Seedling Diversity Considerably Changes Near Localities in Three Salinity Zones of Sundarbans Mangrove Forest, Bangladesh

The status of natural seedlings near localities in the Sundarbans Mangrove Forest was assessed through a stratified random sampling method to observe seedling composition and diversity, importance value index, family importance value, and species evenness. A total of 63 sample plots of 3,990 m 2 area were surveyed in three natural and anthropogenic influenced salinity zones of Sundarbans. A total of 16,166 seedlings of 15 species under 12 families were found. Family Euphorbiaceae showed the highest (59.59%) family relative density and Rhizophoraceae presented the highest (20%) family relative diversity Index (FRDI). The maximum family importance value (FIV) showed by Euphorbiaceae (66.26) and the maximum importance value index (IVI) of species has been observed in Excoecaria agallocha (114.74). Excoecaria agallocha showed the highest relative density (59.6%) and relative abundance (39.87%) where the highest relative frequency (15.27%) was observed by Heritiera fomes. The mean stem density and species diversity index in the whole survey area was 2701 stem ha -1 and 0.0009 correspondingly. The Shannon-Wienner's diversity index was 1.52 where the maximum Shannon-Wienner's diversity index was 2.708. The Simpson's diversity index and Dominance of Simpson index were 0.38 and 0.62 with Simpson's reciprocal index 2.632. The Species evenness index, Menhinick’s , and Margalef's indices were 0.561, 0.118, and 1.445 respectively. The Species diversity index of the three salinity zones were 0.0017, 0.0029, and 0.0035 respectively. The Shannon-Wienner's diversity index of Low Salinity Zone (LSZ), Moderate Salinity Zone (MSZ), and Strong Salinity Zone (SSZ) were 0.887, 1.369, and 1.845 correspondingly where LSZ (0.632) showed the highest Simpson's diversity index follow ed by MSZ (0.394) and SSZ (0.21). The Species evenness index for LSZ, MSZ , and SSZ were 0.346, 0.505, and 0.742 where Menhinick’s Index w ere 0.148, 0.210, and 0.207 respectively. The analysis showed poor diversity indices and the area was dominated by few species with few families. The status is also reduced with increasing salinity.


INTRODUCTION
The flora of the Bangladesh tropical forest is one of the ten global hot spot Copyright: © 2021, J. Tropical Biodiversity Biotechnology (CC BY-SA 4.0) zones for biodiversity (Mittermeier et al. 1998) and retains rich biological diversity due to its exceptional geophysical location (Chowdhury 2001;Hossain 2001;Nishat et al. 2002). The forest land and natural forest cover of this country are 1.442 million ha and 1.204 million ha respectively which are managed by the forest department, land ministry, and other individuals (Sobuj & Rahman 2011). Tropical forests are ecologically and economically important for the livelihood of local communities. For the last few decades, the natural forests of the country are rapidly decreasing at an alarming rate by 1-4% of their current land area (Laurance et al. 1998) due to overpopulation, land use alterations, unsuitable and poor management practices (Khan 2008). The net forest loss in South and Southeast Asia, from 2010 to 2015 was about 25% higher compared to 1990 (Keenan et al. 2015) and Bangladesh is also situated in this zone. Agricultural extension, deforestation, extreme removal of woody and non-woody resources, urbanization, and applying unfitted management tools are the major causes of forest degradation in Bangladesh (Hasan & Alam 2006).
Mangrove forest is one of the most productive vital ecosystems in tropical countries. It provides valuable ecological and economic resources to accelerate sustainable livelihood like nursery grounds, breeding sites for birds, fish, crustaceans, shellfish, reptiles, and mammals with medicinal and aesthetic values (Helal Siddiqui & Islam 2019). It is also a renewable source of wood, accumulation sites for sediments, contaminants, carbon, and nutrients, while also protect against coastal erosion, cyclones, and tsunami (Paolini & Sánchez-Arias 2008;Danielsen et al. 2005;Kathiresan & Rajendran 2005;Badola & Hussain 2005;Alongi 2002;Mazda et al. 1997). Sundarbans mangrove forest is rich in both floral and faunal diversity compared to other mangroves of the world and it is a suitable place for collecting various minor forest products (Islam et al. 2020) and developing tourism . Prain (1903) and (Seidensticker & Hai 1983) recorded 334 species of plants belonging to 245 genera and 75 families where Field (1995) and Tomlinson (1986) included approximately 16-24 families and 54-75 species respectively. In addition, (Karim 1994) reported 123 plant species belonging to 22 families representing 30 genera, and (Helal Siddiqui 2009) reported about 230 species including non-mangroves. Unfortunately, plant species are decreasing gradually due to climatic change, edaphic factors, and various natural and anthropogenic causes. The diminution of native species is hastening at an alarming rate through the quick loss and degradation of forests in Bangladesh (Rahman et al. 2000;Hossain 2001).
Tree diversity, a portray of the natural forest community is fundamental as they offer resources and homes for almost all other forest species (Huston 1994;Richards 1963;Cannon et al. 1998;Hall & Swaine 1976). For understanding the actions and dynamics of forest ecosystems, the knowledge of the floristic arrangement, their quantitative structure, and diversity are vital (Hossain et al. 2015). Effective conservation measures for sustainable management of forests need an appreciation of phytosociological features of tree species diversity (Feroz et al. 2014;Biswas & Misbahuzzaman 2008). Natural regeneration of forest tree species should be enhanced by the appropriate artificial and natural process for defending forest flora and continuing sustainability of yield, goods, and services (Haque & Alam 1988). Potential information for many native tree species is received from various studies that focused on natural regeneration status in different natural forests of Bangladesh (Hossain et al. 2004;Hossain 1999;Hossain et al. 2013;Miah 1999;Motaleb & Hossain 2007;Rahman et al. 2011;Rahaman et al. 2020).
For formulating a conservation strategy of the forest, quantitative information like regeneration, species composition, and distribution is important (Malik et al. 2014;Malik & Bhatt 2016;Sharma et al. 2014). The range of biodiversity loss in the Sundarbans mangrove forest of Bangladesh is not exactly known due to a very poor database and limited information (Hossain et al. 2004). So, the study of biodiversity is a crucial requisite which is the main objective of this study to effectively protect and manage the existing natural forests for sustainable livelihoods (Hossain et al. 2018;Hossain et al. 2020) for future generations. Some studies were done before in the total area of Sundarbans (Prain 1903;Heining 1892;Helal Siddiqui 2009;Chaffey et al. 1985;Hossain 2003;Rashid et al. 2008;Saiful et al. 2014;Saiful et al. 2015), but this study focused only those areas where various natural and anthropogenic influenced were present. The study was designed to provide quantitative information of the seedling composition, diversity, importance value index, family importance value, species evenness, and compares the diversity indices of three natural and anthropogenic influenced salinity zones in Sundarbans mangrove forest.

