Southern Rockies Pinyon-Juniper Woodland

This southern Rocky Mountain ecological system occurs on dry mountains and foothills in southern Colorado east of the Continental Divide, in mountains and plateaus of north-central New Mexico, and extends out onto limestone breaks in the southeastern Great Plains. 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 monosperma dominate the tree canopy. Juniperus scopulorum may codominate or replace Juniperus monosperma at higher elevations. Stands with Juniperus osteosperma are representative of the Colorado Plateau and are not included in this system. In southern transitional areas between Madrean Pinyon-Juniper Woodland (CES305.797) and Southern Rocky Mountain Pinyon-Juniper Woodland (CES306.835) in central New Mexico, Juniperus deppeana may be present. Understory layers are variable and may be dominated by shrubs, graminoids, or be absent. Associated species are more typical of southern Rocky Mountains than the Colorado Plateau and include Artemisia bigelovii, Cercocarpus montanus, Quercus gambelii, Achnatherum scribneri, Bouteloua gracilis, Festuca arizonica, or Pleuraphis jamesii.

Pinus edulis and/or Juniperus monosperma dominate the tree canopy. Juniperus scopulorum may codominate or replace Juniperus monosperma at higher elevations. Stands with Juniperus osteosperma are representative the Colorado Plateau and are not included in this system. In southern transitional areas between Madrean Pinyon-Juniper Woodland (CES305.797) and Southern Rocky Mountain Pinyon-Juniper Woodland (CES306.835) in central New Mexico, Juniperus deppeana becomes common. Understory layers are variable and may be dominated by shrubs, graminoids, or be absent. Associated species are more typical of southern Rocky Mountains than the Colorado Plateau and include Artemisia bigelovii, Cercocarpus montanus, Quercus gambelii, Achnatherum scribneri, Bouteloua gracilis, Festuca arizonica, or Pleuraphis jamesii.

This southern Rocky Mountain ecological system occurs on dry mountains and foothills in southern Colorado east of the Continental Divide, in mountains and plateaus of north-central New Mexico, and extends out onto limestone breaks in the southeastern Great Plains. Elevations range from near 1500 to 2900 m with high-elevation stands restricted to relatively warm, dry ridges and south and west aspects. Lower-elevation stands are often restricted to cooler north- and east-facing slopes.

Climate: Climate is cool-temperate, continental, and semi-arid. Precipitation ranges from approximately 33-46 cm (13-18 inches) annually. Most of the precipitation occurs during the summer growing season. 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.

Soil/substrate/hydrology: Soils supporting this system vary in texture ranging from stony, cobbly, gravelly sandy loams to clay loam or clay.

Both Pinus edulis and Juniperus monosperma are relatively short (generally <15 m tall), shade-intolerant, drought-tolerant, slow-growing, long-lived trees (Meeuwig and Bassett 1983, Little 1987, Anderson 2002, Johnson 2002, Romme et al. 2003). Both tree species are also non-sprouting and may be killed by fire (Wright et al. 1979).

Pinyon-juniper woodlands are influenced by drought, fires, grazing, and insect-pathogen outbreaks (West 1999b). Stands vary considerably in appearance and composition, both elevationally and geographically. Juniper tends to be more abundant at the warmer/drier lower elevations, pinyon tends to be more abundant at the higher elevations, and the two species share dominance within a broad middle-elevation zone (Woodin and Lindsey 1954).

The effect of fire on a stand is largely dependent on the tree height and density, fine-fuel load on the ground, weather conditions, and season (Dwyer and Pieper 1967, Wright et al. 1979). Some large trees may survive unless the fire gets into the crown due to heavy fuel loads in the understory or extreme fire conditions.

Site conditions 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. According to Gottfried et al. (1999) junipers are the first to return in secondary succession but are often followed and replaced by pinyon.

