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.