Regional Studies in Marine Science 55 (2022) 102589 Contents lists available at ScienceDirect Regional Studies in Marine Science journal homepage: www.elsevier.com/locate/rsma Dominant species losing functions to salinity in the Sundarbans Mangrove Forest, Bangladesh ∗ Md. Akramul Islam a,c , , Shamim Ahmed b,c , Tanmoy Dey a,c , Rahul Biswas c , Md. Kamruzzaman c , Shanewas Hossain Partho c , Biplab Chandra Das d a Bangladesh Forest Research Institute, Ministry of Environment, Forest and Climate Change, Bangladesh Forest Growth and Yield Science, Department of Life Science Systems, School of Life Sciences, Technical University of Munich, Hans-Carl-von-Carlowitz-Platz 2, 85354 Freising, Germany c Forestry and Wood Technology Discipline, Khulna University, Khulna 9208, Bangladesh d Environmental Science Discipline, Khulna University, Khulna 9208, Bangladesh b article info Article history: Received 15 May 2022 Received in revised form 5 July 2022 Accepted 25 July 2022 Available online 2 August 2022 Keywords: Stand structure Biomass accumulation Carbon stock Growth reduction E. agallocha H. fomes Productivity Salinity zones a b s t r a c t Globally, mangrove forests are deteriorating due to several natural and anthropogenic factors such as sea level rise, habitat fragmentation, over-exploitation, pollution, etc. Sea-level rise - driven salinity would influence the functional activity of dominant species by declining their structure and functions, which is not well understood. Therefore, we tried to understand the increased salinity impact on the structures and functions of two identified dominant mangrove species (i.e., Excoecaria agallocha and Heritiera fomes) in the Sundarbans Mangrove Forest (SMF), Bangladesh. We test our hypothesis that salinity significantly retards the functions of dominant species structure and functions by evaluating two consecutive years of inventory data from 60 permanent sample plots (100 m2 each) established in three distinct salinity zones in the SMF. The study revealed that structural parameters of dominant species such as tree height, diameter at breast height (DBH), and basal area decreased in higher saline zones than in less saline zones. E. agallocha and H. fomes stored more biomass and carbon in less saline and moderate saline zones compared to high saline zone. Besides, functional variables such as aboveground biomass carbon and total biomass carbon decreased with salinity for both dominant species. This study demonstrated that salinity shapes dominant species by declining their height, DBH, growth, etc., which indicates salinity is a critical discriminating variable for losing species’ stand structures and functions. This information is critica to determine the physiological response of dominant species across the globe, which is crucial to predicting future climate change impacts such as sea-level rise. © 2022 Elsevier B.V. All rights reserved. 1. Introduction Indiscriminate environmental changes with anthropogenic pressures in the mangrove forest of tropical and sub-tropical regions have threatened ecosystem functions and services (Sarker et al., 2019; Rivera-Monroy et al., 2017). However, mangrove forests represent a significant amount of net primary production (i.e., 30%–40% of global carbon cycle) (Mitra et al., 2011; Clark et al., 2001) and act as a critical C sequestrator (i.e., 3– 5 times higher per unit C) than terrestrial ecosystem (Ahmed and Kamruzzaman, 2021; Rahman et al., 2015; Donato et al., 2011). Mangroves playing crucial role as a unique contributor by providing massive tangible and intangible benefits such as thatching materials, medicinal products, coastal shelterbelt etc. to ∗ Corresponding author at: Forestry and Wood Technology Discipline, Khulna University, Khulna 9208, Bangladesh. E-mail address: akramkukhulna@gmail.com (M.A. Islam). https://doi.org/10.1016/j.rsma.2022.102589 2352-4855/© 2022 Elsevier B.V. All rights reserved. the local community and global scale (Hossain et al., 2021; Islam et al., 2020; Iftekhar and Saenger, 2008; Siddiqi, 2001; Sarker et al., 2016). In spite of having such benefits, the widespread global mangrove ecosystem has already been deprived of its 50% coverage since 1950 (Sarker et al., 2019), whilst the present rate of deforestation is about 1%–2% per year in the mangroves (Sarker et al., 2019; Madelon et al., 2015). In addition, magnitude of global warming as well as rising sea-level (Sarker et al., 2016, 2019; Rahman et al., 2021b,a, 2015) alter land use, habitat alteration, and changes in vegetation pattern of mangroves, which extirpate global biodiversity and its functions tremendously over a spatial temporal period (Rahman et al., 2015; Detwiler and Charles, 1987; Sarker et al., 2016, 2019; Mitra et al., 2011; Islam, 2019). On the contrary, 16% of mangrove species are classified as highly endangered or vulnerable, while the remaining 10% are classified as near-threatened (Sarker et al., 2016). In addition, many mangroves are hampered in their growth and development by extreme salt stress (Sarker et al., 2016). Excessive salinity M.A. Islam, S. Ahmed, T. Dey et al. Regional Studies in Marine Science 55 (2022) 102589 levels hinder the structural development of mangrove species that maintained key functions around the globe (Hossain et al., 2021; Hossain, 2015). Another perilous alarm is that by 2050, one-third of total production will have been lost owing to salinity in the Sundarbans mangrove forest (Sarker et al., 2021). Furthermore, the largest dominant single territory mangrove forest, Sundarbans is unquestionably prepotently highly productive and carbon rich forest (Ahmed and Kamruzzaman, 2021; Rahman et al., 2021b,a; Hossain et al., 2019; Rahman et al., 2015) by providing innumerable goods and services in the coastal region locally, nationally and globally (Aziz and Paul, 2015; Dey et al., 2020; Helal Siddiqui and Islam, 2019; Islam, 2019; Islam et al., 2021; Sheikh et al., 2021). This forest is already