock by two allometries varied greatly and didn't follow the tree age-increasing pattern. The differences of ecosystem carbon stocks among plots were more determined by 62.9 -95.6.9% of soil carbon stocks. The percentage of soil carbon decreased when the mangroves growing older from 95.6% at 2-year plot to 62.9% at 30-year plot (Annex 14). The older trees can produce more percentage of vegetation carbon stocks because of increasing its growth parameters. However, the 6-year plot (96.9%) and 12-year plot (85.2%) didn't follow the age plot pattern because those plots consist of higher organic soil stocks compared to their vegetation carbon stocks. In actual condition, the average ecosystem carbon stocks estimated by D 30 allometry (344.1 MgC ha -1 ) was lower than the stocks estimated by D 30 2 H allometry (362.0 MgC ha -1 ), with the differences of 5.2%. The carbon stocks in this study were on the range value of carbon stocks (50 to 800 MgC ha -1 ) in Zambesi river delta, Mozambique reported by [13]; in Mahajamba Bay, Madagascar [27] in Kenya [27]; and at Sofala bay [28] In fact, the stocks of this study (344.1 -362.0 MgC ha -1 ) were lower than the ecosystem carbon stock of in Geza mangrove forest (414.6 MgC ha -1 ) and Mtimbwani (684.9 MgC ha -1 ) -Tanzania where their soil carbon content reached 65% [29]. Furthermore, the carbon in this study was far below the value of ecosystem carbon stocks in Indo-West Pacific that varied from 830 to 1,131 MgC ha -1 [29]; and 18-year R. apiculata (1,117 MgC ha -1 ) in Peninsular Malaysia, 6-year R. apiculata (1,179 MgC ha -1 ) in Southern Vietnam, 25-year R. apiculata (808 MgC ha -1 ) and 3-year of Ceriops decandra (600 MgC ha -1) in Southern Thailand, Kandelia kandel (619 MgC ha -1 ) in Southern China, and Rhizophora stylosa (863 MgC ha -1 ) and Avicennia marina (662 MgC ha -1 ) in Western Australia [29]. Differences of inter-site carbon stocks are due to differences in plant age, vegetation structure, species composition, individual species capability, sediment conditions and local climate of each location.

Soil carbon sequestration
The average sequestration of stored organic soil carbon (organic carbon burial) in the sediment is ideally calculated by the measurement time interval of soil carbon evolution [29]. Due to very long period in collecting sequence data of soil evolution, the clustered/grouped approach was applied in estimating the carbon sequestration rate in this study. The soil carbon sequestration was estimated by grouped areas where have similar soil and environmental conditions, and was not based on the plant age groups.
The sequestration regression of soil carbon based on in Sei Meran and Tanjung Rejo clustering system was Y = 9.34 * X ^ -0.188 (Annex 17). The average soil carbon sequestration of this study was 1.81 ± 0.18 MgC ha -1 yr -1 or equivalent to 6.62 ± 0.66 MgCO 2 e ha -1 yr -1 . The sequestration value of this clustered soil carbon was the same to [30] finding (1.81 MgC ha -1 yr -1) on mangrove forest in Australia.

Ecosystem carbon sequestration
Mangrove ecosystem provides benefits of environmental services in the form of carbon sequestration [31] which is essential to control the concentration of greenhouse gas emissions. The ecosystem is able to accumulate carbon without saturation and store it in the sediment [31]. The process of accumulating C in leaves, fruits, flowers, stems, branches, twigs, stumps and proop-roots is called sequestration process of carbon. This sequestration involves carbon capture and storage of carbon dioxide (CO 2 ) for a long time [31]. The carbon sequestration can be derived from the ability of the mangrove tree themselves and due to support of surrounding environmental factors such as tidal patterns and physical factors of estuary area ( [32]; [6]). The carbon sequestration of restored mangroves estimated by two different allometries can be seen in Figure 4.  Average carbon stocks of 2-30 year restored mangroves estimated by both allometries ranged from 344.1 to 362.0 MgC ha -1 . These values are smaller than the average carbon stock of mangrove forest ecosystems in the tropics (1,023 MgC ha -1 ) where the soil carbon stocks varied between 49 -98% at 0.5 -3 m soil depth [18].
In addition, the average (12-30 years) ecosystem carbon stock estimated by diameter variable of this study (485.3 MgC ha -1 ) is smaller than [19] finding at several natural mangrove forests in Indonesia: Cilacap (592.8 MgC ha -1 ), Kubu Raya (794,2 MgC ha -1 ), Teminabuan (910.9 MgC ha -1 ), Bunaken National Park (938.5 MgC ha -1 ), Tanjung Puting National Park (1,240.0 MgC ha -1 ), Timika (1,275.2 MgC ha -1 ), Sembilang National Park (1,319.1 MgC ha -1 ) and Bintuni Bay (1.396 MgC ha -1 ). The referenced ecosystem carbon stocks were from soil carbon stocks, while the soil carbon in this study was much lower than the referenced stocks. An average ecosystem carbon of natural mangrove forests in the tropics was 1,023 MgC ha -1 that contains 49-98% organic soil carbon [19]. The differences of carbon stocks in different locations may be influenced by soil carbon component, different mangrove age, soil fertility, individual tree growth, species, biotic and abiotic factors, and local climate between restored mangroves and natural mangrove forests.
