Effects of changing climate on game-animal and human use of the Colorado high country (USA) since 1000 BC

Radiocarbon dates from prehistoric campsites and game-drive systems were used to develop a 3000-yr chronology of human population change at high altitude in the Colorado Front Range. Populations increased to record levels during the Late Prehistoric Period and then declined, tracking a probable warming-and-cooling trend. Superimposed on the long-term cycle were short-term oscillations that correlate closely with episodes of lichen snowkill inferred from the diameters of Rhizocarpon subgenus Rhizocarpon thalli in randomly selected 100-m(2) sample plots above tree limit. Transplant experiments and historical accounts of an epic storm in the spring of 1844 help understand the causes of lichen snowkill and its linkage to hunter-gatherers. During five periods of unknown duration between 1000 BC and AD 1230, vast areas above timberline remained snow covered for an average of at least 40 wk yr(-1). Upslope snowstorms triggered by low temperatures and by cut-off low pressure systems in the southwestern United States are thought to have been responsible. The snow caused elk (Cervus elaphus), bighorn sheep (Ovis canadensis), and mule deer (Odocoileus hemionus) to starve on their springtime ranges in the eastern Front Range foothills. It damaged important forage species above timberline, discouraging surviving game animals and their human predators from returning to high altitudes for many decades. Springtime snow became less important after AD 1230, when northwesterly airflow intensified and lichen mortality was restricted to the floors of expanded snowpatches in topographic catchments. Low temperatures, summer drought, and epidemic disease were characteristic of this part of the archaeological record. The study demonstrates that changes in the frequency of archaeological radiocarbon dates in extreme environments can provide useful proxy evidence for climatic change and a precise chronological framework in which to interpret discontinuous paleoenvironmental data. It cautions that the East Slope of the Front Range is vulnerable to upslope snowstorms larger and more frequent than any that have occurred since the beginning of meteorological observations in the region.