Distribution Model

Question: Does Utah juniper occupy all of its potential habitat in Wyoming, or is the regional migration ongoing?

Question: What environmental conditions correlate with the distribution of Utah juniper populations in Wyoming?

This portion of the project centers on the creation of a correlative, GIS-based model to study the distribution of Juniperus osteosperma in Wyoming. Using 141 J. osteosperma herbarium records, we developed a potential-habitat model based on climatic and edaphic factors. We then queried a GIS database of precipitation, soils, and geology for Wyoming to identify areas fitting the model criteria.

Locations of the 141 confirmed Utah juniper collections used in this study (Courtesy of the Rocky Mountain Herbarium, University of Wyoming).

Preliminary results indicate that J. osteosperma may not occupy all of its potential habitat in Wyoming (Figure 1 below). Both geology and soils are limiting throughout the state, with J. osteosperma showing a strong affinity for sandstone outcrops and coarse, frigid soils. Annual precipitation has little influence on distribution patterns below 1800 m, but becomes increasingly important at higher elevations. In eastern Wyoming, Utah juniper is excluded from areas where precipitation reaches a maximum in May or early June (Figure 2 below). Field observations also indicate that overestimates of potential habitat are likely in areas with alkaline soils. Dispersal barriers, especially north-south mountain chains in the state's central region, may limit the migration of J. osteosperma into eastern Wyoming. Alternatively, competition for seasonal moisture, especially in Pinus ponderosa dominated systems, may limit establishment in eastern Wyoming. Efforts are underway to model the effects of soil alkalinity and temperature on the distribution of J. osteosperma.

Figure 1. Final model output (version 1.5) based on annual precipitation, spring precipitation, soils, bedrock type and surficial geology. Red dots show know Utah juniper populations.

Figure 2. Model output showing potential Utah juniper distributions for Wyoming based on soils, bedrock geology and annual precipitation. In the first figure (Figure 2a), May precipitation is included as a variable in the model input. Areas of the predicted distribution that change after May precipitation was added to the model appear in red.