The Sierra Madre Occidental boasts some of the richest biodiversity anywhere in North America, and contains about two thirds of the standing timber in Mexico. Twenty-three different species of pine and about 200 species of oak reside within the Sierra Madre Occidental pine-oak forests ecoregion. Many distinctive species have evolved here as a result of the landforms, altitude, temperature and rainfall. However, over harvesting of the forests in this area since the early part of this century has caused the extinction of the imperial woodpecker (the largest woodpecker on Earth) and has lead to the likelihood of several other species becoming extinct in this ecoregion, such as the Mexican gray wolf. Currently, all but 300,000 acres, or about 2 percent, of the original old-growth forest is gone. Location and General Description
This ecoregion occurs along the Sierra Madre Occidental, a rugged mountain range running from Rio Grande de Santiago, in the state of Jalisco, Mexico, north through the Mexican states of Sonora and Chihuahua, and into southern Texas and the Madrean Sky Islands of coniferous forests. These islands comprise approximately twenty-seven small mountain ranges in southern Arizona, southwestern Mexico, and northwestern Mexico. This ecoregion is very large, and encompasses the intersection of temperate and tropical influences.
Fluctuation in temperatures and rainfall occur due to the distance between the northern and southern most borders of the ecoregion, and to the great variations in elevation, which reaches more than 3,000 m on some parts of the southern escarpments. The heights of these mountains also cause differences in the conditions present on individual sides. For example, the western sides generally receive more rainfall and have a milder winter, although as a whole, the ecoregion is considered to have mild winters and wet summers (Brown 1994). Mean annual rainfall is around 553 mm, mostly falling in August. The temperature varies between extremes of –3 oC and 28 oC (WCMC 1990); some of the highest elevations in the area are snow covered year round.
Pine-oak forests grow on elevations between approximately 1,500 and 3,300 m, and occur as isolated habitat islands in northern areas within the Chihuahuan desert. Soils are typically deep, where the incline allows soil build up and derived from igneous material, although metamorphic rocks also form part of the soils in the west and northwest portions of the sierra. Steep-sloped mountains have shaped some portions of the Sierra, while others are dominated by their deep valleys, tall canyons and cliffs. These steep-sided cliffs have thinner soils limiting vegetation to chaparral types; characterized by dense clumps of Arctostaphylos pungens, Quercus potosinai and Q. rugosa (WCMC 1990). There are also areas of natural pasture with Arisitida spp., Panicum spp., Bromus spp., and Stevis spp.
The diversity in topography also accounts for the diversity of pine-oak communities: Pseudotsuga and Pinus constitute the biggest trees (50-150 cm dbh), and are abundant in the highest parts of the Sierra. A wildlife haven exists on the highest plateaus of the Sierras with pines as the largest trees. Unfortunately, these plateaus are the most severely degraded areas (Lammertink 1997). Pinus lumholtzii, growing at 1,900-2,400 m. above sea level, dominates the east slopes. Associations of Cupressus, Pseudotsuga, and Pinus are abundant near small rivers that form in areas close to canyons. In the drier portions of the sierra, the pine-oak forests gradually transform woodlands into what is known as oak-grassland (Rzedowski 1978). These communities represent an ecological transition between pine-oak forests and desert grasslands. Here, species like Quercus chihuahuensis, Q. grisea, Q. santaclarensis, Q. cordifolia and Q. emoryi, mark a transition zone between temperate and arid environments, growing in a sparse manner and with a well-developed herbaceous stratum resembling xeric scrub (Rzedowski 1978). Cacti are also part of these transition communities extending well into the woodlands. Some cacti species such as the cream cactus (Mammillaria gummifera), the pin cushion (M. orestera), and the hedgehogs (Echinocereus triglochidiatus and E. ledingii) are mainly centered in these biotic communities (Brown 1994). The dominant vegetation in the northernmost part of the ecoregion in the Madrean Sky Islands includes Pinus leiophulla, P. cembroides, P. ponderosa arizonica, Quercus hypodleucodes, Q. arizonica, Q. emoryi, Q. rugosa, Juniperus deppeana, and Arctostaphylos punges (Peet 1988).
