DYNAMICS OF COASTAL WETLANDS RELATIVE TO LATE HOLOCENE LAKE LEVEL CHANGE

Lake-level changes are the driving force behind Great Lakes wetland dynamics. High water levels periodically kill large, canopy-dominating plant species, and low water levels expose sediments to allow revegetation from the seed bank. Lake-level changes are climate-driven; thus, the chosen study sites for evaluation of potential effects of global climate change on Great Lakes wetlands were chronosequences of ridge and swale terrains (Lake Michigan) where wetlands associated with past lake levels are preserved. A 4700-year lake-level history (proxy for climate change) was reconstructed using sedimentological techniques. Long-term changes in vegetation were reconstructed using paleoecological techniques. Modern vegetation was characterized by sampling plant communities along transects in the wetlands between the ridges and then compared with the paleoecological record to develop predictive capabilities. Modern vegetation at one site at the north end of the lake demonstrates classic boreal succession patterns, grading from wet sand-flat communities to sedge/leatherleaf floating mats to tamarack swamp to cedar swamp as wetlands increase in age. However, some of the inland wetlands revert to sedge/leatherleaf dominance with a marked reduction in trees, especially cedar. Paleoecological records also show that wetlands in different swales responded differently to the same climatological events of the past. Differences in the role of ground water in local hydrology may explain some of these differences, as greater or more constant supplies of ground water may have allowed some wetlands to avoid drought-driven vegetation changes during warming periods. High lake levels during cooling periods could have raised ground-water elevations in swales near the lake, thus reflooding some later-stage swamp communities and allowing new floating mats to form. These results suggest that wetland response to climate change may be site-specific and dependent on the role of ground-water hydrology and lake level, as well as the plant community type associated with the climatic region.