Document: STE-3-41-16

Late Quaternary vegetation history of the Flaming Gorge region, Utah/Wyoming.

JACKSON, S.T.* ¹, J.L.BETANCOURT ², M.E.LYFORD ¹ and S.T.GRAY ¹

University of Wyoming, Laramie, WY 82071, USA ¹
Desert Laboratory, U.S. Geological Survey, Tucson, AZ 85745, USA ²

Abstract:
The Flaming Gorge of the Green River marks the boundary between the Colorado Plateau and the high basins of Wyoming. Here, the northernmost populations of pinyon pine (Pinus edulis) form dense woodlands with Utah juniper (Juniperus osteosperma). We collected, analyzed, and dated 48 woodrat middens from sandstone escarpments in pinyon/juniper woodland to determine the antiquity of local pinyon and Utah juniper populations. Middens ranged in age from 26,400 ¹⁴C yr BP to modern. Macrofossil assemblages from Late Wisconsinan middens (26,400 to 16,000 yr BP) are dominated by common juniper (J. communis) and lack tree species, suggesting open vegetation with scattered low shrubs. Late-glacial/early Holocene middens (11,150 to 9100 yr BP) contain a diverse assemblage of trees, including blue spruce (Picea pungens), Douglas-fir (Pseudotsuga menziesii), limber pine (Pinus flexilis), and Rocky Mountain juniper (J. scopulorum). This assemblage now occurs 400-600 m above our study sites, indicating cooler and wetter conditions during the late-glacial/early Holocene. Rapid transition to dry climate is suggested by an 8500-yr-old midden containing only Utah juniper, but dominance of Rocky Mountain juniper between 6300 and 5900 yr BP indicate a return to somewhat moister climate in the mid-Holocene. Woodlands were dominated by Utah juniper between 4700 and 1000 yr BP. Pinyon invaded the region between 1000 and 460 yr BP, evidently dispersing from populations 40 km to the south. We have identified several living pinyons >525 yr in age, suggesting that both the initial colonists and the pattern of colonization can be identified at the site. This ongoing, natural migration of a tree species presents unique opportunities for empirical measurement of local patterns and rates of population spread.

Keywords:

This abstract is being presented at: 10:30 AM in session:
PALEOECOLOGY