Pollen and macrofossil evidence of Late Pleistocene and Holocene treeline fluctuations from an alpine lake in Colorado, USA

High-resolution pollen, plant macrofossil and magnetic susceptibility (MS) data are presented from an alpine lake sediment core from west-central Colorado, recording changes in vegetation and sedimentation for the latest Pleistocene and Holocene (c. the last 12.5 ka; 1 ka = 1000 cal. yr BP). During the Younger Dryas chron (c. 12.9-11.5 ka), Artemisia steppe or tundra grew around the lake, but by the earliest Holocene (10.7-9.5 ka) a subalpine Picea and Abies parkland was established there. Picea remained important through the early Holocene, but also bristlecone and lodgepole pines (Pinus aristata and P. contorta) grew around the lake. Warming conditions are indicated from 9.5 ka, lasting until c. 4.5-3.5 ka, which may have been the warmest period, with greatest development of monsoonal conditions. Trees subsequently retreated downslope from Kite Lake c. 150-200 m during the last 3.5 ka, establishing their present treeline position. A decrease in total Pinus and increases in Artemisia and piñon (P. edulis) indicate a trend toward progressive climate cooling and enhanced winter precipitation. These long-term climatic trends correlate with Holocene changes in summer insolation.