Wetlands characterized by the presence of rare and endangered reed plant communities are seriously threatened by hydrological changes and pollution caused by human activity, e.g., drainage, river regulation, and conversion to agricultural land. Despite numerous studies of wetland communities, the “volume of ecological niche” based on Ellenberg indices, i.e., the ecological preferences of vascular plant species, has rarely been analyzed at the level of entire plant communities. Properly defined indicators of microclimatic and habitat factors (ranges of environmental conditions), appropriate for individual rush and sedge communities (specific communities), are very important for the sustainable management of ecosystems and potential restoration processes in renaturation activities. Therefore, a comprehensive floristic and habitat assessment of wetland communities of the Phragmitetea class was conducted in a Natura 2000 site in southeastern Poland (name and number of the Natura 2000 site—Wolica Valley PLH060058), located within the East European Lowland. The communities were analyzed in the context of the variability of individual Ellenberg indices and designated ecological hypervolumes. These were typical rush communities occurring in wet and fertile soils with a neutral or alkaline pH. The microclimatic conditions were typical for these habitats. The studied communities differ in terms of the variability of Ellenberg ecological indices. Some of them are characterized by low ecological niches, while others are characterized by larger ones. The volume of determined multidimensional hypervolumes allowed us to distinguish two communities (Phragmitetum australis and Caricetum rostratae) to have greater generality compared to the others. They can occur in a greater variety of environmental conditions than other communities that require more specific conditions. Other phytocenoses with low hypervolume values (hypervolumes more than 10 times smaller than mentioned before) were distinguished by high habitat specialization. In turn, the analysis of the overlapping of hypervolumes allowed us to group communities into four clusters with similar ranges of Ellenberg indices’ values: (1) Caricetum distichae and Caricetum gracilis; (2) Glycerietum maximae, Iridetum pseudoacori, Caricetum appropinquatae, and Phalaridetum arundinaceae; (3) Phragmitetum australis and Caricetum rostratae; and (4) Caricetum acutiformis, Caricetum vesicariae, and Caricetum elatae.