Southern hardwood swamp is a forested wetland community type found in insular basins with seasonally high water tables. This type is best developed in glaciated southeastern Wisconsin but was not of large extent even prior to EuroAmerican settlement. Finley (1976) classified less than 1% of southeastern Wisconsin as lowland hardwood forest, and this figure includes bottomland forests along rivers as well as southern hardwood swamps in closed basins. Dominant tree species are silver maple (Acer saccharinum), red maple (Acer rubrum), hybrids of red and silver maples (e.g., Acer X freemanii), and green ash (Fraxinus pennsylvanica). Associate tree species may include swamp white oak (Quercus bicolor), bur oak (Q. macrocarpa) and their hybrids, basswood (Tilia americana), and American elm (Ulmus americana), all of which may be a significant part of the canopy or subcanopy in sites impacted by emerald ash borer. Black ash may be present in southern hardwood swamps but is usually not dominant across the site. The groundlayer is often dominated by species typical of floodplain forests 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). Southern hardwood swamps are also noted for a high component of lianas, including poison ivy (Toxicodendron radicans), Virginia creeper (Parthenocissus quinquefolia), and grapes (Vitis spp.). In the relatively undisturbed sites, there can be a rich spring flora. Microtopographic differences account for the existence of patches of spring ephemerals as well as many wetland species. The exotic reed canary grass (Phalaris arundinacea) has become dominant in the understory of many hardwood swamps.
This Natural Heritage Inventory community type partly includes the "southern wet-mesic forest" of the Curtis (1959) classification, although this Curtis type also includes the more common hardwood swamps usually maintained by groundwater seepage and dominated by black ash, red maple, American elm, and yellow birch. Curtis referred to this conglomerate type as "lacustrine forests" due to their frequent occurrence on lakeplains, both around the margins of larger existing lakes and on extinct glacial lakes, although southern hardwood swamps also occur in lower-lying portions of till plains that may not have held ponded water for any significant length of time during or after glaciation. Southern hardwood swamps are not restricted to southern Wisconsin, the name rather refers to their similarly to swamps more commonly found in the southern Midwest, especially those of the poorly drained till plains along the Ohio River valley.
