Abstract
Aim Ecotones are expected to be sensitive to climate change, but supporting evidence is scarce. Here we test the ecotonal sensitivity hypothesis by reconstructing past movements of a major temperature‐governed forest ecotone (the Tension Zone, separating temperate deciduous forests and northern mixed hardwood‐conifer forests) and comparing the variability of ecotone‐proximal and distal vegetation. Location Michigan, United States. Time Period Holocene. Major Taxa Studied Temperate tree taxa in north‐central U.S. Methods We apply non‐metric multidimensional scaling (NMDS) and empirical Bayesian Kriging (EBK) to a new pollen record from Sunrise Lake, MI located in the Tension Zone and 19 extant fossil pollen datasets from the Neotoma Paleoecology Database to track movements of the Tension Zone during the Holocene and use the R‐Ratepol package to measure compositional variability. Results NMDS Axis 2 clusters southern hardwoods from northern mixed forest, and so can be used to indicate Tension Zone position, while Axis 4 captures shifts in mesic versus xeric taxa, Axis 1 captures the early Holocene decline of Picea woodlands and Axis 3 captures the rise of Ambrosia ‐dominated assemblages after Euro‐American settlement. At Sunrise Lake, which is within the Tension Zone, major changes include expansions of Tsuga canadensis populations at 6.7 ka and Fagus grandifolia at 6.3 ka BP. The Tension Zone shifted northwards by approximately 150 km from 11 and 6 ka during a 3.5°C TJuly warming, then retreated southwards, reaching its current position by 2 ka. Sites close to the Tension Zone ecotone show more compositional variability than distal sites, suggesting that ecotonal populations in Michigan were more sensitive to Holocene climate variability. This relationship varied over time and strengthened after 6 ka. Main Conclusions These findings support the ecotonal sensitivity hypothesis, constrain estimates of climate‐driven forest ecotonal movement, and reinforce concerns about the sensitivity of ecotonal ecosystems to anthropogenic global warming.