The average ecosystem carbon sequestration had consistently increased considerably with age. The carbon sequestration estimated by D 30 allometry for an average age of 2-12 years; 2-30 years and 12-30 years were respectively 29.5; 31.5 and 37.7 MgCO 2 e ha -1 yr -1 , while the average carbon sequesration estimated by D 30 2 H alometry was 36.5; 41.1 and 55.1 MgCO 2 e ha -1 yr -1 . Two alometries show similar trend that the highest carbon production rate of mangroves when they grow after 12 year old. The range of carbon sequestration of this study was higher than [21] finding on carbon sequestration in mangrove ecosystem of Mumbai-India (12.08 MgCO 2 e ha -1 yr -1 ). [33] mentioned that the biomass of Rhizophora apiculata in the Mekong delta, Vietnam still increase until 40-year old. Moreover, mangroves at French Guiana effectively accumulated carbon from 15 to 70-year old [34] modified from [21]. Therefore, it is important to conduct more study whether the restored mangroves in the study sites can still effectively sequester carbon after 30 year old.
Carbon production of mangrove ecosystem is determined by the rates of primary productivity and decomposition. An ability of mangrove ecosystem to sequester carbon is influenced by respiration time. The high rate of carbon sequestration occurred during spring tides rather than neap tides. The ecosystem carbon production is strongly influenced by climate factors such as temperature and precipitation [35]. Therefore, the design of mangrove restoration program should be based on carbon cyclus model and should consider tidal pattern in anticipating sea rise effects in the future [36].
To prevent the decline in ecosystem carbon sequestration, it is necessary to conduct tree thinning or pruning efforts starting from 8 years old. These actions will give more space to the trees who live on the middle site to get enough sunlight for their photosynthetic process. Storing CO 2 in biomass through photosynthetic process will be a carbon source for mangrove ecosystem. In addition, it is important to let the restored mangroves be able to make their natural succession and enrichment. So that carbon sequestration can increase along with the growth of plant life due to enough space and species variation.
The comparison of sequestration values indicates that: (a) the carbon sequestration value of the ecosystem tends to increase with tree age; and (b) the sequestration value estimated by D 30 2 H alometry is relatively greater compared to the sequestration value estimated by D 30 allometry. The use of D 30 2 H alometry is also performed by [37] for carbon sequestration of Kandelia obovata in Okinawa -Japan and [37] for aboveground biomass production of Rhizophora apiculata Blume in Sarawak mangrove forest. For carbon validation and verification purposes, it is recommended that mangrove carbon calculation use D 30 2 H alometry because this calculation includes a tree high element in estimating vegetation carbon stocks.
Ecosystem carbon sequestration at 2-12 years and 2-30 years can be used as reference for proposing carbon credits Afforestation, Reforestation and Re-vegetation (ARR) scheme since the credit value is calculated since 2-year mangroves begin to sequester carbon. While carbon stocks and carbon sequestration at 12-30 years are used for certifying carbon credits Reducing Emissions from Defrorestation and Degradation (REDD) or ARR schemes when the carbon sequestration capabilities are calculated since the mangroves have grown from 12 year old.
The goal of carbon credit program is to prevent carbon emissions from permanently changing mangrove ecosystem cover to permanent non-mangrove vegetation. The program is expected to slow the rate of deforestation and degradation of existing mangrove ecosystems for long-term period in which [37] confirm that Rhizophora mangle mangrove species can continue growing until age around 250 years. Updating annual data is important being carried out in the study sites for monitoring an increase or decrease of mangrove carbon stocks and its sequestration in the future. It is also essential to build a better understanding on how the role of mangrove ecosystem in stabilizing CO 2 in the atmosphere by conducting more study on how mangrove plants and soils absorb, store, and release carbon, also how these activities will affect the carbon cycle. mangrove carbon project. We also thank to the Yagasu field team and volunteers from various university students in Medan that involve on the field data collection. We appreciate to the RISPA laboratory -Medan, IPB Soil Biotechnology -Bogor who allow us to use their laboratory for carbon fraction analysis. The special thanks are given to the research partners (Rahayu Subekti, Melanton Haloho, Hasri Abdilah, Rangga Bayu Basuki, Dany Saragih and Ricky Stiawan) who assist in providing secondary data, mapping study sites, drawing illustrations and analysing statictical data. We also appreciate and thank to Prof. Zoelkifli Nasution and Nirmal Beura for proof-reading and critical review of this manuscript.