Accurate mapping of <i>Spartina alterniflora</i> in Fujian Province using phenological traits derived from Landsat time series

The rapid spread of Spartina alterniflora poses a critical threat to coastal wetland ecosystems in China, requiring accurate monitoring of its spatiotemporal dynamics for sustainable management. Earth observation data are valuable for tracking vegetation dynamics, but frequent cloud cover, tidal flooding, and spectral confusion challenge the accuracy of satellite-derived coastal vegetation maps. To address these limitations, we developed a method using phenological traits derived from Landsat time series to distinguish Spartina alterniflora from intertidal vegetation communities in Fujian Province. Accuracy assessment of the Spartina alterniflora maps showed an Overall Accuracy(OA) of more than 93%, with a kappa coefficient of more than 0.92 over the study period. The resulting accurate maps showed a significant increase in Spartina alterniflora coverage from 21.41 km2 to 107.14 km2 from 1990 to 2020. Bare flats, which accounted for 60% of the total increase in area, were the primary coastal habitats lost to Spartina alterniflora. An increasing trend of the intertidal ecosystem occurred during the same period due to the rapid invasion of Spartina alterniflora into native habitats. It is also found that there are three types of Spartina alterniflora expansion in spatial pattern, driven by morphology, hydrodynamic environment, and biological stress. Notably, human activities emerged as the dominant factors influencing the expansion of Spartina alterniflora, surpassing natural factors. This suggests that Spartina alterniflora, driven by its remarkable reproductive capacity, will continue to encroach on available habitat. Therefore, strategic management of the coastal zone is crucial to maintain the ecological balance.