Long-term Evolution Process And Mechanisms Of Wetland Ecosystem In The Yangtze River Estuary Using Time-series Multi-sensor Remote Sensing Data

Affected by global climate change and human activities,the Yangtze River estuary wetland ecosystem is facing with many degradation risks,such as wetland area shrinking,alien species invasion,environmental pollution and coastal erosion.Whereas there were challenges to analyze long-term changes of wetland ecosystem and its driving mechanisms due to a lack of high quality monitoring data for a long time continuously,which have great impact on the formulation of long-term wetland ecological restoration strategy,evaluation of wetland restoration and reconstruction effects,and reasonable protection and scientific management of wetland resources.With the rapid development of remote sensing technology and the acquisition ability of remote sensing data,it is possible to analyze the long-term evolution processes and their laws,mechanisms and ecological effects of wetland ecosystems based on high-quality time-series remote sensing data.In this dissertation,we chose the Yangtze River estuary wetland ecosystem as study area,aiming at the problems of spatiotemporal discontinuity,poor consistency and low precision of the existed wetland maps,a new method oriented to long-term monitoring of typical estuarine wetland based on multisource remotely sensed data was developed,and consequent mapping of the Yangtze River estuary wetland ecosystem were done(1985-2016).Finally,the long-term evolution process of the structure and function of this wetland ecosystem,characteristics and its driving mechanism were analyzed.The main conclusions were as follows.(1)An object-oriented mapping framework was developed based on long time-series of multi-source remotely sensed data in this study.In this framework,several latest image processing techniques and classification methods(i.e.,object-based hierarchical classification method,updating,backdating and the phonological rules)were integrated to generate annual time-series wetland products.It can not only balance precision and efficiency of wetland product production,but also generate uniformly continuous long time series products.The total accuracies of land use/land cover product,coastal wetland product at community scale and inland water wetland product were all higher than 80% in this study.(2)Based on long-term annual time-series wetland products,the long-term evolution processes,patterns and trend of Yangtze River estuary wetland ecosystems,and its abrupt and gradual changes can be effectively monitored and analyzed,which was usually unable to come true based on bi-temporal or sparse time-series remotely sensed data.The results showed that the structure of the Yangtze River estuary wetland ecosystem had changed significantly spatially and temporally from 1985 to 2016.Overall,the coastal wetlands continued to degrade,the areas of wetlands dominated by native species or bare tidal flat decreased significantly,opposite to what was observed to wetland dominated by alien species.The area of aquaculture water increased from 1985 to 2002 firstly,and decreased from 2003 to 2009,and finally a slight increase from 2010 to 2016.The area of non-aquaculture water was almost stable from 1985 to 2005,whereas a slight increase was detected from 2006 to 2016.(3)Based on the "equivalent factor method",the long-term evolvement characteristics of the ecological service function of the wetland ecosystem in the Yangtze River estuary were evaluated,and the influence of urbanization was also analyzed.The total ecosystem service value(ESV)showed a decrease trend,the overall decline value was approximately 12%.The ESV drop in coastal wetland ecosystem was about 20%,which was significantly higher than that of other ecosystems.(4)Combined with qualitative and quantitative analysis,the driving mechanisms of long-term change of Yangtze River estuary wetland were determined.Climate change,continuous reduction of sediment flux to the sea,large-scale reclamation and alien plant invasions were the main driving forces for the long-tern evolution of Yangtze River estuary wetlands.However,the impacts of each driving factor on different typical wetland ecosystems were different.Reclamation,exotic plant invasion and reduction of sediment flux to the sea were the main driving factors for the long-term evolution of Chongming Dongtan wetland,Nanhui wetland and Qidong wetland;and alien plant invasions and reduction of sediment flux to the sea were the main driving factors for the evolution of Jiuduansha wetland.Our results suggested that the appropriate ecological restoration strategies should be implemented combined with partition management for different wetland ecosystem.