Species composition and mangrove forest structure in Buano Island, Moluccas

Mangrove forests are protected areas that play an important role for coastal communities on small islands. This study aims to convey data and information about the structure and composition of mangrove species on Buano Island. The Importance Value Index (INP) obtained from vegetation analysis is used as an indicator to describe the composition and structure of mangrove forests. The plot-assisted path method is used in vegetation analysis. 9 illustrative paths with a width of 200 m were created which were divided into 9 segments, there were 54 plots to obtain data and types of mangroves. The research results showed that there were 12 types of mangrove trees and forest regeneration (Heritiera littoralis, Rhizopora Mucronata, Rhizopora stylosa, Rhizopora apiculata, Ceriops Tagal, Sonneratia alba, Bruguiera gymnorrhiza, Lumnitzera racemose, Aegiceras corniculatum, Deriss trifoliata, Nypa fruticans, Xylocarpus granatum). The dominant species is R. mucronata. The vegetation structure at both locations also has a similar pattern the same, where the vertical structure has 2 layers/stratum of trees and both are dominated by trees with a height of 6-10 meters. Meanwhile, for the horizontal structure, the distribution of diameter classes at both locations is even, marked by a graph that forms an L curve.


Introduction
Sustainable Development Goal (SDG) 15, Life on Land, highlights the significance of forests in supporting life on our planet and their key role in combating climate change.Forest protection, including mangroves, is one of the SDG goals that highlights the importance of protecting forests.Mangroves offer a variety of ecosystem services, including as safeguarding against severe weather events, controlling the global temperature, and providing sustenance, shelter, resources, and ecotourism opportunities for several coastal populations [1].
Mangrove forests, noticed in coastal areas and small islands, serve multiple purposes such as safeguarding coastlines by diminishing wave and storm velocities, shielding against erosion, preventing the intrusion of seawater, retaining mud, and capturing sediments carried by surface water currents.[2].In addition, physically mangrove ecosystems can function as protected forests that affect the flow of 1315 (2024) 012020 IOP Publishing doi:10.1088/1755-1315/1315/1/012020 2 water masses in the soil.The typical root system in mangrove plants can inhibit water currents and waves, thus keeping the coastline stable and avoiding erosion (abrasion).The condition of the mangrove ecosystem provides protection and fertility and is also safe for marine biota in general.
The mangrove ecosystem is an efficient coastal ecosystem for protecting small island land [3].Physically, the role of the ecosystem is as an erosion prevention, sediment trap, and wave and current barrier.Coastal and small island ecosystems serve as natural protectors, thereby significantly enhancing the adaptive capacity of an island from natural disasters [4].Small islands are a vulnerable and sensitive cluster of islands [5,4,6,7].
However, generally in coastal areas these islands feature mangrove ecosystems as one of the key ecosystems in coastal areas and small islands.The existence of an ecosystem that has a high adaptive capacity can reduce the vulnerability of small islands, but if the adaptive capacity is low then its function and role in reducing the vulnerability of small islands will not be optimal.
Buano Island has a mangrove forest covering an area of 462 ha.Although it has a fairly wide mangrove forest, but the availability of data related to mangroves is still very minimal, including those related to the structure and composition of mangrove forest species in the area.These data are needed in the context of preparing management plans and utilization of mangrove forests so that their functions and benefits can be enjoyed sustainably.Knowing the composition of the forest, the variety of plant species, and the distribution patterns of trees can provide the groundwork for efficiently safeguarding and managing the forest reserve's biodiversity [8,9].For this reason, it is necessary to research the structure and composition of mangrove forests on Buano Island.The results of this research are expected to be used as a reference for related parties in preparing mangrove management plans in the future.

Data collection
The research Procedure is as follows: 2.2.1.Research Preparations.including (1) obtaining research administration permits; (2) collection of secondary data/literature related to research; (3) observation of the field conditions of the research site; and (4) preparation of equipment and materials for field data collection.The material used in this research is all the vegetation found on the observation route and the land cover map of Buano Island.

