There are 15 species of juniper and 4 species of pinyon in the American West and northern Mexico (Miller et al. 2019, table 2-2). However, singleleaf pinyon (Pinus monosperma), twoneedle pinyon (P. edulis), Utah juniper (Juniperus osteosperma), and western juniper (J. occidentalis) are the primary species occurring within the Great Basin and northern Colorado Plateau. Two additional species are typically found growing in cooler temperatures than western and Utah junipers. Rocky Mountain juniper (J. scopulorum) is found primarily in the Rocky Mountains and Mogollon Rim, and Sierra juniper is found on the upper east slopes of the Sierras and high mountain ranges in central Nevada. Pinyon and Utah juniper species grow separately or in mixed stands, whereas western juniper is not associated with pinyon pines. The abundance and seasonal distribution of precipitation and temperature are primary factors that determine the distribution and separation of these tree species.
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.