Spatio-temporal Dynamics Of Poyang Lake Wetland Landscape Patterns Based Time Series Optical Remote Sensing Data In Poyang Lake

Wetland systems at the land-water interface often provide critical ecosystem functions,i.e.,flood reduction,protect surface water storage,fish production,conservation of biological diversity,mitigate climate and energy change,carbon storage,yet they are vulnerable to both human activities and climate changes..However,due to intensive human activities and species invasions,dramatic changes in the wetland landscape have occurred worldwide over recent decades,degrading their ecological functions and triggering numerous environmental and social problems.Thus,accurate monitoring and understanding of wetland changes are of significant importance to both the scientific community and local governments.Poyang Lake,the largest freshwater lake of China,is well known for its ecological importance as a dynamic wetland system.However,due to the significant seasonality of the lake's inundation area,no systematic study has assessed the wetland cover types change and wetland vegetation communities change over the past few decades,not to metion the impact of the climate change and impoundment TGD on the wetland ecosystem.The impoundment of the Three Gorges Dam(TGD)has led to significant inundation shrinkage and water level decrease in China's largest freshwater lake,Poyang Lake.However,little is known about the influence on the lake's wetland landscapes.In this study,several methods were developed to combine in situ data of meteorological and hydrological stations,high spatial resolution Landsat(MSS,TM,ETM+,OLI)data,HJ-1A/1B CCD data,and high temporal resolution MODIS data to:1)Develop a reliable method to classify the major wetland cover types of Poyang Lake using Landsat images,which overcomes the difficulties of sensor associated-differences and atmospheric correction;2)Proposed a phenology-based decision tree method to classify the Poyang Lake Wetland into seven land cover types using MODIS satellite images;3)Document major wetland changes of Poyang Lake in the last four decades in both dry and wet seasons,with particular focus on the changes after the impoundment of the TGD in 2003;4)Learn the impact of the TGD on the Poyang Lake wetland ecosystem.The detail object are as follows:First,Using four decades(ranging from 1973 to 2013)of Landsat data,which were acquired in the same season to ensure similar phenological and hydrological conditions during each year,and training and validation samples collected from high-resolution Quickbird imagery to develop a Support Vector Machines(SVM)method for wetland classification of Poyang Lake.To obtain consistent results from different Landsat instruments,an empirical line correction approach was introduced to adjust the sensor-associated differences in band configurations and spectral responses?Significantchanges in the major wetland cover types in Poyang Lake were revealed from long-term classification maps.The vegetation coverage of Poyang wetland showed astatistically significant increasing trend during the overall period(15.9 km2 year-1),and the vegetation tended to spread into the lake center in the Nanjishan Wetland National Nature Reserve(NWNNR)in recent years.At the same time,out-of-phase variability was observed for the mudflats since 1984,with a significant shrinking trend of-12.1 km2 year-1(p<0.05).Although sand coverage experienced a rapid decrease from 1973 to 1990(from 544.3 km2 to 62.9 km2),it remained at a relative stable low level(<100 km2)in the following period.The two national reserv;s in Poyang Lake shared change patterns similar to those of the entire lake.Although?70%of the long-term changes in the wetland vegetation area appeared to be explained by local temperature,there was rapid increase after 2002 possibly could be due to the impoundment of the Three Gorges Dam(TGD)in 2003.The large sand area in 1973 was potentially linked to previous human activities in China between the 1950s and the 1970s.Then,Using four decades of Landsat and HJ-1A/1B CCD satellites observations and recently developed classification algorithms,long-term wetland changes in Poyang Lake,the largest freshwater lake of China,have been investigated in this study.In dry seasons,while the transitions from mudflat to vegetation and vice versa were comparable before 2001,vegetation area increased by 620.8 km2(16.6%of the lake area)between 2001 and 2013.In wet seasons,although no obvious land cover changes were observed between 1977 and 2003,?30%of the Nanjishan Wetland National Nature Reserve(NWNNR)in the south lake changed from water to emerged plant during 2003 and 2014.The changing rate of the Normalized Difference Vegetation Index(NDVI)in dry seasons showed that the vegetation in the lake center regions flourished,while the growth of vegetation in the off-water areas was stressed.Rapid NDVI increase was also found in the NWNNR in the wet seasons.The relationships between the water levels and vegetation coverage also showed two regimes in both dry and wet seasons for the pre-Three Gorges Dam(TGD)period(before 2003)and post-TGD period(after 2003).Analyses of long-term hydrological and meteorological data clearly indicated that while local precipitation remained stable,the water level of Poyang Lake decreased significantly after the impoundment of the TGD,which is likely the main reason for the wetland expansion in recent years.The Three Gorges Dam(TGD)is the world's largest hydroelectric dam.It has received numerous criticisms since its planning stage due to its environmental and societal impacts.How the different vegetation communities changed over the years or transitioned among each other has remained largely unknown,as have the causes of these changes.Through Moderate Resolution Imaging Spectroradiometer(MODIS)observations and a phenology-based decision tree approach,we present the spatial and temporal changes of the major wetland cover types from 2000 to 2014.Fifteen years of classification data analysis indicated that the total coverage of the vegetation(Carex spp.,Triarrhena lutarioriparia L.-Phragmites community,sparse grass and Zizania latifolia communities)and the area of each vegetation(excluding FAM)showed significant increasing trend during the observed period,and the expansion of vegetation towards the lake center.Of the seven land cover types,water and mudflat have their areas decreased significantly since the TGD impoundment and were mainly replaced by prominently expanded vegetated areas.Based on the studies above,the transition map during 2000-2014 show that wetland vegetation communicates transitions occurred mainly from hydrophilic cover types to those adapted to dryer conditions.Despite the significant changes in wetland cover types,the most preferable water depth for each cover type remained stable before and after the TGD.The relationships between the water levels and vegetation coverage also showed two regimes in both dry and wet seasons for the pre-Three Gorges Dam(TGD)period(before 2003)and post-TGD period(after 2003).Analyses of long-term hydrological and meteorological data clearly indicated that while local precipitation remained stable,the water level of Poyang Lake decreased significantly after the impoundment of the TGD,which is likely the main reason for the wetland expansion in recent years.