Research Sampling
Design.sampling design used in this study is probability sampling with twostage sampling.Two-stage sampling provides an advantage in concentrating measurement work close to the selected primary sample location rather than placing sample units scattered throughout the forest area [10].In this study, the first level of sampling was carried out to determine the observation points.Research observation points are determined purposively with certain considerations based on research objectives and field characteristics.The consideration to determine the observation point in this study is based on area analysis, and land cover analysis using GIS analysis to obtain normalized differences vegetation index (NDVI) values which will identify areas and densities in 3 categories, namely rare, medium, and dense and the results of this identification will be analyzed by the Decree of the Minister of Environment No. 201 of 2014 concerning Standard Criteria and Guidelines for Determining Mangrove Damage, where for land cover criteria dense > 75%, medium > 50-75% and rare < 50%.The second level of sampling in this study was carried out by making six sample plots measuring 20 m × 20 m with a distance of 20 m wide each sample plot.The example tile is created by combining the path method with the grid method.The direction of the sample plot path is made to intersect the contour or perpendicular to the coastline (seaside/strait).

Research data collection.
The primary data collected in this study is in vegetation data, collected using vegetation analysis techniques.Vegetation analysis techniques are aimed at determining the structure and type composition of a forest stand.Vegetation analysis techniques are applied to observation trails with a width of 20 m and a length of 20m or adapted to field conditions, which are divided into several sample subplots for analysis of tree-level vegetation (trees, saplings and seedlings), as well as undergrowth, epiphytes, lianas, and palms.The sample sub-plot sizes for each observed vegetation growth rate are as follows: 1) Sample subplots 2 m × 2 m for measurement of seedling and undergrowth (grasses, herbs, ferns, shrubs); 2) Sample sub-plots 5 m × 5 m for sapling measurement; 3. Sample sub-plots 10 m × 10 m for tree and palm-palm measurements.

Data processing
The results of data collection at the research location were processed to calculate the Important Value Index (INP), Type Dominance Index (C), Diversity Index (H'), Equity Index (E) species richness index (R), and Species Similarity Index (I) and forest structure analysis based on the relationship between tree height and diameter and tree density.b.The Species Diversity Index (H') is calculated using Shannon's species diversity formula [12] as follows: The notation H' expresses the Shannon diversity index, N is the total number of individuals of all species found, ni is the number of individuals of the i-th species, and s is the total number of species found.Shannon-Wiener index (H') values have values between 0 -5.The low value of this index is an indicator of disturbances in the area.The classification of this index value category includes [13]: The Evenness Index shows the level of evenness of individuals per type.If the value of E is closer to 1, then the value of evenness is higher.The value of E is calculated using the following mathematical formula [12]: The notation E expresses the evenness index of the type; H' expresses the species diversity index; S represents the number of species found.According to Magurran (1988): E < 0.3 indicates low evenness, 0.3 < E < 0.6 indicates moderate evenness and E > 0.6 indicates a high level of evenness.d.The Type Dominance Index (C) aims to determine the concentration or mastery of a type in an area using a mathematical formula [14] as follows:

𝑖=1
Where C is Dominance Index Type, ni is i-th Density, N is Total Density.The Type Dominance Index value ranges from 0 ≤ C ≤ 1, if a stand is controlled by only one type if the value of C is close to 1, in other words, there has been a grouping/concentration of a type of plant.the value of C is close to the value of 0, so there is no concentration of species where several species are dominating together.
e. Species Similarity Index (I) This index is used to compare the similarity of the species composition of two communities.To obtain the value of the type similarity index used the formula from Motyka as follows [15]: Where IS is Species similarity index of two communities (%), Mw is a smaller number of INP of species present in both communities, MA is INP count of all species in Huaroa hamlet community, MB is INP count of all species in Huhua hamlet community.
f.The species richness index, calculated using Margallef's formula Clifford and Stephenson 1975 as follows [12]: Where R is Species Richness Index, S is number of species found, N is Total number of individuals.
Belan-Santini in [13] set the lower limit of this index value at 2.05 so that the species richness of an ecosystem is said to be high if it has an index value of more than 2.05.
g. Forest Vegetation Structure.Analysis Forest vegetation structure is defined as a collection of individuals in a space that form a stand and its main elements include growth rates, stratification, and cover [16].Vegetation structure is divided into three components, namely vertical structure, horizontal structure, and quantitative structure (the abundance of a type in a community) [17].In this research, the vertical structure of vegetation is presented in a graph to visualize the relationship between tree height class and tree density per hectare, while the horizontal structure of vegetation is depicted in a graph to visualize the relationship between tree diameter class and tree density per hectare.

