Without a doubt the rarest plant wetland community, and perhaps the rarest of any plant community in North America, even before development, is the calcareous fen. Calcareous fens are seldom over two acres in size. Calcareous fens rely on a calcium-bearing groundwater flow that seeps out of springs, or slowly flows through the soil. Sedges are common, and sometimes mosses. The calcium in the groundwater provides a unique habitat and many species found in calcareous fens are rare in other places.
Like many other grass and sedge based wetlands, calcareous fens suffer from natural invasion of shrubs and trees, as well as non-native species such as Reed Canarygrass (Phalaris arundinacea).