SPECIES: Juniperus monosperma
Oneseed juniper is a native, long-lived, evergreen tree with often shrubby form, 10 to 40 feet (3-12 m) high with several curved limbs arising near the base [32,42,121,127].

Oneseed juniper produces small, "berry-like", glaucous, globular staminate cones, and subglobose ovulate cones [42,127]. Mature cones are dark blue to purple or brownish, and succulent, or at least somewhat fleshy [50,127,132]. Seeds are generally 1 per fruit, more rarely 2, reddish-brown and ovoid to globose [50,127]. Juniper seeds are described as having a semipermeable and thick seed coat with a dormant embryo [41]. The cones are often referred to as "berries" in the literature.

Mature oneseed junipers have both tap and lateral root systems. The taproots in 1 study ranged from 18 inches (46 cm) to more than 12 feet (3.7 m) in length. Of 500 trees examined, 347 had well-developed taproots. Lateral roots were widespread, commonly being 2.5 to 3 times as long as the tree was tall. Most lateral roots were in the surface 3 feet (1 m) of the soil, most of those concentrated below the surface 6 inches (15 cm) [64]. The deep root system of mature oneseed junipers is adapted for growth on dry sites [51,64,107]. Foxx and Tierney [43] reported rooting depths ranging from 16 to 197 feet (5-60 m).

Oneseed juniper is a slow-growing species. Lymbery and Pieper [80] reported an increase in height of approximately 6.3 inches (16 cm) per decade, with a corresponding increase in stem diameter of 0.5 inch (1.2 cm). Growth rate tends to vary according to site characteristics, however. On a hilly site in southwestern Texas, a oneseed juniper 35 years of age was 14 feet (4.3 m) in height with a diameter of 0.5 inch (1.3 cm) [23].

Oneseed juniper has the ability to stop active growth when moisture is limited but can resume growth when moisture availability improves [53]. This growth pattern may represent an important adaptation allowing junipers to survive on harsh, arid sites. Although small trees may be killed by drought, mature oneseed junipers are resistant to drought, especially in comparison to Colorado pinyon [64,107].


Breeding system: Oneseed juniper is dioecious with seed persisting on the plant for 1 to 2 years [32,64,66].

Pollination: No information

Seed production: Trees first produce seed at 10 to 30 years of age, although maximum seed production generally does not occur until 50 to 200 years of age [66,115]. Trees as short as 18 inches (46 cm) in height can produce seed [64]. Oneseed juniper typically produces large seed crops at 2- to 5-year intervals [66].

Seed dispersal: Dispersal of oneseed juniper seeds may occur through water, gravity, or by any of a number of birds and mammals [10,64]. Animal dispersal may be particularly important, as digestive processes may enhance germination [13]. Most seed cones occur on the outer edges of trees where they are most visible and accessible to birds [110]. The brightly-colored, highly-visible cones persist on the trees for much of the year, providing a continually available food source for animals [13,34]. On some sites in New Mexico, as much as 95% of juniper reproduction could be attributed to bird dispersal [,30]. Domestic sheep and cattle may also aid in seed dispersal [64].

Seed banking: According to Johnsen [64], "Since the seed is not harmed by long periods of dry storage, drought probably does not affect seed viability. Viable seed in the soil may endure prolonged drought and still germinate when conditions become favorable."

Germination: Please refer to the section in this report titled "Value For Rehabilitation Of Disturbed Sites", under the MANAGEMENT CONSIDERATIONS heading, for detailed information about germination experiments with this species.

Seedling establishment/growth: Seedling establishment of oneseed juniper is often very poor even when good germination occurs [114]. The growth rate has been characterized as slow with medium vigor. Researchers in some areas have found that only approximately 3% of juniper seeds develop to the seedling stage. Shade may be important for good early growth of oneseed juniper [60]. Emergence appears to be somewhat greater under trees or shrubs than in interspaces where humidity and temperature fluctuations are more extreme [64]. In some areas, small junipers are particularly numerous under the canopy of pinyon or other trees [60,115]. Most seedlings occur some distance from the parent tree, although most seeds are located beneath the source tree [110]. Seedlings seldom establish beneath mature junipers, and an autopathic effect from litter is suspected [115].

Asexual regeneration: Although oneseed juniper is usually regarded as a nonsprouter [135], limited sprouting, mostly from older trees, has been observed [24,129]. Approximately 10% of living oneseed junipers sprouted from the base following an Arizona fire [129].

