LATE-GLACIAL AND HOLOCENE ENVIRONMENTAL HISTORY OF SOUTH-CENTRAL WISCONSIN: A STUDY OF UPLAND AND WETLAND ECOSYSTEMS (POLLEN, CHARCOAL, DIATOMS, LAKE LEVELS, BOG EVOLUTION)

Analysis of pollen, charcoal, and sediment at 5 sites in south-central Wisconsin provides evidence of regional vegetation and climate change since deglaciation as well as the evolutionary history of each site. A chemical analysis of charcoal, developed in this study, was used to determine regional fire history. At about 13,000 yr B. P., there was a change from a cold-dry Picea-herb parkland to a cold-wet Picea-Fraxinus nigra forest. As the climate became warmer and drier, Picea decreased and Pinus banksiana/P. resinosa, Betula, and Abies became more abundant. By about 9800 yr B. P., a mesophytic hardwood-conifer forest with Pinus strobus, Ostrya/Carpinus, and Ulmus developed while Betula, Fraxinus, and Abies decreased. At about 8000 yr B. P., Pinus disappeared from regional forests as the climate became warmer and wetter. Between about 6500-3500 yr B. P., increased percentages of charcoal, and high Quercus and Gramineae pollen percentages, suggest that an oak savanna was dominant in south-central Wisconsin and the climate was warm and dry.; This middle Holocene dry period can be inferred from wetland evidence as well as from regional upland pollen and charcoal changes. A sand lens in cores taken from shallower to deeper water from Lake Mendota in Dane Co. suggests a lake level 2.9m lower than present natural level. Hydrologic budget reconstruction of annual precipitation indicates 12% less precipitation than modern between 6500-3500 yr B. P. Hook Lake Bog in Dane Co. and Washburn Bog in Sauk Co. changed from deeper lakes to shallower Brasenia ponds, and Leopold Marsh, in the Wisconsin River floodplain in Sauk Co, showed decreased sedimentation between 6500 and 3000 yr B. P. A cooler, wetter climate after about 3500 yr B. P. is indicated by closed Quercus forest in the upland, a decrease in regional charcoal abundances, and wetland change to Sphagnum bog.; Comparison of ecosystem dynamics and regional fire history suggests that high fire frequency was required for stability of northern conifer and oak-savanna ecosystems, while low fire frequency was necessary for stability of the mesophytic forest. Late-glacial and early Holocene vegetation was different from modern and had a more frequent rate of change. Changes in the earth's orbital parameters (causing enhanced seasonality), the presence of an ice sheet, and rapid changes in landscape, contributed to these differences.