in a drastically stressed condition of having ongoing freshwater flows reduction and salinity intrusion in the perspective of sea level rise (Dahdouh-Guebas et al., 2020; Aziz and Paul, 2015). Nonetheless, the freshwater supply to the Sundarbans has declined by 90% and the degree of salt has increased by 60% since the Farakka barrage was erected in India in 1975 (Aziz and Paul, 2015); Kumar and Jha. As a result less saline zone (LSZ) is converting to moderate saline zone (MSZ) and moderate saline zone to strong saline zone (SSZ) at the same time (Sarker et al., 2016, 2019; Iftekhar and Saenger, 2008). Due to chronological changes in salinity biodiversity and abundance of mangroves species have altered in the ecological zones viz., LSZ, MSZ and SSZ, respectively (Iftekhar and Saenger, 2008; Sarker et al., 2019). However, dominant species traits are one of the key factors associated with driving ecosystem functioning and as a result, increasing biomass and carbon uptake in the mangroves (Fotis et al., 2017; Lin et al., 2016; Sonkoly et al., 2019). Furthermore, salinity along with vegetation structures of identified dominant species is one of the pivotal components to affecting global biomass– carbon stock (Rahman et al., 2015; Adame et al., 2013) to maintain ecosystem functioning. The Sundarbans, being a Ramser site and a World Heritage site as declared by UNESCO in 1997 are immensely indispensable by considering its multipurpose roles (Rahman et al., 2015), which indicates the inevitability of sustainable conservation of that forest in relation to withstand salinity in salinity zone (Rahman et al., 2015; Sarker et al., 2016). However, different structural and functional parameters of dominant mangrove species such as basal area, mean tree height and diameter etc. affects biomass and carbon potentiality (Rahman et al., 2015; Mitra et al., 2011; Iftekhar and Saenger, 2008). Stand structure of mangrove species greatly depends on tidal inundation and many physiological activities through salinity intrusion (Kumar and Jha, 2010; Khan and Aziz, 2001; Karim and Karim, 1993), which ultimately may affect the functional activities of the mangrove forest. About 10% to 50% variation of salinity in sea water is required for optimal growth (Karim and Karim, 1993; Kumar and Jha, 2010; Khan and Aziz, 2001), increased salinity inhibited plant growth in mangroves (Khan and Aziz, 2001). Although in terms of high and changing salinity and water logging environments, mangroves are facultative (Ayma-Romay and Bown, 2019). Numerous studies have been conducted worldwide, focusing on biomass and carbon storage potential, salt tolerance mechanisms aiming to conserve and manage the mangrove wetland (Ahmed and Kamruzzaman, 2021; Khan and Amin, 2019; Kamruzzaman et al., 2017a, 2018; Sitoe et al., 2014; Rahman et al., 2015; Mitra et al., 2011; McKee, 1995; Khan and Aziz, 2001; Kumar and Jha, 2010; Rashid and Ahmed, 2011; Ahmed et al., 2011) but the role of dominant mangrove species in the salinity zones, as well as salinity gradient, in the context of global climate change and key functional activities adjacent to the coastal wetland communities, remains inconclusive. Meantime, several research studies have been conducted in the Sundarbans mangrove forest focusing on the stand structure, spatial variation of carbon stocks in the vegetation of mangrove species, carbon storage, aboveground productivity, phenological traits, fine root biomass and its contribution, salinity effect on plant growth (Mitra et al., 2011; Rahman et al., 2015; Karim and Karim, 1993; Ahmed and Kamruzzaman, 2021; Ahmed et al., 2021a,b; Kamruzzaman et al., 2017a, 2018; Azad et al., 2020) and so on. However, to our knowledge, no consensus study has yet paid attention to the structural attributes of dominant species with respect to salinity and how salinity shapes these functional variables in the salinity zones and gradient in the Sundarbans. Furthermore, there is a fundamental knowledge gap regarding dominant species functions with salinity, which posits us to know the inevitability of up taking study as tropical forests are the potential terrestrial blue carbon pool (Yu and Chen, 2015) and mangrove provides 25% of C burial to the worldwide coastal zone by maintaining their functional composition and stand attributes over- a spatio-temporal period (Lin et al., 2016; Fotis et al., 2017; Hossain et al., 2021; Sarker et al., 2016). Eventually, such knowledge inadequacies may create an impediment to conserving mangrove ecosystem in this delta (Sarker et al., 2016, 2019; Ahmed et al., 2011). Moreover, it is critical and imperative to observe the effect of salinity on the dominant mangrove species structures and its functions in spite of having challenges and uncertainty simultaneously in the Sundarbans, Bangladesh (Ayma-Romay and Bown, 2019; Masha et al., 2017). Thereafter we comprehensively investigated it through asking two research questions, e.g., 1. How do the salinity gradients and zones impact the structure of dominant species? 2. How does salinity shape the functional variables such as biomass and carbon, and their changes over time of dominant species? Consequently, we hypothesized that increased salinity (higher salinity and salinity zones) has significant and negative impacts on the structure and functions of identified dominant species. To answer these questions and to test the hypothesis, we evaluated stand structure, vertical and horizontal diversity, functional variables such as above and belowground biomass, and carbon stocks of dominant mangrove species from two years of inventory data, highlighting salinity gradients and zones in the Sundarbans mangrove forest, Bangladesh. 