Juniperus scopulorum, the Rocky Mountain juniper, is a species of juniper native to western North America, from southwest Canada to the Great Plains of the United States and small areas of northern Mexico. They are the most widespread of all the New World junipers. They are relatively small trees, occasionally just a large bush or stunted snag. They tend to be found in isolated groves or even as single trees rather than as the dominant tree of a forest. Though they can survive fires, they are vulnerable to them especially when young and this is one of the factors that can limit their spread into grasslands.
Rocky Mountain junipers provide habitat and food for wildlife. They provide cover to a range of species, from small birds and mammals to deer and bighorn sheep. Their berry-like cones are eaten by many animals and their scaly leaves and small twigs are browsed in small amounts by large herbivores. The primary human use is in landscaping for aesthetic purposes, to shelter habitations, or attract fruit-eating birds. They are also used in small amounts for their insect repellent and rot-resistant wood or as firewood for heating.
Description Juniperus scopulorum is a small evergreen tree that in favorable conditions may reach as much as in height. However, on sites with little water or intense sun it will only attain shrub height, and even those that reach tree size will more typically be tall in open juniper woodlands. Younger trees have a narrow pyramidal shape, but develop into a rounded, oval, or spreading and irregular crown when older. They may either have a single trunk or multiple stems. Trunks can be large on mature trees, in diameter. When the subsoil is difficult to penetrate and lacks moisture the roots of Juniperus scopulorum spread out. They are numerous and fibrous in the upper part of the soil. When soils are deep and well drained they will grow to a greater depth.
On twigs between in diameter the bark is smooth. On larger twigs and branches it becomes rough, coming off in thin strips. Juniperus scopulorum has red-brown bark on branches that can weather to grey on the trunk. The texture of the bark is rough and comes away in shreds on the trunk with brown bark showing in the cracks at times. Branches tend to grow outwards a short distance and then curve to growing upwards (ascending). In sheltered to somewhat shady locations the branches may hang downwards and be quite slender. The very ends, the branchlets, can either stand upright or hang down.
Young shoots are very slender. All the leaves are light to dark green, but are often covered in a waxy coating that gives a blue or white cast to the leaves (glaucous) making them appear blue-gray or blue. On immature trees they will be covered in sharp needle like "whip leaves" long that stick out from the shoots. The needles will not have the waxy coating on their upper surface. What appear to be green scales on the shoots of adult trees are the mature leaves which clasp the shoot in opposite pairs with the next pair down or up the shoot rotated a quarter turn (90°) around it (decussate). Occasionally they will be turned one third to make the shoot three sided. The scales either do not overlap or overlap for at most one-fifth of their length of .
The seed cones are berry-like that are round to somewhat irregular in shape with two lobes (globose to bilobed) and in diameter. They are dark blue-black in color, but will be pale blue-white when covered in natural wax. The berries most often contain two seeds, but may contain one or three; they are mature in about 18 months. The pollen cones are long, and shed their pollen in early spring, generally in April. It is usually dioecious, producing cones of only one sex on each tree, but is occasionally monoecious.
Chemistry Rocky Mountain juniper is an aromatic plant. Essential oil extracted from the trunk is prominent in cis-thujopsene, α-pinene, cedrol, allo-aromadendrene epoxide, (E)-caryophyllene, and widdrol. Limb essential oil is primarily α-pinene and leaf essential oil is primarily sabinene. Experiments with deer have found that oxygenated monoterpenes, like sabinene, inhibit the gut bacteria of ruminants and deer show the expected preference for foliage lower in these chemicals.
Similar species Juniperus scopulorum is closely related to eastern redcedar (Juniperus virginiana), and often hybridizes with it where their ranges meet on the Great Plains. It will also form hybrids with alligator juniper (Juniperus deppeana), creeping juniper (Juniperus horizontalis), oneseed juniper (Juniperus monosperma), and Utah juniper (Juniperus osteosperma). The population of juniper trees in Mexico near the former site of Colonia Pacheco, Chihuahua is a hybrid with Juniperus blancoi. There is some disagreement whether hybrids are formed with the oneseed juniper in the wild.
Taxonomy
Juniperus scopulorum was first described and named as a separate species by Charles Sprague Sargent in 1897. Previously trees had been identified as one of its two close relatives, Juniperus virginiana or Juniperus occidentalis. Parts of the species were described as Juniperus virginiana var. montana by George Vasey in 1876 and as Juniperus occidentalis var. pleiosperma by George Engelmann in 1877. Its proper classification has continued to be debated by botanists with Per Axel Rydberg proposing to move it to his new genus as Sabina scopulorum in 1900 and Albert Edward Murray publishing a paper in 1983 that reclassified it as a subspecies under the name Juniperus virginiana subsp. scopulorum.
