Carbon stock estimation of Sonneratia alba in mangrove restoration area in Pasar Rawa, Langkat North Sumatera

Mangrove forests must be protected because they are an important ecosystem for climate mitigation. Mangrove forests are one of the “blue carbon” ecosystems capable of absorbing significantly more carbon than terrestrial forests. Mangrove restoration is being carried out in various regions as part of an effort to rehabilitate damaged ecosystems, one of which being Pasar Rawa Village in Langkat Regency. Sonneratia alba is a native plant that is employed in restoration activities. The purpose of this study is to investigate carbon stock in the S. alba monoculture restoration area in Pasar Rawa Village, Langkat Regency, North Sumatra Province. Purposive sampling was employed in this study, with plots distributed evenly around the research area. Carbon stock estimation is performed non-destructively by estimating carbon estimates based on diameter at breast height (DBH) and total height of trees. According to the research findings, the total carbon stored in the Sonneratia alba restoration area is quite low, at 60.89 ton.ha−1. S. alba produced 192.13 ton.ha−1 of absorbed carbon with CO2 absorption of 19212.84 g/m2 and so was classified as low.


Introduction
North Sumatra has an extensive mangrove area, which is distributed over various districts, including Langkat Regency.North Sumatra's wide mangrove habitat is not resistant to destruction and land change.Mangrove land had a noticed extent of 34,742.12ha in 1990, but this has decreased to 16,765.96ha in 2015.The careless conversion of land into oil palm plantations and aquaculture was responsible for this decline in area [1].Conservation-based management is being applied in an effort to restore lost and damaged mangrove regions, however restoration is currently producing uneven regeneration outcomes.This is due to a variety of conditions, including being performed in a short period of time [2] and being carried in an improper habitat [3].Various initiatives have been attempted to reduce restoration failure, including the implementation of a monoculture restoration system with local species.Pasar Rawa Village is one of the restoration areas where one species is planted with the native species Sonneratia alba.This region has been undergoing restoration for the past six years.The mangrove ecosystem, as a blue carbon area, is known for its potential to reduce CO2 by absorbing carbon from the atmosphere.Mangroves have a much higher capacity to retain and absorb carbon than tropical forest ecosystems.Despite their limited distribution, particularly in tropical and subtropical, mangroves can store three times more carbon than other ecosystems such as seagrass beds, mudflats and sand dunes [4].As a result, mangroves are believed to be prepared for absorbing and storing significant amounts of carbon in the global carbon cycle Because information on carbon stocks in the Pasar Rawa restoration region has not been gathered, this research focuses on Pasar Rawa Village, which has undertaken numerous restoration initiatives, including the use of native species, notably Sonneratia alba.

Time and research location
The research was conducted out between May and July 2022 at Pasar Rawa Village, Langkat Regency, North Sumatra province.Carbon was estimated on mangrove restoration area dominated by the Sonneratia alba species and located on river banks with direct tidal influence.At the highest tide, the water flows 2 to 3 meters.The research site has a slightly deep mud substrate.

Research methods
To assess carbon on restored area dominated by the Sonneratia alba species, measurements of stem diameter at 1.3 meters (DBH) on trees with a minimum diameter of 5 cm were taken.The presence of trees is determined using plots established using a purposive sampling strategy over the observation region.On a predefined path, a 10m × 10m plot is constructed.Figure 1 depicts the observation plot design in greater detail.Aside from the diameter of each tree observed, the projected total height of each tree encountered is also recorded as supporting data.

Data analysis
In this study, equations based on [5] were employed to calculate the basal area.The allometric equation referred to [6] was used to calculate tree basal area.Calculations are carried out to obtain biomass while taking into account the species encountered, with each species having a different allometric calculation.The estimated biomass of Sonneratia alba, being the dominant species, was determined using the general equation developed by [7], which is as follows.: W top = 0.251 pD 2.46 r 2 = 0.98, n = 104, D max = 49 cm (ABG (Above ground biomass (kg)) (1) WR = 0.199 p 0.899 D 2.22 r 2 = 0.95, n = 26, D max = 45 cm (BGB (Below Ground biomass (kg)) Where, W top is the estimated total weight of a tree on the ground expressed by the trunk diameter (D).Meanwhile, WR is the estimated weight of the tree underground in kg.r 2 is the coefficient of determination which explains how far dependent data can be explained by independent data which has a value range of 0 to 1. N is the number of samples while D max is the largest diameter and Ro (P) is the density.
Following the determination of the biomass value, the value conversion is performed to determine the carbon value in the desired units.In the meantime, carbon uptake estimates were carried out using the method referred to in [6].Biomass conversion was carried out by referring to [7].

