Sky Island Pinyon-Juniper Woodland

This system occurs on foothills, mountains and plateaus in the Sierra Madre Occidentale and Sierra Madre Orientale in Mexico, Trans-Pecos Texas, southern New Mexico and Arizona, generally south of the Mogollon Rim. Substrates are variable, but soils are generally dry and rocky. The presence of Pinus cembroides, Pinus discolor, or other Madrean trees and shrubs is diagnostic of this woodland system. Juniperus coahuilensis, Juniperus deppeana, Juniperus pinchotii, Juniperus monosperma, and/or Pinus edulis may be present to dominant. Madrean oaks such as Quercus arizonica, Quercus emoryi, Quercus grisea, or Quercus mohriana may be codominant. Pinus ponderosa is absent or sparse. If present, understory layers are variable and may be dominated by shrubs or graminoids.

Vegetation is characterized by an open to moderately dense tree canopy dominated by pinyon and juniper trees 2-5 m tall. The presence of pinyons Pinus cembroides, Pinus discolor, Pinus remota, or Pinus edulis with Madrean elements in the understory is diagnostic of this ecosystem. Juniperus coahuilensis, Juniperus deppeana, and Juniperus pinchotii are character species that are often present to dominant. Pinus edulis and Juniperus monosperma may be the dominants in the northern distribution in combination with Madrean shrub and/or graminoid elements. Pinus ponderosa is absent or scattered. Understory layers are variable, ranging from sparse to dense grass or shrub layers. If Madrean oak trees such as Quercus arizonica, Quercus emoryi, or Quercus grisea are present, then they do not dominate the tree canopy. Common shrub species may include chaparral, desert scrub or lower montane shrubs such as Arctostaphylos pungens, Canotia holacantha, Ceanothus greggii, Cercocarpus montanus, Mimosa dysocarpa, Quercus turbinella, or Rhus trilobata. Perennial grasses such as Bouteloua curtipendula, Bouteloua eriopoda, Bouteloua gracilis, Muhlenbergia emersleyi, Muhlenbergia pauciflora, Piptochaetium fimbriatum, or Piptochaetium pringlei are present in many stands and may form an herbaceous layer. The vegetation description is based on several references, including Brown (1982a), Gottfried (1992), Dick-Peddie (1993), Muldavin et al. (2000b), and Gori and Bate (2007).

This woodland system is common in foothills, mountains and plateaus in the Sierra Madre Occidentale and Sierra Madre Orientale in Mexico, Trans-Pecos Texas, southern New Mexico and Arizona, generally south of the Mogollon Rim. Elevation generally ranges from 1300-2225 m with high-elevation stands restricted to warmer southern aspects.

Climate: Climate is semi-arid with drought not uncommon. Summers are hot and winters are mild with cold periods and occasional snows. The mean annual precipitation ranges from 40-50 cm with approximately two-thirds occurring during the Arizona monsoon season from July to September, often as high-intensity convective storms. May and June are typically dry. Stands typically occur on nearly level to steep, rocky slopes.

Physiography/landform: Stands occur on cool aspects of steep scarp slopes, in canyons (including alluvial terraces), on gently sloping alluvial fan piedmonts (bajadas), steeper colluvial slopes and ridges, as well as mesatops. Pinyon and juniper woodlands extend down to 760 m elevation in Trans-Pecos ranges. At the lowest elevation, encinal generally occupies the rockier substrates or is restricted to drainages within grasslands (Brown 1982a).

Soil/substrate/hydrology: Soils are variable, but are generally shallow, rocky, calcareous, but may include deeper clay loamy to gravelly loamy soils. Parent materials include andesite, rhyolite, limestone, basalt, colluvium and alluvium (Sullivan 1993c, Pavek 1994b, Tirmenstein 1999i, Hauser 2007b).

