Colorado Plateau Pinyon-Juniper Shrubland

This ecological system is characteristic of the rocky mesatops and slopes on the Colorado Plateau and western slope of Colorado, but these stunted tree shrublands may extend further upslope along the low-elevation margins of taller pinyon-juniper woodlands. Sites are drier than Colorado Plateau Pinyon-Juniper Woodland (CES304.767). Substrates are shallow/rocky and shaly soils at lower elevations (1200-2000 m). Sparse examples of the system grade into Colorado Plateau Mixed Bedrock Canyon and Tableland (CES304.765). The vegetation is dominated by dwarfed (usually <3 m tall) Pinus edulis and/or Juniperus osteosperma trees forming extensive tall shrublands in the region along low-elevation margins of pinyon-juniper woodlands. Other shrubs, if present, may include Artemisia nova, Artemisia tridentata ssp. wyomingensis, Chrysothamnus viscidiflorus, or Coleogyne ramosissima. Herbaceous layers are sparse to moderately dense and typically composed of xeric graminoids.

The vegetation is dominated by dwarfed (usually <3 m tall) Pinus edulis and/or Juniperus osteosperma trees forming extensive tall shrublands in the region along low-elevation margins of pinyon-juniper woodlands. Other shrubs, if present, may include Artemisia nova, Artemisia tridentata ssp. wyomingensis, Chrysothamnus viscidiflorus, or Coleogyne ramosissima. Herbaceous layers are sparse to moderately dense and typically composed of xeric graminoids.

This tree-dominated ecological system is characteristic of the dry, lower elevation sites in the rocky canyons of the Colorado Plateau and Western Slope of Colorado (1200-1600 m elevation), but these stunted-tree shrublands may extend further upslope to 2000 m on locally xeric sites (Stuever and Hayden 1997a).

Climate: Climate is semi-arid to arid with hot summers and cold winters. Based on data from Moab, Utah, average annual precipitation is approximately 25 cm. Precipitation mostly occurs as rain during monsoons (late July to October) and spring (March to May). June is the driest month.

Physiography/landform: Stands occur on the rocky mesatops, canyon rims, and dry slopes and ridges that are too dry for woodlands.

Soil/substrate/hydrology: Substrates are shallow/rocky and shaly soils at lower elevations. Sandstone is the most common parent material.

Pinus edulis is extremely drought-tolerant and slow-growing (Little 1987). It is also non-sprouting and may be killed by fire (Wright et al. 1979, Wright and Bailey 1982a). This shrubland or stunted woodland (<3 m tall) is characteristic of the drier, hotter low-elevation sites (usually <1600 m), rock outcrops and sites with shallow soils that limit tree growth. The understory is typically sparser than Colorado Plateau Pinyon-Juniper Woodland (CES304.767) and this system is more affected by drought than fires; however, occurrences of this system will burn under extreme fire conditions. The effect of fire on a stand is largely dependent on tree height and density, fine-fuel load on the ground, weather conditions, and season (Dwyer and Pieper 1967, Wright et al. 1979, Wright and Bailey 1982a). Trees are more vulnerable in open stands where fires frequently occur in the spring, when the relative humidity is low, wind speeds are over 10-20 mph, and there are adequate fine fuels to carry fire (Wright et al. 1979, Wright and Bailey 1982a). Under other conditions, burns tend to be spotty with low tree mortality. Large trees are generally not killed unless fine fuels, such as tumbleweeds, have accumulated beneath the trees to provide ladder fuels for the fire to reach the crown (Jameson 1962). Closed-canopy stands burn infrequently because they typically do not have enough understory or wind to carry fire (Wright et al. 1979).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has five classes in total (LANDFIRE 2007a, BpS 2311020). These are 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 (shrub-dominated - 5% of type in this stage): Dominated by shrubs, 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 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 (average FRI of 100 years) thins the woody vegetation but does not cause a transition to another class.

C) Mid Development 2 Open (shrub-dominated - 10% of type in this stage): Shrub-dominated community with young juniper and pinyon 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 (conifer-dominated - 35% of type in this stage): Community dominated by young and stunted juniper and pinyon of mixed age structure. Juniper and pinyon becoming competitive on site and beginning to affect understory composition. Duration 300 years with succession to E unless replacement fire (average FRI of 500 years) causes a transition to 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 (conifer-dominated - 45% of type in this stage): Site dominated by widely spaced old and stunted juniper and pinyon. Understory depauperate and high amounts of bare ground and rock present. Grasses present on microsites with deeper soils (>50 cm [20 inches]) with restricting clay subsurface horizon. Potential maximum overstory coverage is greater in those stands with pinyon as compared to those with only juniper. Replacement fire and mixed-severity fires are rare (average FRIs of 500 years). Surface fire every 100 years on average will scar ancient stunted trees. Duration 600 years+.

