Secondary succession in wet grasslands has received surprisingly little attention. We analyzed the vegetation of a eutrophic alluvial wetland of western France and the relationship between its composition and environmental variables. The objectives of our study were twofold: (1) to describe species assemblages of wet grasslands and oldfields and correlate their species composition to hydrologic stress and (2) to study the changes in species composition, species richness, and species diversity when mowed meadows are abandoned. More than 100 species were recorded, and five species assemblages (‘A’ to ‘E’) were identified. Hydrology and land use (mowing versus land abandonment) were the two main ecological factors that affected plant community development. Hydrologic regime contributed to separate ‘A’, ‘B’ (on the wettest end of the gradient) from ‘C’, ‘D’, and ‘E’ (on the driest end of the gradient). Land use helped to discriminate ‘B’, ‘C’ from ‘A’, ‘D’, and ‘E’. Larger-scale disturbances such as mowing maintain meadow species, leading to a high level of species richness, species diversity, and evenness. We compared the species richness, species diversity, and evenness according to the duration of land abandonment since last mowing disturbances (<5 years, 6–10 years, 11–15 years, 16–20 years). No significant differences were observed. Botanical composition of these plots did not differ significantly. In the first five years of land abandonment, there was an increase of litter, which reduces the species richness, diversity, and evenness. Live, above-ground biomass of grasslands was significantly greater than that of oldfields, but total organic matter (biomass litter) was significantly greater for oldfields than for grasslands. Total biomass showed a negative logarithmic relationship with species richness, with species richness being initially high at very low standing crop levels (grasslands) and decreasing at greater levels of standing crop (oldfields).