Dynamics are complicated by the variation in physiognomy and diverse plant communities present in this system. The pinyon-juniper woodlands and savannas included in this system are represented by what Moir and Carleton (1987) classified as the High Sun Mild climate zone (summer precipitation and warm climate). Romme et al. (2003) developed a pinyon-juniper classification with three types based on canopy structure, understory composition, and historic fire regime. All three types, pinyon-juniper grass savanna, pinyon-juniper shrub woodland, and pinyon-juniper forest, are included in this system. For this model an ecologically similar type, pinyon-juniper grass open woodland (with tree canopy >10% cover), was added to the pinyon-juniper grass savanna making this the more widespread type (Landis and Bailey 2005, Gori and Bate 2007). The other types are the pinyon-juniper shrub woodland, represented by pinyon-juniper trees with an understory of shrubs such as Quercus turbinella, and the pinyon-juniper forest type that has a typically sparse understory and is restricted to dry, rocky areas where it is protected from fires (Romme et al. 2003).

Fire dynamics for these types under historical natural conditions (also called natural range of variability (NRV) for pre-1900 timeframe) are summarized below based on (Romme et al. 2003).

The fire regime for the pinyon-juniper grass savanna/pinyon-juniper grass open woodland includes frequent, low-severity surface fires that are carried by the herbaceous layer. The low density of trees (5-20% cover) and high perennial grass cover is maintained by this fire regime. Mean fire interval is estimated to be 12-43 years (Gori and Bate 2007).

The fire regime for the pinyon-juniper shrub woodland is described as moderately frequent, high-severity crown fires that are carried by the shrub and tree layers. After a stand-replacing fire the site begins at early-seral stage and returns to a moderately dense tree layer with a moderate to dense shrub layer. Succession happens relatively quickly if the shrub layer includes chaparral species that recover rapidly from fire by re-sprouting or from fire-scarified seeds in a seed bank. Mixed-severity fires may alter this pattern by creating a mosaic of pinyon-juniper states (early-, mid-, and late-seral). Mean fire interval is estimated to be 23-81 years (Gori and Bate 2007).

The fire regime for the pinyon-juniper forest type is characterized by very infrequent, very high-severity fires that are carried by tree crowns. The stand dynamics are stable with a multi-age tree canopy and with little change in shrub or herbaceous layers.

The historical fire season was probably similar to that of other Madrean woodlands and grasslands, occurring predominantly before the summer monsoon between April and June when vegetation is dry and ignition sources from dry lightning strikes are common (Swetnam and Betancourt 1990).

Other important ecological processes include climate, drought, insect infestations, pathogens, herbivory and seed dispersal by birds and small 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, Tirmenstein 1999i). 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 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. Juniper seeds that pass through the digestive tract of birds and other herbivores germinate faster than uneaten seeds (Johnsen 1962, Tirmenstein 1999i). The primary dispersers of pinyon seeds, i.e., scrub jays (Aphelocoma californica), pinyon jays (Gymnorhinus cyanocephalus), Steller's jays (Cyanocitta stelleri) and Clark's nutcrackers (Nucifraga columbiana), cache hundreds of thousands of pinyon seeds during mast crop years, many of which are never recovered (Balda and Bateman 1971, Vander Wall and Balda 1977, Ligon 1978, Pavek 1994b). 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. In addition, small mammals, such as cliff chipmunk (Neotamias dorsalis) and rock squirrel (Otospermophilus variegatus), compete with birds (Christensen and Whitham 1993).

There are many insects, pathogens, and plant parasites that attack pinyon and juniper trees (Gottfried et al. 1995, Rogers 1995, Weber et al. 1999). For pinyon, there are at least seven insects, plus a fungus (black stain root disease (Leptographium wageneri), and pinyon dwarf mistletoe (Arceuthobium divaricatum). These 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 beetle (Ips confusus) (Rogers 1993).

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

The Madrean pinyon-juniper woodland ecological system has been impacted by human activities over the last century. Historical fire regimes were disrupted following the introduction of livestock (and the 1890s drought). Fire suppression has increased woody species, led to changes in woody species composition and led to an uncharacteristic fire regime in many stands (Barton 1999, Gori and Bate 2007, Muldavin et al. 2002b, Turner et al. 2003). Grazing passively suppresses fire by removing fine fuels needed to carry surface and mixed-severity fires that likely maintained the structure and composition of pinyon-juniper savannas and pinyon-juniper shrub woodlands historically. Active fire suppression was also practiced by the Federal government during the last 100 years (Swetnam and Baisan 1996a). As fire became less frequent, pinyon and juniper trees became denser and subsequent fires became more severe (Gori and Bate 2007).

