Holocene vegetation history of an alpine-subalpine ecotone in the Snowy Range, southeastern Wyoming, USA

Pollen records from lake sediment have yielded a developing record of Holocene vegetation and climate in the central and southern Rocky Mountains, USA. Absent from these records are alpine and subalpine areas of southeastern Wyoming. We recovered sediment cores totaling 2.27 m depth from East Glacier Lake, near alpine treeline at 3280 m elevation in the Snowy Range, on the Medicine Bow National Forest. Present vegetation is open, “parkland” forest dominated by Picea engelmannii (Engelmann spruce). The cores cover the entire Holocene, containing glacial diamict at the base and gytja at the surface. The samples were processed and pollen counted according to standard laboratory methods. Six ¹⁴C dates were obtained, yielding an average sediment accumulation rate of 67.5 yr cm^-1. Deglaciation occurred by at least 15400 cal yr BP, with vegetation dominated by Artemisia and graminoids until ca. 11600 cal yr BP, when these species declined and coniferous pollen (primarily Pinus and Picea) began to increase. These trends continued until ca. 8000 cal  yr BP when vegetation resembled the modern community. The period 6500-5200 cal YBP showed a considerable increase in coniferous pollen and a further decrease in Artemisia, Cheno-am (saltbush species) and graminoid pollen, possibly indicating a closed coniferous forest present on the site, and a higher alpine treeline. The corresponding section of the core contains coarse plant material. Vegetation and inferred climate trends are broadly similar to those found at sites in Colorado's Front Range and other locations in the southern Rocky Mountains.