Sierra Madre Occidental pine-oak forests
The Sierra Madre Occidental pine-oak forests ecoregion 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. This ecoregion is classified as an element of the Tropical and Subtropical Coniferous Forests biome. Many distinctive species have adapted here as a result of the rugged topography, altitude, temperature and rainfall. A total of 648 vertebrate taxa are found in this ecoregion.
Over-harvesting of the forests in this area since the early 1900s has caused the extinction pressure to 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 two percent, of the original old-growth forest is destroyed.
Location and general depiction
This ecoregion occurs disjunctively at higher elevations 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 vegetative sky islands comprise approximately twenty-seven small mountain ranges in southern Arizona in the USA, as well as western 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 3000 meters 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. Mean annual rainfall is around 553 millimeters, mostly falling in August. The temperature varies between extremes of –3 ° and 28 ° Celsius; some of the highest elevations in the ecoregion are snow-covered all year around.
Pine-oak forests typically grow on elevations between approximately 1500 and 3300 meters (m) in this part of the world, 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 Mexican Manzanita (Arctostaphylos pungens), Quercus potosina and Netleaf Oak (Q. rugosa). There are also zones of natural pasture, with grasses from the genera Arisitida, Panicum, Bromus and Stevis.
The extreme variations in topography also accounts for the diversity of pine-oak communities: Pseudotsuga and Pinus genera constitute the largest trees (50 to 150 centimeters diameter at chest height), and are abundant in the highest parts of the Sierra Madre. A wildlife haven exists on the highest plateaus of the Sierra with pine taxa as the tallest trees. Unfortunately, these plateaus are the most severely degraded areas. Lumholtz's Pine (Pinus lumholtzii), growing at altitudes of 1900 to 2400 m above sea level, dominates the eastern versant. Associations of Cupressus, Pseudotsuga, and Pinus are abundant near small rivers that form in the many canyons.
In the more arid parts of the Sierra Madre, the pine-oak forests gradually transform into an oak-grassland vegetative association. Such communities represent an ecological transition between pine-oak forests and desert grasslands.. Here, species such as Chihuahuan Oak (Quercus chihuahuensis), Shin Oak (Q. grisea), Q. striatula and Emory Oak (Q. emoryi), mark a transition zone between temperate and arid environments, growing in a sparse fashion and with a well-developed herbaceous stratum resembling xeric scrub. Cacti are also part of these transition communities extending well into the woodlands. Some cacti species such as the Little Nipple Cactus (Mammillaria heyderi macdougalii), Greenflower Nipple Cactus (M. viridiflora), Mojave Mound Cactus (Echinocereus triglochidiatus), and Leding's Hedgehog Cactus (E. fendleri var. ledingii) are chiefly centered in these biotic communities. The dominant vegetation in the northernmost part of the ecoregion in the Madrean Sky Islands includes Chihuahua Pine (Pinus leiophylla), Mexican Pinyon (P. cembroides), Arizona Pine (P. arizonica), Silverleaf Oak (Quercus hypoleucoides), Arizona White Oak (Q. arizonica), Emory Oak (Q. emoryi), Netleaf Oak (Q. rugosa), Alligator Juniper (Juniperus deppeana), and Mexican Manzanita (Arctostaphylos pungens).
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 considerable richness of endemic species. There are a number of special status taxa that are found in the Sierra Madre occidental pine-oak forests, denoted variously as Near Threatened (NT), Vulnerable (VU), Endangered (EN), or Critically Endangered (CR).
Among fauna, approximately ten percent 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. These forests also host a great diversity of squirrels. A number of Important Bird Areas (IBAs) have 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. In addition, several of the terrestrial priority regions identified by CONABIO overlap in the ecoregion, including Bavispe-El Tigre, Alta Tarahumara-Barrancas, and Rocahuachi-Nanaruchi.
Escalante-Pliego et al., and Collar et al. 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. Endemic bird species include the Thick-billed Parrot (Rhynchopsitta pachyrhyncha EN) which is in danger of extinction, with population estimates as low as 500 pairs; the Tufted Jay (Cyanocorax dickeyi NT), Eared Quetzal (Euptilptis neoxenus NT) and the Green-striped Brush Finch (Buarremon virenticeps). Temperate and tropical influences converge in this ecoregion, forming a unique and rich complex of flora and fauna. Many other birds are found in this ecoregion including the Green Parakeet (Aratinga holochlora), Eared Trogon (Euptilotis neoxenus NT), Coppery-tailed Trogon (Trogon elegans), Grey-breasted Jay (Aphelocoma ultramarina), Violet-crowned Hummingbird (Amazilia violiceps), Spotted Owl (Strix occidentalis NT), and Golden Eagle (Aguila chryaetos). Some species found only in higher montane areas are the Gould's Wild Turkey (Meleagris gallopavo mexicana), Band-tailed Pigeon (Patagioenas fasciata), Mexican Chickadee (Poecile sclateri) and Hepatic Tanager (Piranga flava).