These impacts altered stand dynamics differently depending on stand structure. Fire dynamics under current conditions are summarized below for the three major pinyon-juniper types (pinyon-juniper grass savanna/open woodland, pinyon-juniper shrub woodland, and pinyon-juniper forest) developed by Romme et al. (2003) using canopy structure, understory composition, and historical fire regime.

The fire regime for the pinyon-juniper grass savanna/ open woodland has a fire frequency that is significantly reduced and fire severity has greatly increased from pre-1900, from low-severity surface fires towards high-severity and stand-replacing crown fires. Tree density has increased and herbaceous biomass has decreased from historical conditions with active fire suppression and livestock grazing. Currently stands have some very old trees (>300 years) present but not numerous, but are typically dominated by many young trees (<150 years). This type may also occur on sites with more rock soil and less grasses. This type is outside Historical Range of Variation (HRV) for disturbance regime, structure and composition (Gori and Bate 2007).

The fire regime for the pinyon-juniper shrub woodland has a fire frequency that is reduced and fire severity is somewhat increased from pre-1900, from low to moderately frequent, high-severity stand-replacing fires and moderately frequent mixed-severity fires that likely maintain this type, toward less frequent, higher severity fires (Gori and Bate 2007). Tree density has increased and herbaceous biomass has decreased from historical conditions with active fire suppression and livestock grazing. Currently most stands have a variable mix of tree and shrubs with few or no very old trees (>300 years) present. With fire suppression, this type may be outside HRV for disturbance regime, and possibly for structure and composition as recent fires are likely more severe than historical fire in late 1800s (Romme et al. 2003).

The fire regime for the pinyon-juniper forest type still has infrequent, high-severity fires that are carried by tree crowns. The stand dynamics remain relatively stable with little change in density of tree or shrub and herbaceous layers. Currently stands have numerous very old trees (>300 years) present with a multi-aged structure. Active fire suppression and livestock grazing are thought to have had little impact on fire frequency and severity and the overstory structure and composition with this type remains within HRV for disturbance regime (Gori and Bate 2007).

Historic fuelwood cutting for mining and domestic use and fencepost cutting was common in stands of this system until the late 1800s, and is still common in Arizona, New Mexico and northern Mexico today (Bahre 1991, Bennett 1992). Although fuelwood harvesting had dramatic effects historically, its consequences were generally local and short-lived (Turner et al. 2003). More recently, chemical and mechanical treatments such as chaining and rotochopping have impacted age structure, tree density and cover of many pinyon-juniper woodlands with current demand for these products continuing to increase (Ffolliott et al. 1979, Gottfried 1987, Dick-Peddie 1993, Gottfried and Severson 1993).

Fragmentation from a variety of sources such as construction of roads and secondary homes has occurred in many areas of pinyon-juniper woodlands (Gori and Bate 2007). Additional roads from oil and gas exploration and development is important in some areas. The introduction of non-native species is a threat to this ecosystem and needs to be further investigated (Gori and Bate 2007). Non-native species invasion is an important issue in the Great Basin pinyon-juniper woodlands which has led to increased fire frequency and size in that type (Miller and Tausch 2001). In Mesa Verde National Park, invasive non-native species dominate pinyon-juniper woodland areas post-fire (Romme et al. 2003). Post-fire succession may be altered if invasive non-native species colonize and prevent native grasses and forbs from establishing (Floyd et al. 2006).

This system occurs in the Sierra Madre Occidentale and Sierra Madre Orientale of Mexico, Trans-Pecos Texas, southern New Mexico and Arizona, generally south of the Mogollon Rim. It occurs on the west side of the Sacramento Mountains but may transition into Southern Rocky Mountain Pinyon-Juniper Woodland (CES306.835) or Southern Rocky Mountain Juniper Woodland and Savanna (CES306.834) on the eastern side.