Analysis of mangrove vegetation on Mursala Island

Mangrove forest destruction is a serious problem that is currently occurring in almost all of Indonesia. To ensure the success of mangrove forest rehabilitation and as evaluation material, monitoring the condition of the rehabilitated mangrove ecosystem is very important. The methods used in measuring and observing mangrove vegetation are the path method and the grid line method. Transects were selected using the Purposive Sampling method after conducting an initial survey. The transect locations were chosen to reflect the overall condition of the mangrove forest in the study area. As a result of the research, 30 research plots were built, and 6 types of mangrove species from 3 families were obtained, namely Rhizophoraceae (Rhizophora stylosa, Rhizophora mucronata, Rhizophora apiculata, Bruguiera gymnorhiza), Combretaceae (Lumnitzera racemosa), and Meliaceae (Xylocarpus granatum). Rhizophora stylosa dominates with the highest IVI, namely (118.4), followed by Rhizophora mucronata (78.0) and Rhizophora Apiculata (55.1). This shows that Rhizophora stylosa has the strongest role in the tree community in the study area. At the seedling stage, the diversity index (H’) reached 0.4, indicating a relatively low level of diversity. At the sapling stage, the diversity index increased to 1.0585 and at the tree growth stage, the diversity index was 1.38, indicating a balanced level of diversity.


Introduction
Mangrove ecosystems have an important role in environmental conservation and climate change mitigation [1].Mangroves function as a natural shield that protects beaches from erosion, sediment buildup, and the impact of sea waves [2].Apart from that, mangroves also provide protection from natural disasters such as hurricanes and tsunamis.The biodiversity in mangroves supports environmental sustainability, and this ecosystem also has economic value in terms of forest products, fisheries, and the ecotourism sector.With its main role in maintaining environmental balance and reducing the impact of climate change, mangrove conservation is an important priority in global efforts to protect our planet.Indonesia has the largest mangrove forest in the world with an area of around 4.25 million hectares [3][4][5].However, damage to mangrove forests in Indonesia is also very high, resulting in a reduction in the area of the mangrove ecosystem by 2.15 million hectares from the previous total [4].
Mangrove forest destruction is a serious problem that is currently occurring in almost all of Indonesia.To ensure the success of mangrove forest rehabilitation and as evaluation material, monitoring the condition of the rehabilitated mangrove ecosystem is very important [6].Evaluation of the success of rehabilitation efforts can be done by comparing the vegetation structure between the rehabilitated mangrove area and the natural mangrove ecosystem, especially in terms of biological aspects [6].Under natural conditions, two mangrove ecosystems that have similar environments will have similar mangrove vegetation structures [4,7].Therefore, for comparison in observing the rehabilitation area, the mangrove ecosystem must be in the same area.Damage to the mangrove ecosystem will have an impact on habitat destruction and has the potential to cause the loss of various species of flora and fauna that depend on mangroves.Therefore, the establishment of a marine conservation area such as the KKPD in Central Tapanuli Regency, which includes Tapian Nauli Bay and Mursala Island [8] which has an area of 81,243 Ha. is an important step to anticipate threats to biological resources and the ecological function of ecosystems, as well as maintaining the sustainability of coastal and small island resources in the short and long term.This decision is based on the protection needed for mangrove ecosystems and crucial coastal environments [9].
The status of mangrove biodiversity needs to be communicated to the public, policymakers and business people in the tourism industry.Loss of mangrove biodiversity caused by human activities can be measured through mangrove forest degradation and threats to species.However, this topic is still rarely researched in the context of the sustainability of mangrove ecotourism in Indonesia.Therefore, this research aim to analyze mangrove vegetation on Mursala Island and assess mangrove diversity to support the sustainability of mangrove ecotourism.With a deeper understanding of mangrove biodiversity, it is hoped that it can encourage more effective conservation and management actions.

Method
This research was conducted in September-October 2023 on Mursala Island, Sibolga City, Central Tapanuli Regency, North Sumatra Province, Indonesia (Figure 1).Based on geographical location, Mursala Island is located at the coordinates01°38$15% South Latitude and 98°31$33%East longitude.Tools and materials used in this research are writing tools (pencils, markers, chest boards, rulers), plastic, measuring tape, tape, tally sheet, GPS Garmin GPSMAP 62s, survey location map, camera, machete and Phiband.

