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.