Great Basin Pinyon-Juniper Woodland

This ecological system occurs on dry mountain ranges of the Great Basin region and eastern foothills of the Sierra Nevada extending south in scattered locations throughout southern California. This woodland is typically found at lower elevations ranging from 1600-2800 m. These woodlands occur on warm, dry sites on mountain slopes, mesas, plateaus and ridges. Woodlands dominated by a mix of Pinus monophylla and Juniperus osteosperma, pure or nearly pure occurrences of Pinus monophylla, or woodlands dominated solely by Juniperus osteosperma comprise this system, but in some regions of southern California, Juniperus osteosperma is replaced by Juniperus californica. Cercocarpus ledifolius is a common associate. On the east slope of the Sierras in California, Pinus jeffreyi and Juniperus grandis may be components of these woodlands. Understory layers are variable. Associated species include shrubs such as Arctostaphylos patula, Artemisia arbuscula, Artemisia nova, Artemisia tridentata, Cercocarpus ledifolius, Cercocarpus intricatus, Coleogyne ramosissima, Yucca brevifolia, Quercus gambelii, Quercus turbinella, Quercus john-tuckeri, Juniperus californica, Quercus chrysolepis, and bunchgrasses Hesperostipa comata, Festuca idahoensis, Pseudoroegneria spicata, Leymus cinereus, and Poa fendleriana. This system occurs at lower elevations than Colorado Plateau Pinyon-Juniper Woodland (CES304.767) where sympatric.

These woodlands are characterized by an open to moderately dense tree canopy typically composed of a mix of Pinus monophylla and Juniperus osteosperma, but either tree species may dominate to the exclusion of the other. In some regions of southern California, Juniperus osteosperma is replaced by Juniperus californica. Cercocarpus ledifolius is a common associate and may occur in tree or shrub form. On the east slope of the Sierra Nevada in California, Pinus jeffreyi and Juniperus grandis (= Juniperus occidentalis var. australis) may be components of these woodlands. Understory layers are variable, but shrubs such as Artemisia tridentata frequently form a moderately dense short-shrub layer. Other associated shrubs include Arctostaphylos patula, Artemisia arbuscula, Artemisia nova, Cercocarpus intricatus, Coleogyne ramosissima, Quercus gambelii, and Quercus turbinella. Bunch grasses such as Poa fendleriana, Hesperostipa comata, Festuca idahoensis, Pseudoroegneria spicata, Leymus cinereus (= Elymus cinereus), and Bouteloua gracilis are commonly present and may form an herbaceous layer. In the southern extent, Arctostaphylos patula, Ceanothus greggii, Garrya flavescens, Quercus john-tuckeri, Juniperus californica, Purshia stansburiana, Quercus chrysolepis, Yucca baccata, and Yucca brevifolia are common.

This system occurs on dry mountain ranges of the Great Basin region and eastern foothills of the Sierra Nevada extending south into the Mojave Desert ranges and southwest in to the northern Transverse Ranges and San Jacinto Mountains. Elevations range from 1000 to 2800 m. Upper elevation limits are determined by local climate and/or the presence of competing tree species. Stands generally occur on sites with shallow rocky soils or rock-dominated sites that are protected from frequent fire (rocky ridges, broken topography and mesatops).

Climate: Climate is temperate, continental, and semi-arid with cold winters. Precipitation ranges from 20 to 45 cm annually, mostly occurring during fall and winter months (Brown 1982a). Summers are typically dry and there is usually extreme variation in annual precipitation. 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, ridges, foothills, and upper alluvial fans.

Soil/substrates/hydrology: Soils supporting this system vary in texture, ranging from stony, cobbly, gravelly sandy loams to clay loam or clay. Adjacent upland systems include Inter-Mountain Basins Montane Sagebrush Steppe (CES304.785), Inter-Mountain Basins Curl-leaf Mountain-mahogany Woodland and Shrubland (CES304.772), Inter-Mountain Basins Aspen-Mixed Conifer Forest and Woodland (CES304.776) above and at lower elevations, Great Basin Xeric Mixed Sagebrush Shrubland (CES304.774), Inter-Mountain Basins Big Sagebrush Shrubland (CES304.777), and Mojave Mid-Elevation Mixed Desert Scrub (CES302.742).

