Physiognomy and Structure: This savanna and woodland macrogroup is composed of a typically short (3-15 m), widely spaced to closed canopy (5-60% cover) of evergreen, scale-leaved and/or broad-leaved trees. A sparse to dense shrub layer may be present and dominated by a variety of species such as evergreen broad-leaved oaks and rosette stem succulents. The more open-canopied stands often have moderate to high cover of perennial graminoids (>25% cover). Diverse forb species typically with low cover are often present. However, open woodlands lacking an understory tree because of substrate such as rock outcrops are also included in this macrogroup.

Floristics: This broadly defined Madrean savanna and woodland macrogroup has a typically short (3-15 m), open to moderately dense canopy of trees (5-60% cover) with an understory dominated by shrubs or grasses. The presence of Madrean pinyon, juniper or oak tree species is diagnostic of this macrogroup. Diagnostic trees include conifers Juniperus coahuilensis, Juniperus deppeana, Juniperus flaccida, Juniperus pinchotii, Pinus cembroides, Pinus discolor, Pinus remota, and oaks Quercus arizonica, Quercus emoryi, Quercus gravesii, Quercus graciliformis, Quercus grisea, Quercus oblongifolia, and, in Mexico, Quercus chihuahuensis and Quercus albocincta. At the northern end of the range, communities may be dominated or codominated by northern tree species such as Juniperus monosperma and/or Pinus edulis, but Madrean species will always be present as differential species. Juniperus deppeana trees have a broader range than this Madrean macrogroup, extending north into southern stands of ~Southern Rocky Mountain & Colorado Plateau Two-needle Pinyon - One-seed Juniper Woodland Macrogroup (M027)$$ as an incidental subcanopy tree. In higher elevation stands low cover of other trees may be present, such as Arbutus arizonica and Arbutus xalapensis, along with scattered taller pines such as Pinus arizonica, Pinus engelmannii, or Pinus leiophylla [see ~Madrean Montane Forest & Woodland Macrogroup (M011)$$]. The understory is variable, ranging from absent to dense shrub and/or herbaceous layers, with stands with open tree canopies (<20% cover) generally having a more abundant understory. If present, the shrub layer may be open to dense with chaparral, foothill and montane shrub species, or shrubby oaks. Characteristic species include Acacia angustissima, Arctostaphylos pungens, Ceanothus greggii, Cercocarpus montanus, Dasylirion leiophyllum, Dasylirion wheeleri, Garrya wrightii, Mahonia trifoliolata, Mimosa aculeaticarpa var. biuncifera, Mimosa dysocarpa, Nolina microcarpa, Purshia mexicana, Quercus grisea, Quercus mohriana, Quercus pungens, Quercus turbinella, Quercus toumeyi, Rhus trilobata, Yucca baccata, Yucca madrensis (= Yucca schottii), and Yucca torreyi. The herbaceous layer is typically diverse with variable cover, but is usually prominent, especially in interspaces between trees in open woodlands. Dominant species are typically warm-season perennial grasses such as Aristida spp., Bouteloua curtipendula, Bouteloua rothrockii, Digitaria californica, Hilaria belangeri, Leptochloa dubia, Muhlenbergia emersleyi, Muhlenbergia longiligula, Muhlenbergia pauciflora, Piptochaetium fimbriatum, Piptochaetium pringlei, and Schizachyrium cirratum, species typical of desert grasslands and steppe. Some of the widespread graminoid species are similar to those found in the southwestern Great Plains with Aristida purpurea, Bouteloua gracilis, and Eragrostis intermedia being most common.

Dynamics:  Dynamics are complicated by the diverse plant communities present in this macrogroup. The pinyon-juniper woodlands and savannas included in this macrogroup 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 macrogroup. However, the pinyon-juniper grass savanna and a new, ecologically similar type with tree canopy >10% cover (pinyon-juniper grass open woodland) have the greater aerial extent in the macrogroup (Landis and Bailey 2005, Gori and Bate 2007). 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 historic natural conditions (also called natural range of variability (NRV); for pre-1900 timeframe) are summarized as follows 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 has 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 has very infrequent, very high-severity fires that are carried by tree crowns. The stand dynamics are stable with multi-age tree canopy and with little change in shrub or herbaceous layers.

Pinyon and juniper stands in this macrogroup have been impacted by human activities over the last century. Historical fire regimes were disrupted following the introduction of livestock (and the 1890s drought). 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 1996b). 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 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 historic fire regime and adapted for our use as follows.