Other important ecological processes include drought, insect infestations, pathogens, herbivory and seed dispersal by birds and mammals. Juniper berries and pinyon nut 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 and pinyon seeds are birds, although many mammals also feed on them. 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 waxwing (Bombycilla garrulus), but others include cedar waxwing (Bombycilla cedrorum), American robin (Turdus migratorius), turkey (Meleagris gallopavo), and several species of jays (Scher 2002). Pinyon seeds are a critically important food source for scrub jay (Aphelocoma californica), pinyon jay (Gymnorhinus cyanocephalus), Steller's jay (Cyanocitta stelleri) and Clark's nutcracker (Nucifraga columbiana). These birds are the primary dispersers of pinyon pine seeds and, during mast crop years, cache hundreds of thousands of pinyon seeds, many of which are never recovered (Balda and Bateman 1971, Vander Wall and Balda 1977, Ligon 1978). Because pinyon seeds are heavy and totally wingless, seed dispersal is dependent on vertebrate dispersers that store seeds in food caches, where unconsumed seeds may germinate. This dispersal mechanism is a good example of a co-evolved, mutualistic, plant-vertebrate relationship (Vander-Wall et al. 1981, Evans 1988, Lanner 1996) and would be at risk with loss of trees or dispersers. Many mammals are also known to eat pinyon seeds, such as several species of mice (Peromyscus spp.), woodrats (Neotoma spp.), squirrels (Sciurus spp.), chipmunks (Neotamias spp.), and desert bighorn sheep (Ovis canadensis nelsoni) and, although less effective, they may inadvertently disperse seeds (Anderson 2002).

Although Pinus edulis is drought-tolerant, prolonged droughts will weaken trees and promote mortality by secondary agents. Periodic die-offs of pinyon pine caused by insects, such as the pinyon ips beetle (Ips confusus), or fungal agents, such as blackstain root-rot (Leptographium wageneri), tend to be correlated with droughts (Anhold 2005). These mortality events may be localized or widespread but can result in 50 to 90% mortality of Pinus edulis in affected areas (Harrington and Cobb 1988). There are many insects, pathogens, and plant parasites that attack pinyon and juniper trees (Meeuwig and Bassett 1983, Gottfried et al. 1995, Rogers 1995, Weber et al. 1999). Juniper mistletoe (Phoradendron juniperinum) occurs on junipers and pinyon dwarf mistletoe (Arceuthobium divaricatum) occurs on pines. Both mistletoes reduce vigor and cause dieback but rarely cause mortality (Meeuwig and Bassett 1983). For pinyon and juniper, there are at least seven insects, and fungi such as black stain root-rot (Leptographium wageneri), and pinyon needle rust and pinyon blister rust (Skelly and Christopherson 2003). The insects are normally present in these woodland stands and during drought-induced water stress, outbreaks may cause local to regional mortality (Wilson and Tkacz 1992, Gottfried et al. 1995, Rogers 1995). Most insect-related pinyon mortality in the West is caused by pinyon ips bark beetle (Ips confusus) (Rogers 1993).

Most pinyon-juniper woodlands and shrublands in the Southwest have high soil erosion potential (Baker et al. 1995). Several studies have measured present-day erosion rates in pinyon-juniper woodlands, highlighting the importance of herbaceous cover and biological soil crusts in minimizing precipitation runoff and soil loss (Baker et al. 1995, Ladyman and Muldavin 1996, Belnap et al. 2001).

Conversion of this type has commonly come from catastrophic crown fires and "chaining" or mechanical removal of trees by land management agencies to convert these wooded areas to grasslands for livestock (Stevens 1999, Tausch 1999a, Tausch and Hood 2007). Before 1900, this system was mostly open shrubland restricted to fire-safe areas on rocky ridges and outcrops where the low cover of fine fuels reduced the spread of fires. Over the last 100 years fire regimes were altered by fire suppression and grazing by livestock, which reduces the amount of fine fuels (grasses) that carry fire, thus reducing fire frequency (Pieper and Wittie 1990, Swetnam and Baisan 1996, Miller and Tausch 2001). Consequently, some stands of this system have a more closed canopy. Direct and indirect fire suppression has led to a buildup of woody fuels that increases the likelihood of high-intensity, stand-replacing fires. If exotic species are present, post-crown fire and post-treatment outcomes may result in conversion to exotic species.

In addition, energy exploration and development and mining operations can drastically impact natural vegetation. Road building and power transmission lines continue to fragment vegetation and provide vectors for invasive species. Invasion by introduced annual grass, such as Bromus tectorum and other annuals, provide fine fuels that carry fire (Tausch 1999a, Miller and Tausch 2001, Tausch and Hood 2007), although the sites where this system occurs may be too dry for cheatgrass to become abundant.

Management actions such as chaining pinyon-juniper stands creates a large food source of injured pines for native ips beetles (Ips confusus) to feed on that can quickly multiply, creating epidemic outbreaks of beetles that attack and kill many healthy pinyons (Furniss and Carolin 2002). Increasingly frequent drought stresses pinyons and makes them less able to survive ips attacks (Furniss and Carolin 2002).

Other human development has impacted many locations throughout the ecoregion. High- and low-density urban and industrial developments also have large impacts. For example, residential development has significantly impacted locations within commuting distance to urban areas. Impacts may be direct as vegetation is removed for building sites or more indirectly through natural fire regime alteration, and/or the introduction of invasive species. This system is popular for outdoor recreation (e.g., hiking, camping, mountain biking, and off-road vehicle recreation) in canyons and mesas in southern Utah. Recreationalists are vectors for invasive species and likely degrade these shrublands in other ways such as soil compaction, soil erosion, and damage to biological soil crusts (Schwinning et al. 2008).

This system occurs on rocky mesatops and slopes on the Colorado Plateau.