Sky Island High Mountain Conifer-Oak Forest

This ecological system occurs at the upper elevations in the Sierra Madre Occidental and Sierra Madre Oriental of Mexico with disjunct and limited occurrences at the highest elevations of the Chisos and Guadalupe mountains in Texas. In the U.S., it is restricted to north and east aspects at high elevations (1980-2440 m) in the Sky Islands (Chiricahua, Huachuca, Pinaleno, Santa Catalina, and Santa Rita mountains) and along the Nantanes Rim. It is more common in Mexico and does not occur north of the Mogollon Rim. The vegetation is characterized by large- and small-patch forests and woodlands dominated by Pseudotsuga menziesii, Abies coahuilensis, or Abies lowiana and Madrean oaks such as Quercus arizonica, Quercus emoryi, Quercus grisea, Quercus hypoleucoides, Quercus rugosa, and Quercus toumeyi. If Quercus gambelii is prominent in the shrub layer, then other Madrean elements are present. This system may include stands of Quercus gravesii woodlands. It is similar to Southern Rocky Mountain Dry-Mesic Montane Mixed Conifer Forest and Woodland (CES306.823) which typically lacks Madrean elements.

The vegetation is characterized by large- and small-patch forests and woodlands dominated by Pseudotsuga menziesii, Abies coahuilensis, or Abies lowiana (= Abies concolor var. lowiana), and Madrean oaks such as Quercus arizonica, Quercus emoryi, Quercus grisea, Quercus hypoleucoides, Quercus rugosa, and Quercus toumeyi. In Texas, the characteristic dominants of the system are Pseudotsuga menziesii and Pinus strobiformis, though Pinus ponderosa, Juniperus spp., and Pinus cembroides or Pinus edulis may also be present to common. The shrub and subcanopy are typically sparse and generally dominated by species from the canopy. In some areas, Quercus gambelii may form dense shrub patches. The herbaceous layer is typically dominated by graminoids, including species such as Blepharoneuron tricholepis, Festuca arizonica, Koeleria macrantha, Muhlenbergia pauciflora, Piptochaetium fimbriatum, and Poa fendleriana.

In Texas, this system occurs on Permian limestone in the Guadalupe Mountains, and in the Chisos Mountains, it primarily occurs on Tertiary igneous formations and associated colluvial and alluvial deposits from these formations. In the Chisos Mountains, it occurs on Igneous Hill and Mountain soils; in the Guadalupe Mountains in occurs on Victorio-Lorenz-Rock outcrop complex.

[from M011] Under historic natural conditions (also called natural range of variability, NRV), lower to mid-elevation stands in this macrogroups varied from open woodlands (10-20% cover) with pines dominating the overstory and perennial bunch grass dominating the understory to moderately dense woodlands (20-40% tree cover) with less dense herbaceous layer and more tree and shrub cover. Lower elevation tree line of pines is primarily controlled by dry season water stress (Barton 1993). Fire and drought are the primary disturbances of this ecosystem (USFS 2009).

Information on fire return intervals is varied depending on elevation zone with fires frequently starting at lower elevations and burning upslope into the montane zone. Lower montane elevation pine-oak stands had frequent, low intensity surface fires (mean fire return every 6-14 years) as a result of lightning ignitions primarily between early spring and summer (Bahre 1985, Swetnam et al. 1992, 2001, Kaib et al. 1996, Schussman and Gori 2006, Swetnam and Baisan 1996b). However, minimum fire-free periods of 20-30 years are necessary for pines to establish and become resistant (thick bark) to surface fires (Barton et al. 2001). More frequent fire favors oaks and other sprouting species over pines and other conifers, which can alter stand composition. Less frequent fire (FRI >50 years) results in more conifer recruitment and denser vegetation that can lead to higher intensity, mixed-severity and patches of stand-replacing fires that also favors oaks and other sprouting species (Danzer et al. 1996, Barton 1999, Barton et al. 2001, Schussman and Gori 2006). For stands with inclusions of Ponderosa Pine Woodland in the Madrean Conifer-Oak Forest and Woodland, the historic mean fire-return interval is similar (Smith 2006). In Arizona and New Mexico, Swetnam and Baisan (1996b) found the historic mean fire-return interval ranges from 2 to 17 years for fires scarring one or more trees, and 4 to 36 years for fires scarring between 10% and 25% of trees between the years of 1700 and 1900. However, in the more mesic subalpine fir communities a fire return interval of up to 400 years is not uncommon.

Herbivory by native herbivores in the Madrean montane conifer-oak forests and woodlands is variable in this type. For more open stands with grass-dominated understory herbivores are similar to semi-desert grasslands. Large herbivores include browsers like Coues' white-tailed deer (Odocoileus virginianus couesi), mule deer (Odocoileus hemionus), elk (Cervus elaphus), and rodents such as yellow nosed cotton rat (Sigmodon ochrognathus), white-throated wood rat (Neotoma albigula), southern pocket gopher (Thomomys umbrinus), Apache squirrel (Sciurus nayaritensis), Arizona gray squirrel (Sciurus arizonensis), porcupine (Erethizon dorsatum), Bailey's pocket mouse (Chaetodipus baileyi), and eastern cotton tail (Sylvilagus floridanus) are common in the Madrean pine-oak woodlands (Schussman and Gori 2006, Majka et al. 2007). Southwestern forest trees have been host to several species of insects, pathogenic fungi, and parasitic plants, however there are no accounts of historic insect outbreak, fungi or parasitic plant periodicity (Dahms and Geils 1997).

A good condition/proper functioning occurrence of Madrean Montane Conifer-Oak Forest and Woodland ecosystem is large and uninterrupted; the surrounding landscape is also in good condition with soils that have not been excessively eroded. Weeds are few. There is a diversity of stand age and size classes in response to a functioning natural fire regime. For the majority of the type (lower montane pine-oak woodlands) that is frequent (mean fire return every 6-14 years), low-intensity surface fires with occasional fire free periods of 20-30 years minimum to allow for conifers to establish and become resistant (thick bark) to surface fires. For upper montane conifer oak and mixed conifer forests, the historical fire regime would have less frequent fires, mixed-severity and occasional stand-replacing fires.

A poor condition/non-functioning occurrence is highly fragmented, or much reduced in size from its historic extent; the surrounding landscape is in poor condition either with highly eroding soils, many non-native species or a large percentage of the surrounding landscape has been converted to exurban development. Over time passive (livestock grazing) and active fire suppression would result high density of trees and heavy fuel loading that would lead to large, high-severity, stand-replacing fires in stands of the montane conifer-oak forests.

This system is found in the Sierra Madre Occidental and Sierra Madre Oriental of Mexico. In the U.S., it is restricted to north and east aspects at high elevations (1980-2440 m) in the Sky Islands (Chiricahua, Huachuca, Pinaleno, Santa Catalina, and Santa Rita mountains) and along the Nantanes Rim. It also has limited distribution in Texas on the highest mountain areas of the Guadalupe and Chisos mountains, but is lacking from high elevations of the Davis Mountains.