Although the general sequence of vegetation change in the late-glacial upper Midwest is well understood, details on the timing of major ecological events have been hampered by poor radiocarbon chronologies. Moreover, recent work at Appleman Lake, IN suggests that the late-Pleistocene decline in megaherbivores coincided with and may have contributed to the formation of novel plant communities and more frequent wildfires. In the upper Midwest, these “no-analog” forests were characterized by high abundances of boreal conifers (e.g. Picea, Larix) coexisting with temperate deciduous taxa (e.g. Fraxinus, Ostrya/Carpinus), as well as anomalously low abundances (or absences) of Pinus, Alnus, and Betula. The role of the Pleistocene megafauna in shaping late-glacial vegetation change is largely unknown, due to dating uncertainties and the poor spatiotemporal and taxonomic resolution of fossil data. Sporormiella is a coprophilous fungus with spores preserved in lake sediments, making it an ideal proxy for testing the relationship between local megaherbivore population declines and vegetation change. At Appleman, the coincidence of the Sporormiella decline, the rise in Fraxinus and Ostrya/Carpinus, and a large peak in sedimentary charcoal suggests that keystone megaherbivores may have altered ecosystem structure and function through 1) the release of palatable hardwoods from herbivory pressure and 2) a shift in fire regimes due to increased fuel loads. However, additional records are needed to assess the spatiotemporal pattern of the Sporormiella decline and to investigate whether it is consistently associated with increases in the pollen abundances of broadleaved deciduous taxa.
Results/Conclusions
We present new pollen, charcoal, and Sporormiella records from Kirchner Marsh, MN, Rock Lake, WI, Spicer Lake, IN, and Silver Lake, OH and analyze the spatial and temporal patterns of vegetation change and megafaunal decline along a northwest-southeast transect. The local “functional extinction” of megaherbivores was identified as the point at which Sporormiella values decreased below 2% of the arboreal pollen sum and did not recover during the Holocene. Constrained Incremental Sum of Squares (CONISS) analysis was used to identify shifts in plant community composition relative to the Sporormiella decline, and no-analog communities were identified using the squared-chord distance dissimilarity metric. These high-resolution pollen records provide new details and better age constraints on the late-glacial vegetation history of the upper Midwest. Additionally, the sequencing of vegetation change, fire regime shifts, and megafaunal extinction both within and across sites in the transect sheds new light on the interacting effects of climate change and megafaunal herbivory on late-glacial plant communities.
