Western juniper occurs in the Pacific Northwest, California, and Nevada. Old-growth western juniper stands that established in pre-settlement times (before the 1870s) occur primarily on sites of low productivity such as claypan soils, rimrock, outcrops, the edges of mesas, and upper slopes. They are generally very open and often had sparse understories. Western juniper has established and spread onto low slopes and valleys in many areas, especially areas formerly dominated by mountain big sagebrush. These post-settlement stands (woodland transitional communities) are denser than most pre-settlement and old-growth woodlands. They have substantial shrub understories in early to mid-succession.