THE DISJUNCT COASTAL-PLAIN FLORA IN THE GREAT-LAKES REGION

Three areas in the Great Lakes region are known to harbor species whose primary range is the coastal plain: (1) the southeastern Georgian Bay region of Ontario, (2) the sandplains near Lake Michigan in southwestern Michigan and northern Indiana, and (3) the lacustrine sand deposits in central and northern Wisconsin. Secondary areas with smaller concentrations are found in the southeastern Lake Ontario region of New York, the Lake Erie region, small areas of north-central Illinois, and local areas of northern Michigan. These areas are all sand deposits associated with postglacial lakes and drainage channels. A review of the flora of the Great Lakes region disclosed a list of 62 primarily coastal plain species that are disjunct to the Great Lakes region. The species are largely herbs, nearly half are graminoids, and many are annuals. Their habitats are primarily sandy, gravelly, or peaty emergent shores of shallow, soft-water ponds and small lakes with fluctuating water levels, or sometimes sandy, periodically flooded swales. A few species are aquatic, and fewer still are bog or wet forest species. Most are evident only during years of low water levels, and sometimes their appearance is quite sporadic, making survey and monitoring work difficult. However, the consequences of loss of populations can be severe since, with disjuncts, there may be no seed source nearby to facilitate recolonization of sites. The sandy lakeshore habitats are frequently prime recreational lands, thus creating additional pressures. Previous hypotheses for the occurrence of these disjuncts suggested gradual migration along the shores of postglacial lakes and drainages. However, these species do not typically inhabit shores of rivers and large lakes. As well, habitats along these shores undoubtedly were not continuous. Nevertheless, the localized occurrence of the species at the termini of major postglacial drainages in spite of the more widespread availability of apparently suitable habitats argues against random long distance dispersaL Here proposed is that these species migrated into the Great Lakes region through dispersal jumps of varying distances between substantial areas of suitable habitat created along major postglacial drainage channels. Once in the Great Lakes area, they were most successful in areas with extensive drying shorelines.