GROUND-WATER CONTRIBUTION TO HYDROLOGY-DRIVEN DEVELOPMENT OF WETLAND PLANT COMMUNITIES

Wetland plant communities are very responsive to water depth, and plant successional processes may accompany climate-driven changes in hydrology. Recent work in chronosequences of wetlands occurring in ridge and swale terrains of the Great Lakes suggests that different ground-water flow regimes (shallow, intermediate, deep) also influence the distribution plant communities and may mediate effects of climate and successional processes on those communities. The modern vegetation in a series of wetlands between more than 80 beach ridges along the shore of Lake Michigan near Manistique, MI grades from open shallow marsh in young wetlands near the lake (1-2) to floating sedge mats to shrubs to northern white cedar, then back to floating sedge mat and eventually to hardwood swamp in the oldest wetlands (81-84). Hydrologic studies identified an upwelling of ground water centered in the vicinity of wetlands 58-60. This discharge occurs where an underlying clay-rich aquitard is absent, which allows flow from a confined aquifer (deep flow regime) to reach the surface. These hydrologic conditions are conducive to growth of northern white cedars and explain the prominence of cedar swamp in nearby wetlands, as well as a noticeable presence of cedars in lakeward wetlands that receive this water via the flow-through nature of the shallow flow system. The modern vegetation and the paleoecological record of vegetation changes suggest that this continuous source of cold, calcareous water provides the predominant control on plant communities. Control on plant communities in wetlands outside the influence of this upwelling can be tied more directly to climate and succession. During warm climate phases, plant communities with sufficient ground-water supply may be immune to climate-related water-level reductions that drive vegetation change. Over prolonged periods with no climate-driven flooding to reset the successional stage, plant communities may develop with greater fidelity to ground-water influences than to assumed successional processes.