Colorado Plateau Pinyon-Juniper Woodland

This ecological system occurs in dry mountains and foothills of the Colorado Plateau region including the Western Slope of Colorado to the Wasatch Range, south to the Mogollon Rim, and east into the northwestern corner of New Mexico. It is typically found at lower elevations ranging from 1500-2440 m. These woodlands occur on warm, dry sites on mountain slopes, mesas, plateaus, and ridges. Soils supporting this system vary in texture, ranging from stony, cobbly, gravelly sandy loams to clay loam or clay. Pinus edulis and/or Juniperus osteosperma dominate the tree canopy. In the southern portion of the Colorado Plateau in northern Arizona and northwestern New Mexico, Juniperus monosperma and hybrids of Juniperus spp. may dominate or codominate the tree canopy. Juniperus scopulorum may codominate or replace Juniperus osteosperma at higher elevations. Understory layers are variable and may be dominated by shrubs, graminoids, or be absent. Associated species include Arctostaphylos patula, Artemisia tridentata, Cercocarpus intricatus, Cercocarpus montanus, Coleogyne ramosissima, Purshia stansburiana, Purshia tridentata, Quercus gambelii, Bouteloua gracilis, Pleuraphis jamesii, Pseudoroegneria spicata, Poa secunda, or Poa fendleriana. This system occurs at higher elevations than Great Basin Pinyon-Juniper Woodland (CES304.773) and Colorado Plateau shrubland systems where sympatric.

Pinus edulis and/or Juniperus osteosperma dominate the tree canopy. In the southern portion of the Colorado Plateau in northern Arizona and northwestern New Mexico, Juniperus monosperma and hybrids of Juniperus spp. may dominate or codominate the tree canopy. Juniperus scopulorum may codominate or replace Juniperus osteosperma at higher elevations. Understory layers are variable and may be dominated by shrubs, graminoids, or be absent. Associated species include Arctostaphylos patula, Artemisia tridentata, Cercocarpus intricatus, Cercocarpus montanus, Coleogyne ramosissima, Purshia stansburiana, Purshia tridentata, Quercus gambelii, Bouteloua gracilis, Pleuraphis jamesii, Pseudoroegneria spicata, Poa secunda, or Poa fendleriana.

This ecological system occurs in dry mountains and foothills of the Colorado Plateau region, including the western slope of Colorado to the Wasatch Range, south to the Mogollon Rim, and east into the northwestern corner of New Mexico. It is typically found at lower elevations ranging from 1500-2440 m (Hess and Wasser 1982, Stuever and Hayden 1997a).

Climate: Climate is semi-arid. Annual precipitation is usually from 30-55 cm in the form of rain and snow. Severe climatic events occurring during the growing season, such as frosts and drought, are thought to limit the distribution of pinyon-juniper woodlands to relatively narrow altitudinal belts on mountainsides.

Physiography/landform: These woodlands occur on warm, dry sites on mountain slopes, mesas, plateaus, and ridges. Stands occur on a variety of aspects and slopes. Slope may range from nearly level to steep (up to 80%).

Soil/substrates/hydrology: Soils supporting this system vary in depth and texture, ranging from shallow, stony, cobbly, gravelly sandy loams to often deeper clay loam or clay. Parent materials likewise vary widely from granite, basalt, limestone, and sandstone to mixed alluvium (Springfield 1976). Soil depths may range from shallow to deep.

Key ecological processes are drought, fire, herbivory, and insect/disease outbreaks. Both Pinus edulis and Juniperus osteosperma are relatively short (generally <15 m tall), shade-intolerant, drought-tolerant, slow-growing, long-lived trees (especially Juniperus osteosperma can reach 650 years old) (Meeuwig and Bassett 1983, Little 1987, Zlatnik 1999e, Romme et al. 2003). Both tree species are also non-sprouting and may be killed by fire (Wright et al. 1979). 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, increase diversity and productivity in understory species, and create a mosaic of stands of different sizes and ages across the landscape while maintaining the boundary between woodlands and adjacent shrublands or grasslands (Bradley et al. 1992).