Data collection
The methods used in measuring and observing mangrove vegetation are the path method and the grid line method.Transects were selected using the Purposive Sampling method after conducting an initial survey.The transect locations were chosen to reflect the overall condition of the mangrove forest in the study area.Data collection was carried out using the plotted line method.The direction of the transect is chosen perpendicular to the shoreline or coastline.Route selection is based on land cover which is included in the open area zone, middle zone, zone with brackish water rivers, and zone towards land that has fresh water sources.Observations were made at the seedling level, sapling level and tree regeneration.For vegetation analysis, species data was obtained by creating plots measuring 2 m x 2 m for the seedling level, 5 m x 5 m for the sapling level, and 10 m x 10 m for the tree level in 30 plots.

Data analysis
Observed and calculated mangrove data involves several parameters, including Density (D), Relative Density (RD), Dominance (D), Relative Dominance (RDo), Frequency (F), Relative Frequency (RF), and Importance Value Index (IVI).These data were calculated using the formula described in the 1974 work of Mueller-Dombois and Ellenberg.
! # " Where: FR = Relative Frequency KR = Relative Density DR = DominationRelat e. Diversity index To estimate species diversity, Shannon Index analysis is used.
Where: H' = Diversity index ni = number of individuals n = total number The species diversity index can be defined based on the H' value, where if H' > 3, then species diversity is considered high or abundant; if 1 ≤ H' ≤ 3, then species diversity is considered moderate; and if H' < 1, then species diversity is considered small or low.

Forest composition, diversity and conservation status of mangrove trees on Mursala Island, Tapanuli Tengah Regency
Based on the results of the analysis of mangrove vegetation types carried out in the Mursala Island mangrove forest, it was found that there were 6 mangrove species belonging to 3 families, namely Rhizophoraceae (Rhizophora stylosa, Rhizophora mucronata, Rhizophora apiculata, Bruguiera gymnorhiza), Combretaceae (Lumnitzera racemosa), and Meliaceae (Xylocarpus granatum).The results of observations made in the mangrove forests on Mursala Island found more mangroves from the Rhizophoraceae family, namely 3 types.In accordance with the statement of Muhtadi et al, 2016., the appearance of a large number of Rhizophoraceae families in this study can be explained by the fact that the opportunity to find species from this family is greater in each particular location.In addition, substrate conditions in the study area significantly support the growth of species from the Rhizophoraceae family.The substrate found at the research location is sandy mud, which acts as the main growing medium for this family.This condition makes the area an environment that really supports the growth of the Rhizophoraceae family.In areas with muddy substrates, it appears that Rhizophora species are the dominant plants that dominate the landscape.Their existence makes a major contribution to the ecosystem in this environment, strengthening the importance of preserving the mangrove ecosystem which plays a role in maintaining biodiversity and environmental balance in the area [11].The conservation status of the six types of species observed has the conservation status of "LC" (Least Concern) according to the IUCN assessment."Least Concern" status indicates that currently, these species are not at risk of extinction.This is positive news in efforts to preserve nature on Mursala Island.The table shows the growth stages of mangrove trees, which include the growth of 'Tree', "Sapling", and "Seedling".At the tree growth stage, all types of species observed were found, marked with a positive sign (+) indicating that the tree was found in the tree growth stage, as well as saplings and seedlings.However, at the sapling and seedling growth stages, several tree species such as Lumnitzera racemosa and Bruguiera gymnorhiza were only found in the sapling growth stage, while Xylocarpus granatum was found in both growth stages.This data reveals the diversity of mangrove tree species on Mursala Island, which is an important indicator of the health and sustainability of the mangrove ecosystem.Knowing how many species are present at different stages of growth can help research and environmental monitoring on the island.In this study, data on mangrove populations at three growth stages, namely trees, saplings, and seedlings, have been analyzed.It can be seen in Figure 2. At the tree stage, it was found that Rhizophora stylosa was the dominant species with 81 individuals, followed by Rhizophora mucronata (53 individuals) and Rhizophora apiculata (39 individuals).Lumnitzera racemosa had a much lower number of individuals, only 1 individual, while Bruguiera gymnorhiza and Xylocarpus granatum had 29 and 2 individuals respectively.At the sapling stage, Rhizophora mucronata dominates with 84 individuals, making it the most common species found at this stage.Rhizophora stylosa also has an individual number of individuals.Rhizophora apiculata had 19 individuals, while Bruguiera gymnorhiza and Xylocarpus granatum had 3 and 1 individual, respectively.At the seedling stage, Rhizophora stylosa remained dominant with 37 individuals.Rhizophora mucronata has 4 individuals, while Rhizophora apiculata has 6 individuals, and Bruguiera gymnorhiza has 7 individuals.A total of 44 individuals were identified at the seedling stage.These results illustrate the distribution and dominance of mangrove species at various growth stages.

