Packrat Middens

Migrational History of Utah Juniper in Wyoming

The intent of this research is to document the invasion of Utah juniper (Juniperus osteosperma ) into and throughout Wyoming, where Utah juniper reaches its northeastern limit (see distribution map above). Woodrat (Neotoma cinerea) middens are being collected throughout Wyoming and adjacent Montana and Utah in extant stands of Utah juniper to determine the arrival time of Utah juniper at each site. With collection sites dispersed across Wyoming, will be able to determine the pattern of spatial spread of this species at time scales of centuries to millennia years.

Locations of midden collection sites in Wyoming and adjacent Montana and Utah.

Researcher with a packrat midden.

Initial results from four collection sites located in southern Montana and northern Wyoming indicate that the invasion patterns of Utah juniper throughout Wyoming were complex. At its northern limits, Utah juniper was established at Bighorn Canyon 4630 yrs BP (Figure 2 below), while only 30 km to the west at Big Pryor Mountain it arrived 1785 yrs BP (Figure 3 below). Further south on the central-west slope of the Big Horn Mountains, Utah juniper first appears in the macrofossil record 1405 yrs BP (Figure 4 below). The earliest recorded Utah juniper at Wind River Canyon in north-central Wyoming was 1970 yrs BP (Figure 5 below). However, only the last 3000 years are recorded in middens collected so far from Wind River Canyon, and mid-Holocene occurrence of Utah juniper is possible.

Figure 2. Relative abundance of juniper macrofossils in middens for Bighorn Canyon/East Pryor Mountain. Dots represent AMS dates of juniper material. The site was dominated by Juniperus scopulorum and Juniperus horizontalis during the middle Holocene. Juniperus osteosperma arrived at the site by 4630 yrs BP. The location of the study site is indicated by the red dot on the map.

Figure 3. Relative abundance of juniper macrofossils in middens for Big Pryor Mountain (solid bars) and pollen percentage of Cupressaceae (open bars). Dots represent AMS dates of juniper material. During the early Holocene Juniperus communis and Juniperus scopulorum were present at the site. During the middle Holocene Juniperus communis was no longer present but was replaced by Juniperus horizontalis. Juniperus osteosperma appears in the record 1785 yr BP. The location of the study site is indicated by the red dot on the map.

Figure 4. Relative abundance of juniper macrofossils in middens from Buffalo Creek Lookout (solid bars) and pollen percentage of Cupressaceae (open bars). Juniperus scopulorum was present from 3090 yr BP to 225 yr BP with Juniperus osteosperma arriving at the site 1785 yr BP. The location of the study site is indicated by the red dot on the map.

Figure 5. Relative abundance of juniper macrofossils in middens for Wind River Canyon. Preliminary dates indicate that Juniperus osteosperma arrived during the late Holocene (1970 yr BP) while Juniperus scopulorum was the dominant juniper prior to the arrival of Juniperus osteosperma.

In addition to documenting the invasion of Utah juniper throughout the state, we will determine how the vegetation at each site has changed during the Holocene by identifying other plant material preserved in the woodrat middens. Impacts of Utah juniper invasion on the community will be documented and instances of species replacement can be identified. Preliminary evidence indicates that species composition has changed with the arrival of Utah juniper at a site. We will also conduct a modern midden survey to determine the probability of presence of different species in a midden depending on their abundance in the local flora. With this information, we will be better able to assess the meaning of the presence or absence of a species in a fossil midden.