Document: MAR-3-41-25

The role of dispersal, climate, and topography in the Holocene migration of Utah juniper, Juniperus osteosperma, into Wyoming and southern Montana.

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

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

Abstract:
The Holocene migration of Juniperus osteosperma to its present northeastern limits in Wyoming and southern Montana represents a complex spatial and temporal phenomenon driven by climate, dispersal, and topographic interactions. Plant macrofossils in packrat middens record its arrival at Flaming Gorge Reservoir, Utah (1950 - 2050 m) between 9100 and 8500 ¹⁴C yr BP, contraction of local populations between 6100 and 5500 yr BP, and re-expansion since 4700 yr BP. Dispersal limitations, perhaps combined with low seed source-strength related to the mid-Holocene population contraction, delayed its establishment east of the Continental Divide until 4700 yr BP. At this time, J. osteosperma made a large geographic jump ( 500 km) to its northernmost limits at Bighorn Canyon (1250 - 1500 m), one of the lowest areas in Wyoming and southern Montana. However, climatic constraints prevented establishment at sites between 1500 and 1700 m in the Bighorn Basin and western Pryor Mountains until 2500 - 1500 yr BP due to a late Holocene wet episode (4500 to 2500 yr BP). J. osteosperma arrived at least 3600 yr BP in foothills southeast of the Wind River Mountains. Unfortunately, a lack of older records cannot confirm that area as a stepping stone from Flaming Gorge to the Bighorn Basin. The quasi-synchronous appearance of J. osteosperma at many sites in the Bighorn Basin indicates a climatic control in the expansion of J. osteosperma. In addition, Cercocarpus ledifolius, another Great Basin species, migrated into the Bighorn Basin during the late Holocene, also suggesting a climatic role. The invasion of J. osteosperma to these sites consistently resulted in a decline or disappearance of J. scopulorum populations on south-facing slopes, and woodland densities appear to have increased concurrently. Although grazing and fire suppression have played a role in the 20th century expansion of J. osteosperma onto native rangelands, this expansion is a continuation of the natural late Holocene invasion and consequent population infilling.

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This abstract is being presented at: 4:00 PM in session:
Oral Session #32: Paleoecology.