The pioneer zone of salt marsh,with the physical environment characterized by frequent flooding and wide salinity fluctuation,was significantly affected by tides action.The pioneer zones of the saltmarshes at the Yangtze estuary are dominated by Scirpus mariqueter.Spartina alterniflora,the exotic species that has become invasive along coastline of china,dominates at mid-tidal flat and adjoins the border orienting the seawall of Scirpus mariqueter.The establishment of pioneer vegetation zones is seemingly directly related to physical environment gradients.In this paper,our aim was to discriminate the key factors controlling vegetation zonation at micro-scale (within vegetation zone,the distance is 50 meters) and meso-scale (among vegetation zones,the distance is about2km).The environmental factors data of elevation,water table depth,soil salinity,soil water content and compaction were investigated in the tidal flat zone,Scirpus mariqueter zone,Spartina alterniflora zone and the borders between them.The results showed that soil salinity,soil water content and shallow groundwater determined the vegetation zones structure at mirco-scale.At meso-scale,all environment factors between different vegetation zones showed significant differences.Elevation,soil salinity,soil water content and shallow groundwater determined the dynamics of vegetation spatial distribution.The elevation,especially,determined the dynamic,development and evolution of mudflat through its influences on microtopography,and consequently constrained the dispersal and establishment of pioneer species.The species responses to environment were certainly also one of the main reasons of vegetation zonation.