Mangroves are woody biomes distributed in tropical and subtropical coastal areas, growing in wetlands between the mean tide level (mean sea level) and mean high water springs. The mangrove ecosystem is one of the most productive ecosystems in the coastal zone, with important functions including wind and wave resistance, coastal protection, and carbon storage. The mangrove ecosystem also plays an important role in the adaptation to and mitigation of climate change in coastal areas. However, as mangroves can only survive in intertidal mudflats where the waves are relatively weak, currents are slow, sedimentation is obvious, and tidal range is relatively low, they are vulnerable to sea level changes and extreme events such as strong typhoons and droughts. In recent decades, coastal habitats including mangrove wetlands have been severely threatened by the rapid rise in sea level and strong typhoons as global warming has intensified (Alongi 2007, 2015; Bindoff et al. 2019; Cai et al. 2016; Cai et al. 2020; Dahdouh-Guebas et al. 2005; Gilman et al. 2007; Kathiresan and Rajendran 2005; Li et al. 2020; Long et al. 2016; Accessed et al. 2019; Villamayor et al. 2016). A previous study revealed that if the rate of global sea level rise reaches 6.1 mm year− 1 in the next 30 years, mangroves will very likely struggle to adapt and survive (Saintilan et al. 2020). Studies have shown that mangrove wetlands can adapt to sea-level rise by sediment accretion. However, this is only possible if the vertical accretion rate of the mangrove wetland sediment is greater than or equal to the relative rise in local sea level to address the current threat of rising tides (Alongi 2007; Tan and Zhang 1997). Under the high greenhouse gas (GHG) emissions scenario, representative concentration pathway (RCP) 8.5, only localized sedimentation accretion effects in mangrove wetlands could keep pace with rising sea levels by 2055 and 2070 (Sasmito et al. 2016). For example, in the Caribbean mangrove margin, mangrove growth rates can be equal to the rate of sea level rise. However, if this rate exceeds 5 mm year− 1, the mangrove islands in the Caribbean are unlikely to persist (McKee et al. 2007). In the Indo-Pacific tropics, the current rate of sea level rise exceeds the vertical accretion rate of the mangrove wetland surface at 69% of all sites studied. In areas with low tidal ranges and low sediment supplies, mangroves may be inundated as early as 2070 (Lovelock et al. 2015). In other words, the ability of mangroves to adapt to rising sea levels in the context of global warming is closely related to the relative rate of local sea level rise, as well as the sedimentation accretion of mangrove wetlands and other factors.
In China, mangroves are mainly distributed in the tropical and subtropical coastal areas of Hainan, Guangxi, Guangdong, Fujian, and Taiwan provinces, with an area of approximately 21148–24801 hm2 (Zhao and Qin 2020a) (Fig. 1a). Among them, the largest contiguous mangrove area and richest mangrove species are located in the Dongzhaigang National Nature Reserve of Haikou City, with a small amount distributed in Wenchang City, Hainan Province (hereinafter referred to as Dongzhaigang mangrove). This contiguous area is the earliest established national mangrove wetland reserve in China, with a total of 19 families and 35 species of mangrove plants, accounting for 97% of the mangrove species in China (Hainan Dongzhaigang National Nature Reserve Authority 2015). Possessing gentle terrain and a winding and curved coastline, Dongzhaigang is a drowning valley bay formed by subsidence from the 1605 Qiongzhou Earthquake (Fu 1995; Zhang et al. 1996), which is mainly located in Haikou City, with a small part located in Wenchang City, Hainan Province, respectively. Prior to 1960, the Dongzhaigang mangrove was mainly a natural mangrove forest with an area of 3416 hm2 (Chen and Chen 1985). Nearly 700 million m3 year− 1 of water flows into Dongzhaigang from rivers such as the Yanzhou, Luoya, Yanfeng East, and Yanfeng West. The rivers carry a large amount of sediment to be deposited, forming a wide mudflat marsh wetland, which provides a suitable environment for growth of the mangroves (Wang et al. 2006). By 1980, the area of the Dongzhaigang mangrove had been drastically reduced by more than 50% due to human logging, economic tree planting, and the reclamation of fish ponds (Chen and Chen 1985; Sun et al. 2015; Wang et al. 2006). Since the 1980s, because of the establishment of provincial and national nature reserves, the Dongzhaigang mangrove has been protected, but the area of mangrove forest has not increased, remaining stable at ~ 1600 hm2 (Huang et al. 2015; Li et al. 2020; Wang et al. 2006).
In the context of climate change, the rate of coastal sea level rise in China in recent decades (3.4 mm year− 1, 1980–2020) is higher than the global average (Cai et al. 2020; Ministry of Natural Resources 2021). The sea level rise in the Dongzhaigang area was 4.6 mm year− 1 from 1980–2017, which was much higher than the coastal China average. Moreover, the sea level rise is expected to accelerate in the future (Kopp et al., 2014; Yan et al., 2019). As a result, the impact of rapidly rising sea levels on the mangroves in Dongzhaigang and other areas is becoming increasingly apparent. Whether the Dongzhaigang mangroves can adapt to these changes and whether countermeasures are necessary to adapt to them are currently unknown. However, few studies have been conducted in this area. The ability of mangroves to adapt to the effects of rising sea levels is closely related to the sedimentation accretion of the mangrove wetlands. Although historical data on sedimentation rates in Dongzhaigang mangrove wetlands have been investigated from observation sites such as Linshi and Daoxue villages (Wang 2011; Zhang et al. 1996), spatial representation is somewhat lacking. Accordingly, this study first analyzed the changes in the Dongzhaigang mangrove area over the past 60 years and the reasons for these changes based on historical literature, spatial remote sensing distribution data, and topographic elevation data. A supplementary survey of sediment vertical accretion rates in two mangrove wetlands at Hegang village, Yanfeng and Sanjiang Farm, Sanjiang was also utilized. Then, based on the study of historical and future relative sea level changes in Haikou City, where Dongzhaigang is located, the impacts and risks of sea level rise on mangroves in Dongzhaigang under three different climate scenarios, RCP 2.6, 4.5, and 8.5, were determined. Finally, the measures needed to adapt to rising sea levels in the Dongzhaigang mangrove were analyzed and proposed for mangrove conservation and management in China.