Quantitative Assessment Of Tidal Wetlands Degradation And The Identification Of Priority Areas For Ecological Restoration In Shanghai City

Tidal wetlands are coastal ecosystems developed in the global land-sea transition zone,providing various invaluable ecosystem services(ESs)to human society,and are vital to coastal biodiversity.The reclamation of tidal wetlands provides essential land resources for the development of human society and economy.Ironically,the threat of marine disasters to coastal communities has been increased due to unreasonable development activities.Healthy tidal wetlands play an important role in achieving the United Nations Sustainable Development Goals(SDGs).Thus,tidal wetlands restoration has become a focus of many researchers from domestic and abroad.However,there is still a lack of the study on identifying the restoration priority area for tidal wetland at regional scale,which is helpful to environment managers in allocating limited restoration resources.Shanghai's tidal wetlands in the Yangtze River Estuary of East China was used as a case study.Based on historical remote sensing images and auxiliary data,we mapped the spatial distribution of tidal wetland land use/land cover(LULC)in four time series(1990,2000,2008 and 2018).The features and drivers of tidal wetland evolution of different periods(1990–2000,2000–2008,2008–2018 and 1990–2018)were detected by using continuous LULC transformation analysis.NDVI data of pixel by pixel of long time series combined with linear regression model were used to reveal the relationship between the edge erosion and expansion of tidal marsh and the variation of the growth status of vegetation.‘Vigor-Organization-Resilience'assessment framework was adjusted and applied to map the spatial features of the ecosystem status of Shanghai's remaining tidal wetlands.Finally,the identification framework of restoration priority area for tidal wetlands was established by introducing the theory and methods of systematic conservation planning,and the effects of human demand were incorporated in restoration planning for tidal wetlands under this theoretical framework.The results are as follows:(1)From 1990 to 2018,Shanghai's tidal wetlands decreased by 18.1%,with Spartina alterniflora marshes accounting for 42%of the area.Coastal reclamation is the main driver causing tidal wetlands loss in different coastal sections of Shanghai except Jiuduan shoal.A total of 693.4 km² of tidal wetland and coastal waters were reclaimed in the past 29 years,which is equivalent to 1.2 times the area of the tidal wetland in 1990.The newly increased tidal wetlands generated from progressive succession can offset at least48%of the wetland loss caused by reclamation in different periods.The invasion of Spartina alterniflora was the main natural driver that threatened the structural integrity of tidal wetlands at present because the newly invaded area had exceeded the degraded area caused by retrogressive succession after 2008.In 2018,about 85%of Spartina alterniflora marshes are distributed in Chongming North Beach and Jiuduan shoal.(2)There is no correlation between the tidal marsh edge erosion and the changes of the vegetation growth status in the interior of tidal marsh.The rate of tidal marshes edge erosion along southern Chongming coast is about 1.3m·yr^-1(sd=5.1 m·yr^-1)during 1988 and 2018.The overlapping area of the tidal marsh between 1988–1990 and 2016–2018 was 697.4 hm²,with only 2%of the vegetation of the overlapping area showed significant degradation in growth status.On the one hand,there is no significant correlation between the rate of erosion and expansion at the edge of the vegetation zone and the degradation of the internal vegetation,indicating that the rate of tidal marsh edge erosion cannot represent the change in its internal vegetation growth status.On the other hand,we found that there is a significant correlation between the rate of expansion at the edge of the vegetation zone and the improvement rate of the internal vegetation growth status(Spearman's r=0.4,p<0.01),indicating that controlling the tidal marsh edge erosion has a positive effect on improving the growth of the entire tidal marsh vegetation.(3)It is not sufficient to use the change of tidal wetlands area as the only basis for the implementation of wetland restoration actions.The design of wetland conservation and restoration goal should include two core components:the loss and gain of tidal wetlands area and ecosystem health status.Tidal wetlands ecosystem health was assessed with the adjusted‘Vigor-Organization-Resilience'framework.The results showed that the remaining Shanghai's tidal wetlands were in the middle health status.Only 13.6%of the natural tidal wetlands are in health status,mainly distributed along the coast near Niupenggang of Chongming island and the northwest of Jiuduan shoal.There is no significant linear correlation relationship between the area changes of the tidal wetlands and the quality of the remaining part.Of the 592 units assessed,27.1%of them showed increase in tidal wetlands area from 1990 to 2018,but the ecosystem health status of their current natural tidal wetlands was below the middle level,which suggested that area changes(1990–2018)of the tidal wetlands and the quality of the remaining part(2018)showed high spatial mismatch.(4)Incorporating human demand factors into the identification framework of the restoration priority area can avoid missing restoration opportunities for coasts that really need restoration,such as the coasts of Jinshan and Baoshan districts.The priority areas for ecological restoration was identified based on the theory and algorithm of systematic conservation planning.When considering the human demand for ecosystem services,the number of planning units selected as priority areas for ecological restoration along the coastal reclamation zone of Jinshan,Fengxian,and Baoshan increased by at least 93%compared to the scenario of ignoring human demand.Compared with restoration priority areas identified without considering the human demand,accounting for human demand factors in the selection algorithm does not necessarily reduce or increase the potential supply of ESs and the cost of the priority areas for tidal wetlands restoration.It depends on the spatial coupling degree of restoration cost,benefit and human demand.The results of the identification of the priority area for restoring the Spartina alterniflora marshes showed that compared to the situation with human demand ignored,the human demand considered scenario resulted in an increase in the restoration cost(?9%),with small increase of the supply of some ecosystem services.This study quantitatively mapped the spatial patterns of tidal wetlands loss over the past 29 years and the ecosystem health status of the remaining tidal wetlands around Shanghai,which can provide basis for the implementation of site-specific restoration actions.Based on ecosystem services supply,human demand and restoration cost,a systematic framework on identifying the priority areas for tidal wetlands restoration was proposed in this study,which can provide an important reference for the implementation of the coastal restoration planning at national scale.