As modeled by LANDFIRE (2007a), the fire regime is characterized by somewhat frequent mixed-severity mosaic fires (mean FRI of 150-200 years) with very infrequent replacement fires (mean FRI of 200-500 years) (Rondeau 2001). Surface fire occurs only in the earliest succession class every 200 years on average (LANDFIRE 2007a). There is frequent fire spread from adjacent types (LANDFIRE 007a). Severe climatic events occurring during the growing season, such as frosts and drought, are thought to limit the distribution of pinyon-juniper woodlands to relatively narrow altitudinal belts on mountainsides. Weather-related stress thins trees every 145 years on average in more closed stands (LANDFIRE 2007a). Insects/disease has a similar effect, but with a greater frequency in closed stands (mean return interval of 100 years) than open ones (mean return interval of 1000 years) (LANDFIRE 2007a). Competition from grasses and older trees in late-open stands is also included as a disturbance that maintains stand openness (LANDFIRE 2007a).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has five classes in total (LANDFIRE 2007a, BpS 2310160). These are summarized as:

A) Early Development 1 All Structures (10% of type in this stage): Grass/forb/shrub/seedling - usually post-fire. Cover is 0-30%. Shrub height 0.5 m. Both replacement fire and surface fire occur in this class (mean FRI of 200 years for both). The dominant succession path is to class C (mid, open) after 60 years, although the model allows for an alternate succession pathway to class B (mid, closed) 1/100 times to represent tree invasion.

B) Mid Development 1 Closed (20% of type in this stage): Tree cover is 40-70%. Tree height <5 m. Mid-development, dense (>40% cover) pinyon-juniper woodland; understory is sparse. Replacement fire occurs every 400 years on average. Three disturbances cause a transition to class C (mid, open): mixed-severity fire (mean FRI of 150 years), insects/disease (mean return interval of 100 years) and weather-related stress (mean return interval of 150 years). Succession to class E, late-closed, after 120 years.

C) Mid Development 1 Open (25% of type in this stage): Tree cover is 10-40%. Tree height <5 m. Mid-development, open (<40% cover) pinyon-juniper stand with mixed shrub/herbaceous community in understory. The mean FRI for replacement fire is 500 years. Mixed-severity fire (mean FRI of 200 years) and insects/disease (mean return interval of 1000 years) maintain stand structure. Primary succession pathway to class D, late-open, after 100 years, although an alternate succession pathway to class B 2/100 times is included to represent tree invasion;

D) Late Development 1 Open (35% of type in this stage): Tree cover is 10-40%. Tree height 5-10 m. Late-development, open juniper-pinyon stand with "savanna-like" appearance; mixed grass/shrub/herbaceous community. Replacement fire is infrequent (mean FRI of 500 years). Mixed-severity fire (mean FRI of 200 years), insects/disease (mean return interval of 1000 years) and competition (1/100 prob/year) maintain vegetation in class D, which is the primary succession endpoint. Alternate succession to class E, late-closed, occurs 1/200 times to represent tree invasion;

E) Late Development 2 Open (conifer-dominated - 35% of type in this stage): Tree cover is 40-70%. Tree height 5-10 m. Dense, old-growth stands with multiple layers. Late-development, closed pinyon-juniper forest. May have all-aged, multi-storied structure. Moderate mortality within stand. Occasional shrubs with few grasses and forbs and often rock or bare soil. The mean FRI of replacement fire is 500 years. Mixed-severity fire (mean FRI of 150 years), insects/disease (mean return interval of 100 years) and weather-related stress (mean return interval of 100 years) thin tree cover, therefore causing a transition to class D. Succession maintains vegetation in class E.

Other important ecological processes include drought, insect infestations, pathogens, herbivory, and seed dispersal by birds and mammals. Juniper berry 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). Large mammals, such as mule deer (Odocoileus hemionus), white-tailed deer (Odocoileus virginianus) and elk (Cervus elaphus), eat leaves and seeds of both species and they browse woodland grasses, forbs and shrubs, including Artemisia tridentata, Cercocarpus montanus, Quercus gambelii, and Purshia stansburiana (Short and McCulloch 1977).