Methods
A total of 63 sample plots of 3990 m 2 (21 sample plots of 1330 m 2 area for each salinity zone) area were surveyed followed by a stratified random sampling method in April and May 2019 because most of the phonological behavior (flowering, fruiting, seed dispersal, etc) occurs in April and start to accelerate seed dispersal with seedling recruitment gradually which become more stable gradually to affect seedling diversity. The relative abundance (RA), relative density (RD), relative frequency (RF), and Important Value Index (IVI) were calculated by (Dogra et al. 2009;Nebel et al. 2001;Shukla & Chandal 2000). Family relative density (FRD) and family relative diversity (FRDI) were calculated by following . Various diversity and richness indices were scrutinized by following, (Clifford & Stephenson 1975;Kent & Coker 1992;Margalef 1958;Michael 1990;Odum & Barrett 1971;Pielou 1995;Shannon 1948;Simpson 1949;Whittaker 1977).

Seedling status
A total of 16,166 seedlings of 15 species under 11 families were found in a 3990m 2 area within three salinity zones of Sundarbans. A total of 7723 seedlings of 13 species under 9 families were found in the Low Salinity Zone (LSZ) where Aegiceras corniculatum and Sonneratia apetala was not found. Besides, 5075 seedlings of 15 species under 11 families were found in the Moderate Salinity Zone (MSZ). In addition, a total of 3368 seedlings of 12 species under 9 families were found in the Strong Salinity Zone (SSZ) where Cynometra ramiflora, Rhizophora mucronata, and Sonneratia apetala was not found in the Strong Salinity Zone (SSZ). About 59% were represented by one family (Euphorbiaceae) and about 26% by two families (Rhizophoraceae and Sterculiaceae). Maximum of three species were found in family Rhizophoraceae and two species in Arecaceae and Meliaceae where other families had one species (Table 1). Family Euphorbiaceae showed the highest (59.59%) family relative density followed

Biological diversity indices for seedling
The mean stem density and species diversity index in the whole survey area was 2701 stem ha -1 and 0.0009 correspondingly (  (Table 3).

Biological diversity indices of three salinity zones
The total number of seedlings and mean density for Low Salinity Zone (LSZ), Moderate Salinity Zone (MSZ), and Strong Salinity Zone (SSZ) were 7723 and 4466 stem ha -1 , 5075 and 2543 stem ha -1 and 3368 and 2110 stem ha -1 separately (

