LB 23-196 - History in the Margins: Ecotonal Dynamics and Holocene Abrupt Change in Lower Michigan

Abrupt changes in ecological systems are of increased importance in the face of climate change and other mounting anthropogenic pressures. The role of ecotonal transitions in either buffering or amplifying ecological responses to drivers is poorly understood, but often populations near to ecotones are hypothesized to be more sensitive to environmental variability. Studies of past climate change, ecotonal dynamics, and sensitivity of near and distal populations can help test this expectation. This research focuses on the Michigan Tension Zone (MTZ) in Lower Michigan, which separates southern broadleaf deciduous forests from northern Laurentian mixed-pine forests. While periodic collapses of mesic tree taxa such as American beech (Fagus grandifolia) and Eastern hemlock (Tsuga canadensis) over the mid- to late-Holocene have been well-documented throughout the Upper Midwest and northeastern US, Lower Michigan remains understudied. Here we present a new fossil pollen record for Sunrise Lake, MI and combine it with non-metric multidimensional scaling (NMDS) of 20 extant fossil pollen datasets from the Neotoma Paleoecology Database to track the MTZ’s movements since its establishment roughly 10ka BP (thousand years before present). At Sunrise, Tsuga arrived at 6.7 ka and Fagus at 6.4 ka BP. Tsuga abruptly declined at 5.0 ka BP, and both taxa display anti-phased behavior with oak (Quercus) and other hardwoods. The NMDS Axis 1 captures the decline of spruce (Picea) woodlands at the Pleistocene-Holocene transition, while Axis 2 clusters southern hardwoods from northern mixed-forest, indicating a signal of MTZ position. Maps of the regressed MDS2 scores reproduce the current position of the MTZ, and maps of this axis allow analysis of shifts in the MTZ over time. Analyses are currently underway to assess the variability of Holocene community dynamics for sites near to or far from the MTZ.