Isolated from warmer environments by geological processes that produced the second largest mountain range in Mexico, the Sierra Madre Occidental pine-oak forests are characterized by a distinctive biota, and richness in endemic species. Escalante-Pliego et al. (1993), and Collar et al. (1992) recognized this as an important area for bird richness and bird endemism. Likewise, virtually all of the ecoregion is included in the Sierra Madre Occidental and trans-mexican range Endemic Bird Area (Stattersfield et al. 1998). Endemic bird species include the thick-billed parrot (Rhynchopsitta pachyrhyncha) which is in danger of extinction, with population estimates as low as 500 pairs (Lammertink1997), the tufted jay (Cyanocorax dickeyi), eared quetzal (Euptilptis neoxenus) and the green-striped brush finch (Atlapetes virenticeps) (Stattersfield 1998). Temperate and tropical influences converge in this ecoregion, forming a unique and rich complex of flora and fauna (Lammertink et al. 1997). Many other birds are found in this ecoregion including the green parakeet (Aratinga holochlora), eared trogon (Euptilotis neoxenus), coppery-tailed trogon (Trogon elegans), Mexican jay (Aphelocoma ultramarina), violet crowned hummingbird (Amazilia violiceps), spotted owl (Strix occidentalis), and golden eagle (Aguila chryaetos) (Brown 1994). Some species found only in higher montane areas are the Gould's turkey (Meleagris gallopavo meciacanaI, band-tailed pigeon (Columba fasiata), mexican chickadee (Parus sclateri) and hepatic tanager (Piranga flava) (Brown 1994).
This large ecoregion varies greatly in altitude, temperature and habitat types, allowing it to house a diversity of mammal species. The Mexican gray wolf (Canis lupus baileyi) and Mexican grizzly (Ursus horribilis), although considered by most to be extinct from this ecoregion, once roamed these mountains (Lammertink 1997). Mammals also present include white-tailed deer (Odocoileus virginianus), black bear (Ursus americanus), Buller’s chipmunk (Tamias bulleri), endemic Zacatecan deer mouse (Peromyscus difficilis), rock squirrel (Spernophilis variegatus), Zacatecas harvest mouse (Reithrodontomys zacatecae) and coati (Nasua nasua), to name a few (Brown 1994).
Reptiles and amphibians are also numerous in this ecoregion. There are at least three species of rattlesnakes including the rock, twin-spotted, and ridgenose rattlesnakes (Crotlaus lepidus, C. pricei, C. willardi). Clark's spiny (Sceloporus clarki), Yarrow's spiny (S. jarrovi), bunchgrass (S. scalaris), and striped plateau (S. virgatus) are just a few of the lizards in this ecoregion (Brown 1994). Along springs and streams the barking frog (Hylactophryne augusti) or the Tarahumara frog (Rana tamahumarae) may be found (Brown 1994).
Plant endemism is particularly high for a number of groups in the isolated portions of the ecoregion that occur as habitat islands within the Chihuahua desert. At least two endemic species of oak are found here, including Quercus carmenensis and Q. deliquescens (Nixon 1993). Pine and oak species richness is also greatest in these northern portions of the ecoregion. The Mexican state of Chihuahua, for example, is home to 15 species of Pinus and 25 of Quercus, representing 30% and 20% of Mexican pines and oaks, respectively (Styles 1993, Nixon 1993). It is also recognized as the area of highest diversity for the plant genus Agave in Mexico (Tambutti et al. 1995).
Among animals, roughly 10% of birds in the northern parts of the ecoregion are endemic. Similarly, about one-quarter of the reptiles and over half of the amphibians are endemic to these isolated ranges. In the winter, the number of birds increases due to the migration of thousands of ducks and geese that fly from the colder winter of the United States and Canada to the warmer forests of Mexico (Challenger 1998). These forests also host a great diversity of squirrels (Yensen & Valdés-Alarcón 1999). A number of important bird areas have recently been identified in this area, including Parte Alta del Rio Humaya, Pericos, Rio Presidio-Pueblo Nuevo, San Juan de Camarones, and Sistema de Islas Sierra Madre Occidental (Benitez 1999). In addition, several of the terrestrial priority regions recently identified by CONABIO overlap in the ecoregion, including Bavispe-El Tigre, Alta Tarahumara-Barrancas, and Rocahuachi-Nanaruchi (Arriaga 2000).