Results and Discussion
Mangrove forests on Buano Island are found in Huaroa hamlet, which includes an area of 131 ha, and Huhua hamlet, which comprises an area of 65 ha.The mangrove forests of Buano Island are divided into two categories: primary mangrove forests, which are mostly found in Huaroa hamlet, and secondary mangrove forests, which are found in Huhua hamlet.This secondary forest is a mangrove forest ecosystem resulting from restoration of damage caused by human intervention (illegal logging).

Forest Type Composition
The results of vegetation analysis at both observation sample plots show that in the Buano Island area there are 8 families with 12 species, both major and minor tree species (Table 1).In the mangrove forest of Huaroa Hamlet, 12 tree species were found (7 major mangrove species, 3 minor mangrove species and 2 associated mangrove species), while in the Huhua Hamlet mangrove forest, 8 tree species were found (5 major mangrove species, 2 minor mangrove species and 1 species associated mangrove).As stated by [18] the primary mangrove forest, also known as the major mangrove, is comprised of a single species.Minor mangroves, on the other hand, are composed of a mixture of different species of mangroves.Associated species are mangrove vegetation that rely heavily on salt levels and typically located in the areas that are only submerged by seawater during high tides.The density, frequency, dominance, and important value index (IVI) of trees, sapling, seedlings, and undergrowth at both observation sample plots are presented in Table 2.At the location of Huaroa hamlet, 10 species of seedling were found, with the dominant species being the R. mucronata (density of 13,402 individuals/ha and IVI 103.27%).As for the location of Huhua hamlet, the number of species found is less, namely 5 species with the dominant species is R. mucronata (density of 18,888 individuals/ha and IVI 130.22%).The number of species at the sapling found at Huaroa hamlet was 11 species with the dominant species R. mucronata with a density of 1555.55 individuals/ha and IVI 88.09%, while in the location of Huhua hamlet as many as 5 species with density of 3133.3 individuals/ha and IVI 112.50%.At the tree stage, the number of species found at the Huaroa is 9 species, while at the Huhua there are 6 species.The dominant species at both sites was R. mucronata with a density of 131 individuals/ha and an IVI of 127.859% at the Huaroa and a density of 254 individuals/ha and an IVI of 160.17% at the Huhua.R. mucronata dominates at this location because this species can grow on fine mud substrates even slightly coarse and can tolerate slightly high salinities such as those found in this study areas.The conditions of the species found at the location of Huaroa hamlet and Huhua hamlet are generally not much different.Based on the species composition in both locations, the Huaroa area has a higher composition of mangrove species compared to Huhua, even though the number of major mangroves in Huaroa is more than in Huhua.The Huaroa area is very close to the open sea, compared to the Huhua area which is in the strait area (Valentine Strait).This causes the Huaroa area to be exposed to tides more often than Huhua, causing salinity conditions in Huaroa to be higher (40%) than in Huhua (35%).The results of the analysis of the Shannon-Wiener diversity index (H'), Pielou species evenness index (E), Simpson type dominance index (C), and Margalef species richness index (R) are presented in Table 3.Based on Table 3, the species diversity index (H') values in Huaroa and Huhua hamlets is in the poor/low category.As for the value of the evenness index (E) shows that the species in Huaroa Hamlet and Huhua Hamlet have moderate evenness.This is indicated by the value of E at all growth stages having values in the range of 0.55 -0.68.In Huaroa, there is a concentration on one speccy.This can be seen from the value of the dominance index (C) in the area which has a value close to 1 or even reaches 2 while in Huhua hamlet it can be seen that there is dominance by one particular species with an index value of 0.69-0.76for each growth stages.The results of the species richness index (R) in Huaroa Hamlet are relatively high for saplings (> 2.05), but relatively low for seedlings and trees (< 2.05), while the species richness index (R) in Huhua Hamlet is low for all growth stages (< 2.05).Mangrove forest areas tend to have homogeneous species and grow to form zoning, that is what makes the value of diversity, species evenness, and species richness low [19]; [20].Both locations have forest compositions that tend to be similar.To see the similarity of type composition in both locations, a type similarity index (IS) was used.The results of the IS analysis are presented in Table 4. Based on the Table 4, it can be seen that the species similarity index (IS) of seedlings, saplings, and trees is categorized as high because it is more than 50%.When viewed from the similarity of species index (IS) at each growth stages in both locations, almost all growth stages and undergrowth have a high IS value, which is more than 50%.This shows that both locations have similar forest compositions.This is because both locations are still on one coastline and are still located on the same island.