Importance Value Index (IVI)
The magnitude of the IVI value of a species is in line with the extent to which the species controls the community.This is in accordance with [12] statement, that the Importance Value Index (IVI) reflects the role and dominance of a species in the vegetation community.Dominant species are able to utilize their environment more efficiently than other species, as reflected in the highest IVI [12].In this research, the Important Value Index is presented in Table 2.The IVI value of a species is calculated by combining information about density (RD), frequency (RF), and coverage (RDo).The higher the IVI value, the more dominant the species is in the mangrove community, while a lower value indicates a smaller role.In this study, it can be seen that at the tree growth stage, Rhizophora stylosa dominates with the highest IVI, namely (118.4),followed by Rhizophora mucronata (78.0) and Rhizophora apiculata (55.1).This shows that Rhizophora stylosa has the strongest role in the tree community in the study area.Meanwhile, the lowest IVI value at tree level was for the Lumnitzara racemosa tree (0.9).At the sapling level, Rhizophora mucronata was the most dominant with the highest IVI (138.87), while Rhizophora stylosa also remained significant with an IVI of 122.22.Meanwhile, at the seedling level, Rhizophora stylosa again dominated with the highest IVI (121.12),followed by Xylocarpus granatum (24.27).

Diversity Index
Hidayatullah and Eko [13] explained that the higher the diversity index value, the more species there are in an area, and this contributes to ecosystem stability [14].Therefore, the diversity index is an important factor in research on vegetation [15].The diversity index (H') values for mangroves at each growth level at the research location are listed in Table 3.This table depicts the diversity index (H') for various types of mangroves at three different growth stages, namely: seedlings, saplings, and trees.At the seedling stage, the diversity index (H') reached 0.4, indicating a relatively low level of diversity, which may be caused by the dominance of certain mangrove species, especially Rhizophora stylosa.However, at the sapling stage, the diversity index increased to 1.0585, indicating higher diversity with more mangrove species represented in the samples.Meanwhile, at the tree growth stage, the diversity index was 1.38, indicating a balanced level of diversity.Variations in mangrove types in the samples at the tree stage tend to be more even.This diversity index is an important parameter in vegetation research because it provides insight into the diversity of types and ecosystem structure.The higher the H' value, the more diverse the community, which may indicate greater ecosystem stability.These data provide valuable insight into mangrove ecology at various stages of growth.

Conclusion
Mursala Island offers a variety of ecological richness with six species of mangroves from three different families, including Rhizophoraceae, Combretaceae, and Meliaceae.Observations of mangrove populations at the tree, sapling, and seedling growth stages showed consistent dominance by Rhizophora stylosa.In calculating the Important Value Index (IVI), Rhizophora stylosa was the most dominant at all growth stages, with the highest value at the sapling stage (138.87).Although there was a certain dominance at the seedling stage, the Diversity Index (H') value increased at the sapling stage, reaching 1.0585, indicating higher diversity in the mangrove community.The conservation status of these six mangrove species, according to the IUCN, is currently stated as "Least Concern" (LC), indicating their resilience to the risk of extinction.These data provide an in-depth understanding of the distribution, dominance, and diversity of mangrove species, providing an important basis for maintaining a stable ecosystem on Mursala Island.

Figure 1 .
Figure 1.Map of Research Location and Distribution of Mangroves in Musala Island.

Figure 2 .
Figure 2. Number of individuals at each growth stage on Mursala Island.

Table 1 .
List of tree species on Mursala Island, Central Tapanuli Regency along with their conservation status.
Note: LC: Least Concern

Table 2 .
Density, frequency, coverage and important value index (IVI) of mangrove species.

Table 3 .
Mangrove Species Diversity Index on the growth rate of seedlings, saplings and trees