2. Materials and methods 2.1. Study site and forest inventory This research was carried out in the Sundarbans Mangrove Forest (SMF) of Bangladesh across three salinity zones. In the Ganges–Brahmaputra estuary, the Sundarbans of Bangladesh (21◦ 30′ –22◦ 30′ N and 89◦ 00′ –89◦ 55′ E) are part of the world’s largest river delta (Sarker et al., 2016). Bangladesh accounts for over 60% of the world’s mangrove forest, with India sharing the remaining 40% (Siddiqi, 2001). Diverse oligohaline or Less Salinity Zone (LSZ), (<2 dsm−1 ), mesohaline or Moderate Salinity Zone (MSZ) (2–4 dsm−1 ), and polyhaline or Strong Salinity Zone (>4 dsm−1 ) zones subsist in the Sundarbans (Siddiqi, 2001) (Fig. 1). The yearly precipitation averages 1700 mm (range: 1474 to 2265 mm) and the soil type is silty–clay–loam (Iftekhar and Saenger, 2008; Aziz and Paul, 2015). The maximum yearly temperature ranges between 29.4 ◦ C and 31.3 ◦ C (Azad et al., 2020; Ahmed et al., 2011). In the Sundarbans, salinity plays an significant role in shaping species distribution (Iftekhar and Saenger, 2008; Sarker et al., 2016; Ahmed et al., 2018). Plants from 22 families and 30 genera can be found in this mangrove forest, making it a very productive and diversified ecosystem (Iftekhar and Saenger, 2008; Hossain et al., 2021). An extensive survey was 2 M.A. Islam, S. Ahmed, T. Dey et al. Regional Studies in Marine Science 55 (2022) 102589 Fig. 1. Map of the study area indicating location of the sample plots (red circles) across three distinct salinity zones viz., LSZ (less salinity zone), MSZ (moderate salinity zone) and SSZ (strong salinity zone) of Sundarbans mangrove forest, Bangladesh. . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) However, in some areas, positive ecosystem functioning is confined to two species mixes stand in terms of productivity more than mono species stand (Sonkoly et al., 2019). Again, E. agallocha and H. fomes are dominant (Hossain, 2015), most prominent species from habitat suitability analyses and spatial density maps (Sarker et al., 2016). Both this forest type had increased, and were considered as the climax species, and occupied the largest areas (Chaffey et al., 1985; Iftekhar and Saenger, 2008; Forestal, 1960). The stand types of H. fomes - E. agallocha and E. agallocha - H. fomes, account for 29.7% and 14.8% of the forest area, respectively in the Sundarbans (Siddiqi, 2001). Both these species possess 39% stem per ha and also dominant in the lower size classes (>2.5 cm DBH) (Iftekhar and Saenger, 2008). However, H. fomes takes up 18.8% and 45.48% of the forest’s vegetative area in monospecific and mixed stands with E. agallocha, respectively. This stand type accounts for 67% of the forest’s vegetated area (Iftekhar and Saenger, 2008). In terms of abundance, distribution, and commercial value, H. fomes and E. agallocha are two most important resources (Iftekhar and Saenger, 2008; Siddiqi, 2001). In addition as discussed earlier, based on Importance Value (IV) this two species is dominant in the Sundarbans, Bangladesh (Ahmed et al., 2021a,b). carried out by establishing sixty plots of size 10 × 10 m (total area of 6000 m2 ) in each salinity zone (2000 m2 in each salinity zone). A multistage random sampling technique was adopted purposively to fulfill the objectives of the study. During the study period, all trees greater than 5 cm DBH in the study plots were taken and their height (H) (measured by Haga Altimeter) and DBH (cm) (taken by diameter tape) were measured. Soil salinity (ppt) data was collected through Handheld salinity meter (Model No. BK8391) from all equal size sample plots in the consecutive years. Soil salinity was measured in the month of June (monsoon period) and December (winter season) in each consecutive year of the study period (twice in every year) e.g., from 2020 to 2021 in the study area. 2.2. Description of the selected dominant species Mangrove species develop distinct unique physiological and ecological characteristics (Supplementary Table 1) (Sarker et al., 2016; Iftekhar and Saenger, 2008; Rahman et al., 2015; Hossain et al., 2019). In the study area, several mangrove species can be found viz., Ceriops decandra Griff. Ding Hou, Bruguiera sexangula lour. Poiret, Excoecaria agallocha L., Xylocarpus moluccensis Pierre, Avicennia officinalis L, Sonneratia apetala Buch.-Ham, Heritiera fomes Buch. –Ham, Nypa fruiticans Wurmb, Rhizophora apeculata Lam, Aegiceras corniculatum L. Blanco, Aglaia cucullata Roxb, Phoenix paludosa Roxb, Cynometra ramiflora L, Lumnitzera racemose Willd, Sonneratia caseolaris L. But, only two species such as E. agallocha and H. fomes were selected regarding their distinct unique characteristics (Supplementary Table 1) and dominancy in the stands compared to salinity zones in the Sundarbans (Iftekhar and Saenger, 2008; Hossain et al., 2021; Ahmed et al., 2021b). Based on abundance, this species is also dominant in the study area (Islam et al., 2020; Hossain et al., 2021; Khan et al., 2021). 2.3. Biomass and carbon stocks estimation and changes of functional variables Several allometric equations have been developed for biomass and carbon estimation in mangrove forest (Hossain et al., 2015; Chave et al., 2005; Komiyama et al., 2005; Dahdouh-Guebas and Koedam, 2006; Kathiresan et al., 2013). We used species-specific allometric model developed by Rahman et al. (2021b,a) for E. agallocha and H. fomes which is most up to date and best fitted species wise equation for measuring above-ground biomass in the Sundarbans mangrove forest, Bangladesh (Rahman et al., 3 M.A. Islam, S. Ahmed, T. Dey et al. Regional Studies in Marine Science 55 (2022) 102589 47 cm and 0.018 m2 ha−1 ) respectively than the rest two other zones (Table 1). However, density (trees ha−1 ) of E. agallocha and H. fomes varies from 295–585 and 80–450 corresponding LSZ– MSZ and SSZ–LSZ consecutively (Table 1). All structural attributes such as height, DBH, BA of both species showed significant difference (p < 0.05) (except BA of E. agallocha, p > 0.05) and negatively correlated with increasing salinity (Fig. 2). Again, CV of height (vertical tree diversity) and CV of DBH (horizontal tree diversity) of these dominant species was significantly differ (p <0.05) and negatively correlated with salinity (Fig. 2). 