Intermountain Juniper Savanna

This ecological system occupies dry foothills and sandsheets of western Colorado, northwestern New Mexico, northern Arizona, Utah, and west into the Great Basin of Nevada and southern Idaho. It is typically found at lower elevations ranging from 1000-2300 m. This system is generally found at lower elevations and more xeric sites than Great Basin Pinyon-Juniper Woodland (CES304.773) or Colorado Plateau Pinyon-Juniper Woodland (CES304.767). These occurrences are found on lower mountain slopes, hills, plateaus, basins and flats often where juniper is expanding into semi-desert grasslands and steppe. The vegetation is typically open savanna, although there may be small-patch inclusions of juniper woodlands. This savanna is typically dominated by an open canopy of Juniperus osteosperma trees with high cover of perennial bunchgrasses and forbs, with Bouteloua gracilis, Hesperostipa comata, and Pleuraphis jamesii being most common. In the southern Colorado Plateau, Juniperus monosperma or juniper hybrids may dominate the tree layer. Pinyon trees are typically not present because sites are outside the ecological or geographic range of Pinus edulis and Pinus monophylla. It has been suggested that all Juniperus osteosperma stands in Wyoming be placed in Colorado Plateau Pinyon-Juniper Woodland (CES304.767). This savanna system does not occur in Wyoming. Extensive Juniperus osteosperma woodlands should be included in one of the pinyon-juniper woodland systems or Rocky Mountain Foothill Limber Pine-Juniper Woodland (CES306.955).

The vegetation is typically open savanna, although there may be small-patch inclusions of juniper woodlands. This savanna is typically dominated by an open canopy of Juniperus osteosperma trees with high cover of perennial bunchgrasses and forbs, with Bouteloua gracilis, Hesperostipa comata, and Pleuraphis jamesii being most common. In the southern Colorado Plateau, Juniperus monosperma or juniper hybrids may dominate the tree layer. Pinyon trees are typically not present because sites are outside the ecological or geographic range of Pinus edulis and Pinus monophylla.

This widespread ecological system occupies dry foothills and sandsheets of western Colorado, northwestern New Mexico, northern Arizona, Utah, and west into the Great Basin of Nevada and southern Idaho. It is typically found at lower elevations ranging from 1000-2300m, but may extend up to 2650 m.

Climate: Climate is cool, semi-arid, and continental. Summers are generally hot and dry. Winters are typically cold with occasional snow and there can be extended periods of freezing temperatures. Mean annual precipitation is 25-35 cm, but the seasonal distribution varies across the range of the system. Generally, winter precipitation in the form of westerly storms is maximal along the northwest edge of the range, and summer moisture increases to the east and south (monsoons). Annual precipitation on the Colorado Plateau has a bimodal distribution with moisture peaking in winter and summer.

Physiography/landform: Stands occur on lower to middle elevation mountain slopes and foothills of the many ranges and plateaus of the region.

Soil/substrate/hydrology: Substrates are typically moderately deep to deep, coarse- to fine-textured soils that readily support a variety of growth forms, including trees, grasses, and other herbaceous plants (Stuever and Hayden 1997a, Romme et al. 2009).

Juniperus osteosperma is a relatively short (generally <10 m tall), shade-intolerant, drought-tolerant, slow-growing, long-lived tree (up to 650 years old) (Meeuwig and Bassett 1983, Zlatnik 1999e). Juniperus osteosperma is non-sprouting and may be killed by fire (Wright et al. 1979). Litter from juniper has an allelopathic effect on some grasses such as Bouteloua gracilis, Festuca idahoensis, and Poa secunda (Jameson 1970, Zlatnik 1999e).

Within a given region, the density of juniper trees, both historically and currently, is strongly related to topo-edaphic gradients. Less steep sites, especially those with finer-textured soils are where savannas, grasslands, and shrub-steppes have occurred in the past. Stands in this system occurred on these gentler slopes and historically may have been large and savanna-like with a very open upper canopy and high grass production. Juniper savanna is usually distributed across the landscape in patches that range from 10s to 100s of acres in size (LANDFIRE 2007a). In areas with very broken topography and/or mesa landforms, this type may have occurred in patches of several hundred acres (LANDFIRE 2007a). In Utah and Nevada pinyon and juniper landscape patches tended to be 10-100s of acres in size (LANDFIRE 2007a).

Key ecological processes are fire, climate fluctuations, grazing/herbivory, and insect/disease outbreaks. The effect of a fire on these stands is largely dependent on the tree height and density, fine-fuel load on the ground, weather conditions and season (Wright et al. 1979). Large trees generally survive unless the fire gets into the crown due to heavy fuel loads in the understory. In this system fire acts to open stands, kill young trees, increase diversity and productivity in understory species, and create a mosaic of stands of different sizes and ages across the landscape (Bradley et al. 1992).

Uncertainty exists about the fire frequencies of this ecological system, though it is predominantly Fire Regime Group III (fire frequency 30-100 years) (LANDFIRE 2007a); the fire regime is primarily determined by fire occurrence in the surrounding matrix vegetation (LANDFIRE 2007a). Lightning-ignited fires were common but typically did not affect more than a few individual trees. Replacement fires were uncommon to rare (average FRI of 100-500 years) and occurred primarily during extreme fire behavior conditions (LANDFIRE 2007a). Mixed-severity fire (average FRI of 100-500 years) was characterized as a mosaic of replacement and surface fires distributed through the patch at a fine scale (<0.1ac) (LANDFIRE 2007a). Surface fires could occur in stands where understory grass cover is high and provides adequate fuel. Surface fires were primarily responsible for producing fire scars on juniper trees and killing juniper seedlings and saplings (average FRI of 100 years).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has five classes in total (LANDFIRE 2007a, BpS 2411150). The model was reviewed and references to pinyon were removed, then summarized as:

A) Early Development 1 Open (herbaceous-dominated - 5% of type in this stage): Initial post-fire community dominated by annual forbs. Later stages of this class contain greater amounts of perennial grasses and forbs. Duration 10 years with succession to class B, mid-development closed. Replacement fire occurs every 100 years on average. Infrequent mixed-severity fire (average FRI of 300 years) thins vegetation.

