PJ101-Distribution & Climate
Distribution: Pinyon and juniper woodlands occupy over 70,000 square miles (181,000 km2) of the Great Basin and Colorado Plateau, extending across a significant climatic gradient from eastern Oregon to the Four Corners of Utah, Colorado, Arizona, and New Mexico. Estimates of area occupied vary widely, largely as a result of the method used to estimate area, the geographic area or species included, the criteria used to define woodland (often based on % canopy cover), and and how wooded scrubland and savannas are defined. Woodlands occupy a wide range of elevations between 1,000 to 8,000 ft, and variety of soils and parent materials. Pinyon and juniper woodlands often project the illusion of being homogeneous, but they vary in age, structure, composition, and often intermingle with other plant communities. The broad heterogeneity of these woodlands and the sites they occupy results in large spatial and temporal variations in ecohydrological processes, disturbance regimes, resilience to disturbances, response to vegetation management, and resistance to invasive species. The most common semi-arid conifer species in the Great Basin and northern Colorado Plateau are Utah juniper (Juniperus osteosperma), western juniper (J. occidentalis), singleleaf pinyon (Pinus monophylla), and twoneedle pinyon (P. edulis). Common but less abundant within this region are Rocky Mountain (J. scopulorum) and Sierra (J. grandis) junipers.
Precipitation and Temperature: The majority of these semiarid woodlands occupy precipitation zones between 10-16 inches (250-400 mm). There is a large range in the distribution of seasonal precipitation along a gradient from eastern Oregon to southeast Utah and southwest Colorado, where summer precipitation ranges from <5% to >30% of the total annual precipitation. The quantity and seasonal availability of moisture, especially in the summer, are major factors influencing the geographic distribution of these semi-arid conifers. Temperature is also an important variable influencing conifer species distribution. Rapid warming periods interrupted by cold periods during late winter and early spring in the Northwest, limit the northwestern distributions of Utah juniper and singleleaf pinyon. Of the four species, Utah juniper is the most drought adapted. Western juniper typically occurs in cooler and somewhat wetter environments than Utah juniper. And, the two pinyons are separated by amounts of summer precipitation.
For more detailed information and citations refer to: Miller, R.F. and others. 2019. The Ecology, History, Ecohydrology, and Management of Pinyon and Juniper Woodlands in the Great Basin and Northern Colorado Plateau of the Western U.S. US Forest Service, Rocky Mountain Research Station. RMRS-GTR-403. 284 pg.