Mangrove community and abundance of Avicennia and Sonneratia species
The habitat diversity in the study areas contributes to the Avicennia and Sonneratia species diversity confirmed as present. In contrast to environmental factors such as water salinity and substrate in each study area, habitat characteristics determined the diversity, distribution, and abundance of Avicennia and Sonneratia species. In particular, the study areas exhibited dissimilarities. As defined by Rani et al. (2018), the similarity index value ranges from 0 to 1, where the higher value of the index of species similarity explains the higher level of similarity of species between the two communities compared and can also be interpreted as the higher the index value of the similarity of species. Avicennia alba and A. marina thrive in a wide salinity range, from 15 to 32 at Teluk Buih and as low as 12 to 19 at Pengkalan Nangka lagoon, which was similarly reported by Purwanto et al. (2022). Avicennia rumphiana (synonyms: A. lanata, A. officinalis and A. officinalis var. spathulata) was notably found thriving further inland with a salinity of 15 to 32 at Teluk Buih, which differs from the habitat preferences of other Avicennia and Sonneratia species. Conversely, Sonneratia alba was observed in mangrove habitats such as Pulau Burong and Teluk Buih, where water salinity levels ranged from 31 to 35. A similar trend of S. alba thriving in high-salinity areas was reported in the Philippines (Raganas et al. 2020). However, S. caseolaris inhabits the Pengkalan Nangka lagoon with a lower salinity range of 12 to 19. Ecologically, S. caseolaris thrives in brackish to freshwater environments in the upstream estuarine zone with lower salinity and deep muddy soil (Tatongjai et al. 2021). Thus, A. rumphiana and S. caseolaris exhibited adaptability to lower salinity in landward areas compared with other species, potentially explaining the limited species diversity in this study.
Ecological diversity and habitat characteristics
The environmental characteristics and habitat diversity exhibited notable differences. At Pulau Burong, mangroves were observed thriving in the challenging environmental conditions of this small island, a phenomenon previously undocumented on small Malaysian islands. Selvam and Karunagaran (2019) used the term "shrub mangroves" to describe a similar habitat in extreme environments with limited nutrient resources and infrequent freshwater availability. In such areas, trees are typically stunted, rarely exceeding a height of 1 meter. Consequently, the novel structure of these mangroves on the small island presents a unique opportunity to investigate previously unrecognized ecological and biogeochemical processes that require in-depth exploration. Notably, the mangroves within the Pengkalan Nangka lagoon were distinctly classified as an inland lagoon habitat while the coastal mangrove area observed in Teluk Buih encompassed a combination of landward and seaward habitats. The mangrove fringe in Teluk Buih corresponds to habitats described in previous studies, where usual models of mangrove area mixing typically distinguish between two distinct end associates: freshwater and seawater.
Salinity levels within the mangrove environment vary from brackish to seawater. The salinity ranges in this study were categorized according to the classification outlined by McLusky and Elliott (2004) and Den Hartog (1960). Lower salinity was evident in the Pengkalan Nangka inland lagoon and the landward region of Teluk Buih. In contrast, higher salinity levels were documented along Teluk Buih’s open coast and the isolated island of Pulau Burong. The Pengkalan Nangka lagoon's lower salinity is caused by seawater and freshwater intrusion. This salinity decrease extends gradually from the open water gate to the nearby shallow coastal area, indicating that more significant upstream freshwater input leads to a low-salinity gradient than other study areas. The data presented here emphasize the crucial role of water salinity stress in shaping mangrove ecosystems within the study areas. Water salinity is an important ecological factor influencing mangrove flora and fauna's distribution, survival, and species diversity (Irawan et al. 2021). In the study conducted by Shaltout et al. (2020), it was observed that the productivity of mangroves can be impacted by salinity. The present study reveals that different mangrove species, particularly Avicennia and Sonneratia species, exhibit distinct salinity preferences and thrive optimally at varying salinity levels, as shown by Sonneratia alba and S. caseolaris. Although many mangrove species can thrive fluctuating salinity, their tolerance levels vary considerably among species. Nonetheless, Asaeda and Barnuevo (2019) advise that growth inhibition and mortality can occur when salinity levels change significantly from specific limits, either too high or too low.
Vegetative and reproductive morphology adaptation of Avicennia and Sonneratia
True or exclusive mangrove plants exhibit well-developed structural and physiological adaptations across diverse geographical regions. These have been mentioned on mangroves, e.g., Latif (2012) and Chapman (1976). As a result, mangrove tree species have evolved diverse adaptive mechanisms to cope with the mangrove environmental challenges (Krauss et al. 2023). This study identified two key variables influencing the morphology patterns of Avicennia and Sonneratia species in the study areas: water salinity and substrate.