This large ecoregion varies greatly in altitude, temperature and habitat types, allowing it to house a diversity of mammal species. The Sierra Madre Mantled Ground Squirrel (Spermophilus madrensis NT) is an endemic to the Sierra Madre Occidental pine-oak forests, restricted to southwestern Chihuahua, Mexico. The Mexican Gray Wolf (Canis lupus baileyi) and Mexican Grizzly Bear (Ursus horribilis), although considered by most to be extinct from this ecoregion, once roamed these mountains. Mammals also present include White-tailed Deer (Odocoileus virginianus), American 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 set forth a subset of mammals present.
Reptiles and amphibians are also numerous in this ecoregion. Fox´s Mountain Meadow Snake (Adelophis foxi) is an endemic taxon to the ecoregion, only observed at the type locality at four kilometers east of Mil Diez, about 3.2 kilometers west of El Salto, in southwestern Durango, Mexico. There are at least six species of rattlesnakes including the Mexican Dusky Rattlesnake (Crotalis triseriatus), Mojave Rattlesnake (C. scutulatus), Rock Rattlesnake (C. lepidus), Western Diamondback Rattlesnake (C. atrox), Twin-spotted Rattlesnake (C. pricei), and Ridgenose Rattlesnakes (C. willardi). Clark's Spiny Lizard (Sceloporus clarkii) and Yarrow's Spiny Lizard (S. jarrovii), Bunchgrass Lizard (S. scalaris), and Striped Plateau Lizard (S. virgatus) are several of the lizards found in the Sierra Madre Occidental pine-oak forests.
Along springs and streams the Western Barking Frog (Craugastor augusti) and the Tarahumara Frog (Rana tamahumarae) are two anuran taxa occurring in the ecoregion. Other anuran taxa found here include: Bigfoot Leopard Frog (Lithobates megapoda), Northwest Mexico Leopard Frog (Lithobates magnaocularis) and the Blunt-toed Chirping Frog (Eleutherodactylus modestus VU). The Sacramento Mountains Salamander (Aneides hardii) is an endemic salamander found in the Sierra Madre Occidental pine-oak forests, restricted to the Sacramento Mountains, Capitan Mountains, and Sierra Blanca in Lincoln and Otero Counties within southern New Mexico, USA.
Plant endemism is particularly high for a number of groups in the isolated portions of the ecoregion that occur as habitat islands within the Chihuahuan Desert. At least two endemic species of oak are found here, including the Mexican Oak (Quercus carmenensis) and Q. deliquescens, the latter known solely from the valley of Río Concho and north along the Río Grande River. 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 fifteen species of Pinus and 25 of Quercus, representing 30 percent and 20 percent of Mexican pines and oaks, respectively. Chihuahua is also recognized as the area of highest diversity for the plant genus Agave in Mexico..
The original forests of the 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 percent of the original vegetation remains intact. 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 CR), the largest woodpecker in the world; Gray Wolf (Canis lupus); American black Bear (Ursus americanus); and Mountain Lion (Felis concolor) once inhabited these forests, but are now almost certainly extinct or very close to it due to human activities such as massive logging operations and hunting. Exploitation of dead trees for paper fabrication has also driven to near extinction other species dependent on these trees for nesting or seed storage such as the Thick-billed Parrot (Rhynchopsitta pachirhynca).
The only protected area in the Mexican Sierra Madre Occidental is la Michilía Biosphere Reserve, with an area of 350 km2. 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. Some protected areas within 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..
Ecological threat profile
Deforestation caused by logging, overgrazing by livestock, and conversion of land for cultivation threatens both plants and wildlife of the Sierra Madre Occidental ecoregion. There are also ongoing threats to this ecoregion from trampling of portions of the habitat by use for illegal drug trafficking and illegal immigration corridor; however, due to the higher elevations here compared to the desert floor, most of the latter impacts of illegal use are occurring on the desert floor. Due to the remoteness and ruggedness of the ecoregion topography, law enforcement on both sides of the border is weak.
Logging provides income, and with two-thirds of all standing timber in Mexico located in this ecoregion, the threat of continued logging by indigenous peoples is quite real. 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. However, the small trees do not generally survive due to exposure to the elements, which are usually shielded by the larger trees. The forest is also cleared for cultivation of crops, including plants which yield illegal drugs such as opium, heroin and marijuana. 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 groundwater supplies. Some of the Madrean Sky Islands including Kit Peak, Mount Graham and the Catalina Mountains have experienced major localized 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 faunal movement 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 around twenty years ago. This reckless destruction of species is clearly reducing 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 utilize the Sierra Madre Occidental Mountains to catch the air currents which lift them high into the atmosphere, allowing flight with less biological energy expenditure.
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 USA (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. Linework was reviewed by experts during ecoregional priority setting workshops in Mexico.
- L. Arriaga, J.M. Espinoza, C. Aguilar, E. Martínez, L. Gómez y E. Loa (coordinadores). 2000. Regiones terrestres prioritarias de México. Escala de trabajo 1:1 000 000. Comisión Nacional para el Conocimiento y uso de la Biodiversidad, México.