Pinus monophylla, Juniperus osteosperma, and Juniperus scopulorum are slow-growing, long-lived trees (about 650 years for Juniperus osteosperma, 300 years for Juniperus scopulorum, and 800 years for Pinus monophylla, although older individuals are known) (Burns and Honkala 1990a, Zlatnik 1999e, Zouhar 2001b, Scher 2002, Sawyer et al. 2009). These trees are killed by severe fire because of thin bark and lack of self-pruning; however, mature trees can survive low-intensity fires (Zouhar 2001b, Sawyer et al. 2009). Although there is variation in fire frequency because of the diversity of site characteristics, stand-replacing fire was uncommon in this ecological system historically, with an average fire-return interval (FRI) of 100-1000 years occurring primarily during extreme fire behavior conditions and during long droughts (Zouhar 2001b) (LF BpS model 1210190). Mixed-severity fire (average FRI of 100-500 years) was characterized as a mosaic of replacement and surface fires distributed through stands in patches at a fine scale (<0.1 acre) (LF BpS model 1210190).

Fire rotation in the San Bernardino Mountains was determined to be 480 years (Wangler and Minnich 2006). These woodlands have a truncated long fire-return interval of 200+ years with surface to passive crown fires of medium size, low complexity, high intensity, and very high severity (Sawyer et al. 2009). After a stand-replacing fire, the site is usually colonized by herbaceous plants and shrubs. The shrubs act as nurse plants, with Pinus monophylla seedlings establishing 20-30 years post fire after shrub density increases, and then a tree canopy forms after 100-150 years (Minnich 2007). As tree canopy becomes denser there is a decline in shrub cover (Minnich 2007). Fires are associated with herbaceous fuel buildup following a wet period (Minnich 2007).

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 principal 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 five 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 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 singleleaf pinyon seeds, including 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) (Christensen and Whitham 1993, Zouhar 2001b). Because singleleaf 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 seed 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 (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, there are at least seven insects, and fungi such as blackstain root-rot (Leptographium wageneri), pinyon needle rust (Coleosporium ribicola), and pinyon blister rust (Cronartium occidentale) (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). The current epidemic of ips beetles in many areas that has killed numerous pinyons has created high fuel loads that further threaten stands (Thorne et al. 2007).

LANDFIRE modelers predict severe weather (usually drought), insects and tree pathogens are coupled disturbances that thin trees to varying degrees and kill small patches every 250-500 years on average, with greater frequency in more closed stands (LF BpS model 1210190).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has five classes in total (LANDFIRE 2007a, BpS 1210190). These are summarized as:

A) Early Development 1 Open (herbaceous-dominated - 5% of type in this stage): Herbaceous cover is 0-15%. Shrub cover is 0%. Initial post-fire community dominated by annual grasses and forbs. Later stages of this class contain greater amounts of perennial grasses and forbs. Evidence of past fires (burnt stumps and charcoal) should be observed. Duration is 10 years with succession to class B, mid-development closed. Replacement fire occurs every 300 years on average.

B) Mid Development 1 Open (shrub-dominated - 5% of type in this stage): Shrub cover is 5-20%. Dominated by shrubs, perennial forbs and grasses. Tree seedlings starting to establish on favorable microsites. Total cover remains low due to shallow unproductive soil. Duration is 20 years with succession to class C unless infrequent replacement fire (FRI of 200 years) returns the vegetation to class A. It is important to note that replacement fire at this stage does not eliminate perennial grasses. Mixed-severity fire (average FRI of 200 years) thins the woody vegetation but does not change its succession age.

C) Mid Development 2 Open (shrub-dominated - 20% of type in this stage): Tree cover is 5-20%. Tree height <5 m. Shrub- and tree-dominated community with young juniper and pinyon seedlings becoming established. Duration is 70 years with succession to class D unless replacement fire (average FRI of 250 years) causes a transition to class A. It is important to note that replacement fire at this stage does not eliminate perennial grasses. Mixed-severity fire as in class B. Mortality from insects, pathogens, and drought occurs at a rotation of approximately 500 years and causes a transition to class B by killing older trees.

