Document: STE-3-37-15

Rates and dynamics of Utah Juniper (Juniperus osteosperma) invasion in arid shrublands.

GRAY, S.T.* ¹, S.JACKSON ¹ and J.BETANCOURT ²

University of Wyoming, Laramie, WY 82071-3165 ¹
USGS Desert Laboratory, Tucson, AZ 85745 ²

Abstract:
Utah juniper (Juniperus osteosperma) is currently expanding into sagebrush (Artemisia tridentata var.wyomingensis) communities in much of southwestern Wyoming, We studied the timing of juniper recruitment and the relationship between seedling densities and community structure at four such sites with similar soils, aspect, and elevation. Three of the sites were located in the same watershed, but separated by > 2 km; the fourth site was located 20 km north of these sites. We established paired, 30 by 50 m plots in the juniper woodlands and sagebrush at each site. Shrub cover by species and J. osteosperma seedling (<1m height and <1m crown diameter) densities were measured before all J. osteosperma seedlings where harvested from the plots (n=182). We then counted annual rings at the root-shoot interface to determine establishment dates (+/- 1 yr.). Seedling densities were significantly higher in all of the sagebrush plots than in the mature juniper woodlands. Seedlings were found in significantly greater numbers within or adjacent to A. tridentata canopies than in the space between A. tridentata canopies, and what appeared to be individual junipers were often tight clumps of several (up to 8) individuals. Although recruitment occurred in almost every 5-yr period from 1950-1995, the majority of young J. osteosperma germinated in the mid-1950s, and, to a lesser degree, the early 1970s. Both recruitment events were observed at all sites, indicating the possible influence of regional factors on the timing of J. osteosperma establishment. These pulses also coincided with extended dry periods, indicating a possible link between drought and J. osteosperma recruitment or seedling survival. The low numbers of seedlings in mature woodlands, coincidence of seedlings with sagebrush canopies, and occurrence of multiple individuals in a single shrub also suggest the importance of community and microsite factors.

Keywords: Juniperus osteosperma, woody-plant invasion

This abstract is being presented at: 10:30 AM in session:
RESTORATION ECOLOGY AND INVASIONS