LATE PLEISTOCENE CLIMATE AND VEGETATION RECONSTRUCTION USING POLLEN FROM A SEDIMENT CORE, EAST GLACIER LAKE, WYOMING, GLACIER LAKE ECOSYSTEM EXPERIMENT SITE (GLEES)

We present pollen results from a 15,500 cal yr BP, 2 m core recovered from East Glacier Lake (3170 m) located in the Snowy Range, southeastern Wyoming, USA, on the Medicine Bow National Forest. The sedimentation rate was 68 years per cm and thirty-five samples were analyzed with an average age between samples of 450 years.

A ratio of spruce and pine (Picea plus Abies) to Poaceae was calculated as a potential proxy for changes in treeline. At the time of deglaciation, 15,500 years ago, grassy alpine meadows surrounded the lake and the absence of fir pollen and low quantities of spruce and fir pollen suggest that treeline was well below the elevation of the lake. At the end of the Pleistocene (11,600 cal yr BP) the forest began shifting upslope, juniper woodland declined, and sagebrush steppe gave way to coniferous forest. Pollen percentages of coniferous trees suggest that treeline was higher than at the present time. The greatest percentages of subalpine forest pollen, particularly spruce, occur between 5600 and 5100 cal yr BP, after the dates typically associated with the thermal maximum. The high percentages of organic matter in the lake suggest that there may have been a closed canopy forest surrounding the lake and treeline advanced to its highest elevation during the Holocene. Modern conditions appear to have become established by about 3000 cal yr BP, with treeline moving back down slope to a position roughly similar to today.