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Wildlife, Animals, and Plants
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BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Juniperus occidentalis
| Western Juniper
Western juniper is a tree that typically grows 15 to 30 feet (4.6 to 9.1 m) [87]. It rarely exceeds 60 feet (18.3 m) in height. The largest recorded specimen is 86 feet tall (26 m), with a circumference of 480 inches (1,219 cm) and crown spread of 58 feet (18 m) [12]. Trees develop full crowns and heavy limbs at maturity [22].
Varieties differ with respect to growth characteristics. The variety Juniperus occidentalis var. australis reaches an average height of 26 feet (7.9 m), with a maximum height of 50 feet (15.2 m), whereas J. occidentalis var. occidentalis typically grows to 23 feet (7.0 m) in height, and rarely grows over 40 feet (12.2 m). Average circumference of the largest stem 5 feet (1.5 m) above the ground of J. occidentalis var. australis is 84 inches (213 cm), while only 42 inches (107 cm) for J. occidentalis var. occidentalis [98].
Bark is furrowed and shreddy [31]. Taproots average 51 inches (130 cm) in depth [55]. Root-to-shoot ratios decrease with age giving western juniper the ability to outcompete species such as big sagebrush [55].
Western juniper is slow growing and long lived [31]. Individuals can survive for 1,000 years or longer [87]. The Bennett juniper, which grows near Sonora Pass, California, is believed to be 3,000 to 6,000 years old [4,12]. Annual growth of mature western juniper is as follows [28]:
Stand Height Diameter
inch cm inch cm
Open, dominant 3.5 9 0.3 0.8
Closed, subdominant 3.5 9 0.1 0.4
Closed, young 1.2 3 0.05 0.2
Highest rate 4.3 11 0.5 1.3
Phanerophyte
Western juniper regenerates through seed. Green pistillate megastrobili form seed cones in abundance [87,92]. Female cones first develop 2 weeks after the male cones, and seed cones remain on the tree for 2 years until mature [72,84]. Seed cones generally ripen in mid-September. Seed cones are blue-green prior to ripening, and bluish-black and glaucous when ripe [50]. Cone-berries contain 1 to 4 (more rarely up to 12) brownish seeds [69]. Western juniper is monoecious or dioecious [92]. Many plants have the capacity to shift their sex from year to year, depending on conditions. Trees bordering roads or clearings typically produce much greater numbers of female cones than do trees within stands [28].
Seed production typically begins at 20 years of age or more, but few seed cones are produced until the tree reaches 50 to 70 years of age [28,29]. Full reproductive potential is achieved at 50 years or older [73]. Western juniper produces an abundance of seed cones nearly every year after maturity [87].
The germination of most species of juniper is delayed due to embryo dormancy, impermeable seedcoats, or temperature constraints. Western juniper seed is dormant when freshly harvested. Germination of juniper seed has been described as "erratic and unpredictable" [104]. Specific information on germination characteristics is available [50,104]. Tueller [91] reports that germination in junipers (Juniperus spp.) "is not a straight-forward process, but one that requires a specific sequence of environmental conditions for natural germination and seedling establishment." Mineral soil may be required for best seedling establishment [92].
Seed of western juniper is dispersed by birds, mammals, water, and gravity [69]. In a southwestern Idaho study, western juniper seed dispersed an average of 4.7 feet (1.42 m) downslope and 2.0 feet (0.60 m) upslope during a 4-month summer period, and an average of 4.2 feet (1.29 m) during a 6-month winter period. More distant seed dispersal in summer is due to animals. Most, if not all, uphill seed movement can be attributed to animal dispersal. Livestock may aid in dispersal by kicking or rolling seeds [19]. The seeds of many juniper species are thought to germinate faster after the seed cone has been consumed by animals [88]. Rabbits, ground squirrels, woodrats and other rodents, mule deer, elk, coyotes, and domestic livestock are all dispersal agents for western juniper [19,84]. Schupp and others [84] report that although rabbits, deer, and other mammals consume cone-berries; coyotes are the most important of the mammalian dispersers.
Wintering birds such as the American robin, Stellar's jay, scrub jay, and Townsend solitaire eat and disseminate large numbers of western juniper cone-berries [28,68,75]. Burkhardt and Tisdale [19] maintain that the proportion of long-range seed dispersal, at least in southwestern Idaho, is low as evidenced by the lack of disjunct stands of western juniper.
Water dispersal accounts for a large percentage of downslope seed movement. Spring runoff traveling across frozen soil compacted by livestock may account for high densities of western junipers along waterways [28]. Seed establishment is favored in deeper valleys or in areas rich in forbs [19].
Western juniper seedlings establish more rapidly under big sagebrush, bunchgrasses, or under existing trees. In a central Oregon study, seedlings established as follows [28]:
with big sagebrush - 47%
with western juniper - 15%
with bunchgrasses - 14%
in the open - less than 1%
Burkhardt and Tisdale [19] found that most seedlings established on the north side of existing trees where the young plants were protected from intense solar radiation. Seedlings are often found along fences, hedges, or under taller trees where large numbers of birds perch [69,87,103].
