Sky Island Pine-Oak Forest

This ecological system occurs on mountains and plateaus in the Sierra Madre Occidental and Sierra Madre Oriental in Mexico, Trans-Pecos Texas, southern New Mexico and Arizona, generally south of the Mogollon Rim. These forests and woodlands are composed of Madrean pines (Pinus arizonica, Pinus engelmannii, Pinus leiophylla, or Pinus strobiformis) and evergreen oaks (Quercus arizonica, Quercus emoryi, or Quercus grisea) intermingled with patchy shrublands on most mid-elevation slopes (1500-2300 m elevation). Other tree species include Hesperocyparis arizonica, Juniperus deppeana, Pinus cembroides, Pinus discolor, Pinus ponderosa (with Madrean pines or oaks), and Pseudotsuga menziesii. Subcanopy and shrub layers may include typical encinal and chaparral species such as Agave spp., Arbutus arizonica, Arctostaphylos pringlei, Arctostaphylos pungens, Garrya wrightii, Nolina spp., Quercus hypoleucoides, Quercus rugosa, and Quercus turbinella. Some stands have moderate cover of perennial graminoids such as Muhlenbergia emersleyi, Muhlenbergia longiligula, Muhlenbergia straminea, and Schizachyrium cirratum. Fires are frequent with perhaps more crown fires than ponderosa pine woodlands, which tend to have more frequent surface fires on gentle slopes.

These forests and woodlands are composed of Madrean pines (Pinus arizonica, Pinus engelmannii, Pinus leiophylla, or Pinus strobiformis) and evergreen oaks (Quercus arizonica, Quercus emoryi, or Quercus grisea) intermingled with patchy shrublands on most mid-elevation slopes (1500-2300 m elevation). Other tree species include Hesperocyparis arizonica (= Cupressus arizonica), Juniperus deppeana, Pinus cembroides, Pinus discolor, Pinus ponderosa (with Madrean pines or oaks), and Pseudotsuga menziesii. Subcanopy and shrub layers may include typical encinal and chaparral species such as Agave spp., Arbutus arizonica, Arctostaphylos pringlei, Arctostaphylos pungens, Garrya wrightii, Nolina spp., Quercus hypoleucoides, Quercus rugosa, and Quercus turbinella. Some stands have moderate cover of perennial graminoids such as Muhlenbergia emersleyi, Muhlenbergia longiligula, Muhlenbergia straminea (= Muhlenbergia virescens), and Schizachyrium cirratum.

Texas occurrences are typically dominated by Pinus ponderosa (or Pinus arizonica in the Chisos), but oak species such as Quercus emoryi, Quercus grisea, Quercus x pauciloba, and Quercus gambelii may be present to codominant. The subcanopy and shrub layer are typically not dense and may include species of the canopy as well as Quercus hypoleucoides, Juniperus deppeana, Cercocarpus montanus, Holodiscus dumosus, Symphoricarpos spp., Nolina spp., Cylindropuntia imbricata, and Mimosa aculeaticarpa var. biuncifera. Pinus cembroides and, in the Guadalupe Mountains, Pinus edulis becomes a common component, particularly at the lower elevational limits of this type and in more xeric situations. The herbaceous layer is typically dominated by graminoids, including Andropogon gerardii, Blepharoneuron tricholepis, Bothriochloa barbinodis, Bothriochloa laguroides ssp. torreyana, Bouteloua curtipendula, Bouteloua gracilis, Bouteloua hirsuta, Eragrostis intermedia, Hesperostipa neomexicana, Heteropogon contortus, Koeleria macrantha, Muhlenbergia dubia, Muhlenbergia emersleyi, Muhlenbergia montana, Muhlenbergia pauciflora, Muhlenbergia rigida, Panicum bulbosum, Piptochaetium fimbriatum, Schizachyrium cirratum, and Schizachyrium scoparium.

This system is found on mountains and plateaus, on gently rolling landscapes or rugged slopes. In the Davis Mountains of Texas, it occurs on Tertiary igneous substrates, but may also occur on sandstone and limestone substrates, such as in the Guadalupe Mountains region. Soils are often rocky but also include mountain loams.

[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, Trans-Pecos Texas, southern New Mexico and Arizona, generally south of the Mogollon Rim.