Both bare soil and bedrock communities are characterized by bare mineral soil or a thin to no soil over bedrock. Bare soil communities include beaches, dunes, and temporary mudflats along rivers and lakes formed by natural or managed drawdowns. Many bare soil communities have an abundance of annual plants, which sprout from the seedbank or are spread along the shoreline, while dunes are dominated by deep-rooted perennial grasses and occasional low-growing shrubs.
Clay seepage bluffs which have steep open slopes of raw, moist, eroding clay, and occur along rivers in the Superior Coastal Plan as well as stretches of Lake Michigan, have a mix of both annuals and perennials. Weeds are common in many bare soil communities due to the prevalence of suitable seedbeds, abundant sunlight, and relative lack of competition.
Bedrock communities occur on a variety of substrates ranging from porous sandstone (Moist Cliff) to limestone or dolomite (Moist Cliff, Dry Cliff, Alvar, Algific Talus Slope, Cave) to metamorphic (Felsenmeer, Bedrock Glade). Associated vegetation is strongly influenced by a site's microclimate (cool and moist versus hot and dry), as well as by available moisture. Due to the typically thin substrates and exposed conditions, habitat specialists are often associated with these community types.
Some of these communities are exceptionally rare and unique. Algific Talus Slope, for example, occurs on steep north- or east-facing slopes with a substrate of fractured limestone (dolomite) bedrock that retains ice and emits cold air throughout the growing season. The community is dependent on water entering gaps in the dolomite, freezing in winter, and then slowly melting during the summer months and producing a steady outflow of cold air. Cold microhabitats support and enable the persistence of disjunct northern plant species, and "periglacial relicts" such as northern monkshood and globally rare terrestrial snails. Alvar, in contrast, has a thin, discontinuous soil overlying horizontal beds of limestone or dolomite that supports an unusual blend of boreal and prairie species.
