The Surface Elevation Changes Of Mangrove Forests In China And Impacts Of Sea-level Rise On Mangrove Forests

Sea-level rise threatens the stability of mangrove habitats and the ecological services they provide by increasing the risk and severity of tidal inundation.Vulnerability of mangrove to sea-level rise(SLR)can be evaluated by making comparison between surface elevation change and SLR.However,such efforts are hindered by a lack of site-specific data on surface elevation changes.Additionaly,mangrove forest responses to SLR are likely misestimated in many studies because current assessment methods often assume a constant rate of surface elevation change along the intertidal profile.Most studies also neglect the ecogeomorphic feedback process known to accelerate soil building with SLR.In this study,we analyzed the spatial variability in surface accretion and surface elevation change across different mangrove forests worldwide through literature review,quantied the surface accretion and surface elevation changes in Chinese mangrove forests using Rod Surface Elevation Table-Marker Horizon(RSET-MH)methodology,evatulated the vulnerability of mangrove forests in China to SLR,and constructed an empirical,dynamic model at the intertidal zone of a mangrove bay in Hainan,China to explore the responses of mangrove forest to different rates of SLR.The main results and conclusions are as follows:(1)Due to the soil subsidence,the rates of mangrove surface elevation change are typically smaller than that of surface accretion worldwide.Comparision of rates between surface accretion and SLR failes to evaluate the vulnerability of mangrove forests to SLR.Surface elevation changes show obvious differentiation among mangrove forests under different disturbances.Mangrove rehabitation can increase the surface elevation change,while mangrove mortality will lead to negative surface elevation change.(2)Significant variability in mangrove surface accretion and surface elevation change across different hydro-geomorphic settings was observed.Mangrove forests in river delta,tidal esturary and regions with macrotidal amplitude typically have high surface elevation gains,indicating a strong resistance to SLR.In contrast,mangrove forests in carbornate reef and regions with microtidal amplitude tend to experience small surface elevation changes,and is severely threatened by SLR.(3)During the measuring period from 2015 to 2019,the surface accretion ranged from 15.4 to 56.0 mm a-1 at six study sites(i.e.,Dongzhai Bay,Yingluo Bay,Pearl Bay,Shenzhen Bay,Zhangjiang Estuary and Jiulongjiang Estuary).Meanwhile,the surface elevation changes were 8.5-34.1 mm a-1。Considerable soil subsidence did occur below ground with a rate of 9.2 mm a-1 in low-elevation mangrove and 9.7 mm a-1 in high-elevation mangrove.However,mangrove forests in our study sites appear to have the strong capacity to offset elevation losses by way of sufficient sedimentation to mangrove forests.(4)Due to rapid sediment accretion,mangrove forests had higher surface elevation gains than the findings in Australia,FSM islands,USA and Caribbean region.The high surface accretion and surface elvation gians are resulted from human-induced high sediment supply indicated by rich total suspended matter in coastal water.(5)Sediment accretion and surface elevation change were sensitive to the elevation in the intertidal zone.Accretion rates and thereby surface elevation gain could more than double from high-elevation mangrove to low-elevation mangrove in the intertidal zone,indicating a feedback process known to accelerate soil building with SLR.(6)The RSET data were broadly consistent with that derived from radiometric measurements,thus can denote the longer term rate of surface elevation change.(7)Mangrove landward margins in China were dominated by concrete seawall(83.0%),earthen seawall(10.0%),road(4.0%),and steep slope(3.0%),indicating no possibility of landward migration of mangrove forests under SLR.However,mangrove forests in China can adapt to SLR through high surface elevation changes.Mangrove forests in our study sites can generally survive relative SLR rates of up to 12.6-86.3 mm a-1.Hence,the current and anticipated SLR by 2100 will not pose a considerable threat to the stability of mangrove forests,assuming current-day levels of sediment availability.(8)We introduced a dynamic model of wetland response to SLR.Our model showed that SLR(at rates of 4 and 16 mm a-1)would not lead to the loss of mangrove forest area but that it could alter mangrove species zonation along the intertidal profile by 2100.An increase in the area of high-inertidal mangrove community was expected under low and moderate SLR.However,rapid SLR and landward barriers can pose a considerable threat to the high-intertidal community and lead to the loss of mangrove biological and structural diversity along the intertidal profile.This study quantifies the surface elevation changes of mangrove forests in China unprecedentedly,improves our predictions of sea-level rise impacts on mangrove forests and provides the baseline data for the proper management.The predition of mangrove responses to sea-level rise based on dynamic surface elevation changes at Dongzhaigang Bay expands our current understanding of the impacts of SLR on mangrove forests in sediment-rich systems.