Late Holocene Vegetation History of the Forest Tension Zone in Central Lower Michigan, USA

Abstract

This research reconstructs a 2000-year vegetation history of the forest tension zone, an ecotone between the mixed coniferous-deciduous (Laurentian) forest and the eastern broadleaf forest, in the Lower Peninsula of Michigan, USA. Different forest communities relative to the ecotone are represented by pollen analysis of sediment cores from three lakes: Hicks Lake (75 km north of the ecotone), Cowden Lake (within), and Morrison Lake (55 km to the south). Pollen data delineate the presence of the ecotone during the past 2,000 years. North-south shifts in the location of the ecotone occurred along with changes in tree dominance in response to two climatic oscillations over the past two millennia: the Medieval Warm Period (MWP) from ~ 1000 to 700 cal yr BP and the Little Ice Age (LIA), ~ 500-150 cal yr BP. During the MWP, Thuja increased in the forest around the northern lake, Quercus replaced Fagus in the forest around the central lake, and Quercus forest succeeded a Fagus-Acer forest near the southern lake. Abrupt shifts in the abundance of several important taxa were associated with the LIA cooling, beginning at 500 cal yr BP. Thuja declined from 40% to just under 20% as Pinus and Tsuga increased around the northern lake; Quercus declined whereas Pinus, Tsuga, and Fagus expanded in the vicinity of the central lake; and Fagus and Acer became more prevalent near the southern lake. This research demonstrates the sensitivity of forest communities to late Holocene climate change as well as documents how species dominance changed in response to small variations in temperature (1-2° C) and precipitation over the last 2000 years. We can expect that this and similar systems will respond greatly to the temperature and precipitation fluctuations projected for the Great Lakes Region.