ECOTONAL POPULATIONS SHOW HIGHER COMPOSITIONAL VARIABILITY DURING HOLOCENE SHIFTS OF THE MICHIGAN TENSION ZONE

Abrupt changes in ecological systems are of increasing concern, given on-going climate change and other mounting anthropogenic pressures. Ecotonal populations are believed to be more sensitive to environmental variability, but the capacity of ecotones to either buffer or amplify ecological responses to drivers remains poorly understood. Here we present new maps of the shifting Holocene position of the Michigan Tension Zone (MTZ), which separates southern broadleaf deciduous forests from northern Laurentian mixed-pine forests, and we test whether distance to ecotone is a predictor of compositional variability. For these analyses, we present a new fossil pollen record record for Sunrise Lake, MI and combine it with non-metric multidimensional scaling (NMDS) and empirical Bayesian Kriging (EBK) of 19 extant fossil pollen datasets from the Neotoma Paleoecology Database to a) track the MTZ’s movements during the Holocene and b) test whether distance to ecotone is a predictor of vegetation compositional variability. The 16 calibrated ¹⁴C dates at Sunrise include a basal date of 10.9 ka BP and all but two are in stratigraphic order. At Sunrise, Tsuga canadensis populations expanded at 6.7 ka and Fagus grandifolia at 6.3 ka BP. NMDS Axis 2 clusters southern hardwoods from northern mixed-forest, indicating a signal of MTZ position, while Axis 1 captures the decline of spruce (Picea) woodlands at the Pleistocene-Holocene transition and Axis 3 captures the rise of Ambrosia-dominated assemblages after EuroAmerican settlement and logging. The MTZ shifted northwards by approximately 150 km from 11 and 6 ka, then retreated southwards, reaching its current position by 2 ka. Sites close to the MTZ ecotone show significantly more compositional variability than distal sites, suggesting that ecotonal populations in Michigan were more sensitive to Holocene climate variability. This in turn reinforces concerns that ecotonal systems may be more sensitive to Anthropogenic Global Warming.