INTRODUCTION
Mangrove forests are woody plant communities composed of evergreen shrubs or trees with mangrove plants as the main component. They grow in upper intertidal zones of tropical and subtropical low-energy coasts and are periodically flooded by tidal water (Doughty et al., 2017; Duke et al., 2007; Palacios and Cantera, 2017; Sheng, 2017). Mangroves grow on the tidal flats at the junction of land and ocean and constitute a transitional ecosystem from land to ocean. Mangroves are one of the main forest types serving as a carbon sink in tropical regions and provide unique habitats, diverse species, obvious ecological functions, and good social benefits; therefore, mangroves possess high values for scientific research, education, and ecological protection (Donato et al., 2011; Mcleod et al., 2011). Natural plant communities and their composition diversity are severely deteriorating in the current stage of intensive land use and global warming. The settlement, distribution, extinction, and survival of biological species in natural communities have become a critical global topic (Pimm et al., 2014; Martin et al., 2016). Specifically, due to the simultaneous impact of human disturbance, global warming, and sea-level rise, mangroves are undergoing significant changes (Blasco et al., 2001; Carugati et al., 2019). The mangroves in the intertidal zone are a dynamic ecosystem. Since they appeared 75 million years ago, climate change, rising sea levels, and human disturbance have significantly changed the distribution of mangroves (Friess et al., 2019). At present, sea-level rise will continue to affect the global distribution of mangroves, bringing great uncertainty to the future survival and development of mangroves (Apichaya et al., 2019, Friess et al., 2019).
A large number of studies on the ecology, biology, and restoration of mangrove ecosystems have tried to reveal the uniqueness of mangrove communities from various aspects and to predict the future of mangroves (Ilka et al., 2017; Carugati et al., 2019). At present, the research and discussion topic of most importance is how sea-level rise affects the survival of mangroves; however, the discussions mainly focus on whether sea-level rise leaves any space for mangroves to grow. Mangroves cannot survive in the oceans that rise more than 7 mm per year. At present, the global sea level is rising at an average annual rate of 3.4 mm, which is close to half the speed needed to threaten the survival of mangroves (Rebecca et al., 2020). Some researchers predicted that if CO2 emissions are not reduced, mangrove survival will be in danger by 2050 (Gholami et al., 2020). However, research on the physiological and ecological responses of mangrove species to rising sea levels is scarce. It is still unclear whether the physiological and ecological responses to rising sea levels are different between mangrove communities in the high tidal zone and the low tidal zone.
The future developmental trend and distribution of mangrove species are restricted by many factors, such as temperature, CO2, salinity, light, nutrients, and flooding, which will affect the survival and growth of mangrove seedlings (Ken et al., 2007). Flooding is considered the most crucial factor, and the physiological and ecological adaptation of mangrove trees caused by flooding is also considered important (Ariel et al., 2007; Gonasageran, 2016; Friess et al., 2017). It is not yet clear whether it is applicable to study the future developmental direction of mangroves by investigating the responses of different mangrove seedlings to rising sea levels. Over the years, many studies have shown that mangrove plants, like land plants, have evolved to adapt to changes in habitat through long-term adaptation to environmental conditions, which promote the physiological and ecological differences among species in the community (Ball et al., 1984; Rout et al., 2013). The differences in the survival, growth, or death of seedlings of terrestrial tree species are mainly caused by the inconsistent physiological responses to low light-induced stress (Bazzaz et al., 1979; Han, 2017). However, the driving factors for mangrove tree seedlings’ survival, growth, or death are far more complex than those of terrestrial forests. Mangroves may have to simultaneously endure the stress of low light, long or short flooding time, and high salinity. It is challenging to carry out experiments by simulating and simultaneously controlling three factors to study the physiological and ecological characteristics of mangrove tree seedlings in the field. In contrast, it is possible to simulate sea-level rise in the field (Aaron, et al., 1997) or to control light and salinity factors separately in the field and indoors. By comparing and analyzing the survival, growth, and photosynthetic and physiological responses of mangrove seedlings, the data obtained allow for a better interpretation of the physiological and ecological adaptation characteristics(Laura et al., 2006; 2007). However, it is not feasible to control salinity and light factors at the same time. More studies focus on the physiological and ecological adaptability, survival, growth, and photosynthesis of one to two mangrove species and their response to flooding or salinity stress. It is also urgent to study the changing patterns of enzyme activities in mangrove plants under flooding and salt stresses and to interpret the changes in the activity of these antioxidant enzymes in mangrove plants to reduce cell damage. This can improve the tolerance of mangrove plants to stresses (Parida and Jha, 2010) or allow them to adapt to unfavorable ecological environments through changes in stomata and coordinating water functions (Huai-Tong, 2017). Moreover, the performance of tree seedling growth in different tidal zones and the changes in antioxidant enzyme activities under adverse environments are different. These changes will not only extensively affect the future dynamics of mangrove seedlings in different tidal zones but also help in understanding how mangrove species in different tidal zones respond to global factors and regional influences, as well as the survival and growth of mangrove seedlings, which in turn affects the future of the mangrove community (Ken 2007; Friess et al., 2017; Ilka et al., 2017).
In the past 50 years, due to urban development, aquaculture, and other human activities, a large area of ​​coastal space has been invaded, and about 73% of the mangrove planting area in China has disappeared (Fan et al., 2017). There are 25 species of mangrove plants distributed in Hainan, which is a province with the most species of mangrove plants in China (Chen et al., 2017; Yang et al., 2019). For this study, the representative mangrove plants, including Bruguiera sexangula andCeriops tagal at high tidal levels, Kandelia obovata andRhizophora stylosa at medium tidal levels, and Aegiceras corniculatum and Avicennia marina at low tidal levels were selected. The growth and antioxidant enzyme activities of these seedlings were investigated under various light, flooding, and salinity conditions to compare and analyze their physiological and ecological responses to sea-level rise. This study tried to determine the differences in the growth and physiological responses of mangrove plant seedlings to sea-level rise at different tidal levels to predict the developmental trend of mangrove plants distributed across various tidal levels.