ALGIFIC TALUS SLOPES IN IOWA'S PALEOZOIC PLATEAU AND THE CENTRAL APPALACHIANS
Algific (cold-air producing) talus slopes in the Paleozoic Plateau of Iowa share many similarities with less famous analogs in the Central Appalachians, despite profound differences in underlying bedrock geology and reported dissimilarity in air-flow mechanisms. Algific slopes in Iowa are developed consistently on carbonate bedrock, whereas those documented in the Appalachians are associated with various sandstone units. Air circulation within Midwestern slopes has been described as a seasonal two-way system (Frest 1981, 1984), but Appalachian slopes appear dominated by a one-way density-driven natural refrigeration system first proposed by Hayden (1843), and more fully described by Balch (1900).
Multi-year, multi-site temperature logging efforts show a wide diversity in the annual thermal cycles at disparate sites within both regions, but remarkable similarities between similar sites separated by over 1200 km. Generally, Iowa sites have slightly lower mean annual temperatures, principally because of longer and colder winters. A typical annual cycle in either area includes a thermally stratified summer season, at the end of which temperatures reach a maximum of ~10-11°C in late September or early October. Thermal stratification abruptly gives way to an open system, and several months of overall declining temperatures are dominated by precipitous (<24 hr) 4-10°C cooling events and somewhat slower temperature rebounds. Temperature minima in the talus typically are very close in time and temperature to the coldest ambient conditions in winter. Maximum ice formation occurs in early spring. While ice persists, temperatures may remain at 0°C for many weeks, eventually followed by progressive 0.1 to 0.2°C/day warming until the next annual maximum. The similarity of annual thermal cycles at sites in the two regions suggests air-flow mechanisms may be more alike than the literature implies.
Algific talus slopes in both regions provide refugia for relict flora and fauna that were more widespread in their respective regions during the Pleistocene, and they may have served as sources for regional expansion of species and ecological assemblages during past climatic cooling. In the future, these sites may serve as bellwethers for the impacts of regional climate change.