DIFFERENTIATING CLIMATIC AND SUCCESSIONAL INFLUENCES ON LONG-TERM DEVELOPMENT OF A MARSH

Comparison of long-term records of local wetland vegetation dynamics w ith regional, climate-forced terrestrial vegetation changes can be use d to differentiate the rates and effects of autogenic successional pro cesses and allogenic environmental change on wetland vegetation dynami cs. We studied Holocene plant macrofossil and pollen sequences from Po rtage Marsh, a shallow, 18-ha marsh in northeastern Indiana. Between 1 0 000 and 5700 yr BP the basin was occupied by a shallow, open lake, w hile upland vegetation consisted of mesic forests of Pinus, Quercus, U lmus, and Carya. At 5700 yr BP the open lake was replaced rapidly by a shallow marsh, while simultaneously Quercus savanna developed on the surrounding uplands. The marsh was characterized by periodic drawdowns , and the uplands by periodic fires. Species composition of the marsh underwent further changes between 3000 and 2000 yr BP. Upland pollen s pectra at Portage Marsh and other sites in the region shifted towards more mesic vegetation during that period. The consistency and temporal correspondence between the changes in upland vegetation and marsh veg etation indicate that the major vegetational changes in the marsh duri ng the Holocene resulted from hydrologic changes forced by regional cl imate change. Progressive shallowing of the basin by autogenic accumul ation of organic sediment constrained vegetational responses to climat e change but did not serve as the direct mechanism of change.