Oneseed juniper occupies xeric sites in semiarid climatic zones [38,51,102]. A typical Arizona site occupied by oneseed juniper receives 10 to 15 inches (250-380 mm) of precipitation annually, and has an average growing season of approximately 120 days [95,134]. Unlike several related species, the distribution of oneseed juniper does not appear to be limited by temperature inversions [59].

Oneseed juniper grows on dry, rocky, open flats, and slopes [32,50,52]. It commonly occurs in canyons or on middle-elevation foothills [50,134]. In many areas this juniper occurs in a zone below ponderosa pine (P. ponderosa) or alligator juniper, but above oak (Quercus spp.)-mountain-mahogany (Cercocarpus spp.) shrublands [96,134].

Because soil moisture is limited on many oneseed juniper sites, competition with others species may significantly influence the occurrence of this tree on a particular site. Grasses can compete effectively with oneseed juniper seedlings for moisture and can limit its distribution in some areas [64,135]. Many oaks also compete for soil moisture, although oneseed juniper appears to be capable of outcompeting them on shallow soils [115]. Where oneseed juniper occurs with Colorado pinyon, junipers show much more adaptation to drought stress than do the pines [107]. These observations were based on tissue water potentials and metabolic activity during the hottest part of the day.

Oneseed juniper grows on a variety of soil textures including gravelly, rocky, or sandy soils [50]. Parent materials include basalt, limestone, and sandstone [38]. Soil characteristics, combined with temperature, moisture and topography influence the upper and lower elevational extent of oneseed juniper [18,134]. Elevational ranges of oneseed juniper reported in the literature are:

Arizona 3,000 to 7,000 feet (914-2,130 m) [32,72]
Colorado 4,000 to 7,600 feet (1,220-2,315 m) [52]
New Mexico > 5,000 to 7,500 feet (1,525-2,285 m) [125]

Populations of oneseed juniper have been classified as climax [54,113], seral [44], late seral [27], and postclimax [64]. Schott and Pieper [113,115] examined secondary succession in pinyon-juniper several decades following cabling and concluded that re-established stands of pinyon and oneseed juniper were climax. Johnsen [64] concluded that within northern Arizona grasslands are numerous postclimax oneseed juniper stands with soil and microclimate conditions different from adjacent grasslands. The oneseed junipers maintain themselves on these areas but are not invading the surrounding grassland. Francis [44] described 4 seral phyto-edaphic community types in northwestern New Mexico where oneseed juniper is a codominant indicator species.

According to Gottfried [48] junipers are the 1st to return in secondary succession but are often followed and replaced by pinyon. "Habitat type affects the successional pathway following a disturbance. Succession on a site is influenced by the severity and size of the disturbance, and by the composition, longevity, and density of any surviving plants and propagules within the disturbed area and the characteristics of plant communities in adjacent undisturbed areas. Climatic conditions also influence the nature and speed of succession."

The subsection of this report titled " Other Management Considerations" in MANAGEMENT CONSIDERATIONS describes the analysis of a drought-induced ecotone shift of oneseed junipers into a declining ponderosa pine forest in Arizona.

New Mexico studies comparing plant growth in zones extending out from the boles of oneseed juniper and pinyon pine showed differences between the zones and also between the two tree species [6,112]. Grasses and other plants were consistently sparser beneath the tree canopies. The author's review of possible explanations includes allelopathy, shade, precipitation interception by the canopy, and litter cover forming a physical barrier to germinating plants. Other explanations offered to explain the reduced under-canopy vegetation include root competition for soil moisture, and possible chemical properties of oneseed juniper litter [9,62].

Based on a study conducted near Flagstaff Arizona, annual leader elongation of oneseed juniper generally begins in April [53]. Detailed phenological development from that study is:

phenological state date
bark begins to slip March 25
pollen shedding and female flowers open March 25
approximate start of leader elongation April 20
1st conspicuous formation of male flowers April 19
leader elongation ceases October 26

Flowering generally occurs in March or April [50,66], but can occur as early as January or as late as June, depending on geographic location [41]. Fruit matures in 1 season, ripening from August through November [64,66]. Seed may remain on the tree for 1 to 2 years [66]. Lymbery and Pieper [80] reported that in the northern Sacramento Mountains, flowering occurred from March to April, fruit ripening from August to September, and seed dispersal from October to November.

Species Index