CLIMATE-CHANGE AND THE DEVELOPMENT OF COASTAL-PLAIN DISJUNCTIONS IN THE CENTRAL GREAT-LAKES REGION

Paleoecological studies at Portage Marsh in northwestern Indiana provi de a Holocene record of local populations of four species disjunct fro m the Gulf and Atlantic Coastal Plains: Rhynchospora macrostachya, R. scirpoides, Fuirena pumila, and Eleocharis equisetoides. The populatio ns were established at Portage Marsh when water levels dropped owing t o regional climatic drying during the mid-Holocene. They persisted unt il the late Holocene; only R. macrostachya still occurs in the marsh. Populations of these and other Coastal Plain disjuncts probably were e stablished in the southern Lake Michigan region following mid-Holocene drying and formation of extensive shallow wetlands by coastal process es. Seed sources for these populations may have been in the eastern Gr eat Lakes region. We propose that development of Coastal Plain disjunc tions in the central Great Lakes region consisted of a sequential seri es of westward range-shifts in response to climate-induced habitat cha nges, rather than a single late-glacial migration along a corridor of suitable habitat.