Site conditions influence the stand density. Sites with fewer trees typically have relatively deep soils and support a dense herbaceous level; those with more trees have shallow, rocky soils and often occur on steeper slopes. Stands may range from even-aged to uneven-aged stands. Some stands may have closed canopies with little or no understory, but many stands are open with widely scattered trees with a wide variety of understory vegetation (Rondeau 2001).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has four classes in total (LANDFIRE 2007a, BpS 2710590). These classes are summarized as:

A) Early Development 1 Open (10% of type in this stage): Total cover is 0-20% (grass cover <20%, shrub cover <15%, tree cover <10%). Shrub height 0-0.5 m. There would be very little of this class historically. Initial post-fire community grass- and shrub-dominated, consisting of mountain-mahogany with Gambel oak sprouts, perennial grass and various forbs. Pinyon and juniper seedlings and saplings will be in low density. Evidence of past fires may be observed, including charcoal and resprouting woody plants. Duration 50 years with succession to class B, mid-development stand of small trees. Trees exert very little influence until about 50 years in this system. Replacement fire occurs every few centuries. Drought occurs every 30 years and maintains the class but does not set it back to the beginning.

B) Mid Development 1 Open (20% of type in this stage): Tree cover is 0-40%. Tree height <3 m. Young juniper saplings are increasing and growing. Grass and shrubs are still dominant. Grass species that would be present are: blue grama, little bluestem, western wheatgrass, and needlegrass. Pinyon seedlings delayed until shade occurs for better growth. Mixed-severity fire also occurs because sometimes grass density is sufficient to result in pinyon and juniper scorch as well as mortality. Mixed fire occurs every 100-200 years. Replacement fires every several hundred years. This class probably lasts approximately 100 years, i.e., 50 to 150 years. Might remain in class until 10-20-year heavy moisture cycle; this increases seedling production, and juveniles mature. Drought occurs every 30 years but does not cause a transition.

C) Mid Development 1 Closed (45% of type in this stage): Tree cover is 21-70%. Tree height 5.1-10 m. Junipers reaching pole-size, and pinyon pine seedlings and saplings are growing dependent on rainfall patterns and shade. Pinyon having rapid growth in this stage. Gambel oak is also forming stand patches. Thinning effect for mountain-mahogany due to space/nutrient competition. Very little recruitment of junipers in this stage. This class lasts from approximately 150-250 years of age, so spending 50-100 years in this class. For the model, this class will last 75 years. Replacement fire unlikely in this class due to open canopy. Mixed fire also modeled infrequently. Drought occurs every 30 years, also maintaining this class.

D) Late Development 1 Closed (25% of type in this stage): Tree cover is 10-40%. Tree height 5-10 m. Mature juniper mixed with maturing pinyon. Understory declining due to canopy closing. Small amount of fine fuels. There is a shift in dominance from juniper to pinyon. This class can persist. Pinyon would be susceptible to drought mortality, disease, and insects. Drought creates conditions for insect disturbance to occur in pinyon pine. Drought itself, however, can impact the understory separate from the insect component. Optional 1 is drought plus insect effect. This takes it back to class C, because pinyon lost but still have mature junipers. Modeled at every 50 years, or 2% of the class each year. Regular drought modeled as every 30 years, as in other classes, not causing a transition. Mistletoe might also be influenced by the drought but not being modeled due to lack of information.

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 (Anderson 2002, Johnson 2002, 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, Evans 1988, Hall and Balda 1988, Ronco 1990). 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 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.

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 biological 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. Since then the distribution and density of pinyon and juniper and accompanying native understory have been significantly altered (Stevens 1999, West 1999b, Romme et al. 2009). Altered fire regimes, overgrazing, and tree cutting can all affect stand quality and fire behavior (Anderson 2002, Johnson 2002). These factors can also disturb microbiotic soil crusts and lead to increased soil erosion and habitat/species loss.

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.

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 Witte 1990, Swetnam and Baisan 1996a, Miller and Tausch 2001). Currently, much of this system's 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. Long-term heavy grazing reduces perennial 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. Livestock are also vectors for invasive species and disturb biological soil crusts. 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).

This system occurs on dry mountains and foothills east of the Continental Divide in southern Colorado, in mountains and plateaus of northern New Mexico and Arizona, and extends out onto breaks in the Great Plains. It extends south to the Sacramento Mountains, especially the eastern side. The western side of the Sacramento Mountains has Madrean elements (Quercus grisea) and may be classified as Madrean woodland.