- Brown, David E. ed. 1994. Biotic Communities: Southwestern United States and Northwestern Mexico. University of Utah Press, Salt Lake City, Utah. ISBN: 0874804590
- Benitez, H., C. Arizmendi y L. Marquez. 1999. Base de datos de las AICAS. CIPAMEX, CONABIO, FMCN y CCA, Mexico.
- A. Challenger. 1998. Utilización y conservación de los ecosistemas terrestres de México. Pasado, presente y futuro. Conabio, IBUNAM y Agrupación Sierra Madre, México.
- Collar, N.J., Gonzaga, L.P., Krabbe, N., Madroño-Nieto A., Naranjo, L.G., Parker, T.A. & Wege, D.C. 1992. Threatened birds of the Americas. The ICBP/IUCN Red Data Book, Part 2. Third edition. International Council for Bird Preservation, Cambridge, England. ISBN: 1560982675
- CONABIO Workshop, 17-16 September, 1996. Informe de Resultados del Taller de Ecoregionalización para la Conservación de México.
- CONABIO Workshop, Mexico, D.F., November 1997. Ecological and Biogeographical Regionalization of Mexico.
- Escalante-Pliego, P., Navarro, A.G. & Peterson, A.T. 1993. Un análisis geográfico, ecológico e histórico de la diversidad de aves terrestres de México. T.P. Ramamoorthy, R. Bye, A. Lot, & J. Fa, editores. Diversidad Biológica de México. Orígenes y Distribución. Instituto de Biología, UNAM, Mexico.
- Galster, Geoff (1996) Mexican Deforestation in the Sierra Madre. TED Case Studies. 5:2.
- INEGI Map (1996) Comision Nacional Para el Conocimiento y Uso de la Biodiversidad (CONABIO) habitat and land use classification database derived from ground truthed remote sensing data Insitituto Nacional de Estastica, Geografia, e Informática (INEGI). Map at a scale of 1:1,000,000.
- Lammertink, J.M., Rojas-Tomé, J.A., Casillas-Orona, F.M. & Otto, R.L. 1997. Situación y conservación de los bosques antiguos de pino-encino de la Sierra Madre Occidental y sus aves endémicas. Consejo Internacional para la preservación de las aves, Sección Mexicana, D.F. México.
- Nixon, K.C. 1993. El género Quercus en México. T.P. Ramamoorthy, R. Bye, A. Lot, & J. Fa, editores. Diversidad Biológica de México. Orígenes y Distribución. Instituto de Biología, UNAM, Mexico.
- Peet, R. K. 1988. Forests of the Rocky Mountains. In: M. G. Barbour and W. D. Billings, editors. North American Terrestrial Vegetation. Cambridge University Press, Cabridge, England. ISBN: 0521559863
- Ricketts, Taylor H., Dinerstein, Eric, Olson, David M., Loucks, Colby J., et al. 1999. Terrestrial Ecoregions of North America. Island Press Inc, Washington D.C., USA. ISBN: 1559637226
- P. Robles-Gil, Ceballos, G. & Eccardi, F. 1993. Mexican Diversity of Fauna. Cemex & Sierra Madre, México.
- J. Rzedowski. 1978. Vegetación de Mexico. Editorial Limusa. Mexico, D.F., Mexico.
- J. Rzedowski. pers.comm. at CONABIO Workshop, 17-16 September, 1996. Informe de Resultados del Taller de Ecoregionalización para la Conservación de México.
- Stattersfield, A.J., Crosby, M.J., Long, A.J., and Wege, D.C. 1998. Endemic Bird Areas of the World: Priorities for Biodiversity Conservation. BirdLife International, Cambridge, UK. ISBN: 0946888337
- B.T. Styles. El género Pinus: su panorama en México. T.P. Ramamoorthy, R. Bye, A. Lot, & J. Fa, editores. Diversidad Biológica de México. Orígenes y Distribución. Instituto de Biología, UNAM, Mexico.
- T.W. Swetnam and P.M. Brown. 1992. Oldest known conifers in the Southwestern United States: Temporal and Spatial patterns of Maximum Age, In M.R. Kaufmann, W.H. Moir, and R.L. Bassett, eds., Old-Growth Forests in the Southwest and Rocky Mountain Regions, Proceedings of a Workshop. USDA Forest Service General Technical Report RM 213: 24-38
- M. Tambutti, Silva, A., García-Mendoza, A. & Eguiarte, L. 1995. Patrones de distribución geográfica del género Agave: ¿posible hipótesis histórica? XIII Congreso Mexicano de Botánica: Diversidad vegetal de México. Cuernavaca, Morelos, del 5 al 11 de noviembre de 1995 Libro de resúmenes. Universidad Autónoma del Estado de Morelos y Sociedad Botánica de México, México.
- E. Yensen & Valdés-Alarcón, M. Family Sciuridae. In: Alvarez-Castañeda, S.T. & Patton, J.L. 1999. Mamíferos del noroeste de México. CIBNOR, México.
- WCMC. 2000. Data Sheet on Reserva de la Biosfera La Michilia.
Disclaimer: This article contains some information that was originally published by the World Wildlife Fund. Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the World Wildlife Fund should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.