Wetlands can be divided into many different types. The simplest may be that of swamp, marsh and bog. Swamps are wetlands dominated by trees or shrubs. A marsh is dominated by grass-like (graminoid) vegetation like the true grasses and members of the sedge family. However, a marsh with mostly open water will be dominated by a variety of aquatic plants. The simple definition of a bog is a wetland whose ground cover is dominated by mosses. Plant community designation is further complicated by differences in regional terminology. Fen, swamp, mire, bracken, glade, marsh, meadow, carr, bog, bayou, billabong are just a few of the terms, and many of these are further divided. A plant community is an artificial construct, but an important first step in studying and managing wetland ecosystems.
For the most part I will use the communities listed by Eggers and Reed in Wetland Plants and Plant Communities of Minnesota & Wisconsin. The book is now out of print, but it is available here for viewing on line.
What plants occupy a wetland depend a myriad of factors: soils, hydrology, time since disturbance, fire, grazing, agriculture, climate, etc. A wetland plant community may appear timeless, but it is not static. A sedge meadow will not remain a sedge meadow indefinitely; left unattended it will usually transition to shrub wetlands and then forested wetlands. This natural process, called succession, is often sped up by man due to fire suppression and other factors. Muskrats may move in, digging a tiny pond in the meadow and creating a bit of shallow marsh. Beavers may flood a forested wetland, killing the trees. When the dam is abandoned and the water recedes it may become sedge meadow, and then an alder thicket. These and many other natural changes create and rejuvenate many different types of wetland plant communities.