The vast Mexican plateau is flanked on the west by the Sierra Madre Occidental, on the east by the Sierra Madre Oriental, and to the south by the Sierra Madre Sur.

These ranges reflect rich plant and animal diversity resulting from their proximity to the tropics and the island-like character resulting from their tempestuous geologic origins.

Many distinctive species have evolved here because mechanisms such as landform, altitude, temperature, and rainfall have created conditions for genetic isolation.

These ranges are very important as corridors of dispersal for  tropical plants and animals testing their tolerance for cooler temperatures and drier climates. 

 Oak woodland vegetation is widely distributed at elevations above desert, grassland, thornscrub, or tropical deciduous forest, but below pine-oak woodland or pine forest.  The species composition and tree density in oak woodland changes both with elevation and latitude.  Although these oak zones have been called Madrean Evergreen Woodland (Brown 1982), many of the oaks and associated species are drought-deciduous during the late spring dry season.  Autumn colors associated with failing leaves in temperate regions are seen in our region during the pre-summer drought.

 Extensive areas in the northern part of the region are dominated by open woodlands of Emory oak or bellota (Quercus emoryi).  The acorns are harvested in considerable quantity in northern Sonora in early summer and sold locally.  This is one of the few remaining commercial, wild food harvests in the region.  The acorns are eaten fresh and are often consumed in cantinas-the floors becoming littered with the empty shells.

Emory oak, Mexican blue oak (Q. longifolia), and Arizona oak (Q. arizonica) are the most common low-elevation oaks in the northern part of the our region.  At lower elevations these oak zones border grassland or desertscrub.  There is sometimes a broad ecotone between oak woodland and grassland where the oaks become widely spaced and grasses predominate.  Such areas have been termed oak-grassland or oak-savanna.  In mountains in southeastern and east-central Sonora, oak woodland sometimes occurs as islands on acidic, hydrothermically altered soils within tropical deciduous forest.  The ecotone between these two plant communities is often only a few meters.

Oak woodland in southeastern Sonora and Southwestern Chihuahua, called Oak Forest by Gentry (1942), shows considerable tropical affinity, At its lower limits it borders tropical deciduous forest.  Their boundaries are often remarkably well defined, apparently maintained by fire.  Across the Rio Mayo and Rio Fuerte mountain drainages it is common to see low fires creeping almost harmlessly through dry grasses, forbs, and leaf litter among the barren oaks in May and June.  These fires destroy small TDF trees and shrubs but not the oaks and their associated vegetation.

 The southern oak woodland is host to a rich array of subtropical or Mexican oak species.  There is considerable elevational and habitat zonation among the diverse oaks.  Many of the oaks in southeastern Sonora and nearby southwestern Chihuahua are strikingly large-leaved (e.g., the hand-basin oak, Q. tarahumara) as compared to those of the northern part of the region.  The southern oaks often support tropical epiphytes such as bromeliads (Tillandsia spp.) and orchids (e.g., Encyclia microbulbon, Laelia autumnalis, and Oncidium cebolieta).

 Extensive areas of pine-oak woodland occur along the east side of the continental divide in western Chihuahua.  Along the western slope of the Sierra Madre Occidental the climate is generally somewhat wetter, with presumably milder winter temperatures, resulting in a more diverse flora with more tropical elements including Apache pine (Pinus engelmannii), Durango pine (P. durangensis), egg-cone pine (P. oocarpa), pino chino (P. herrerae), and Mexican tropical-montane oaks.

Towards southeastern Sonora and adjacent Chihuahua the pine-oak woodland is floristically and structurally akin to the Mexican pine-oak woodland of central and southern Mexico.

Pine-oak woodland is continuous with oak woodland at lower elevations.  In pine-oak woodland the pines form the overstory while the oaks generally form an understory.  There are extensive areas of pine-oak woodland in the mountains of our region.  Pine-oak woodland is included within the concept of Madrean Evergreen Woodland (Brown 1982), and the pine forest has been called Madrean Montane Conifer Forest (Brown 1982).  For our purposes of this study it is not practical to distinguish pine-oak woodland from pine forest.  Especially in the southern part of our region oaks are a major part of the forests containing pines.  The abundance of oaks may be in part a consequence of overharvesting of pines.  However, especially in the northern part of the region a distinctive pine forest is distinguishable.  In these  communities Douglas Fir (Pseudotsuga menziesii) is often locally common in an otherwise pinedominated forest, thus blurring the boundary with mixed conifer forest.

At higher elevations within the pine-oak zones the pines become increasingly conspicuous and the tree density increases so that the vegetation could be called forest rather than woodland.  Pine forest is characteristically dominated by one species of pine, usually Arizona pine (Pinus ponderosa var. arizonica), ponderosa pine (P. ponderosa var. scopulorum, or white pine (P. strobiformis), with scattered individuals or small groups of oaks, especially Gambel oak (Q. gambelii) and net-leaf oak (Q. rugosa).  Gambel oak is the
 only winter-deciduous oak in our region.  Pine forest is more widespread in Chihuahua and Durango than in Sonora.  Ponderosa pine replaces Arizona pine at the higher elevations in Chihuahua and on the northernmost sky islands.  These closely-related pines can be found intermixed in the Santa Catalina Mountains in southern Arizona.  Mountains ranges to the south have only Arizona pine, while the ranges to the north have only ponderosa pine.

Mixed conifer forest is restricted to the highest mountain tops.  Winters are cold and summers cool and moist.  It is most extensive in the northern sky islands and at the highest elevations in Chihuahua.  Southward in Sonora, mixed conifer forest occurs in extremely limited areas on northfacing slopes and riparian canyons on north slopes.  Northeastern Sonora and adjacent Chihuahua support mixed conifer forests at elevations mostly above 2135 m (7000 ft).  Mixed conifer forest barely extends into southeastern Sonora from Chihuahua above 2100 m (6890 ft) in the upper reaches of the Rio Mayo Drainage.  Because there are more extensive areas of higher elevation in Chihuahua the mixed conifer forest is more common there.

Three coniferous genera, Abies (fir), Pinus, and Pseudotstiga (Douglas fir), define this vegetation.  These trees are commercially valuable for lumber.  Most of the old growth forest has been logged, but in some places it is recovering from extensive logging in the mid-twentieth century, Most of the broadleaf (dicot) trees found here are winter-deciduous, e.g., Gambel oak (Quercus gambelii), capulin or wild cherry (Primus serotina), ash (Fraxinus papillose), aspen (Populus tremtzloides), and New Mexico locust (Robinia
 neomexicana).

Riparian canyons are shaded with tall forests that may include big-tooth maple (Acer grandidentatum) and alder (Alnus obgngifolia) sometimes towering to 20 meters or more in height.  The two highest sky island peaks, the Pinaleno and Chiricahua mountains in southeastern Arizona, support spruce-fir forest (Picea and Abies) as do a few localities on cold, north-facing slopes at the highest elevations in southwestern Chihuahua.