Using Allometric Equations to Estimate Mangrove Biomass and Carbon Stock in Demta Bay, Papua Province, Indonesia

The mangrove ecological services as carbon sinks and storage are very useful in the efforts to mitigate global warming and climate change. In this study, the above and below-ground biomass, carbon stock, as well as carbon sequestration by the mangroves in Demta Bay, Papua Province, Indonesia were estimated. Allometric equations were used to determine the mangrove biomass in 36 observation plots. The biomass value was used to determine carbon stock and estimate carbon sequestration. Nine mangrove species were found in Demta Bay, with the contribution of mangrove species to biomass (AGB and BGB) in the following order: Rhizophora apiculata > Rhizophora mucronata > Bruguiera gymnorhiza > Bruguiera cylindrica > Heritiera Littoralis > Xylocarpus molucensis > Rhizophora stylosa > Avicennia marina > Sonneratia caseolaris. The average mangrove biomass was estimated at 174.20 ± 68.14 t/ha (AGB = 117.62 ± 45.68 t/ha and BGB = 56.58 ± 22.49 t/ha). The carbon stocks in mangroves at the Ambora site were higher than the Tarfia and Yougapsa sites, averaging 123.57 ± 30.49 t C/ha, 81.64 ± 25.29 t C/ha, and 56.09 ± 39.03 t C/ha, respectively. The average carbon stock in the mangrove ecosystem of Demta Bay is estimated at 87.10 ± 34.07 t C/ha or equivalent to 319.37 ± 124.92 t CO2 e/ha. The results of this study indicate that the mangrove ecosystem in Demta Bay stores quite high carbon stocks, so it is necessary to maintain it with sustainable management. Therefore, climate change mitigation is not only done by reducing the carbon emission levels but also needs to be balanced by maintaining the mangrove ecosystem services as carbon sinks and sequestration.


INTRODUCTION
Some of the most important mangrove ecological services for humans and other living creatures are absorption and storage of carbon which is very useful in mitigating global warming and climate change. The carbon stored in the mangrove ecosystems began to show significant economic value after the emergence of carbon markets [Jaikishun et al., 2017]. Mangroves are one of the important "blue carbon" parameters in reducing the effects of greenhouse gases as a mitigation of climate change, because they can reduce CO 2 through the mechanism of carbon sequestration from the atmosphere [Komiyama in the global carbon cycle [Estrada and Soares, 2017;Kauffman et al., 2020].
Geographically, the potential of carbon stocks and the level of carbon sequestration by mangroves varies greatly due to large variations in local factors [Estrada and Soares, 2017;Kusumaningtyas et al., 2019]. The ability of mangroves to absorb and store carbon is very high and greater than that of tropical forests [Donato et al., 2011;Kauffman and Donato, 2012;Murdiyarso et al., 2015]. Globally, the estimates of mean carbon storage in mangrove ecosystems approximate 885 tons C/ha [Kauffman and Bhomia, 2017] to 1.023 tons C/ha [Donato et al., 2011]. The mangrove ecosystems in Indonesia have great potential to absorb carbon from the atmosphere and store it as biomass. This can be seen from the total area of mangroves in Indonesia, which reaches 22.4% of the total area of mangroves in the world [Giri et al., 2011]. Various methods have been developed to estimate and quantify the mangrove biomass. One method that can be used involves the use of allometric equations [Komiyama et al., 2005]. Biomass estimation is done by measuring the diameter at breast height (DBH) which is then used as an input to the allometric equation. Allometric equations have been widely used to estimate mangrove biomass in several studies, especially for estimating the above-and below-ground biomass. Given the importance of the ecological function of mangroves as carbon sinks in nature, the study of estimated biomass, carbon stocks, and carbon sequestration are needed to determine the ability of mangroves in Demta Bay as one of the important environmental components in climate change mitigation. Besides, the data on biomass, carbon stocks, and carbon sequestration in mangrove ecosystems are important to support the management of conservation of mangrove ecosystems. Therefore, this study aimed to estimate and evaluate the potential of above-and belowground biomass, carbon stocks, and carbon sequestration in the mangrove ecosystems in Demta Bay, Papua Province, Indonesia.

