Coastal salt marshes are transitional ecosystems shaped by strong land-sea interactions. Their vegetation patterns are influenced by multiple interacting factors, including tidal forcing and associated inundation, groundwater-level fluctuations, porewater salinity, redox conditions, soil properties, and microtopographic heterogeneity. However, the relative contributions of these factors across different microhabitats remain uncertain. The Lingang coastal tidal flat in Shanghai, located in the Yangtze River Estuary, is a typical muddy tidal flat-salt marsh system in the Yangtze River Delta and is sensitive to hydrological and ecological changes. As a common dominant species in this area, Phragmites australis occurs as a heterogeneous mosaic of core patches, patch margins, bare patches, tidal-creek-influenced zones, and unvegetated flats, potentially indicating microhabitat-scale differences in root-zone environmental conditions. Fine-scale observations linking salt-marsh vegetation patterns with root-zone environments remain limited.
This study investigates root-zone environmental constraints on Phragmites australis distribution in the Lingang coastal salt marsh by integrating UAV-derived vegetation indices, in-situ sensor monitoring, and porewater sampling. NDVI derived from UAV multispectral imagery was used for spatial analysis and for selecting 12 representative observation sites across Phragmites core zones, patch margins, bare patches, tidal creeks, and unvegetated flats. Sensors were installed at these sites to record groundwater level, temperature, electrical conductivity (EC), and related variables, with the aim of characterizing spatial differences in root-zone hydrological and salinity-related conditions.
The vegetation-site sensors have been deployed, and complete field records will be incorporated for further quantitative verification. Based on existing NDVI results, field observations, and salinity measurements, Phragmites core zones may be associated with relatively lower porewater salinity and more stable root-zone hydrological conditions. In contrast, patch margins, bare patches, and unvegetated flats may experience higher salinity and more frequent inundation, indicating potential environmental stresses on Phragmites establishment and expansion. Sites near tidal creeks are expected to show pronounced fluctuations in groundwater level and salinity under tidal connectivity. These observations suggest that Phragmites distribution in the Lingang salt marsh is unlikely to be determined by salinity alone, but may be jointly associated with porewater salinity, inundation regime, groundwater dynamics, and microtopographic conditions.
 

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