The zonation of Scirpus mariqueter and Scirpus tabernaemontani communities is conspicuous in salt marshes of the Changjiang estuary. However, diking, which partly restricted tidal fluctuation in a salt marsh, resulted in the invasion of S. tabernaemontani into S. mariqueter communities. To explore the role of the interaction between the two species in the vegetation change, the structure of S. mariqueter tussocks at low elevation and S. tabernaemontani clones at medium elevation were investigated in the diked marsh, and biomass allocations to different components of the two species was also estimated. Scirpus mariqueter performed better at the edges of the tussocks but aged and declined in the central areas of the tussocks with low density of living shoots, large proportion of flowering shoots, and low aboveground dry mass. Scirpus taberaemontani grew vigorously in the center areas of large S. taberaemontani clones with high density of shoots, and those clones were surrounded by a residual ring of S. mariqueter, whose structure was similar to the pattern observed in the tussocks of S. mariqueter. Comparison of the vegetation structure along the elevational gradient of the diked marsh suggests that S. taberaemontani invades the center area of the Scirpus mariqueter tussocks, possibly due to changed topography and its well-developed aerenchymatous tissue. Scirpus tabernaemontani allocates a significantly greater proportion of biomass to photosynthetic shoots and rhizomes than S. mariqueter does, both above ground and below ground, a potential competitive advantage. Once established in the central areas of the S. mariqueter tussocks, S. taberaemontani successfully replaced S. mariqueter. We hypothesize that the competitive displacement of S. mariqueter by S. taberaemontani plays an important role in the vegetation change after the short-term tidal restriction and also in generating plant zonation in the salt marshes.