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 historic conditions with active fire suppression and livestock grazing. Currently stands have some very old trees (>300 years) present but not numerous, and 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 Historic 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 historic 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 maybe outside HRV for disturbance regime, and possibly for structure and composition as recent fires are likely more severe than historic fire in the 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 remaining within HRV for disturbance regime (Gori and Bate 2007).

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.

Madrean encinal stands included in this macrogroup also vary considerably under historic natural conditions in tree density ranging from very open woodlands and treed savannas (5-15% cover) with a perennial grass-dominated understory in uplands, to moderately dense oak woodlands (20-40% tree cover) in drainages and on north-facing slopes. The understory of good-condition stands generally has high cover of perennial grasses and low cover of shrubs such as Mimosa, and this good condition of the stand is maintained with frequent fires. Turner et al. (2003) documented a trend from more open woodlands and savannas to denser woodlands with higher cover of species of Juniperus and Prosopis over the last 150 years. Regeneration of oaks following disturbance is from resprouting rather than acorns because of the dry conditions (Germaine and McPherson 1998).

Although there is not much encinal-specific information on fire-return intervals (FRI) available, it is thought to be similar to adjacent ecosystems, primarily the semi-desert grassland (FRI of 2.5-10 years) (Wright 1980, Bahre 1985, McPherson 1995, Kaib et al. 1996) and the pine-oak woodlands (FRI of 3-7 years) (Wright 1980, Bahre 1985, Swetnam et al. 1992, McPherson 1995, Kaib et al. 1996, Swetnam and Baisan 1996b). Fire season in encinal 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). Post disturbance regeneration (such as after stand-replacing fire) mostly occurs from resprouting from trees roots. Successful regeneration from acorns is related to annual precipitation (Germaine and McPherson 1998). The understory of poor-condition stands with less frequent fires or experiencing extended drought may have significant shrub invasion by species of Arctostaphylos and Juniperus and reduction of perennial grass cover (Schussman 2006a).

Over the last century, the woody component in encinal has increased in density over time in the absence of disturbance such as fire (Burgess 1995, Gori and Enquist 2003, Turner et al. 2003, Schussman 2006a). This is correlated to a decrease in fire frequency that is related to a reduction of fine fuels that carry fire because of extensive livestock grazing. Frequent, stand-replacing fire was likely a key ecological attribute prior to 1890 (Wright 1980, Bahre 1985, McPherson 1995, Kaib et al. 1996). The oak woodlands and savannas included in this macrogroup are characterized by a strong perennial grass layer and are driven by many of the same ecological processes as semi-desert mixed grassland, primarily frequent fire and drought (USFS 2009). It is generally agreed that fire regime has been altered for encinal by passive fire suppression via removal of fine fuels through livestock grazing, as well as active suppression over the last 100 years. This has reduced the number of surface fires, permitting a buildup in woody fuels, resulting in increased fire severity when fires occur in encinal and adjacent vegetation types such as semi-desert grasslands and pine-oak woodlands across much or the southwestern U.S. and adjacent Mexico (Kaib et al. 1996, Swetnam and Baisan 1996). Reduced fire frequency is a disturbance of the natural fire regime and results in increased cover of woody plants (Barton 1999, Muldavin et al. 2002b, Gori and Enquist 2003, Turner et al. 2003). The increase in woody species in the Madrean encinal has changed species composition, in some areas, from oak-dominated woodlands or savanna to mesquite- and/or juniper-dominated woodlands (Turner et al. 2003).

Livestock grazing in encinal is currently a common practice in both the United States and Mexico, with grazing occurring in virtually all of Mexico''s and in roughly 75% of the United States'' oak woodlands (McPherson 1995). Livestock grazing can affect the structure and composition of Madrean oak woodlands, as well as soil structure and water infiltration (USFS 2009).

The introduction of the invasive non-native, perennial grasses Eragrostis lehmanniana and Eragrostis curvula has greatly impacted many semi-desert grasslands and encinal in this ecoregion (Cable 1971, Anable et al. 1992, Gori and Enquist 2003). Anable et al. (1992) and Cable (1971) found Lehmann lovegrass is a particularly aggressive invader and alters ecosystem processes, vegetation composition, and species diversity.

Historic fuelwood cutting for mining and domestic use and fencepost cutting was common in stands of this macrogroup in southeastern Arizona until the late 1800s, and is still common in Arizona 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).