Based on these findings, Avicennia and Sonneratia have exhibited distinct and prominent root adaptations to thrive in challenging environments. Despite facing turbulent and saline coastal conditions, mangroves demonstrate survival through their unique root systems used for respiration (Pratolongo 2022). The most typical adaptations observed in mangrove species within the study areas encompass various specialized root types: stilt/prop roots (Avicennia marina), cable roots with pneumatophores (A. alba, A. marina, and S. alba), pencil roots (A. rumphiana), and cone roots (Sonneratia caseolaris). Both species have developed functional, modified, aboveground root systems, such as stilt roots/prop roots and aerial roots, as opposed to the conventional belowground tap root system. Interestingly, in A. marina from Pengkalan Nangka lagoon, pneumatophores' distribution suggests a possible connection to gravity sensing mechanisms linked to nutrient uptake and responses to waves and currents. This observation aligns with previous research by Thampanya et al. (2002), highlighting how mangroves use aerial root structures to absorb the impact of waves and currents, allowing growth in intertidal regions. As documented in this study, the unique, specialized prop roots emerging from the branches of A. marina in Pengkalan Nangka lagoon correspond with previous findings reported by Muta Harah and Japar Sidik (2020). This suggests that pneumatophores failed to adapt to sediment deposition, leading to the development of prop/stilt roots from the trunk. In addition, our results highlight the influence of substrates on pneumatophore growth. Moreover, the branching of cable roots was more distinct in rocky fissures than in muddy and sandy substrates. In the pneumatophores discussed in this study, lenticels emerge down the pneumatophores of both A. marina and A. rumphiana. Although the presence of lenticels on pneumatophores was not extensively explored in this study, as the primary focus was on root anatomy, the numerous pores known as pneumathotodes were observed at the tips of S. alba pneumatophores on Pulau Burong. In particular, mangrove roots serve as critical nursery habitats for juvenile fish, providing sanctuary from larger fish and birds in shallow aquatic environments. Hence, a greater abundance of pneumatophores indicates improved environmental quality, potentially leading to the proliferation of spawning aquatic organisms beneficial to human interests.
This study examined variations in leaf length, width, and thickness as responses to salt stress, focusing on evaluating plant salt tolerance. One-way ANOVA and Duncan’s post hoc results revealed significant differences (p<0.05) in all leaf parameters for both genera, indicating the influence of different habitats and salinity levels on these morphological characteristics. Avicennia species, notably A. alba and A. marina exhibited larger leaf sizes (both in terms of length and width) in environments characterized by lower salinity levels, such as Pengkalan Nangka lagoon and Teluk Buih, in contrast to Pulau Burong, which had the highest salinity. These results align with previous research by Naskar et al. (2021), where salinity stress can interfere with plants' physiological and metabolic processes, decreasing leaf area and increasing leaf thickness. Similar changes were observed in Sonneratia alba leaves from the low salinity area of Teluk Buih, which exhibited longer length and wider width than leaves from the higher salinity of Pulau Burong. The correlation matrix and linear regression results showed statistically significant associations (p<0.05) between leaf measurements (length, width, and thickness) of A. alba, A. marina, and S. alba with the water salinity gradient. These findings highlight the significant impact of water salinity on mangrove leaf morphology, emphasizing the value of the developed linear regression models. Consequently, further investigation into the factors and influences of water salinity is necessary. This study also addresses a research gap by comparatively examining the leaf size of A. rumphiana across varying salinity habitats, offering insights into this less-explored aspect of mangrove ecology.
The morphological diversity of Avicennia branchlets showed the combination of both circular and quadrangular shapes at Pulau Burong, Pengkalan Nangka lagoon, and Teluk Buih, challenging the standards described by Chan et al. (2022) and Watson and Dallwitz (1994). Although previous research often described the quadrangular stem or twigs as a typical feature of Avicennia due to survival in saline and waterlogged environments (McKee and Vervaeke 2017), our findings suggest intraspecific variability, with both circular and quadrangular branchlets reported in different habitats. These findings highlight the modification branchlet characteristics of Avicennia species and the potential further interpretation of their structural adaptations in diverse environments.
Notably, in this study, all three Avicennia species, A. alba, A. marina, and A. rumphiana, showed flowering responses. According to Raju et al. (2012), Avicennia will initiate flowering in the following monsoon showers in June and continue until August. This study highlights the cyme-like spike inflorescence of Avicennia characterized by a central peduncle that branches out, with each branch further extending and growing from the axillary or terminal position. The spike inflorescence is referring to the flowers closely attached to the peduncles. The axillary and terminal position allows for maximum exposure of the flowers to pollinators and facilitates pollen dispersal for successful reproduction (Thatoi et al. 2016). However, regarding inflorescence, we’ve observed variations in arrangement and type highlighted by several studies, such as Borg and Schönenberger (2014) and Primavera et al. (2004) among the Avicennia individuals. The variations in flower branch density results among study areas indicate a possible impact of habitat-specific factors on the reproductive success of flowers. Further research could investigate the ecological factors contributing to these patterns.