This lowland hardwood forest community type occurs along large rivers--usually 3rd order streams or higher. Most of these rivers originate in northern Wisconsin and flow southward, growing in size as the volume of water they carry increases. As the stream gradients diminish, the floodplains become broader. Periodic floods, particularly in the spring, are the key natural disturbance event to which species of this community have adapted. Silt deposition and development of microtopography during flood events create suitable sites for tree germination and establishment. Floods also carry seeds and propagules of plant species. The most extensive occurrences of floodplain forest are found along the large rivers of southern Wisconsin, but the community also occurs at scattered locations in the north. This community was uncommon historically, occupying only about 3% of the Western Coulees and Ridges Ecological Landscape and even smaller percentages of other ecological landscapes (Finley 1976). Canopy dominants vary, but may include silver maple (Acer saccharinum), river birch (Betula nigra), green ash (Fraxinus pennsylvanica), black ash (Fraxinus nigra), hackberry (Celtis occidentalis), swamp white oak (Quercus bicolor and its hybrids with bur oak), and eastern cottonwood (Populus deltoids). Black willow (Salix nigra), basswood (Tilia americana), red oak (Quercus rubra), and red maple (Acer rubrum) are associated species found in these forests. Historically, elms were highly significant components of the floodplain forests, but Dutch elm disease has eliminated most large elm trees that formerly provided supercanopy structure, snags and den sites, and large woody debris. Northern occurrences of this type tend to be less extensive, are often discontinuous, and relatively species-poor compared to those in the south. Silver maple and green ash are still dominant, but balsam-poplar (Populus balsamifera), bur oak (Quercus macrocarpa), and box elder (Acer negundo) replace some of the southern tree species.
Understory composition is also quite variable and follows the pattern exhibited by the canopy species-the most extensive stands and highest plant species diversity occur in southwestern Wisconsin. Buttonbush (Cephalanthus occidentalis) is a locally dominant shrub that may form dense thickets on the margins of oxbow lakes, sloughs, and ponds, which are often important aquatic habitats in these forests. Wood nettle and stinging nettle (Laportea canadensis, Urtica dioica), sedges (e.g., Carex grayi, C. lupulina, C. hystericina, C. tuckermanii), native grasses (e.g., Cinna arundinacea, Elymus villosus, Leersia virginica), ostrich fern (Matteuccia struthiopteris), and green-headed coneflower (Rudbeckia laciniata) are important understory herbs, and lianas such as Virginia creeper (Parthenocissus quinquefolia), grapes (Vitis spp.), Canada moonseed (Menispermum canadense), and poison-ivy (Toxicodendron radicans) along with the shrub form of poison ivy (T. rydbergii) are often common. Among the more striking herbs of this community are cardinal flower (Lobelia cardinalis), fringed loosestrife (Lysimachia ciliata), and green dragon (Arisaema dracontium).
The sprawling floodplains found along the largest rivers sometimes consist of several terraces capable of supporting forests that are subject to floods with differing frequencies and levels of inundation, and support patches of varying floristic composition depending upon local elevation differences, edaphic factors, and disturbance history. The lower terraces experience the most frequent, severe, and long-lasting floods while the uppermost terraces flood infrequently, and the rich alluvial soils can support mesophytic trees species and rich groundlayers similar to those of the mesic hardwood forests.