2021b,a). Therefore, we used equations for E. agallocha (Eq. (1)) and H. fomes (Eq. (2)) are given belowLn(AGB) = −2.57 + 0.862 ln (DBH2 H) (1) Ln(AGB) = −1.99 + 2.46 ln (DBH) (2) Again, to measure belowground biomass we used common allometric equation which was developed by (Komiyama et al., 2005). BGB = (0.199.ρ ′0.899 D2.22 ) where, AGB = above-ground biomass (kg), DBH = diameter at breast height (cm), H = Height (m), BGB = below-ground biomass (kg), ρ ′ = wood density (kg m−3 ). The wood density data of this selected two species were obtained from Rahman et al. (2021b,a) to calculate biomass and carbon. Following Gifford’s guidelines, to convert biomass to carbon, we used the 0.5 thresholds as a conversion factor (Gifford, 2000). However, the present study only calculates all living tree biomass and carbon. Again, in spite of having more mortality for E. agallocha than H. fomes in the study area, this was insignificant or minute in terms of biomass and carbon potential. Again, changes of functional variables of selected dominant species (i.e., E. agallocha and H. fomes) were calculated through an approach of Forrester (2014) with the following equations in case of AGBC, BGBC and TBC {i.e., AGBC (Eq. (3)), BGBC (Eq. (4)) and TBC (Eq. (5)) in the Sundarbans, Bangladesh (Forrester et al., 2014). 3.2. Biomass, carbon stocks and their changes with salinity Table 2 showing the mean of all functional variables in the salinity zones where, the TB and TC production of H. fomes is maximum (10.87 Mg ha−1 ranged from 2.30–5.11 Mg ha−1 ) and (5.43 Mg ha−1 ranged from 1.15–2.55 Mg ha−1 ) corresponding to SSZ–MSZ compare to E. agallocha (4.51 Mg ha−1 ranged from 1.06–1.95 Mg ha−1 ) and 2.26 Mg ha−1 ranged from (0.53–0.98 Mg ha−1 ) corresponding to SSZ–LSZ in the study area. Furthermore, increment of AGBC, BGBC and TBC of E. agallocha and H. fomes was likewise LSZ > MSZ > SSZ in the study area, where these functional variables significantly differ (p < 0.05) in all salinity zone (except in LSZ, p < 0.05) (Fig. 3a). Consequently, BGBC and TBC of both dominant species decreased with salinity, but these functional variables significantly differ only for H. fomes (p < 0.05). Although, AGBC of H. fomes showed no significant difference (p > 0.05) with salinity, this was increased with salinity for both dominant species in study area (Fig. 3b). However, BGBC significantly differ (p < 0.05) with salinity contrast to AGBC and TBC (p > 0.05) in case of E. agallocha in the study area (Fig. 3b). Furthermore, majority functional variables complementary such as AGBC (LSZ and MSZ) and BGBC, TBC (MSZ and SSZ) of E. agallocha were decreased by reversing increasing pattern of AGBC (SSZ) and BGBC, TBC (LSZ). However, all functional variables complementary in two zones (out of total three zones) viz. MSZ and SSZ were decreased except in LSZ of the study area (Fig. 3c). Species complementary (AGBC) (%) = {(AGBCSSZ − AGBCMSZ )/AGBCMSZ }×100 (3) Species complementary (BGBC) (%) = {(BGBCSSZ − BGBCMSZ )/BGBCMSZ }×100 (4) Species complementary (TBC) (%) = {(TBCSSZ − TBCMSZ )/TBCMSZ }×100 (5) where, AGBC is the mean Above Ground Biomass Carbon, BGBC is the mean Below Ground Biomass Carbon (BGBC), TBC is the mean Total Biomass Carbon and LSZ is the Less Salinity Zone, MSZ is the Moderate Salinity Zone, SSZ is the Strong Salinity Zone consecutively. Gradually, species complementary of the dominant species was also calculated thought rest other zones to unravel the complementary effect in the salinity region in the Sundarbans mangrove forest of Bangladesh. 4. Discussion 4.1. Variation in structure of H. fomes and E. agallocha across the zones and with salinity This study demonstrates the distinctive structural parameters of dominant mangrove species and their changes with salinity. However, significant variation of structural parameters of dominant species was observed across salinity zones, and again, increasing salinity had a negative impact on those structures over the two years, which strongly supports our hypothesis in this study. Furthermore, structural parameters, such as, height (m), DBH (cm) and basal area (m2 ha−1 ) of E. agallocha and H. fomes, were maximum in LSZ and MSZ consecutively followed by rest of the other zones (Table 1). The reason behind this variation could be due to the presence of mature stands of large-sized dominant E. agallocha and H. fomes in LSZ and MSZ, compared to other zones (E. agallocha in MSZ, SSZ, and H. fomes in LSZ, SSZ). Again, H. fomes showed a higher growth rate than E. gallocha, which could be due to the prepotent nature of height and DBH of H. fomes in the study area, and this natural phenomenon of dominancy in terms of height and DBH was mentioned by Rahman et al. (2015) in the Sundarbans. Furthermore, the mean density (trees ha−1 ) chronology in salinity zone of E. agallocha and H. fomes was MSZ (585) > SSZ (362) > LSZ (295) and LSZ (450) > MSZ (180) > SSZ (80) consecutively and E. agallocha gained top position in this aspect 2.4. Statistical analysis To check the variations of stand structure, biomass and carbon stocks changes across salinity zones, we demonstrate the two-way-ANOVA followed by Post-Hoc (Tukey HSD) test, when significant variation were found. In addition, we applied correlation regression to test the salinity gradients impacts on structure and functional variables such as biomass and carbon stocks. All statistical analysis and visualization have been done in R (version 4.1.1) (R Core Team, 2020). 3. Results 3.1. Variation in structure of H. fomes and E. agallocha with salinity (i.e., zones and gradient) Studied dominant species stand structural attributes across three different salinity zones were presented in Table 1. Mean height, DBH and basal area of E. agallocha and H. fomes is more in LSZ (8.52 m, 11.21 cm and 0.008 m2 ha−1 ) and MSZ (9.25 m, 14. 