| As the largest freshwater lake in China,Poyang Lake is one of the key flood plain wetland ecosystems connected with the trunk stream in the middle and lower reaches of the Yangtze river.As an important habitat for wintering migratory birds in the world,maintaining its ecological characteristics is of vital importance for global biodiversity conservation.However,in recent years,many hydrological transformations have had a potential impact on the wetland ecosystem of Poyang Lake.In recent years,due to the combined influence of climate and human factors,the extremely low winter water level in Poyang Lake frequently occurs.The hydrological drought greatly affects the submergence and outcrop of the lake shoal,which leads to the change of distribution and phenology of wetland vegetation in different hydrological connectivity areas,and has a potential impact on the habitat of migratory birds in winter.In order to explain the response of vegetation of Poyang Lake shoal to the change of hydrological situation,the following scientific questions are put forward in this study:What is the difference between the vegetation cover and landscape located in sub-lakes of Poyang Lake with different hydrological connectivity?Within hydrological variation,what changes have taken place in the phenology and productivity of wetland plants located in sub-lakes of Poyang Lake with different hydrological connectivity?What is the difference in the response of different hydrological interconnected sub-lake vegetation to environmental stress?In this study,Landsat and MODIS images were adopted from 2000-2016,and EVI datasets with high spatial and high temporal resolution from 2000-2016 Poyang Lake were established by using FSDAF model algorithm,and the following research contents were carried out based on this data source:The differences of wetlands vegetation cover and landscape located in sub-lakes with different hydrological connectivity,The differences of wetlands vegetation phenology and productivity located in sub-lakes with different hydrological connectivity,The variation trend of wetland vegetation located in sub-lakes with different hydrological connectivity and their differences of resistance and resilience to environmental stress.The results showed that:(1)Under similar water level conditions,landscape features of wetland vegetation in sub-lakes with different hydrological connectivity are significantly different.The maximum plaque index of freely connected sub-lakes is obviously higher than that of the partially hydrological controlled lakes,indicating that the area of the freely connected sub-lake in the Poyang Lake area is relatively large or has a large area of continuous grass plaques.There is no significant difference of total edge length for the two types of lakes.While freely connected sub-lakes have higher value in terms of the landscape shape,split index,spread index and diversity index,and lower value in correlation index and aggregate index compared to partially hydrological controlled sub-lakes,which proved that vegetation landscape in freely connected sub-lakes has more polymerizable large patches,but the overall distribution is more fragmented and separated.With the trend of water level decrease,different hydrological connectivity to the lakes leads to different landscape change of wetland vegetation.The wetland vegetation landscape in freely connected sub-lakes will continuously fragmented as the water level decreases,while partially hydrological controlled sub-lakes will be stable after the water level is below 13m,which is determined by the intervention of its hydrological management measures.(2)During the entire 15-year span,wetland vegetation in land and isolated sub-lakes have similar growth cycle.The partially hydrological controlled sub-lakes have no significantly different indicators in addition to lower EVI peak and productivity,while the freely connected sub-lakes are significantly different compared to other sub-lakes in terms of coverage and productivity of wetland vegetation.Combined with field investigations,we found that food plants for overwintering geese are distributed both in freely connected and partially hydrological controlled sub-lakes,while they grow higher in freely connected sub-lakes,which is mainly due to the highest wetland vegetation species diversity there.By splitting the tendency into two periods of before and after 2006,we found that due to changes in flow regimes,the difference in the growth features of double-growth cycle wetland vegetation in freely connected sub-lake in the two periods is obvious.To be specifically,the spring growing season is delayed and the autumn growing season is advanced,which extended beach exposure time,and resulted in excessive accumulation of biomass and overgrowth of food plants.However,since geese prefer to feed in low and fresh vegetation,such changes mean they will miss the good palatability stage of food plants when they arrive,causing the newly migrated geese in short food supply,and threatening to the health of endangered species.There is no such difference for wetland vegetation in partially hydrological controlled lakes,which explains when hydrological extreme drought occurs,wintering geese will gather to the partially hydrological controlled lakes.(3)From 2000 to 2016,the periodicity and trend of wetland vegetation in freely connected and partially hydrological controlled sub-lakes showed significant differences.In terms of the trend of EVI time series,the freely connected sub-lakes showed a continuous growth,while the partially hydrological controlled sub-lakes had a trend of increase first and then decrease(unimodal),which means the two types of sub-lakes had the same vegetation productivity tread before 2006,while showed the opposite trend after 2006.In terms of periodicity,the freely connected sub-lakes show a tendency of increasing periodic fluctuation range,while that of the partially hydrological controlled sub-lakes is just the opposite.Fully synchronized but opposite trend and periodicity are reflected in the water level time series data of Xingzi Station,indicating the negative impact of hydrological factors on the growth of wetland vegetation,while this relation does not exist in the EVI time series of freely connected sub-lakes,especially for the productivity shown in autumn growing season which keeps increasing rapidly growth "by itself".With these evidences,we believe that the grass plaques of freely connected sub-lakes has reached its ecological critical point,or is undergoing a steady state transition.(4)Combining the environmental stress factors of temperature,water level and precipitation with EVI time series of wetland vegetation in freely connected and partially hydrological controlled sub-lakes,we established an AR model and found that:EVI time series of wetland vegetation in the two types of hydrologic connected sub-lakes showed significant autocorrelation;temperature was not correlated with EVI time series;both water level and precipitation were negatively correlated with EVI time series,indicating that higher water levels or higher precipitation will result in lower wetland vegetation productivity or freshness.In addition,the influence of water level on the growth dynamics of wetland vegetation is more sensitive than precipitation.We judged the resilience and resistance to environmental stress(such as hydrology and precipitation)of sub-lakes with different hydrological connectivity through absolute value of correlation coefficient.The results show that:1)Wetland vegetation in freely connected sub-lakes has significantly higher resilience;2)Wetland vegetation in partially hydrological controlled sub-lakes has higher resistance to water level abnormality;3)The two types of sub-lakes have very little differences in resistance to precipitation abnormality.In conclusion,affected by years of declined water level in dry season,the vegetation productivity and biomass of the freely connected sub-lakes in Poyang Lake have continuously increased,reaching the critical steady transition point.Vegetation in sub-lakes with different hydrological connectivity has different responses to the environmental stress of hydrological regime change,which will affect the distribution of herbivores wintering birds on,driving the birds gradually concentrating to partially hydrological controlled sub-lakes.It is urgent to take effective management measures to ensure the survival of wintering migratory birds.At the regional scale,the impact of the operation of Three Gorges Dam on vegetation phenology and wintering migratory birds should be minimized.For example,to optimize time for autumn water withdrawal or summer water level rise,and simulate natural water level fluctuations.Since the resilience of wetland vegetation in partially hydrological controlled sub-lakes will be decreased in this situation,it is suggested to restore the hydrological rhythm at macro scale instead to build water conservancy projects to regulate runoff at local scale.Restricted by workload,this study did not monitor the submerged vegetation,nor did it conduct a deeper study on the mechanism that caused the differences in various aspects of sub-lakes with hydrological connectivity.It is suggested that hyperspectral remote sensing data can be used in the future to further analyze the dynamic trend of growth for submerged vegetation in sub-lakes with hydrological connectivity,so as to comprehensively explain the temporal and spatial succession law of wetland vegetation in flood plain.Combined with field investigation and control experiment,the effects of different hydrological connectivity on its succession mechanism can be studied,and the driving factors causing the dynamic trend will also be further explained. |
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