| Wetlands, together with the forests and oceans, are regarded as the treasures of human and the most important ecosystems on the earth. They have some special ecological functions, such as adjust the globe climate, control the soil erosion, protect the biodiversity, conserve the water resource, prevent the floods and drought, degrade the pollutions and prevent other the potential natural disasters. Thus, wetlands are called "the natural reservoir", "the kidney of the Earth" and "the gene pool". Wetlands are very sensitive because they associate with Atmosphere, Lithosphere, Biosphere and Hydrosphere, so wetlands research will improve our understanding of the globe climate change and the sustainable utilization of wetlands. In this work, four different types of wetlands are chosen, including Hongze wetland (lake wetland), Yancheng wetland (costal wetland), Chongming Dongtan wetland (river mouth wetland) and Everglades wetland (marsh wetland), according to the wetlands classification systems of China (Tang & Huang,2003). History records, field work data, local meteorological data, agricultural activities records and 10 years Moderate Resolution Imaging Spectroradiometer (MODIS) 250-m resolution images are used to study the formation, the development, the driving factors and the trend of the vegetation coverage change of these four wetlands under the interactions caused by both anthropogenic (e.g., agricultural activities) and natural (e.g., climate change) effects. Three vegetation indices (VIs) including Normalized Difference Vegetation Index (NDVI), Enhanced Vegetation Index (EVI) and Floating Algae Index (FAI) are compared to evaluate their effectiveness in assessing relative changes of these four wetlands. AVCAs (accumulated vegetative coverage area) and SVCDs (significant vegetative coverage days) are calculate by the selected VIs. By evaluating the difference of the geographical environments, some suggestions and conclusions for the sustainable utilization of these four wetlands' vegetation are finally given out:(1) Hongze wetland is the typical lake wetland in the watershed of Huaihe and Yangtze River impacted by rivers, lake and quaternary marine transgression. The history records show that the process of the formation and the development of Hongze wetland are affected by both natural and anthropogenic effects. Depression caused by crustal fault, the quaternary marine transgression and deposition of sediment in Huaihe River channel are the main natural effects, and the large scale agricultural activities and water control program are the main anthropogenic effects. Yancheng wetland is the largest coastal wetland impacted by both river and ocean. The history records show that the process of the formation and the development of Yancheng wetland are affected by both natural and anthropogenic effects. The shoreline change is the main natural effect, and the channel change of Yellow River and the spread of Spartina are the main anthropogenic effects. Chongming Dongtan wetland is a typical river mouth wetland impacted by both river and ocean. The history records show that sediment deposition of Yangtze River by river-sea interactive is the main natural effect and affects the process of the formation and the development of Chongming Dongtan wetland. Everglades wetland located among Okeechobee Lake, the Atlantic Ocean and the Gulf of Mexico, so it was affected by rivers, lake and oceans. The history records show that the process of the formation and the development of Everglades wetland are affected by both natural and anthropogenic effects. The rising of Florida platform caused by crustal movements is the main natural effect, and the implementation of Everglades Drainage District (EDD) plan and the construction of the channels are the main anthropogenic effects.(2) The AVCAs and SVCDs of Hongze wetland in both the whole and the core areas show degradation trends in the past 10 years. For the whole area of Hongze wetland, the AVCAs and SVCDs decreased quickly between 2000 and 2008. The AVCAs changed from 13.63×104 km2 d to 13.17×104 km2 d and the SVCDs changed from 360 d to 282 between 2000 and 2008. For the core area, the AVCAs changed from 3.62×104 km2 d to 3.18×104 km2 d and the SVCDs changed from 78 d to 23 between 2000 and 2008. For the whole area, correlation analysis shows that cropland acreage and annual precipitation collectively played significant negative roles in affecting the AVCAs and SVCDs. However, annual sunshine duration played a significant positive role. For the core area of Hongze wetland, correlation analysis shows that annual sunshine duration played a significant positive role in affecting the AVCAs and SVCDs. FAI shows that there are no large scale agricultural activities in the core area of Hongze wetland during the past 10 years. Hongze Lake is the largest Lake in the Huaihe watershed, and the wetland is nearby the Hongze Lake. Thus, flooding event caused a sharp drop of AVCAs and SVCDs in 2003, and the affect continued in the following years. Therefore, the degradation trends of the vegetation coverage of Hongze wetland are mainly caused by both the anthropogenic and the natural effects.