Study Site
Demta Bay is located in the Demta District, which is one of the districts located in the northern coastal area of Jayapura Regency, Papua Province, Indonesia ( Figure 1). The coastal area of Demta Bay is quite rich in natural resources, such as mangroves, coral reefs, seagrass beds and fishery resources [Kalor et al., 2019]. The mangrove ecosystem in Demta Bay is unique because it is directly connected to the Pacific Ocean in the north and the tropical forests of Papua Island in the south. The morphological, ecological, and biological conditions in Demta Bay form the main ecosystem zone in the coastal area consisting of coral reef ecosystems, seagrass ecosystems, and mangrove ecosystems. The existence of the man-

Data collection
Non-destructive methods through quadratic sampling techniques used in this study were conducted in June 2020 at three study sites in Demta Bay, namely in the villages of Ambora, Yougapsa, and Tarfia ( Figure 1). Data collection was carried out by stretching a 50 m transect from the edge of the water perpendicular to the mangrove forest (4 transects at each study site). A 10 m × 10 m plot was installed along the transect at 10 m intervals (a total of 3 plots in each transect). Thus, there were 12 observation plots at each station (a total of 36 plots in all stations). All mangrove trees in the observation plots were identified at the species level, counted in number, and measured DBH (at a height of 1.3 m) for estimating above-and belowground biomass. Referring to Abino et al. (2014), an inventory of mangrove species is only for trees with a minimum DBH of 5 cm and it was used to calculate the mangrove biomass. The identification of mangrove species in the observation plot refers to Noor et al. [1999]. A global positioning system (GPS, Garmin 78s) was used to mark the coordinates of each data collection location.

Data analysis
The mangrove carbon stock estimates were determined from the mangrove biomass. The mangrove biomass was obtained with a non-destructive method based on the data from the measurement of DBH. The data was then converted into the above-and below-ground biomass using allometric equations according to mangrove species. In turn, several other mangrove species use a general allometric equation for mangrove species in Southeast Asia developed by Komiyama et al. [2005]. The allometric equations used to determine the above-ground biomass (AGB) were as follows:  Table 1.
The values of the above-and below-ground mangrove biomass were summed to obtain the total biomass for all observation plots. Then, the biomass value was averaged to obtain the mean total of mangrove biomass (t/ha). The carbon content (C) in organic matter is usually 50%, so the carbon stock can be calculated by multiplying the total weight of the biomass by the percentage of carbon content (0.5) [IPCC, 2006].

The characteristics of the mangrove ecosystem in Demta Bay
There are nine species of true mangroves found in Demta Bay (  Figure 2). The number of true mangrove species found in this study was higher than the results of the study at the same location reported by Kalor et al. [2019], where H. Littoralis was not found, but one species of mangrove association Derris trifolia was reported in his study. Major mangroves or true mangroves can form pure stands and release saltwater, so that they can grow in standing water, Note: (+) = was found; (-) = not found.
minor mangroves grow on the edge of mangrove habitats and do not form pure stands, while associated mangroves tend to only grow in terrestrial habitats [Tomlinson, 1986]. The diameter of mangrove trees (DBH) in Demta Bay ranged from 5.09 to 24.50 cm with an average of 8.75 ± 2.78 cm ( Table 2). The composition of DBH mangrove was dominated by DBH 5 to 10 cm as much as 74.50%. Meanwhile, the DBH composition of 10.01 to 15 cm, 15.0 to 20 cm, and 20.01 to 25 cm is 23.17%, 1.44%, and 0.89%, respectively. The mangrove species diversity index (Shannon-Wiener diversity index) ranges from 1.54 to 1.62 which are classified as medium diversity categories [Magguran, 1991] ( Table 2). The mangrove vegetation density level in Demta Bay ranges from 1750.00 to 2991.67 trees/ha ( Table 2). Mangrove vegetation with a density of >1500 trees/ha is classified as very dense, >1000 to <1500 trees/ha are classified as dense, and <1000 trees/ha are classified as rare [Kementerian Negara Lingkungan Hidup, 2004]. On the basis of these criteria, the mangrove density level of Demta Bay is classified as very dense. The level of mangrove density can be maintained by issuing binding regulations for the community so that mangrove destruction is not carried out.

Above-and Below-ground Biomass
The mangrove biomass in Demta Bay ranged from 112.17 ± 29.14 to 247.14 ± 61.28 t/ha, with an average of 174.20 ± 68.14 t/ha (    [Syafruddin et al., 2018]. The difference in the contribution of each mangrove species to biomass in different areas can be related to the peculiarities of the mangrove vegetation structure in each region. Generally, the mangrove biomass is different for each mangrove species, which is strongly influenced by tree diameter, tree height, mangrove density, soil fertility, and sequestration ability [Kusmana et al., 1992]. Sintayehu et al. [2020] added that species richness, diversity index, and functional diversity can have a significant effect on the storage of aboveground carbon, so that it also directly affects the biomass. Besides, the presence of species dominance also affects the carbon storage above the soil surface.