D) Late Development 1 Open (conifer-dominated - 35% of type in this stage): Tree cover is 5-40%. Tree height <10 m. Community dominated by young to mature juniper and pine of mixed age structure. Juniper and pinyon becoming competitive on site and beginning to affect understory composition. Duration 200 years with succession to class E unless replacement fire (average FRI of 1000 years) causes a transition to class A. Mixed-severity fire is less frequent than in previous states (500 years). Surface fire (mean FRI of 500 years) is infrequent and does not change successional dynamics. Tree pathogens and insects such as pinyon Ips become more important for woodland dynamics occurring at a rotation of 250 years, including both patch mortality (500-year rotation) and thinning of isolated individual trees (500-year rotation).

E) Late Development 2 Open (conifer-dominated - 35% of type in this stage): Tree cover is 5-50%. Tree height 5-25 m. Some sites dominated by widely spaced old juniper and pinyon, while elsewhere there are dense, old-growth stands with multiple layers. May have all-aged, multi-storied structure. Occasional shrubs with few grasses and forbs and often much rock. Understory depauperate and high amounts of bare ground present. Grasses present on microsites with deeper soils (>50 cm [20 inches]) with restricting clay subsurface horizon. Potential maximum overstory replacement fire and mixed-severity fires are rare (average FRIs of 1000 and 500 years, respectively). Surface fire occurs when especially dry years follow wet years (500-year rotation) and will scar ancient trees. Tree pathogens and insects associated with drought conditions kill patches of trees (1000-year rotation), with succession to class C, and individual trees (1000-year rotation) with succession to class D. Duration 800+ years.

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.

Threats to pinyon-juniper woodlands include invasion by introduced annual grasses, livestock grazing, development, and fire suppression. Before 1900, this system was mostly open woodland restricted to fire-safe areas on rocky ridges, etc., where the low cover of fine fuels reduced the spread of fires. Over the last 100 years fire regimes were altered because of fire suppression and grazing by livestock, which reduces the amount of fine fuels (grasses) that carry fire thus reducing fire frequency (Swetnam and Baisan 1996a). Currently, much of this system has a more closed canopy than historical conditions. 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 (Swetnam and Baisan 1996a).

These woodlands have been expanding into adjacent steppe grasslands and shrublands in many areas, reportedly in connection with livestock grazing and altered fire regimes (Blackburn and Tueller 1970, Tausch et al. 1981, Chambers 2001, Wangler and Minnich 2006, LANDFIRE 2007a, Weisberg et al. 2007). Historical fire suppression has resulted in denser tree canopies and a pinyon-juniper woodland expansion especially into big sagebrush shrublands (Wangler and Minnich 2006) and shrub-steppe and grassland (Blackburn and Tueller 1970). This may also allow the presence of relatively fire-intolerant species such as Artemisia tridentata, Coleogyne ramosissima, or Larrea tridentata in stands of this system in relatively mesic sites (Keeler-Wolf and Thomas 2000).

Denser canopies in pinyon-juniper woodland can also increase fire severity, as well as increasing soil erosion because of reduction in ground cover with shading by tree canopy (Tausch and West 1988, Zouhar 2001b). Recently, 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 (Thorne et al. 2007).

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, which kill many pinyons in turn creating very high fuel loads throughout much of the system's range (Furniss and Carolin 2002, Thorne et al. 2007). In addition, many of these communities have been severely 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 native understory species are poorly known (Thorne et al. 2007).

Human development has impacted some locations throughout the Great Basin. 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. 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 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 attacks (Furniss and Carolin 2002, Thorne et al. 2007).

This system occurs on dry mountain ranges of the Great Basin region and eastern foothills of the Sierra Nevada, typically at lower elevations ranging from 1600-2800 m. It extends southwest in California to the northern Transverse Ranges (Ventura County) and San Jacinto Mountains (Riverside County).