Isolated populations of junipers occur close to sea level in the Puget Sound area in Washington Park near Anacortes and southwestern British Columbia in a park called Smugglers Cove. In both locales, there are a considerable number of young and old specimens. A 2007 paper showed that they are genetically distinct, and proposed that it be recognized as a new species Juniperus maritima. If valid, it is a cryptic species barely distinguishable on morphology, though it does differ in phenology, with the cones maturing in 14–16 months, and often has the tips of the seeds exposed at the cone apex. However, as of 2024 it is listed as a synonym by both Plants of the World Online (POWO) and World Flora Online (WFO).
As of 2024 Juniperus scopulorum is listed as an accepted species with no subspecies by POWO, WFO, and World Plants.
Names The genus name Juniperus is classical Latin, rather than botanical Latin, and was the name used in antiquity for this type of tree. The species name (specific epithet), scopulorum, derives from Latin with the meaning "of rocky cliffs", a reference to its frequent occurrence in rocky areas. The most common of its English names, "Rocky Mountain juniper", was at first applied to Juniperus occidentalis in 1841. Because J. scopulorum was at first largely considered the same at the Eastern red cedar, no unique common name was required for it and when it was recognized as a species it was most often called "Rocky Mountain red cedar", a common name now applied to Thuja plicata. Other common names used in the United States include "river juniper", "mountain red cedar", "Colorado red cedar", "weeping juniper", and "Rocky Mountain redcedar". In one unusual locality in Spring Valley, Nevada they are known as "swamp cedar" for growing in a relatively wet canyon bottom. In Canada it is also known as the "western red-cedar" and similar variations in English and "genévrier des Montagnes Rocheuses" (literally juniper of the Rocky Mountains) and genévrier des montagnes du Colorado (juniper of the mountains of Colorado) in French. In casual conversation the trees will usually simply be called "cedars" or "junipers" without qualification by residents of the western United States.
Distribution and habitat
Rocky Mountain junipers are found across a wider range than any other new world juniper species, though it is almost nowhere a common species. More often they are scattered widely across the landscape in isolated groups, grove, or stands. The species is native to western North America, in Canada in south British Columbia and southwest Alberta, in the United States sporadically from Washington east to North Dakota, south to Arizona and also locally western Texas, and northernmost Mexico from Sonora east to Coahuila. It grows at elevations of on dry soils, often together with other juniper species. It requires at least of annual precipitation, though the average for its range is and it survives on Vancouver Island with as much as of precipitation. Though it grows in very dry environments in western North America and has great drought endurance, it is not as adapted to dry conditions as other western juniper species.
The trees are very numerous in the lower mountains and foothills where grasslands or scrublands transition to low forests. In the Southern Rocky Mountains, the Colorado Plateau, and parts of Nevada Juniperus scopulorum is associated with the various species of piñon pine as a key species of the piñon-juniper woodland. At edges and lower elevations the junipers are more numerous with a gradual transition to all piñons at higher elevations. It is also a minor part of forests above this such as ponderosa pine forests (Pinus ponderosa) and areas dominated by Gamble oak (Quercus gambelii). Starting in northern Colorado and northern Utah the Rocky Mountain Juniper dominates a woodland type named for the species and found through Idaho and Montana into southern Canada.
Though tolerant of a wide range of soil conditions, Juniperus scopulorum strongly prefers soils that are alkaline and high in calcium. They grow to their maximum size on deep, moist, but well draining soils with plenty of organic matter. More often they are found on poor, dry soils especially ones formed from basalt, limestone, sandstone, lavas, and shale. They are also tolerant of soils with a significant amount of clay or that have a subsoil that is naturally cemented together like hardpan. Though obtaining a greater size in more sheltered locations they will successfully grow on rock outcrops with no soil and on high ridges. In the mountains to the north of Colorado and Utah the trees grow on relatively dry sites, often south facing slopes. In the south it grows in more sheltered locations and canyons, with the transition occurring in Colorado.
In one instance it has adapted to quite extreme conditions for a juniper, growing on wet clay soils in Spring Valley, Nevada. There it grows in the valley bottom as an almost riparian species and also survives moderately salty water. A similar pattern is also found in the farthest south populations of the species found in Mexico. There it largely grows near streams in canyons.
Pleistocene distribution Towards the end of the Last Glacial Period, from 13,500 to 10,000 years before the present, Rocky Mountain juniper grew at much lower elevations in what is now the great basin and desert Southwest. Evidence from pack rat middens show that plant vegetation zones were 300 to 1100 meters lower in elevation than they are at present. In the Southern Rockies in what is now Colorado and Wyoming juniper woodlands were about 600 meters lower than in the modern Holocene epoch. The relic groves still found on the Great Plains and the Laramie Basin in Wyoming are likely remnants of this older distribution. During the ice age the north of its present range was largely covered in glaciers and far too cold for it in areas not covered in ice with populations only reaching as far north as present day south-eastern Wyoming, southeastern Oregon, southern Idaho, and northern Colorado insolated refuges.
Ecology and conservation