Western juniper does not reproduce vegetatively [22].
Western juniper occurs in open stands on mountain slopes and high plateaus [87]. It grows as scattered individuals on rimrock or rock outcrops and in higher densities along streams, on scablands, and lower slopes where water has dispersed the seed [28]. It has remained dominant in shifting sand dune communities of south-central Washington where fire is unlikely [1].
The growth of western juniper is favored by long, dry summers and cold winters with little moisture [87]. Western juniper often occurs in a zone between desert, shrub-steppe, or grassland below and ponderosa pine (Pinus ponderosa) or sagebrush (Artemisia spp.) desert above [37,87]. Cold winter temperatures may restrict its occurrence at higher elevations [73]. It occurs in the most xeric tree-dominated zone of the Pacific Northwest [37]. Soil, climate, topography, fire, and biotic factors all contribute to the distribution of western juniper, but moisture is thought to be the primary determinant [23].
Average annual precipitation ranges from approximately 10 to 13 inches (250-330 mm), with the bulk occurring as winter snow. Temperature ranges from 36 to 105 degrees Fahrenheit (2-41oC), with an average July temperature of 66 degrees Fahrenheit (19oC). The growing season rarely exceeds 130 days [87].
Western juniper generally grows on soils that are shallow, stony, and low in organics [82,95]. Western juniper grows on soils derived from basalt, andesite, rhyolite, pumice, volcanic ash, tuff, welded tuff, colluvial, alluvial, or eolian mixtures [23]. In the Sierra Nevada, Sierra juniper grows on granitic crevices on windswept ridges [87]. Surface soils supporting western juniper are often slightly acid to moderately acidic sandy loams or coarse sands with little organic matter (1 to 4%) [37,95]. Western juniper also grows on finely-textured calcareous soils [3]. Levels of calcium, potassium, and pH are higher under mature western juniper than in the interspaces [22,23]. Soil depth ranges from 10 to 15 inches (25-38 cm) to more than 48 inches (122 cm). The subsoil is typically broken and indurated with cracked bedrock below [23]. Western juniper is best adapted to soils exhibiting rapid infiltration, deep percolation, low evaporation, and low soil moisture tension [5]. Western juniper is often found on a perched water table [37].
Western juniper grows from near sea level to more than 10,000 feet (3050 m) [87]. Elevational ranges are as follows [42,87]:
3,000 to 10,000 feet (915-3050 m) in California
3,000 to 7,500 feet (915-2288 m) in central Oregon
600 to 1,800 feet (183-549 m) in eastern Washington
Western juniper is a highly competitive invasive species [81]. It is found on all exposures and slopes but is relatively intolerant of shade [22,23]. This long-lived species has been expanding its range into adjacent shrub-steppes, grasslands, and savannas during the past 100 to 150 years [8]. It has doubled its range in central and eastern Oregon during the past 80 to 100 years [81]. Expansion into low sagebrush communities has been slower than into big sagebrush communities [2]. Expansion into shrub steppe communities in Nevada has reduced understory diversity and productivity [7]. Western juniper expansion has been attributed to livestock grazing and associated reductions in fine fuels, climatic changes (mild temperatures and above average precipitation in the late 1880s and early 1900s), and reduction in fire frequency due to fire suppression and cessation of Native American burning [8,29,74,75].
Climax stands are generally restricted to rimrock and the edges of mesas, whereas seral communities can dominate slopes and valley bottoms adjacent to older western juniper stands [95]. Old growth western juniper is generally found in "firesafe" spots [103]. This tree was formerly restricted to rocky ridges and outcrops, deep pumice soils and on fractured bedrock. Western juniper remains a climax dominant in rimrock and similar sites due to the rocky substrate and lack of fuels needed to carry an intense fire [1,18]. Western juniper is an indicator of climax in a number of sagebrush-grassland, shrub-steppe, and drier conifer habitat types. Some communities represent topo-edaphic climaxes [65].
Western juniper survives on harsh sites and is increases in many early seral communities. In seral communities near Prineville, Oregon, young classes of western juniper predominate with a maximum age of nearly 100 years in these seral stands. In non-seral communities western junipers are at least several hundred years in age [96].
In Oregon, leaf elongation begins in June with 15 to 20% annual leaf turnover [72]. Western juniper cones develop from mid-April to mid-May and pollen is shed in May [50,72,87]. Seed cones ripen in mid-September after the second growing season [50]. Cones may persist on the trees for 2 to 3 years [69]. Most germination occurs during the spring [87]. In Oregon, most natural germination occurs during April [22].
Related categories for
SPECIES: Juniperus occidentalis
| Western Juniper
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