B) Mid Development 1 Open (herbaceous-dominated - 5% of type in this stage): Dominated by perennial forbs and grasses. Total cover remains low due to shallow, unproductive soil. Duration 20 years with succession to class C unless infrequent replacement fire (FRI of 100 years) returns the vegetation to A. It is important to note that replacement fire at this stage does not eliminate perennial grasses, thus, succession age in A after this type of fire would be older than zero and <10. Mixed-severity fire (average FRI of 100 years) thins the woody vegetation.

C) Mid Development 2 Open (15% of type in this stage): Shrub-dominated community with young juniper seedlings becoming established. Duration 70 years with succession to class D unless replacement fire (average FRI of 200 years) causes a transition to class A. It is important to note that replacement fire at this stage does not eliminate perennial grasses, thus, succession age in class A after this type of fire would be older than zero and <10. Mixed-severity fire as in class B.

D) Late Development 1 Open (tree-dominated - 35% of type in this stage): Community dominated by young juniper of mixed age structure. Juniper becoming competitive on site and beginning to affect understory composition. Duration 300 years with succession to class E unless replacement fire (average FRI of 500 years) causes a transition to class A. Mixed-severity fire is less frequent than in previous states (200 years), whereas surface fire every 100 years on average becomes more important at this age in succession.

E) Late Development 2 Open (tree-dominated - 40% of type in this stage): Site dominated by widely spaced old juniper trees. Grasses (e.g., Bouteloua gracilis, Hesperostipa comata) present on microsites sites with deeper soils (>20 in) with restricting clay subsurface horizon. Replacement fire and mixed-severity fires are rare (average FRIs of 500 years). Surface fire every 100 years on average will scar ancient trees. Duration 600+ years.

Drought is an important ecological process which limits seedling recruitment and survival and causes mortality of mature trees (Romme et al. 2009). Other important ecological variables include insect infestations, pathogens, herbivory, and seed dispersal by birds and mammals. Juniper berries crops are primarily utilized by birds and small mammals (Johnsen 1962, McCulloch 1969, Short et al. 1977, Salomonson 1978, Balda 1987, Gottfried et al. 1995). The most important dispersers of juniper seeds are birds although mammals also feed on them (Scher 2002). These animals consume juniper berries and excrete viable scarified juniper seeds, which germinate faster than uneaten seeds, over extensive areas (Johnsen 1962, Meeuwig and Bassett 1983). Primary juniper seed dispersers are Bohemian waxwings (Bombycilla garrulus), but cedar waxwings (Bombycilla cedrorum), American robins (Turdus migratorius), turkeys (Meleagris gallopavo), and several species of jays are also dispersers (Scher 2002).

There are several insects, plant parasites and pathogens (Cercospora sequoiae, a blight, and Gymnosporangium spp., stem rusts) that attack juniper trees (Burns and Honkala 1990a, Rogers 1995). Two insects, western cedar borer (Trachykele blondeli) and juniper twig pruner (Styloxus bicolor), damage mature trees and can cause mortality (Rogers 1995). Juniper mistletoe (Phoradendron juniperinum) occurs on junipers where it reduces vigor and causes dieback, but rarely causes mortality (Meeuwig and Bassett 1983).

Biological soils crusts (BSC) are important for soil fertility, soil moisture, and soil stability in many semi-arid ecosystems and may be important on juniper savanna sites, especially on those with more exposed soil surface and less herbaceous and litter cover, and low disturbance (Belnap et al. 2001, Belnap and Lange 2003). Cyanobacteria (especially Nostoc) fix large amounts of soil nitrogen and carbon (Evans and Belnap 1999, Belnap 2001).

Numerous threats influence juniper savannas, including warming climate, heavy livestock grazing, tree harvest, and insect-pathogen outbreaks (West 1999b). The altered fire regime (intensity and frequency) in this savanna system in the form of fire exclusion has also allowed for juniper infill in some stands as well as expansion of juniper trees into the surrounding grasslands (West 1999b, Romme et al. 2009). Heavy grazing by livestock reduces fine fuels and indirectly decreases fire frequency, favoring fire-sensitive woody species such as Juniperus osteosperma. This results in uncharacteristically high cover of trees that shade out the grassy understory as it transitions from savanna to woodland. Some people confuse these younger juniper woodlands with true woodlands dependent on naturally fire-protected features such as rock outcrops. Lacking understory to carry fire, these woodlands only burn under extreme fire conditions resulting in high-intensity, high-severity stand-replacing fires. With loss of perennial grass cover with tree shading, these stands may have difficulty re-establishing the native perennial grass-dominated juniper savanna. Additionally, these stands are vulnerable to invasion by non-native annual grasses such as Bromus tectorum that can increase fire frequency beyond the natural fire regime.

Many stands within this system have been impacted by past range practices of chaining, tilling, and reseeding with exotic forage grasses. Although the dominant trees appear to regenerate after such disturbances, the effects on understory species are poorly known.

This juniper savanna occurs from northwestern New Mexico, northern Arizona, western Colorado, Utah, west into the Great Basin of Nevada and southern Idaho. Where it occurs in California, it is found only in the far eastern edges of the state adjacent to other Great Basin systems.