The original forests Sierra Madre Occidental pine-oak forests ecoregion have been almost completely eliminated. Logging of the forests started as early as 1880, and proceeded continuously through the 20th century, until recently. Only 0.61% of the original vegetation remains intact (Lammertink et al. 1997). Two protected areas exist in the region, but they do not come close to maintaining representative fragments of the distinct ecological conditions throughout the Sierra. Imperial woodpecker (Campephilus imperialis), the largest woodpecker in the world, gray wolf (Canis lupus), black bear (Ursus americanus) and mountain lion (Felis concolor) once inhabited these forests (Lammertink 1997), but are now almost certainly extinct or very close to it due to human activities such as massive logging operations and hunting (Robles Gil et al. 1993. Stattersfield 1998). Exploitation of dead trees for paper fabrication has also driven other species dependent on these trees for nesting or seed storage (i.e. thick-billed parrot - Rhynchopsitta pachirhynca) to near-extinction.
The only protected area in the Mexican Sierra Madre Occidental is la Michilía Biosphere Reserve, with an area of 350 km2 (Stattersfield et al. 1998). Some priority areas for conservation of the thick-billed parrot include the old-growth forests of El Carricito del Huichol in northern Jalisco, the Bufas in central-west Durango and Sierra Tabsco-Río Bavispe in northern Sonora (Lammertink et al. 1997). Some protected areas in the Madrean Sky Islands include the Chircahua National Monument and Wilderness Area, Galiuso Wilderness Area, Saguaro National Monument East, Rincon Wilderness Area, Huachuca Mountains Wilderness Area, Pusch Ridge Wilderness Area, Santa Teresa Wilderness Area, Pajarito Wilderness Area, and Gray Ranch.
Types and Severity of Threats
Deforestation caused by logging, overgrazing by livestock, and conversion of land for cultivation threatens both plants and wildlife of the Sierra Madre Occidental ecoregion. Logging provides income, and with two- thirds of all standing timber in Mexico located in this ecoregion, the threat of continued logging is very real (Galster 1996). With logging operations come roads to transport the felled trees, and small sporadically placed towns also have an impact on the area. Some areas are being more selectively logged, leaving the small trees to replenish the forests. These small trees however, do not survive due to exposure to the elements, which are usually shielded by the larger trees (Galster 1996). The forest is also cleared for cultivation of crops, including plants which yield illegal drugs such as opium, heroin and marijuana (Galster 1996). One of the after effects of these types of habitat destruction is erosion, which causes siltation and drying up of the river beds while preventing the infiltration of water to replenish ground water supplies. Some of the Madrean Sky Islands including Kit Peak, Mount Grahm and the Catalina Mountains have experienced major high-elevation development. The valley bottoms, lower slopes, and riparian zones at lower elevations in this area have been easy to access and develop, thus impeding movements of the fauna among the Sky Islands.
The present rate of deforestation also threatens the fauna of the ecoregion. The Mexican wolf (Canis lupus baileyi) is one of the most endangered species that inhabits this region. Its distribution once included the cold regions of Chihuahua and Durango, but has been reduced to small, inaccessible areas of the sierra. Hunting for food and simple sport are also contributors to the extinction and reduction of species populations including the extinction of the imperial woodpecker; Mexican wolves that have been almost eliminated from this ecoregion; and black bear whose numbers have been greatly reduced by hunting that was only banned about 15 years ago (Lammertink 1997). This reckless destruction of species is seriously lowering the biologic diversity of this ecoregion as it is compounding species loss from habitat destruction due to logging and removal of snag trees.
This ecoregion is also a main artery for the migration of monarch butterflies. On their way to spend winter in central Mexico these invertebrates use the Sierra Madre Occidental Mountains to catch the air currents which lift them high into the atmosphere making flying easier.
Justification of Ecoregion Delineation
These montane pine and oak forests of the Sierra Madre Occidental occur along ridge tops, high valleys, and isolated peaks and slopes in a patchwork distribution from the southern United States (Madrean Sky islands of Arizona) to central Mexico (Jalisco) and are host to a number of endemic species (see description above for details). Linework for this ecoregion follows the INEGI (1996) current landcover maps, encompassing all "pine-oak forests", "oak with pine forests", and "pine forest" classifications within the Sierra Madre Occidental region, as well as portions of "low open forest", "mesophyll montane forest", "low deciduous forest", "matorral", and agricultural activities which fall within these parameters. Classification and justification follow Rzedowski (19789). Linework was reviewed by experts during ecoregional priority setting workshops (CONABIO 1996 and 1997) in Mexico.
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Prepared by: Alejandra Valero, Jan Schipper, Tom Allnutt, and Christine Burdette
Reviewed by: In process