4 M.A. Islam, S. Ahmed, T. Dey et al. Regional Studies in Marine Science 55 (2022) 102589 Fig. 2. Showing the increments of variables changes over time and their response to salinity. [Height (m), coefficient of variation of height (CVH), DBH (cm), coefficient of variation of DBH (CVDBH) and basal area (m2 ha−1 )]. To increase clarity, vertical and horizontal lines across the axis denote values were shown. Table 1 Structural parameters (±standard deviation) of H. fomes and E. agallocha in the salinity zones of Sundarbans, Bangladesh. Species Zone Mean height (m) Mean DBH (cm) Mean density (trees ha−1 ) Mean basal area (m2 ha−1 ) E. agallocha LSZ MSZ SSZ 8.52 ± 2.01 7.19 ± 1.02 6.96 ± 1.31 11.21 ± 2.94 9.49 ± 1.88 8.53 ± 2.67 295 ± 88 585 ± 195 362 ± 113 0.008 ± 0.005 0.006 ± 0.002 0.004 ± 0.003 H. fomes LSZ MSZ SSZ 9.24 ± 3.04 9.25 ± 3.68 5.27 ± 4.52 12.58 ± 4.65 14.47 ± 6.61 7.52 ± 7.13 450 ± 156 180 ± 88 80 ± 64 0.012±0.006 0.018±0.013 0.008±0.011 Table 2 Biomass (Mg ha−1 ) and carbon stocks (Mg ha−1 ) (±standard deviation) across salinity zones in the study area. Species Zone AGB BGB AGC BGC TB TC E. agallocaha LSZ MSZ SSZ 0.26 ± 0.09 0.25 ± 0.041 0.20 ± 0.05 1.70 ± .18 1.25 ± 0.54 0.86 ± 0.68 0.13 ± 0.03 0.12 ± 0.02 0.10 ± 0.02 0.84 ± 0.58 0.62 ± 0.27 0.43 ± 0.34 1.96 ± 1.27 1.50 ± 0.57 1.06 ± 0.73 0.98 ± 0.62 0.75 ± 0.28 0.53 ± 0.36 H. fomes LSZ MSZ SSZ 0.34 ± 0.11 0.39 ± 0.15 0.25 ± 0.20 3.12 ± 1.85 4.71 ± 3.58 2.05 ± 3.07 0.16 ± 0.05 0.19 ± 0.07 0.12 ± 0.10 1.56 ± 0.92 2.35 ± 1.79 1.03 ± 1.54 3.46 ± 1.94 5.11 ± 3.72 2.30 ± 3.25 1.73 ± 0.97 2.55 ± 1.86 1.15 ± 1.62 (Table 1). This variations of tree density of E. agallocha could be happened due to preference of high salinity and downstream habitat (MSZ and SSZ), but in contrast with this H. fomes can withstand more in less salinity and upstream habitat (LSZ) (Sarker et al., 2016; Iftekhar and Saenger, 2008; Kamruzzaman et al., 2017b; Aziz and Paul, 2015). Again, such variation of tree density might have happened due to presence or absence of mature stand in each zone and this finding was repealed earlier by Ahmed and Kamruzzaman (2021) and Kamruzzaman et al. (2017b) in the Sundarbans. Again, all structural parameters showed significant variation with increasing salinity (except BA of E. agallocha) of both dominant species (Fig. 2). In this study, excessive salinity intrusion (Rahman et al., 2015; Crooks et al., 2011) parsed out of such depicting stand structures. In addition to this, an inappropriate balance of fresh water influx interchanged salinity zone (Sarker et al., 2016; Rahman et al., 2015; Sarker et al., 2019; Ahmed and Kamruzzaman, 2021; Hossain et al., 2021; Iftekhar and Saenger, 2008; Kamruzzaman et al., 2018) simultaneously, which might be responsible for variation of growth with salinity, and these findings are identical with Ghagramari (higher salinity forest site) in the Sundarbans (Ahmed and Kamruzzaman, 2021). But, might be due to coppicing potentiality, basal area of E. agallocha may not significantly differ in the study area. Whereas, another study showed, plants growth could be shrunken as saline levels rise (Ahmed and Kamruzzaman, 2021; Karim and Karim, 1993; Sarker et al., 2016; Hossain et al., 2021; Iftekhar and Saenger, 2008; Rahman et al., 2015; Mitra et al., 2004) and these findings is analogous with this study especially in SSZ to MSZ (Fig. 2). In addition, another study conducted in the mangrove species of Pakistan indicated that tidal inundation was an important parameter of balancing water as well as salinity variation in the mangrove forest floor (Khan and Aziz, 2001; Ward et al., 2016) and this may be responsible for diminishing structures in SSZ as this zone acquires more salinity due to its prelocated downstream habitat. Nonetheless, excessive salinity induces stomatal conductance and water absorption potential, which awfully hinders the physiological mechanism and growth of mangroves (Khan and Aziz, 2001). This strongly corroborates our findings of losing structures of dominant species with salinity. However, mangrove growth is ideal in the western (fresh water) area of the Indian Sundarbans compared to the center region (hypersalinity zone) (Mitra et al., 2004, 2011) and this unfavorable salinity hinders the growth of mangroves through early leaf fall (Mitra et al., 2011). Again, these findings are repeated from LSZ to SSZ in this study. Furthermore, several studies (Ahmed and Kamruzzaman, 2021; Khan and Aziz, 2001; Mitra et al., 2004) stated that increasing salinity reduces nutrient availability and ion toxicities and thus ultimately hampers the structure of mangrove species with salinity, which is repeated in the case of dominant species in this study. Furthermore, the CV of height and CV of DBH of observed species were negatively related and 5 M.A. Islam, S. Ahmed, T. Dey et al. Regional Studies in Marine Science 55 (2022) 102589 Fig. 3. Biomass and its changes across salinity zones and salinity gradient. Above ground biomass carbon (AGBC), below ground biomass carbon (BGBC), total biomass carbon (TBC) ((a) across salinity zones (b) salinity gradient (c) complementarity). LSZ, MSZ, SSZ denotes less, medium and strong saline zones, respectively. Letters on the top of the boxes representing the results of post-hoc (Tukey HSD) test. Same letter denotes no significant difference. significantly varied with salinity (Fig. 2), which indicates that observed species had less diverse heights and more uniform spacing of similar-sized trees. Such variation in CV of height and CV of DBH with salinity illustrates that the diversity of that species in terms of height and DBH was stunted. This could be due to the effect of increasing salinity and variation in habitat conditions. The irregular chronology of attaining mature tree stands from seedlings may also be responsible for such variation. The Ganges freshwater flow into the Sundarbans has declined from 3700 m3 s−1 to 364 m3 s−1 since the completion of the Farakka dam in India in 1974 (Mahmood et al., 2021; Sarker et al., 2016). In the Indus river basin of Pakistan, diversion of fresh water flow increased salinity, which has resulted in stunted growth of mangroves (Khan and Aziz, 2001). This type of decreasing scenario of stand structures of mangrove species with increasing salinity, as in this study, was observed earlier in different parts of the world (Banerjee et al., 2017). So, the dominant species of the Sundarbans have faced such an increasing pattern of salinity and thus have been acclaimed to be losing their structures tremendously. Thus, the findings of this study significantly inferred that the structural composition of dominant mangrove species in the SRF, Bangladesh varies with salinity, and this condition is really indicates to foster the management strategies of dominant mangrove species. 