CONCLUSION
This study shows that human disturbances, such as deforestation,
destruction of habitats by the shoreline hardening, and animal breeding,
can have fatal effects on mangroves. Their impacts are even more severe
in the context of global warming and rising sea levels. The main reason
is the special environment required for the survival, growth, and
development of mangrove plants. There are only 86 mangrove species
worldwide and 27 species in China, of which 25 are distributed in
Hainan. It is not possible to cultivate mangrove species that adapt to
the rapidly changing environment. The findings of this study indicate
that most of the existing mangrove species require full sunlight or only
prefer 20% shading. Under 40% shading treatment, all six mangrove
species showed poor growth, and the overall niche differentiation of
light demand was low. Mangrove communities could not form a long-lasting
and stable ecosystem like terrestrial forests. The survival, growth,
development, and dynamics of mangrove communities are mainly affected by
the changes in seawater salinity and flooding time. Plants that are
unsuitable for long-time seawater flooding, such as K. obovata ,C. tagal , and B. sexangula at medium to high tidal levels
will retreat to the land as the sea level rises. However, the offshore
area has been occupied by humans, and their retreat space has been
occupied or blocked by urban construction, animal breeding, and
seawalls. These species may disappear. Plants growing at medium to low
tidal levels such as A. marina , A. corniculatum , andR. stylosa may first invade the current distribution area ofK. obovata , C. tagal , and B. sexangular . However,
it is uncertain whether these mangrove species would advance towards the
sea or retreating towards the land, and they may eventually disappear.
If the natural offshore environment is not restored and mangroves are
not allowed to advance and retreat freely, mangroves may be one of the
forest ecosystems that disappear due to global warming and rising sea
levels.