Discussion
Sundarbans has very rich floristic diversity compared to other mangrove forests of the world where half of the total number of mangrove species found in the world occurs in the Sundarbans (Bangladesh and Indian Sundarbans) (Patil 1962). But this study revealed the poor forest condition of Sundarbans Mangrove Forest (SMF) and about 59% comprised of one species (Excoecaria agallocha) where about 85% comprised of four species (Excoecaria agallocha, Heritiera fomes, Ceriops decandra, and Bruguiera sexangula). Excoecaria agallocha was increased in a highly disturbed area of Sundarbans because its leaves were poisonous and not suitable for grazing. A total of 15 species were found which was higher than the mangrove swamp forest in southern Nigeria (Asuk et al. 2018) and lower than Puerto Princesa Bay (Dangan-Galon et al. 2016) and some other studies carried out in whole Sundarbans (Hossain 2003, Rashid et al. 2008, Saiful et al. 2014, Saiful et al. 2015. The SMF was predominated by Euphorbiaceae and Rhizophoraceae and the dominancy of Rhizophoraceae was also found by (Asuk et al. 2018;Igu et al. 2017;Ogar & Asuk 2015). The total number of species found in this study was lesser than other forests of Bangladesh (Hossen et al. 2021;Malaker et al. 2010;Motaleb & Hossain 2011;Rahaman et al. 2020). This poor forest condition may be the effect of natural and anthropogenic disturbance whereas most of the people are dependent on Sundarbans for their livelihood directly or indirectly (Islam 2019). They entered these areas for collecting fuel, fodder, fruits, golpata, and so on through legal permission of the authorities but they are doing this hamper in their subconscious mind or intentionally. Islam et al. (2020) stated that most of the villagers adjacent to Sundarbans collect fish, honey, golpata, fuel wood, hogla, prawn, hantal, crab, nall, keora fruit, malia, goran stick, shrimp fry, and medicinal plants from Sundarbans but only 31.11% respondents had equipment facilities. Besides this, the government has declared officially the Sundarbans as a pirate-free area in 2018 which improved the destination's image perception to tourists (Haque et al. 2016) which affects diversity negatively. Besides, Sonneratia apetala was found only Moderate Salinity Zone (MSZ) because of high palatability and collection of Sonneratia apetala seeds for making pickles, it was declined drastically. The average mean density was 2701 stem ha -1 where a maximum of 4466 stem ha -1 for LSZ and a minimum of 2110 stem ha -1 for SSZ. The same rich density was found in Puerto Princesa Bay, Palawan Island, Philippines (Dangan-Galon et al. 2016). The density of SMF was higher than other forest parts of Bangladesh (Motaleb & Hossain 2011;Rahaman et al. 2020).
The species richness of this study advocated that many plant species represented by few families may lead to the extinction of many of the species in SMF. The opposite result was found by (Olowokudejo & Oyebanji 2016) in southeastern Nigeria. Species evenness index (0.561) was lower than natural sal forest (0.8) (Rahaman et al. 2020 In this analysis, it was found that the number of seedlings and mean density gradually decreased with the increase of salinity. The highest number (7723 stem) and mean density (4466 stem ha -1 ) were found in LSZ in the same area of 1330 m 2 followed by MSZ (5075 stem, 2543 stem ha -1 ) and SSZ (3368 stem, 2110 stem ha -1 ). The Shannon-Wienner's diversity index (0.887, 1.369, and 1.845) was steadily increased and Simpson's diversity index (0.632, 0.394, and 0.21) was gradually decreased from LSZ to MSZ and SSZ. Simpson's diversity index indicated that the diversity was higher in SSZ than in the other two zones. The highest species evenness was found in SSZ (0.742) where LSZ showed the lowest (0.346). The Menhinick's Index and Margalef's/Species richness index of all three zones indicated the lower diversity but the value was various with the location. It may be the effect of salinity in water and soil, inundation type, time and duration, grazing, erosion, etc. that is observed during field data collection. But the suggested hypothesis of Ball (1998) was the opposite of this observation; he denied the effect of salinity (water and soil) on diversity declination. All these variables may affect the germination and survival rate of species. Further research needs to identify the degree of effect of these factors in SMF.
It was found that most of the species were on the least concerned list of IUCN conservation status where Ceriops decandra and Phoenix paludosa were near threatened and Heritiera fomes was listed endangered. It indicated that this area only consisted of abundant least concern species like Excoecaria agallocha, Bruguiera sexangula, Aegiceras corniculatum, Avicennia officinalis, and Xylocarpus mekongensis. Anthropogenic factors, grazing, tourism, increased number of water vehicles, and excessive extraction of food, fodder, and fuel wood with various natural calamities may affect the diversity and richness of species. Helal Siddiqui and Islam (2019) were recommended that the sustainable enrichment plantation will be carried out which showed satisfactory growth and survivability to improve the species richness of these areas. This study had many limitations like differences in methods of study, the intensity of sampling, area covered, habitat types, and plot size used which may affect species richness obtained by other studies (Hossain 2003, Rashid et al. 2008, Saiful et al. 2014, Saiful et al. 2015.

CONCLUSION
The natural regeneration status of forests is very vital for the sustainable management of forests and forest resources. Bangladesh is still lucky to have a high percentage of forest cover in the Sundarbans Reserve Forest (SRF). However, this rich forest is decreasing with time particularly near human habitation to meet the basic needs of fuel, fodder, and small timber where the degree of causes is still unknown. The study will provide quantitative information about the seedling composition which may be very helpful for understanding the Sundarbans Mangrove Forest. The analysis showed poor diversity indices and the forest was dominated by few species with few families. It was recommended that the sustainable enrichment plantation will be carried out to improve the species richness of these areas. The Forest Department is concerned and to lessen this situation has designed comanagement committees as well as providing alternative livelihoods to the Sundarbans user groups in cooperation with local NGOs. The study may be supportive and pave the way for advanced research on regeneration potentials of the mangrove species for safeguarding and heightening forests in the future.