Monitoring spatial and temporal dynamics of wetland vegetation and their response to hydrological conditions in a large seasonal lake with time series Landsat data

Abstract

Periodic hydrological processes of seasonal lakes are the dominant factors leading to the development of vegetation communities and ecological processes. Dongting Lake is a typical seasonal lake in the middle and lower reaches of the Yangtze River and its hydrological conditions have changed significantly due to the construction of the Three Gorges Dam (TGD). However, there are knowledge gaps in the responses of wetland vegetation dynamics to the changes of hydrological conditions, especially in cloudy and rainy regions, where continuous large-scale monitoring is still challenging. In this study, by using time series of Landsat data on Google Earth Engine, a long-term cloudless remote sensing images dataset of Dongting Lake during the dry season was constructed. The multi-year wetland vegetation from 2003 to 2020 was mapped using the random forest model. Relationships between wetland vegetation dynamics and hydrological conditions (e.g., water level and inundation character) were examined using correlation and linear analysis. Results revealed that wetland vegetation in the Dongting Lake experienced significant inter-annual spatial heterogeneity and temporal dynamic equilibrium with a slight increase during the post-TGD period. Only Carex communities at elevations above 28 m and below 26 m displayed apparent changes. Reed communities have relatively high selectivity to inundation frequency, and are affected mainly by changes in water levels during the dry season of the current year. Hydrological conditions during the rising period have an important impact on the first growth and reproduction stage of Carex, while higher water levels or more rapid surface submergence lead to a decrease in seed density in the soil of low-elevation floodplains. This study deepens our knowledge of changes of wetland vegetation and their response to hydrological conditions, and is of great significance to the ecological protection and restoration of Dongting Lake.