Western juniper is a single species overstory in many northern stands. In ecotones or transitions, ponderosa pine (Pinus ponderosa) and curlleaf mountain-mahogany (Cercocarpus ledifolius) are the most common tree associates at the lower edge of the conifer zone (5,6). At upper elevations, western juniper often grows in narrow ecotones where deep, forested soils grade into shallow, rocky scab flats. Small stands or groups of trees commonly grow where rock outcrops produce shallow soil inclusions in ponderosa pine, Douglas-fir (Pseudotsuga menziesii), white fir (Abies concolor), lodgepole pine (Pinus contorta), and other forest types (5,6,11). In the Sierra Nevada, western juniper may be found on shallow soils with Jeffrey pine (Pinus jeffreyi), California red fir (Abies magnifica), whitebark pine (Pinus albicaulis), mountain hemlock (Tsuga mertensiana), or lodgepole pine (24). At the southern extension of its range in San Bernardino County, it generally grows at a higher elevation than California juniper (Juniperus californica) and Utah juniper (J. osteosperma) (20). This is the only documented area where western juniper and singleleaf pinyon (Pinus monophylla) grow together in a pinyon-juniper woodland vegetation type, although distributions are known to overlap geographically near the west edge of Nevada and from east-central to southern California (10,13). Western juniper is the associate of singleleaf pinyon only in the high altitude section of the type, primarily near Big Bear Lake, CA (13).
Western juniper is recognized in five forest cover types (9). It is the dominant species in Western Juniper (Society of American Foresters Type 238); an associate species in Interior Ponderosa Pine (Type 237) and Jeffrey Pine (Type 247); and a minor or occasional species in Blue Oak-Digger Pine (Type 250) and California Mixed Subalpine (Type 256).
Big sagebrush (Artemisia tridentata) is the most common shrub species associated with western juniper throughout its range. Other shrubs common to western juniper communities in the northern portion of its range are gray rabbitbrush (Chrysothamnus nauseosus), green rabbitbrush (C. viscidiflorus), antelope-brush (Purshia tridentata), wax currant (Ribes cereum), and horsebrush (Tetradymia spp.). Less common shrubs are low sagebrush (Artemisia arbuscula), stiff sagebrush (A. rigida), spiny hopsage (Atriplex spinosa), broom snakeweed (Gutierrezia sarothrae), prickly phlox (Leptodactylon pungens), and desert gooseberry (Ribes velutinum) (2,5,8).
Common grass or grasslike species in northern areas are bluebunch wheatgrass (Agropyron spicatum), cheatgrass (Bromus tectorum), Idaho fescue (Festuca idahoensis), prairie Junegrass (Koeleria cristata), Sandberg bluegrass (Poa sandbergii), bottlebrush squirreltail (Sitanion hystrix), and Thurber needlegrass (Stipa thurberiana). Less common are threadleaf sedge (Carex filifolia), Ross sedge (C. rossii), sixweeks fescue (Festuca octoflora), needle-and-thread (Stipa comata), and western needlegrass (S. occidentalis). Forb species common to northern communities include western yarrow (Achillea millefolium), milkvetch (Astragalus spp.), littleflower collinsia (Collinsia parviflora), obscure cryptantha (Cryptantha ambigua), lineleaf fleabane (Erigeron linearis), woolly eriophyllum (Eriophyllum lanatum), spreading groundsmoke (Gayophytum diffusum), lupine (Lupinus spp.), a suffrutescent wild buckwheat (Eriogonum spp.), and tufted phlox (Phlox caespitosa). Less common associates are sulfur eriogonum (Eriogonum umbellatum), small bluebells (Mertensia longiflora), and Hooker silene (Silene hookeri) (2,5,8).
Major western juniper associations in central Oregon include Juniperus/Artemisia/Festuca, Juniperus/Artemisia/Festuca-Lupinus, Juniperus/ Festuca, Juniperus/Artemisia/Agropyron-Chaenactis, Juniperus/ Artemisia/Agropyron, Juniperus/Artemisia/Agropyron-Astragalus, Juniperus/Artemisia-Purshia, Juniperus/Agropyron, and Juniperus/ Agropyron-Festuca (8).
In one treatment of vegetation types in the conterminous United States, western juniper is considered the dominant species in the Juniper Steppe Woodland (Juniperus-Artemisia-Agropyron), number 24, and is a secondary species in the Juniper-Pinyon Woodland (Juniperus-Pinus), number 23 (8,10,17).