A 2000-year lichen-snowkill chronology for the Colorado Front Range, USA

Abstract

Measurements of Rhizocarpon subgenus Rhizocarpon thalli in fixed-area (100-m 2) sample plots at 295 randomly selected localities above timberline in the Colorado Front Range suggest six episodes of widespread, shallow, lichen snowkill during and immediately following the Audubon glacial interval. Recolonization of lichen-free areas began in cal. AD 210, 375, 570, 700, 1025, and 1255. In contrast to the 'Little Ice Age', when snowkill was restricted to narrow bands peripheral to modern late- lying snowpatches, Audubon snowkill destroyed all crustose lichens in more than 90% of the study area. Heavy April or May snowstorms, occurring after redistribution of snow by wind had diminished, are thought to have been responsible; cool, cloudy summers and warm subnivean winter temperatures may also have been involved. Audubon snowkill episodes lasted for relatively brief periods, and were superimposed on the broader pattern of late-Holocene climatic change. They did not necessarily trigger glaciation or cause geomorphic instability, but are thought to have temporarily discouraged human use of the Front Range alpine region through their adverse effects on large-game-animal populations. Lichenometric dates for landforms in areas of potential snowkill should be evaluated using rock- weathering criteria or other independent age evidence.