The influence of historic fire on the Midwestern Tension Zone

The Tension Zone (TZ) is a prominent Midwestern ecotone where the flora of two major forest types (Northern Mixed Forests and Southern Broadleaf Forests) commingle, striking diagonally across the states of Wisconsin and Minnesota. Here, we evaluated the TZ's original placement as demarcated by previous scholars relative to the synecological and autecological characteristics of witness trees from US Public Land Survey records. Witness trees were categorized by temperature and fire relations, point data analyzed in GIS, and spatial outputs compared with the original TZ. Our temperature-based line, representing temperature relations of witness trees, generally corresponded with the original TZ. However, in the lee of firebreaks, isolated pockets of cool/cold mesophytic genera occurred south of the TZ, indicating that other environmental factors were involved in TZ expression. A pyrogenic-based line, created by classifying witness trees by fire relations, had two major northward departures from the original TZ where cold-adapted yet pyrophilic northern pines occurred on sandy glacial deposits. Fire was found to play a contributing role, being pervasive south of the TZ, whereas edaphically restricted to dry, sandy landscapes to the north. Climate change and fire suppression will exert future "tension" on the line, forcing uncertain movement across the landscape from these two divergent forces, with projected higher future temperatures pushing the TZ northward and fire suppression and accompanying mesophication (comprised of primarily cool/cold-based trees) pulling the TZ southward. An endpoint to the temperate-based Midwestern TZ was identified in northwest Minnesota, converting over to a Boreal-Prairie TZ northward.