CHRONOLOGY AND DYNAMICS OF VEGETATION CHANGE IN THE PRAIRIE‐WOODLAND REGION OF SOUTHERN MINNESOTA, U.S.A. *

Abstract

SUMMARY: The pollen and sediment stratigraphy of Wolsfeld Lake and French Lake in the Big Woods of southern Minnesota was investigated. Wolsfeld Lake is in the northeastern Big Woods, and French Lake is in the western Big Woods about 1.5 km from the prairie‐woodland border. During the mid‐postglacial, most of the present Big Woods was prairie. Oak woodland began invading prairie about 5000 years ago in the eastern Big Woods, then expanded westward, becoming established near French Lake 2400 years ago. Topography and water bodies, which control fire frequency, strongly influenced where and when woodland invaded prairie. Oak woodland persisted until 300 years ago, when Ulmus, Ostrya virginiana, Tilia americana and Acer saccharum rapidly expanded and became dominant. The changes from prairie to oak woodland and from oak woodland to ‘bigwoods’ must have required reductions in fire frequency, which were probably caused by increased precipitation and possibly decreased temperatures. About 100 years before the expansion of bigwoods, sediment influx increased at the coring site in Wolsfeld Lake, probably because of an increase in runoff and, therefore, precipitation. A conceptual model is presented in which climatic changes trigger vegetation changes, but interactions among the existing vegetational pattern, fire, and topography cause climatic thresholds for vegetation change to vary in space and time.