Northern hardwood swamps are characterized by relatively constant water levels, often with a groundwater component, and dominance by deciduous hardwood species, especially black ash. Relatively stable water levels lead to saturated soils that inhibit organic matter decomposition and the development of peat (muck) soils. They can be distinguished from southern hardwood swamps, which can also occur in northern Wisconsin, by their relatively stable water levels and higher prevalence of species that prefer saturated soils (see characteristic species above) while southern hardwood swamps are characterized by seasonally fluctuating water levels that usually drop by mid-later summer (similar to floodplains), higher proportion of mineral soils, and a higher prevalence of species that prefer these conditions such as Virginia wild-rye (Elymus virginicus), white grass (Leersia virginica), common wood-reed (Cinna arundinacea), wood nettle (Laportea canadensis), false nettle (Boehmeria cylindrica), and Ontario aster (Symphyotrichum ontarionis). Northern hardwood swamps can be differentiated from floodplain forests by their occurrence around lakes, basins in till plains, or headwater streams rather than along major rivers, and their prevalence of species that prefer saturated rather than seasonally inundated soils.
Northern hardwood swamps have many similarities with northern wet-mesic forests (cedar swamps), especially as both black ash and northern white-cedar can be canopy associates in both communities. In general, sites that are dominated by hardwoods are best typed as hardwood swamps, while areas dominated by conifers are best typed as northern wet-mesic forest, though both types may occur and intergrade across large wetland complexes.