(3) The AVCAs and SVCDs of Yancheng wetland in both the whole and the core areas show degradation trends in the past 10 years. For the whole area of Yancheng wetland, the AVCAs changed from 200.15×104 km2 d to 188.84×104 km2 d and the SVCDs changed from 142 d to 81 between 2000 and 2009. For the core area, the AVCAs changed from 12.54×104 km2 d to 11.72×104 km2 d and the SVCDs changed from 27 d to 5 between 2000 and 2009. For the whole area, correlation analysis shows that cropland acreage played significant negative roles in affecting the SVCDs. However, annual precipitation played a significant positive role. For the core area of Yancheng wetland, correlation analysis shows that accumulated air temperature played a significant positive role in affecting the AVCAs and SVCDs. FAI shows that there are no large scale agricultural activities in the core area of Yancheng wetland during the past 10 years. The coastal area of Yancheng wetland is generally in erosion condition and the erosion affected the change of AVCAs and SVCDs in this region. Thus, the degradation trends of the vegetation coverage in the core area of Yancheng wetland are mainly caused by both the anthropogenic and the natural effects.(4) The AVCAs and SVCDs of Chongming Dongtan wetland in the whole areas show degradation trends in the past 10 years. For the whole area of Chongming Dongtan wetland, the AVCAs changed from 11.20×104 km2 d to 10.90×104 km2 d and the SVCDs changed from 124 d to 109 between 2000 and 2009. For the core area, the AVCAs increased from 2.34×104 km2 d to 2.39×104 km2 d and the SVCDs increased from 216 d to 280 between 2002 and 2009. For the whole area, correlation analysis shows annual precipitation played a significant positive role in affecting the AVCAs and SVCDs. For the core area, correlation analysis shows that there are no significant roles among all of them. FAI shows that there are no large scale agricultural activities in the core area of Chongming Dongtan wetland during the past 10 years. Chongming Dongtan wetland is generally in accretion condition and the accretion rate is about 150-230 m yr-1. The accretion condition caused the AVCAs and SVCDs of the whole area decrease slowly and the AVCAs and SVCDs of the core area even increase. Thus, the change trends of the vegetation coverage in Chongming Dongtan wetland is mainly caused by the natural effects.(5) The SVCDs of Everglades keep stable between 365 d and 366 d during the past 10 years. However, the AVCAs changed from 155.06×104 km2 d to 153.77×104 km2 d between 2000 and 2009. Correlation analysis shows that annual accumulated air temperature and annual precipitation played significant positive roles in affecting the AVCAs. FAI shows that there are no large scale agricultural activities in the Everglades wetland during the past 10 years. There are many mangroves distributed alone the shoreline of the Everglades and they protect the shoreline from the erosion. Thus, the change of the AVCAs is mainly caused by the natural effects.(6) The slopes of the AVCAs and SVCDs plots show that, for the whole area, degradation trends of Yancheng wetland is listed the 1st and degradation trends of Chongming Dongtan wetland is listed the last. For the core area, degradation trends of Hongze wetland is listed the 1st, and the AVCAs and SVCDs plots of Chongming Dongtan wetland even shows increase trends. FAI shows there are no large scale agricultural activities in the core area of Hongze wetland, Yancheng wetland and Chongming Dongtan wetland. However, the agricultural activities still affect the area outside the core.(7) Three vegetation indices (VIs) are compared to evaluate their effectiveness in assessing relative changes. These are the Normalized Difference Vegetation Index (NDVI), the Floating Algae Index (FAI), and the Enhanced Vegetation Index (EVI). EVI is less sensitive than others in assessing croplands or the area including croplands. Thus, EVI is more suitable in assessing the vegetation of the whole area of Hongze wetland, Yancheng wetland and Chongming Dongtan wetland. FAI is less sensitive than NDVI and EVI to aerosol effects and shows less statistical error than NDVI and EVI in assessing lower vegetation coverage area, such as the area covered by natural vegetation. Therefore, FAI is chosen to study vegetation of the core area of Hongze wetland, Yancheng wetland, Chongming Dongtan wetland and the whole area of Everglades wetland.(8) Depending on the different process of the formation and the development of the four wetlands, following strategies are given out in order to protect and sustainable utilize the vegetation resource of the wetlands. For Hongze wetland: reconverting the farmland into the wetland; keeping the wetlands area stable; optimizing and adjusting the current agricultural structures; prohibiting agricultural activities in core area; moderate exploring the tourism; improving the water quality of Hongze lake. For Yancheng wetland:optimizing and adjusting the current agricultural structures; keeping species diversity;controlling the pollution; moderate exploring the eco-tourism. For Chongming Dongtan wetland:segmenting ecological security boundary dynamically; improving species diversity; utilizing and protecting water resource rationally. For Everglades wetland:controlling the water resource; recovering the natural outlet of the surface runoff; improving the water quality.
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