Stand Structure
The structure of forest vegetation can be divided into three components, namely vertical structure seen from the stratification of the tree canopy, horizontal structure seen from the distribution of spatial distribution, and abundance of species [17].In this study, horizontal structure is seen through individual densities at each growth stages, and also through individual densities at each diameter class.

Horizontal structure.
The horizontal structure can be seen from the density of the type at each growth stages, and also from the class of the diameter of the rod.The results of the analysis of the individual density of trees, seedlings and poles are presented in Figure 2. Figure 2 shows that in Huaroa hamlet the highest number of individual densities is seedling (23,125 individuals/ha), then saplings (3,322 individuals/ha) and trees (268 individuals/ha).While in Huhua hamlet the highest individual density is at seedling stage (27,500 individuals/ha) followed by saplings (5,156 individuals/ha) and trees (464 individuals/ha).So if the exponential line is drawn on the density graph, it will form an L curve in both locations.This shows that the mangrove tree population at this location tends to become a forest with an uneven age balance, namely the larger the diameter, the fewer individuals.Forest structure is a life form of vegetation with complex characteristics, which can be used to determine stratification (vertical and horizontal) and to determine dominant, codominant and suppressed species [21].

Vertical
Structure.The vertical structure can be seen from the height class of trees in both locations showing that the highest density value is trees with a height of 6-10 meters, which is 143 individuals/ha (Huaroa and 65 individuals/ha (Huhua) (Figure 4).This indicates that the Buano Island area is a relatively young forest and will continue to grow.Based on the graph presented, the trend of the relationship of tree height class with individual density forms a bell curve following a normal distribution.Another formation seen from the graph of the relationship between individual density and trunk diameter class (Figure 6) is the Buano island mangrove forest in the developmental stage [22], the mangrove forest is categorized as a balanced uneven age forest.This is because at both locations the graph shows a trend forming an L curve.This condition is a good condition to achieve sustainability, because of the existence of abundant youth (small diameter) [23], but sustainability will occur if there is no significant disturbance.The stage of development and growth of mangrove forests on Buano Island can also be seen from its vertical structure (Figure 4).Tropical forests are usually divided into five layers/stratum, namely stratum A, B, and C for tree species, stratum D and E for shrubs and undergrowth [17].The mangrove area on Buano island in both Huaroa and Huhua locations has 3 layers/stratum, namely stratum D for trees with a height of 1 -5 meters, stratum C for trees with a height of 6 -10 meters, stratum B for trees with a height of 11 -15 meters.

