Interdunal wetlands occupy damp hollows within active dune fields along the Great Lakes shores. They may also occur where moving sand encroaches on nearby wetlands, surrounding and isolating all or portions of them. The vegetation is variable and is highly dependent on the size and shape of the wetland. Water depth in the wetland also strongly influences plant composition, and water levels fluctuate with Great Lakes water levels. The same site can have deep pools of standing water some years and be nearly dry in others.
The wettest zones tend to have species such as twig-rush (Cladium mariscoides), hard-stem bulrush (Schoenoplectus acutus), and soft-stem bulrush (S. tabernaemontani). Sedges are also often a component of the standing water zone (eg, Carex aquatilis, C. lasiocarpa, C. oligosperma). Sites on the Apostle Islands may also support the rare Michaux’s sedge (Carex michauxii). Wetter zones are also prone to invasion by aggressive non-native species such as narrow-leaved cattail (Typha angustifolia) and Phragmites (Phragmites australis ssp. australis). If deep standing water is present, aquatic plants such as water-shield (Brasenia schreberi) and duckweed (Lemna spp.) may also occur.
The saturated soil zone can be extensive, supporting indicators such as little green sedge (Carex viridula), golden-fruited sedge (Carex aurea), Baltic rush (Juncus balticus), northern green rush (Juncus alpinoarticulatus), silverweed (Potentilla anserina), spike-rushes (especially Eleocharis elliptica and E. quinqueflora), ladies-tress orchids (Spiranthes cernua and S. magnicamporum), and false-foxgloves (Agalinis paupercula and A. tenuifolia).
Finally, the driest zone grading up to the dune is often dominated by hairy panic grass (Dichanthelium acuminatum var. fasiculatum). Sites on the drier end of the spectrum may lack the wetter species and be predominantly dominated by Baltic rush, silverweed, and hairy panic grass. Depth to water table can be quite variable in these drier sites, ranging from six inches to well over 24 inches. Nevertheless, these sites can be recognized as interdunal wetlands due to the presence of at least moderately hydrophytic vegetation that does not grow in the surrounding sand dunes.
Notably, due to the proximity to water, interdunal wetlands are often ringed by trees such as white pine, northern white-cedar, paper birch, balsam poplar, and the non-native invasive Scots pine. Shrubs are also occasional but rarely dominant. During prolonged periods of low water, young trees and shrubs can invade and establish in the wetland but are quickly killed when water levels return to normal levels.
Overall, interdunal wetlands and the dune systems that they occur in are rare and not well developed in Wisconsin compared to Michigan where the prevailing winds and nearshore currents are conducive to moving large quantities of sand along stretches of shorelines with active dune fields. High-quality examples of interdunal wetlands in Wisconsin are known from fewer than ten locations. Despite their rarity and limited distribution, these wetlands provide important habitat for many uncommon plant species and provide resting and feeding areas for migrating and resident water birds.
