Abstract: Wetland areas maintain a high level of moisture at all times and experience flooding at regular intervals. High groundwater levels help create wetland areas, as does a relative lack of surface water loss. The paper bases on the hypothesis that wetland areas are characterized by seasonally high water levels and drought. These changes in water content markedly affect the presence of rare plant communities. The purpose of this paper is to determine the effect of changes in the level of groundwater on selected plant communities in wetland areas. The paper also aims to determine if any other determinants affect these plant communities: (1) relief, (2) climate conditions (precipitation, temperature), (3) human impact. Furthermore, the paper provides a detailed hydrographic analysis of wetland areas including information on water migration pathways, water recharge systems, and sources of water loss. The study area consists of Piaśnickie Łąki – a protected natural area in northern Poland, close to the Baltic coastline. It is also a designated “Nature 2000” area, and it is abundant in rare plant communities such as reed grass (Molinietum medioeuropaeum), which thrives in variable moisture areas. The majority of the research work consisted of literature analysis and fieldwork, which included the installation of a groundwater monitoring system, groundwater and surface water level gauging, and discharge gauging for larger streams found adjacent to the study area. The fieldwork was done in the period 2014 – 2015. The collected research data indicate that groundwater levels did vary during the study period. These conditions helped produce a high rate of plant growth and an increased rate of evapotranspiration across the surface of the study area, which then helped decrease the level of groundwater. One basic condition for the functioning of variable-moisture reed grass communities (Molinietum medioeuropaeum) is a specific pattern of variability in groundwater levels. Small depressions in the studied nature reserve offer the best conditions for the occurrence of reed grass communities.