Abstract

Pollen and plant macrofossils from eight sedimentary basins on the west slope of the Colorado Rocky Mountains document fluctuations in upper and lower timberline since the latest Pleistocene. By tracking climatically sensitive forest boundaries, the moisture-controlled lower timberline and the temperature-controlled upper timberline, paleoclimatic estimates can be derived from modern temperature and precipitation lapse rates. Pollen data suggest that prior to 11 000 yr B.P., a subalpine forest dominated by Picea (spruce) and Pinus (pine) grew 300–700 m below its modern limit. The inferred climate was 2–5 °C cooler and had 7–16 cm greater precipitation than today. Abies (fir) increased in abundance in the subalpine forest around 11 000 yr B.P., probably in response to cooler conditions with increased winter snow. Pollen and plant macrofossil data demonstrate that from 9000 to 4000 yr B.P. the subalpine forest occupied a greater elevational range than it does today. Upper timberline was 270 m above its modern limit, suggesting that mean annual and mean July temperatures were 1–2 °C warmer than today. Intensification of the summer monsoon, coupled with increased summer radiation between 9000 and 6000 yr B.P., raised mean annual precipitation by 8–11 cm and allowed the lower limit of the subalpine and montane forests to descend to lower elevations. The lower forest border began to retreat upslope between 6000 and 4000 yr B.P. in response to drier conditions, and the upper timberline descended after 4000 yr B.P., when temperatures cooled to about 1 °C warmer than today. The modern climatic regime was established about 2000 yr B.P., when the summer precipitation maxima of the early and middle Holocene were balanced by increased winter precipitation.

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