DISCUSSION
The environment can determine which plants in the regional germplasm
bank can enter and stay in this environment. Specifically, the
adaptability of these plant seedlings to the environment affects the
composition and development of the community (Kraft et al., 2015). The
antioxidant enzyme activities can reflect the differences in the
adaptability of plants to the environment to a certain extent (Krauss et
al., 2014).
Light is a key factor affecting the dynamic changes of terrestrial
forests. With the changing light intensity, the forest succession will
experience dynamic changes from the
pioneer
community to the mid-term transitional community, and finally to the
climax community (Bazzaz et al., 1979; Yang et al., 2002; Han, 2017).
However, our results show that the six mangrove species are all early
pioneer species or early mid-term species in the succession of light
environment changes. This suggests that the mangroves, represented by
only a few species, have not been formed for a long time and do not
require light differentiation. The study of Yang et al. (2003) showed
that the relative growth rate of K. obovata seedlings decreased
with decreasing light intensity. This is consistent with our results and
indicates that most mangroves are sun-loving plants, and that low light
will reduce the relative growth of the mangrove seedlings. We believe
that the main factor affecting the dynamic changes of mangrove
communities is not the light intensity in Hainan, China. The survival,
growth, development, and dynamic changes of mangrove plant communities
are mainly affected by changes in seawater salinity and flooding time
(Lin, 1997; Fan, et al., 2017; Yang et al., 2019). The physiological and
ecological responses of plants should become stronger when exposed to
both seawater salinity and flooding times compared to just one single
factor (Liao et al., 2009; Shiau, 2017). Our results demonstrate that
the effects of two or more factors on mangrove seedlings are stronger
than one single factor. Tan et al. (2014) studied the effects of
seawater salinity and flooding times on K. obovata seedlings and
found that the damage to K. obovata seedlings was more severe
under both factors, indicating that the damage to the seedlings is more
serious when exposed to multiple factors.
The first manifestation of global climate change and rising sea levels
is that the flooding times of mangrove seedlings will increase during
mangrove regeneration in areas where the sedimentation rate is slower
than that of rising sea levels. The flooding times will exceed the
suitable time lengths, and the mangrove seedlings end up in a flooded
environment (Krauss et al., 2014). Our results show that mangrove
species distributed at high tidal levels are more susceptible to the
increase in flood time caused by rising sea levels than species
distributed at medium and low tidal levels. As the sea level rises,
mangroves will move farther inland (Traill et al., 2011; Di Nitto et
al., 2014). However, as the offshore areas have been occupied by humans,
and the retreat space of mangroves has been occupied or blocked by urban
construction, animal breeding, and seawall construction (Phan et al.,
2015; Fan et al., 2017), species distributed at high tidal levels will
be negatively affected faster than species distributed at medium and low
tidal levels and distinct. When the rate of soil sedimentation is
greater than that of rising sea levels, mangrove plants can still
survive in the original area (Woodroffe et al., 2016).
Rising sea levels and salinity change in the original area (Gilman et
al., 2008) will inevitably affect the settlement, growth, and survival
of mangrove seedlings. Under normal conditions, the production of active
oxygen free radicals in plant cells takes place by homeostasis. When
plants are stressed, the balance is broken, and the active oxygen free
radicals will accumulate in the cells. This leads to an increase in
antioxidant enzyme activities, and the accumulated active oxygen free
radicals will be eliminated to ensure normal physiological processes in
plants. Plant resistance to adverse environments is often closely
related to antioxidant enzyme activities. The stress response of plants
is generally manifested by increasing the antioxidant enzyme activities
to eliminate the accumulation of free radicals and protect the membrane
structure (Hodgson et al., 1987). Our results demonstrate that within
the gradient ranges set in this experiment, mangrove species distributed
at low and medium tidal levels are more tolerant to salinity than those
distributed at high tidal levels in terms of seedling growth indices and
antioxidant enzyme activities. As the sea level rises and the flooding
time increases, the salinity stress of species increases, and species
distributed at high tidal levels, such as E. agallocha , L.
littorea , L. racemose , B. sexangula , and C. tagalin Hainan, China, are more susceptible to stress. In areas with no or
small amounts of freshwater supplements, species distributed at high
tidal levels will be immediately damaged by the increasing salinity
stress, resulting in restricted growth and population renewal, and even
death or extinction. For species distributed at medium and low tidal
levels, the increase in salinity is within their tolerance range, and
they can still grow. When the salinity exceeds their tolerance range,
their growth and
population
renewal will be restricted. In areas with sufficient freshwater supply,
the mangroves’ tolerance to flooding is the limiting factor.