Discussion
The structure of forest vegetation can be divided into three components, namely vertical structure seen from the stratification of the tree canopy, horizontal structure seen from the distribution of spatial distribution, and abundance of species [17].In this study, horizontal structure is seen through individual densities at each growth stages and also through individual densities at each diameter class.In the graph of the individual density of each growth stages (Figure 3), the locations of Huaroa and Huhua have a graph trend that forms an L curve.
The slightly open condition of Huhua causes gaps to provide space for poles stage to grow better [24].In addition, from the graph, it can also be seen that at the Huaroa and Huhua locations, the abundance of seedlings is very high and decreases sharply when it reaches the poles stage.This shows that only a few seedlings survive and reach the next level of growth.
The same thing can also be seen in the graph of the relationship between diameter class and tree density (Figure 4), where it can be seen that at the Huaroa location there was mortality at tree stages which reduced the density of individuals from the 30-40 cm diameter class to the 40-50 cm diameter class.This indicates that there is considerable mortality in trees with a larger diameter.Conditions like this can be a warning of the influence of tree cutting for firewood needs for the community which is quite high on the growth and development of mangrove forest in the locations which can destroy the existence of mangrove vegetation in the future.

Conclusions
The composition of the species at the two locations does not look too different.In both locations, the dominant tree species is R Mucronata.Both sites have the same type of composition, especially on the growth stages of seedlings, poles, and trees.Likewise species diversity in both locations, which both have low species diversity.The vegetation structure in both locations also has the same pattern, whereas in the vertical structure, there are 2 layers/stratum trees equally dominated by trees with a height of 6 -10 meters.As for the horizontal structure, the distribution of diameter classes at both locations of the spread is balanced, characterized by graphs that form the L curve.
This research was conducted from May to June 2023, this study was carried out in Huhua Hamlet, North Buano Village and Huaroa Hamlet, South Buano Village, Huamual Belakang Subdistrict, West Seram District, Maluku Province.The oceans surrounding of Buano Island are proposed to be designated as a Marine and Small Islands Protected Area (KKP3K) with an area of 31,820.89ha.The area of Small Island Park (TPK) Buano Island and surrounding waters include Kasuari Island, Kelapa Island, Pua Island, Nusa Tea Island, Nusa Esuna Island, and Serani Island.Buano Island has a land area of ±127.55 km or 12,755 ha.Based on the 2020 Forest Area Map of Maluku Province (SK.6604/MenLHK-PKTL/KUH/PLA.2/10/2021 dated October 27, 2021), Buano Island has a Protection Forest covering an area of ±92 ha, a convertible Production Forest covering an area of ±395 ha and Other Use Areas covering an area of ±67 ha.The area of mangrove forest in Buano village is 462 ha consisting of Primary Mangrove Forest covering an area of ±265 ha and Secondary Mangrove Forest covering an area of ±197 ha.The location of the study can be seen in Figure 1.

Figure 1 .
Figure 1.Map of the study area a. Important Value Index.INP can also describe the type composition and level of mastery (dominance) of species in a community (Indriyanto 2008) by summing the values of relative density (KR), relative frequency (FR), and relative dominance (DR) of a type [11].To calculate INP, several formulas are used as follows: -Important value index (INP) -For tree and pole level: INP = KR + FR + DR -For sapling and seedling levels: INP = KR + FR KR notation expresses the relative density value obtained from the equation: K = Number of individuals of type (N/Ha) area of the plot 4

Figure 2
Figure 2 Graph of individual densities of seedlings, saplings, and trees

Figure 3 .
Figure 3. Graph of the relationship between individual density and rod diameter class Based on Figure 3. the trend of individual density in Huaro and Huhua hamlets forms an L curve where the highest density at the location is in the diameter class of 10-20 cm at 190 individuals/ha (in Huaroa hamlet) and 176 individuals/ha (in Huhua hamlet).The individual density value gets smaller with an increase in the diameter of the stem.

Figure 4 .
Figure 4. Relationship between individual density and tree height

Table 1 .
Species of plants found in the Buano Island Mangrove Forest area

Table 2 .
Density, frequency, dominance, and important value index (IVI) of all species at Huaroa Hamlet and Huhua Hamlet.

Table 4 .
Type similarity index (IS) values at all growth levels