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 over extensive areas, which germinate faster than uneaten seeds (Johnsen 1962, Meeuwig and Bassett 1983). Primary juniper seed dispersers are Bohemian waxwing (Bombycilla garrulus), 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 western 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 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). 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.), deer, black bear (Ursus americanus), and desert bighorn sheep (Ovis canadensis nelsoni) (Anderson 2002). 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.

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). For pinyon and juniper, there are at least seven insects, plus a fungus (blackstain root-rot (Leptographium wageneri)), juniper mistletoe (Phoradendron juniperinum) and pinyon dwarf mistletoe (Arceuthobium divaricatum). Both mistletoes reduce vigor and cause occasional dieback but rarely cause mortality (Meeuwig and Bassett 1983). The insects are normally present in these woodland stands, and during drought-induced water stress periods, 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 beetle (Ips confusus) (Rogers 1993). Pinyons cannot repel pinyon ips beetles when weakened by drought and many are killed. During the drought of 2002-2003, populations of ips beetles increased to epidemic levels that killed millions of pinyon trees in the southwestern U.S. (Thorne et al. 2007).

Most pinyon-juniper woodlands 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 cryptogamic soil crusts (Baker et al. 1995, Belnap et al. 2001) in minimizing precipitation runoff and soil loss in pinyon-juniper woodlands.

Before 1900, this system was mostly open woodland restricted to fire-safe areas on rocky ridges and outcrops where the low cover 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 1996a, Miller and Tausch 2001). Currently, much of this system distribution has a more closed canopy than historically. Fire suppression has led to a buildup of woody fuels that in turn increases the likelihood of high-intensity, stand-replacing fires. Heavy grazing, in contrast to fire, removes the grass cover and tends to favor shrub and conifer species. Fire suppression combined with grazing creates conditions that support invasion by pinyon and juniper trees into adjacent shrublands and grasslands. Under most management regimes, typical tree size decreases and tree density increases in this habitat.

Other common stressors include invasive species, insect/disease outbreaks, fuel wood cutting, and increased soil erosion, all of which affect stand quality and fire behavior. Significant losses in pinyon-juniper woodlands are a result of shortening of fire-return intervals (FRI) because of invasion by introduced Bromus tectorum and other annuals that provide fine fuels that carry fire (Tausch 1999, Miller and Tausch 2001, Tausch and Hood 2007). Livestock are also vectors for invasive species and disturb biological soil crusts.

Currently, epidemics of the native pinyon ips beetle (Ips confusus) often occur during drought periods when mature trees are weakened and vulnerable to ips beetle attacks killing many pinyons and creating very high fuel loads throughout much of the system's range (Furniss and Carolin 2002). In addition, many of these communities have been severely impacted by past range practices of chaining, tilling, and reseeding with exotic forage grasses.

Human development has impacted some locations throughout the distribution of this type. For example, residential development has significantly impacted locations within commuting distance to urban areas. Impacts may be direct as vegetation removed for building sites or more indirectly through natural fire regime alteration, and/or the introduction of invasive species. Mining operations can drastically impact natural vegetation. Road building and power transmission lines continue to fragment vegetation and provide vectors for invasive species. Management actions such as chaining pinyon-juniper stands creates a large food source of injured pines for 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). Drought stresses pinyon trees and makes them less able to survive ips beetle attacks (Furniss and Carolin 2002).

Conversion of this type has resulted from catastrophic crown fires and "chaining" or mechanical removal of trees by land management agencies to convert woodlands to grasslands for livestock (Stevens 1999, Tausch 1999, Tausch and Hood 2007). If exotic species are present, post crown fire and post-treatment outcomes may result in conversion to exotic species.

This system occurs on dry mountains and foothills of the Colorado Plateau region from the Western Slope of Colorado to the Wasatch Range, south to the Mogollon Rim, and east into the northwestern corner of New Mexico. It is typically found at lower elevations, ranging from 1500-2440 m. In Wyoming, it would occur only in the southern portions of mapzone 22.