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.