Traditional grasslands are often of high conservation value, but depend on non-intensive management like mowing for their preservation. During the 20th century, traditional agricultural usage was either heavily intensified or abandoned due to socio-economic reasons. In Eastern Europe, land abandonment mainly took place in regions with qualitatively bad soils. This large scale land use change lead to secondary succession. In fens and fen meadows, this may lead to a decrease in species richness and a replacement of specialist species by more generalist ones. The main objective of the present study is to examine if and how mowing cessation interacts with hydrology in determining species and trait distribution in a fen meadow. In the Upper Course of the Biebrza National Park, Poland, we selected 15 sites along four transects, with plots in mown and abandoned parcels. In these plots we measured plant abundance, aboveground biomass and relative light intensity, while plant traits were selected from different trait databases. The relationship between these plot characteristics and the different traits was assessed using concordance analysis. Mowing cessation resulted in reduced moss cover and light availability, while vegetation height increased and higher litter deposition and tussock development were observed. This altered environment not only resulted in decreased species richness and evenness in abandoned plots but also caused shifts in plant trait distribution. Most of the significantly linked traits responded more strongly to mowing cessation than to the hydrologic gradient. Traits related to light competition, such as light requirements, plant height and shoot growth form, especially responded to mowing cessation. This stresses the importance of light competition as a major factor determining species and trait distribution in fen systems.