POSTGLACIAL TIMBERLINE FLUCTUATIONS, LA PLATA MOUNTAINS, SOUTHWESTERN COLORADO

Abstract

A paleoecological study of the La Plata Mountains was initiated to develop further a dated and continuous environmental sequence for the San Juan River headwaters to which discontinuous archaeological and alluvial data from the Four Corners region might be compared. Routine pollen analysis of a 4-m core from a subalpine meadow adjacent to Twin Lakes, 3290 m (10,790 ft), including 11 radiocarbon dates and pollen ratios, provides the chronology of climatic change as reflected by postglacial timberline fluctuations. The timberline was lower than that of the present about 9800 BP, then advanced upward at least twice to higher elevations prior to 6000 BP. The timberline retreated to lower elevations shortly after 4000 BP; this retreat was followed by another significant advance upward about 2500 BP. Mining, logging, and grazing, which began in the 1870s, may be represented by a sharp decrease in the relative frequencies of Pinus and Picea pollen, with subsequent secondary succession represented by increased Ranunculaceae and Salix pollen and then a return to conifer pollen dominance. These changes may also result from a significant lowering of the timberline within the last few hundred years.