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.
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 (Fig. 2), while only 30 km to the west at Big Pryor Mountain it arrived 1785 yrs BP (Fig. 3). Further south on the central-west slope of the Big Horn Mountains, Utah juniper first appears in the macrofossil record 1405 yrs BP (Fig. 4). The earliest recorded Utah juniper at Wind River Canyon in north-central Wyoming was 1970 yrs BP (Fig. 5). 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.
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.