Basal area
Several studies have found an important correlation between carbon stocks and tree height [8] and basal area [9].According to the research findings, there are 5 (five) true mangrove species on the restoration land, which is dominated by Sonneratia alba, with varying basal area values (Table 1).According to the research findings, S. alba is the species with the largest basal area value, which is 352.15.The high basal area obtained is assumed to be mainly due to the large number of individuals.The dominance of S. alba at the research location shows its success as a species planted in a restoration program 7 years ago.The ability of this species to adapt to its surroundings undoubtedly contributes to its success in growing and developing.According to [10], S. alba is a plant with a particular morphology that allows it to survive in high salinity.
Aside from S. alba, several more species can be discovered at the research site.Even though the numbers are small, the appearance of multiple species at the research site indicates the formation of a complex ecosystem.The results revealed that Scyphyphora hydrophylacea produced the highest basal area value, followed by S. alba.This species' high basal area value suggests that it is beginning to grow and develop.Given that this species is not among those planted in the restoration program, the existence of this and several other species may be due to reasonably good seed dispersal supported by natural mechanisms such as natural propagule dispersal by water.

Biomass and carbon stock
The allometric equation is used in this study to calculate tree diameter.According to the research findings, above ground biomass contributes a higher value to the total biomass obtained of 43833.60 kg than below ground biomass (Table 2).According to [11], the majority of carbon stocks are contained in tree biomass above the ground surface.The large value of biomass found above ground cannot be separated from the numerous components that store carbon in organic form from photosynthetic results.According to the IPCC [12], aboveground biomass includes primary stems, twigs, leaves, branches, fruit, and flowers.
According to the research findings, the biomass value obtained in restoration regions dominated by the S. alba species is often low.Furthermore [13] reported that the total biomass value in the S. alba restoration area was 327.26 ton.ha -1 .There are numerous elements that influence biomass output in a given area.The biomass value gained is assumed to be mainly affected by the number of individuals, age, stem size, and area.Almulqu and Kleruk [14] stated the same thing, that biomass value is determined by numerous parameters such as wood species, stem diameter, and the quantity of individuals in an area.
In terms of biomass, the carbon absorbed in plants can be assessed by measuring the biomass of standing vegetation, which can then reflect the vegetation's local and regional carbon value [15].According to table 3, the total carbon sequestered at the research site is 60.89 tons.ha - .As with biomass, the results of the investigation demonstrate that carbon above the ground surface has a higher value than carbon below the ground surface.Carbon stored in tree trunks is assumed to be the primary cause of this.Surface carbon calculations, which are typically based on trunk diameter measurements, have a significant impact on total surface carbon in the soil, as trees with bigger diameters will also have larger carbon stocks.
According to the findings, the total carbon stock acquired in this study is classified into low.In contrary to the results obtained, [13] in the S. alba-dominated restoration area resulted in a total carbon increase of 153.81 ton.ha -1 .Meanwhile, research conducted in Bali by [16] revealed that total carbon recovery was 131.05 ton.ha -1 and carbon stock was 70.72 ton.ha -1 .The elements most responsible for the very considerable disparities in carbon stocks are assumed to be plant age and plant quantity.Plants in very young restoration zones (about 7 years old) are typically tiny.This has an impact on the diameter of the last stem.Aside from that, the number of individual plants that are not too close together also adds to the production of a low carbon stock.This was also mentioned by [13], who noted that growing biomass increases carbon storage, which is influenced by the number of individual trees and stem diameter.

Carbon Dioxide (CO2) uptake
Plants collect CO2 from the air during photosynthesis, which is transformed into carbohydrates and travels throughout the plant body, eventually accumulating within the plant [11].Carbon absorption is the first step in the formation of carbon reserves in the plant body.According to the research findings, the S. alba species had the largest carbon uptake, namely 192.13 ton.ha -1 (Table 4).Carbon is absorbed and stored in the body of Sonneratia alba, particularly in the main stem, therefore the larger the tree diameter, the greater the biomass storage resulting from carbon conversion.Sonneratia alba is a pioneer plant that is known for its strong vitality and resilience to excessive salinity and high tidal waves [18].If the restoration area is not disturbed, the plants will develop well, allowing them to absorb a large amount of carbon dioxide (CO2) [19].According to the research findings, the total CO2 absorption at the research site was calculated to be 223.43ton.ha -1 (Table 4).This finding is significantly higher than the carbon uptake results at the research site.Calculate the potential absorption of carbon dioxide (CO2) gas according to [13] by multiplying the proportion of carbon by the quantity of carbon dioxide (CO2) absorption.The density of mangrove trees impacts biomass, amount of carbon, and their capability for absorption CO2.The biomass value of a mangrove stand grows in proportion to its density.This impacts both the amount for carbon content and its capacity for absorption carbon.This has an impact on the amount of carbon content and carbon absorption capacity which also occurs in various restoration locations, such as in the A. Marina restoration area as explained by Harefa [20].

Conclusion
The overall amount of carbon stored in the Sonneratia alba restoration area in Pasar Rawa Village, Langkat Regency is slightly low, at 60.89 tons.ha - .S. alba produces 192.13 ton.ha -1 of absorbed carbon with CO2 absorption of 19212.84g/m 2 and is classed as low.Low carbon stocks were caused by restoration actions done out 6-7 years ago in the Sonneratia alba restoration region in Pasar Rawa Village, Langkat Regency.

Figure 1 .
Figure 1.Design of research plot

Table 1 .
Basal area value of mangrove species in Pasar Rawa Village.

Table 2 .
Carbon stock estimation in research area.

Table 3 .
Carbon dioxide (CO2) absorption of the restoration forest of Pasar Rawa village.