Potential Carbon Stock
The average carbon stock in the mangrove ecosystem in Demta Bay is estimated to be 87.10 ± 34.07 t C/ha (Table 4). These carbon stocks come from the above and below ground carbon stocks averaging 58.81 ± 22.84 t C/ha and 28.29 ± 11.24 t C/ha, respectively. The average above and below ground carbon stock at the Ambora site is higher than the Tarfia and Yougapsa sites estimated at 166.90 ± 42.09 t C/ha and 80.24 ± 18.99 t C/ ha, 109.28 ± 36.18 t C/ha, and 54.01 ± 14.59 t C/ ha, and 76.69 ± 56.28 t C/ha and 35.48 ± 21.81 t C/ ha, respectively. The high carbon stock at the Ambora site can be attributed to the higher density of mangrove trees compared to the other two sites. Besides, although the DBH range at the Ambora site is smaller (5.09 to 16.55 cm), the average is higher (9.41 ± 2.76 cm) than the Yougapsa site (DBH range 5.09 to 24.50 cm, average DBH 8.18 ± 2.79 cm) and Tarfia site (DBH 5.09 to 20.36 cm range, average DBH 8.44 ± 2.65 cm). The differences in environmental settings and conditions can affect the local variations in carbon stock, including the influence of hydrodynamic processes, landform, and vegetation conditions of the mangrove ecosystem (canopy cover and mangrove density) [Hinrichs et al., 2009;Li et al., 2015]. Therefore, disturbed mangrove ecosystems (for example: a large number of mangrove cutting activities) and small tree DBH will result in very low carbon stocks [Kusumaningtyas et al. 2019].  The results of the analysis of the average carbon stock for each mangrove species in Demta Bay based on biomass according to the area was in the following order: Figure 4). The contribution of mangrove species R. apiculata, R. mucronata, B. gymnorhiza, and B. cylindrical at the Ambora site was very high was 57.43 t C/ha, 39.06 t C/ha, 12.88 t C/ha, and 10.57 t C/ ha, respectively. Likewise, at the Tarfia site, the four mangrove species also contributed highly, i.e. 28.70 t C/ha, 13.38 t C/ha, 12.50 t C/ha, and 21.75 t C/ha, respectively. In turn, at the Yougapsa site, B. gymnorhiza, R. apiculata, H. Littoralis, and R. mucronata contributed significantly at 14.82 t C/ha, 14.67 t C/ha, 9.07 t C/ha, and 8.81 t C/ha, respectively. The carbon stock assessment can provide an overview of CO 2 uptake in the air. The mangroves in Demta Bay have a high potential in reducing global warming because of their ability to absorb CO 2 . In this study, the estimated carbon sequestration (CO 2 equivalent) was only calculated from the mangrove biomass with an average of 319.37 t CO 2 e/ha, where the highest carbon uptake at the Ambora location was 453.09 t CO 2 e/ha and the lowest was at the Yougapsa site, reaching 205.66 t CO 2 e/ha (Table 4). This carbon sequestration value is interpreted as an indicative value to highlight the importance of mangrove conservation and the estimation of mangrove ecosystems in the world [Donato et al., 2011;Kauffman et al., 2014]. Climate change and anthropogenic disturbances can impact not only the loss of biodiversity and coastal protection but also the loss of function of mangrove ecosystem services as carbon sequestration and storage [Nellemann et al., 2009]. Mangrove ecosystems need to be conserved as a strategy for climate change mitigation [Pendleton et al., 2012;Murdiyarso et al., 2015]. Thus, it is imperative to emphasize and implement the importance of Reduced Emissions from Deforestation and Degradation (REDD+) as a key and relatively low-cost option for mitigating climate change.

CONCLUSIONS
A total of nine mangrove species are found in Demta Bay and contribute to biomass and carbon stock in the mangrove ecosystem. The contribution of mangrove species to biomass and carbon stock was in the following order: R. apiculata > R. mucronata > B. gymnorhiza > B. cylindrical > H. littoralis > X. molucensis > R. stylosa > A. marina > S. caseolaris. Mangroves in Demta Bay store a fairly high carbon stock, estimated at 87.10 ± 34.07 t C/ha, where the aboveground carbon and belowground carbon contribute 67.52% and 32.48%, respectively. Among the three research sites in Demta Bay, the mangrove ecosystem at the Ambora site was found to store higher carbon because it has a higher mangrove density and an average of DBH than the other two sites. The results of this study have provided an overview of the important contribution of the mangrove ecosystem in Demta Bay to climate change mitigation. Therefore, the climate change mitigation efforts are not only carried out by reducing the level of carbon emissions but also need to be balanced by maintaining the mangrove ecosystem services as carbon storage and sink.