AN ECOLOGICALLY-SIGNIFICANT INCREASE IN MOISTURE AVAILABILITY IN CENTRAL MINNESOTA DURING THE LATE-HOLOCENE

Vegetation patterns arise in response to climatic conditions and ecological disturbances (e.g., land clearance, disease, and fire). Current human activities are affecting vegetation change through both pathways, such that future vegetation responses to atmospheric change need to be understood in the context of broad land use, and vice versa. Our study contributes to understanding the interaction of climate and disturbance by studying the combined roles of wildfire and climatic change in the development of the “Big Woods” in central Minnesota during the past 500 years. Here, we present a test of the hypothesis that an increase in moisture availability allowed the dense Big Woods prairie to replace earlier oak savannas. We tested the hypothesis by reconstructing the late-Holocene water levels of a small kettle pond in the Wood-Rill Scientific Natural Area near Minneapolis. In kettle lakes, such as our study site, the water table of the surrounding aquifer is exposed at the surface, and the lake-level generally reflects the climate-controlled water-budget of the aquifer. Stratigraphic analyses of such lakes, therefore, can provide accurate records of past moisture balance change. Surveys of the lake with ground-penetrating radar (GPR) identified changes in the geometry of the sediments indicative of a long-term transgression of the shoreline position. Likewise, sandy layers and unconformities extend from shore in a transect of sediment cores, and are overlain by late-Holocene sediments. These data indicate that water levels rose over the past few centuries as the “Big Woods” formed, and submerged tree stumps indicate that the water level rise likely continued until the early 20th century. A complementary component of our study evaluated the regional fire history by counting sedimentary charcoal in the lake cores.