6 M.A. Islam, S. Ahmed, T. Dey et al. Regional Studies in Marine Science 55 (2022) 102589 this study and thus creates variation in BGBC, TBC, and AGBC in this study (Fig. 3a). The total biomass of dominant species in this study is 15.38 Mg B ha−1 (Table 2), which is more than R. mangle’s stand (12. 5 Mg B ha−1 ) at Florida, USA Mitra et al., 2011; Coronado et al., 2004) and less than secondary mangrove forest (C. tagal) in Southern Thailand (Mitra et al., 2011). Again, the calculated biomass is comparable to that of other studies, such as dominant mangrove species in the Indian Sundarbans (1.8–23.97 Mg B ha−1 ), young plantations of Rhizophora apiculata in Thailand (2.1–15.0 Mg B ha−1 ), a Rhizophora patch in Matabungkay Beach, Batangas Province (13.3 Mg B ha−1 ), and a mature Rhizophora forest in Southern Thailand (8.1 Mg B ha−1 ) (Mitra et al., 2011; Cruz and Banaag, 1967). Again, these dominant species produce 7.69 Mg C ha−1 and as mentioned earlier, a significant amount of carbon is stored in the vegetation types of E. agallocha- H. fomes (Vegetation Type-VT6) and H. fomes E. agallocha (Vegetation Type-VT2) in the Sundarbans mangrove forest (Rahman et al., 2015). Furthermore, the current study demonstrated that the functional variables complementary to dominant species are negative in different salinity zones (Fig. 3c), which strongly supports our hypothesis. However, both dominant species’ production in terms of AGBC and salinity were increased (Fig. 3b). This increasing pattern of AGBC may be due to the presence of large trees, and similar findings were also observed in S. apetala by Ahmed and Kamruzzaman (2021) in the Sundarbans. But the decreasing pattern of BGBC and TBC (Fig. 3b) could be due to variation in salinity in the forest floor and irregularity in fresh water influx in the study area. Again, increasing salinity induces forest growth and structure by fostering soil anaerobic conditions and thus reducing functional productivity (i.e., BGBC and TBC), which was repeated in the Indian Sundarbans (Chowdhury et al., 2019). Therefore, soil organic carbon is a predominant indicator of soil fertility (Begam et al., 2020) and productivity (viz., BGBC and TBC) may be restricted by decreasing soil organic carbon with increasing salinity (Rogers et al., 2019), which is also repeated in this study. On the other hand, the BGBC of E. agallocha exhibits significant differences with salinity, which contrasts with AGBC and TBC (Fig. 3b), and this difference in BGBC may be caused by the influence of salinity in the salinity zone, nutrient variation, and tidal inundation. Structural parameters viz. height, DBH, basal area, followed a similar chronology of growth pattern and through which BGBC was not varied with salinity, which ultimately repeated in TBC in the study area. On the contrary, H. fomes also postulates similar growth phenomena with no variation of AGBC, but this dominant species functional variation (i.e., BGBC and TBC) is significant with salinity (Fig. 3b). This variation may be due to adaptive traits of dominating species (Ayma-Romay and Bown, 2019) and competition for resources in soil (Ahmed and Kamruzzaman, 2021). Similarly, another observation by Sonkoly et al. (2019) in terms of potential growth rate based on species abundance may be responsible for biomass and carbon growth. However, AGBC is increased by the dominance of particular species (Ayma-Romay and Bown, 2019; Sonkoly et al., 2019), and this study also repeated these findings. Another observation in the C. tagal dominated mangrove forest of Pakistan revealed that alteration and increasing salinity reduce stomatal conduction and water absorption capacity, which are responsible for diminishing biomass and carbon productivity (Khan and Aziz, 2001). This could be another reason for E. agallocha’s function loss. Salinity increase in mangrove ecosystem is crucial mechanism from salinity zone to zone which may propel functional activities of mangrove species. Therefore, E. agallocha indicates negative complementary in LSZ, MSZ (in terms of AGBC) and MSZ, SSZ (in terms of BGBC and TBC) in the study area. This phenomenon 4.2. Biomass, carbon stocks and their changes with salinity The comprehensive study investigated the functional variables of dominant species with salinity. The results show that the functional productivity of E. agallocha (BGB) and H. fomes (BGBC and TBC) significantly varies with salinity, while the rest of the others do not (Fig. 3b), and these findings mostly support our hypothesis. The present study observed more biomass and carbon of dominant E. agallocha and H. fomes in LSZ and MSZ consecutively compared to other zones, and H. fomes was more productive than E. agallocha (Table 2). The reason for this variation could be the greater vigor of structural attributes (i.e., height, DBH, and basal area) in LSZ and MSZ compared to other zones (Table 1). The prepotent nature of E. agallocha and H. fomes in those two zones may also responsible of such variation (Mahmood et al., 2021; Sarker et al., 2016; Iftekhar and Saenger, 2008). Such functional trait dominance in terms of stand structural attributes has been observed in temperate deciduous forest of the eastern