Quantifying latitudinal variation in land surface phenology of Spartina alterniflora saltmarshes across coastal wetlands in China by Landsat 7/8 and Sentinel-2 images

Spartina alterniflora is an aggressive invasive plant spreading along the coastal China, spanning a latitudinal range of 20 degrees N-39 degrees N, and its invasion resulted in dramatic decline in both native plant diversity and ecosystem functioning. Phenology of S. alterniflora saltmarshes is a critical feature to elucidate the invasion dynamics over geographical regions but has not been well understood yet. In this study, we examined the variation of S. alterniflora saltmarsh phenology across coastal China during 2018-2020 by using time series Landsat 7/8 and Sentinel-2 images. Combined Landsat 7/8 and Sentinel-2 images provided more good-quality observations in a year, which could facilitate phenological retrieval. We applied and assessed three widely used phenology retrieval methods (i.e., NDVI-based pixel-specific statistical threshold, NDVI-based double logistic mathematical equation, and LSWI-based biological threshold) for retrieving the start and end of season (SOS and EOS) as well as the length of season (LOS) of S. alterniflora saltmarshes. The SOS and EOS dates derived from three phenology retrieval methods showed similar patterns in latitudinal phenology variation: SOS became later and LOS became shorter as latitude increased, and the latitudinal trend of EOS was not as large as that of SOS. This study shows the potential of Landsat 7/8 and Sentinel-2 to quantify land surface phenology of S. alterniflora saltmarshes, which not only enhances our understanding of the spatial-temporal dynamics of coastal saltmarshes in China but also improves the management of this plant invader that threatens native saltmarshes in the world.