The Medieval Warm Period in the Great Lakes Region: Spatial Patterns of Drought and Terrestrial Ecosystem Response

Tree-ring and other proxy records from western North America and the Great Plains document a series of extensive multidecadal-scale droughts during the Medieval Warm Period (MWP), and recent evidence suggests that these droughts extended well into eastern North America. We have conducted tandem investigations of pollen, charcoal, and hydroclimate proxies (testate amoebae, humification) from peatlands in the Great Lakes region of North America. Our objectives were to 1) determine whether multidecadal droughts of the MWP were spatially and temporally coherent, and 2) assess whether the magnitude of these droughts was sufficient to force ecological change in terrestrial ecosystems. Our site network includes three raised bogs and five kettle peatlands located in Minnesota, Wisconsin, Michigan, Indiana, and New York. Results indicate that the MWP was characterized by high-magnitude fluctuations in water balance, affecting peatland surface-moisture conditions throughout most of the region. The highest magnitude droughts of the last 2000 years occurred during the MWP, with individual drought events centered on 1000 BP, 800 BP, and 700 BP. Pollen and charcoal records indicate that these multidecadal droughts were of sufficient magnitude to force major ecological changes, including abrupt changes in forest composition. In particular, widespread fires and drought appear to have negatively affected Fagus grandifolia (beech) populations, and facilitated the expansion of pine- and oak-dominated ecosystems throughout the central and portions of the western Great Lakes. However, sites located in the lake-effect snow belt of Lakes Superior and Michigan did not record the MWP droughts or evidence of drought-induced vegetation change during the MWP, suggesting that lake-effect snow may have increased during the MWP and buffered these areas from the widespread droughts. Our study demonstrates that even in humid regions like the Great Lakes, multidecadal-scale droughts have the potential to dramatically transform terrestrial ecosystems.