United States and northern Taiwan (Fotis et al., 2017), in upper Peninsula of Michigan, United States (Fahey et al., 2015), and in case of A. saccharum and L. tuliptera (Fahey et al., 2015; Fotis et al., 2017). However, some species, viz., C. alba and P. boldus, may capture more light with the increase in height in the Mediterranean region of Central Chile (Ayma-Romay and Bown, 2019), even though under in stress condition (Prado-Junior et al., 2016; Fotis et al., 2017); this acquisitive phenomenon of functional activities was repeated in dominant species (Ayma-Romay and Bown, 2019; Prado-Junior et al., 2016; Rahman et al., 2015) and viceversa (Prado-Junior et al., 2016; Fotis et al., 2017), which reflects our findings in terms of TB and TC in the salinity zone (Table 2). Besides, vegetation pattern with species association and succession could be another mechanism of such variation of functional traits in this study (LSZ SSZ). Being the tallest species (except Avicennia officinalis and Sonneratia apetala) (Rahman et al., 2015), H. fomes dominated forest was more productive (Rahman et al., 2015) and these results are identical in this study. In this study, the total AGBC, BGBC and TBC increments were greater in LSZ, followed by MSZ and SSZ of both dominant species (Fig. 3a). Similar results were found by Rahman et al. (2015) and Crooks et al. (2011) by indicating that more vegetation produces more biomass and carbon in LSZ. In addition, tree diameter is the key determinant in regulating functional productivity (Rahman et al., 2015; Ruiz-Jaen and Potvin, 2011) and this study was also conducted by taking tree diameter into account in the allometric equation. So, this reason may work to increase AGBC, BGBC, and thus TBC productivity. Basal area of dominating species (RuizJaen and Potvin, 2011; Rahman et al., 2015), and density of mature tree species (Rahman et al., 2015) with less mortality (Ahmed and Kamruzzaman, 2021) may accelerate more dominant stands in terms of diameter in this study, and all of these posit regular annual growth in LSZ to increase annual increment of functional variables. However, in SSZ to MSZ, this growth pattern of dominant species contrasts with that of LSZ, and along with this, salinity might be responsible for losing functional variables from SSZ and MSZ. However, the AGBC increment of E. agallocha and H. fomes did not significantly differ in LSZ compared to MSZ and SSZ (Fig. 3a), which could be due to the presence of vigorous stand structural attributes in LSZ compared to other zones of both species (Table 1). On the contrary, BGBC and TBC increments differ significantly across all zones (Fig. 3a), with salinity being a key factor in this variation. On the contrary, regardless of how tall or little the mangroves are, they store a substantial quantity of carbon in their sediment (Rahman et al., 2015; Ahmed and Kamruzzaman, 2021), and because of the high root-shoot ratios, greater soil, and root c in mangrove forests, BGB is more than AGB (Ahmed and Kamruzzaman, 2021; Komiyama et al., 2008), which is identical in 7 M.A. Islam, S. Ahmed, T. Dey et al. Regional Studies in Marine Science 55 (2022) 102589 to nutrient exchange capacity and water solubility (Ahmed and Kamruzzaman, 2021; Khan and Amin, 2019) functional variables are positive in LSZ of the study area. Wind exposition, water runoff, topographic factors (slope and aspect) may also responsible productivity (Dawud et al., 2016) and these factors may also be accountable for the loss of functional variables in this study. Nevertheless, the interaction of different environmental factors, salinity exchange and zonation (Ahmed and Kamruzzaman, 2021; Rahman et al., 2015; Khan and Aziz, 2001; Karim and Karim, 1993) may be one of the key factors of deterring functional variables in these zones. However, above ground competition, changing temperature, geographical location, organic matter (Ahmed and Kamruzzaman, 2021; Macro et al. 2018), crown area (Macro et al. 2018; Prado-Junior et al., 2016), microclimate influences (Fotis et al., 2017; Ahmed and Kamruzzaman, 2021), trees physical condition (i.e. tree size distribution), human pressure (Kamruzzaman et al., 2018) and biological attributes (i.e. leaf traits) (Fotis et al., 2017), stand age, management, species dilution could also affect combined stand structure and thus functional variables in the Sundarbans mangrove forest, Bangladesh. may be due to the less preferred upstream habitat of this species (Hossain et al., 2021; Sarker et al., 2016) and also inability to withstand chronological changes of salty environment (Sarker et al., 2019; Hossain et al., 2015, 2021; Iftekhar and Saenger, 2008). Again, massive production of seedling and insufficient survival of E. agallocha in these two zones may cause such a negative impact, and this pattern is vice-versa in SSZ by indicating positive complementary in the study area. Additionally, positive complementary in LSZ (viz. BGBC, TBC) may be due to the vigoras stand, influence of other species in terms of belowground productivity, and litterfall in this zone. Furthermore, having more tree density in MSZ and SSZ, the observed negative complementary indicates that the forest stand is young (Table 1), which may be responsible for the loss of functions with salinity. In addition, salinity may affect the environment of each salinity zone, which impedes functional productivity (Rahman et al., 2021b,a; Sarker et al., 2016). On the other hand, due to diminishing stand structure (Table 2), excessive stress with salty environment (Rahman et al., 2015; Ahmed and Kamruzzaman, 2021; Sarker et al., 2016), and nutrient availability variation in soil (Ahmed and Kamruzzaman, 2021) such losing functions in this study could be possible factors. Another noteable phenomenon may be the presence of smaller trees of E. agallocha in the study area, which contribute less biomass by losing their function, as was earlier mentioned by Binkley et al. (2006). Like in