Abstract: Quasi-circular vegetation patches (QVPs) occur in the area with a distance of 25 km from the Bohai Sea to the land in the Yellow River Delta, which are featured by a fast succession rate and high biodiversity. QVPs are ideal targets for studying water-salt interaction and vegetation succession in the Yellow River Delta. Revealing formation processes and mechanisms of the QVPs is of important practical significance for restoring vegetation in the saline tidal flat in the Delta. Based on the water-salt thresholds and living habits of Suaeda salsa, Tamarix chinensis, and Phragmites australis, which are the dominant plant species in QVPs, and the distribution patterns of halophyte vegetation communities in the gentle lowland of the Delta, we analyzed the formation processes and mechanisms of QVPs. The results showed that the local redistribution of water-salt caused by waterlogged microtopography in the gentle lowland was the basis for the formation of QVPs. Suaeda salsa was located in the outermost edge of QVPs with high soil salinity (1.0%-2.1%) and humid zone. To the center of QVPs, short and small Phragmites australis occurred, where was a medium-high soil salinity (0.12%-0.79%) and low water depth. Further to the center of QVPs, Tamarix chinensis scattered near the water mark line, which is a local area with a medium soil salinity (0.16%-0.70%) and low water depth. Phragmites australis and Imperata cylindrica are located in the center of the QVPs where was a low soil salinity (0.1%-0.32%) and high water depth. In spring and winter, the gray white salt belt with high salinity appeared outside of Suaeda salsa where was a local high salinity (1.6%-3.4%) and dry zone, which was caused by the movement and accumulation of salt caused by evaporation. Therefore, a quasi-circular vegetation patch with ring structure and relatively stable area was formed. We can thus build an ecological restoration model of microtopography construction in combination with water and salt control to promote the progressive succession of vegetation, which provides a nature-based solution for vegetation restoration of saline tidal flat in the Yellow River Delta.

Key words: Suaeda salsa, Tamarix chinensis, Phragmites australis, water-salt threshold, quasi-circular vegetation patch, formation process.