this study, a similar phenomenon was repeated in the mangrove forest of Myanmar (Ward et al., 2016). All the functional variables (i.e., AGBC, BGBC and TBC) of H. fomes in this study indicates negative complementary in MSZ and SSZ (Fig. 3c). In these two zones, fresh water supply has been dropped significantly (Sarker et al., 2016) which may have restricted the physiological responses of this species. Besides, habitat preference of H. fomes with salinity is inversely related to zones (i.e., MSZ and SSZ) (Sarker et al., 2016). So, these factors simultaneously affect the functional variables of H. fomes in MSZ and SSZ. Another devastating warning by Iftekhar and Saenger (2008) and Sarker et al. (2016), top dying disease affects roughly 70% of the surviving H. fomes trees, which is repeated in this study as low density (viz., MSZ and SSZ) (Table 1), resulting negative competition, indicating that this species is losing functions. While several reports also mentioned slow growth of this species in terms of height and DBH increment per year, it was observed earlier due to increasing salinity (Sarker et al., 2016), which resembles our finding by hindering growth and ultimately to accelerating lose functions in MSZ and SSZ. But, in contrast with that, this dominant species complementary is positive with salinity in LSZ (Fig. 3c). It could be because of the connection of fresh water-dominated Baleshwar river in eastern region, which provides convenient environment to H. fomes (Sarker et al., 2016). Environmental conditions in LSZ may aid to faster growth by adding more biomass, as mentioned by Temmerman et al. (2012). Habitat fragmentation (Sarker et al., 2016), disease and pest, pollution such as, water and soil pollution etc., Iftekhar and Saenger (2008) and Hossain et al. (2021) may be responsible for such a loss of growth and functional variables in salinity zones. Similarly, loss of CV of height and CV of DBH of these dominant species may be another factor of such negative complementary with salinity. Several experiments repeated this phenomenon by mentioning salinity negatively impact mangrove forest growth (Yoshikai et al., 2021) and confines functional activities (Chowdhury et al., 2019). Therefore, the mortality of mature trees in the study area is another prime factor in reducing growth and variation of dominant E. agallocha and H. fomes in each salinity zone, which was observed at Ghagramari forest sites in Sundarbans conducted by Ahmed and Kamruzzaman (2021). Besides, due 5. Conclusions This study of dominant mangrove species (i.e., E. agallocha and H. fomes) revealed that increasing salinity declined the stand structure of highlighted dominant species in the Sundarbans mangrove forest. Furthermore, the loss of functional variables of dominant mangrove species with salinity indicates that these species, as well as the mangrove ecosystem, are severely vulnerable to salinity. The results of this study illustrated that this chronological pattern of reducing growth with increasing salinity may promote dwarf mangrove species, which indicates increasing salinity could lead dominant species to the verge of extinction in the near future, especially H. fomes. Thus, this ultimately reduces the productive potential of the mangrove ecosystem in terms of biomass and carbon. However, results of this study suggest a significant amount of biomass and carbon is stored in the dominant mangrove species, which must be a major concern when selecting mangrove species for restoration. In addition, considering global climate change as well as sea level rise, the loss of structures and functions of mangrove species is really a pivotal management concern. However, proper water management planning, combined with reduced anthropogenic causes, may play a coherent role in optimizing the salinity effects on the mangrove delta. As a result, additional research may be required to encourage more research by including more sample plots in the deeper portion of the forest and comparing it to coastal plantation. Besides, forest managers and policy makers should be emphasized more to maintain the functions of dominant species with respect to salinity. This study concludes that, in spite of the potential role of dominant E. agallocha and H. fomes in mangrove forest communities locally and globally, these species are losing functions with salinity, which corroborates the inevitability of their sustainable management. 6. Limitations of the study To conduct studies in natural ecosystems have some inherent problems which may occur due to different biotic and abiotic variables to influence functional variables. We accredit that, we consider only trunk living trees to estimate biomass and carbon, but we are able to postulate a explanation of losing functions with salinity which mostly supports our hypothesis. At the same time, geographic and climatic variation may affect the variables with salinity, but repercussion is relatively small as we used best fitted allometric equation to calculate functional variables e.g., biomass and carbon. 8 M.A. Islam, S. Ahmed, T. Dey et al. 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Akramul Islam: Conceptualization, Data collection, Formal analysis, Writing - original draft, review and editing. Shamim Ahmed: Conceptualization, Formal analysis, Software, Visualization, Writing - original draft, Writing - review & editing. Tanmoy Dey: Data collection, Final review. Rahul Biswas: Data collection, Final review. Md. Kamruzzaman: Data collection, Final review. Shanewas Hossain Partho: Data collection. Biplab Chandra Das: Data collection. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgments We are grateful to the field personnel’s support to collaborate throughout data collection. Our sincere gratitude goes to Forestry and Wood Technology Discipline, Khulna University, Bangladesh, to provide technical support with data collection. Appendix A. 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