Location and General Description
This ecoregion is found in the Sierra Madre Oriental mountain range of eastern Mexico and southern Texas, and is dominated by pine-oak forests growing at altitudes between 1,000 and 3,500 m above sea level. This mountain range arose as a result of the upward folding of Cretaceous deposits, and resulted in a region of abrupt topography, with valleys, deep canyons and ravines that allow the persistence of a unique and diverse community of plants and animals. The mountains run north to south along the eastern half of Mexico, including the states of Coahuila, Tamaulipas, Nuevo León, San Luis Potosí, Queretaro and Guanajuato, and less significantly just over the Rio Grande in several isolated mountain massifs in southern Texas. The tallest peaks are Potosí (3,625 m) and Peña Nevada (3,480 m) (Jímenez et al. 1999).
The climate is temperate humid on the northeastern slope, and temperate subhumid on the western slope and highest portions of the mountain range. Pine-oak forests cover most of the region. However, the wettest portions house a community of cloud forests that constitute the northernmost patches of this vegetation in Mexico. The forests grow on soils derived from volcanic rocks that have a high content of organic matter. The soils of lower elevations are derived from sedimentary rocks, and some of them are formed purely of limestone (Cuanalo de la Cerda & Ojeda-Trejo 1989). In the northernmost portions of the ecoregion, the forests occur on irregular hummocks that constitute biological "islands" of temperate forest in the middle of the Chihuahuan Desert. To the south, from Nuevo León southward until Guanajuato and Queretaro, the ecoregion is more continuous along the main stem of the Sierra Madre Oriental. Average annual rainfall ranges from 250 to 300 mm in the north near Big Bend, Texas, U.S. and from 900 to 1,500 mm in the southern parts of the ecoregion near Nuevo León, Mexico (Perry 1991).
The dominant species include the pines Pinus nelsonii, P. cembroides, P. pseudostrobus, and P. arizonica, and the oaks Quercus castanea and Q. affinis. In mesic environments, the most common species are P. cembroides, and Juniperus deppeana, but in more xeric environments on the west slopes of the mountains, the endemic P. pinceana is more abundant although considered by Perry (1991) to be near endangered in status (Passini 1982). Nelson pine (P. nelsoni) is also endemic to this ecoregion and considered near to endangered in status, while P. greggii and P. patula are simply endemic (Perry 1991). The associations between Pinus and Quercus with other species vary depending on the altitude and humidity of the areas they inhabit along the Sierra Madre Oriental.
The Sierra Madre Oriental pine-oak forests represent an island of temperate environments surrounded by more humid and tropical ecoregions to the south, and xeric ecoregions to the north. This positioning has been a major factor contributing to the ecoregion’s diversity and high numbers of endemic species. From the small islands of temperate forests in the middle of the harsh Chihuahuan Desert, to the cloud forests of Tamaulipas on the east slope of the ecoregion, endemic and endangered species thrive at high altitudes throughout the mountains. The ecoregion is a center of diversification for the genus Quercus (Nixon 1993), and is also recognized as the area of highest diversity for the genus Agave (Tambutti et al. 1995).
The forests are considered a zone of remarkable zoological content (Gaona-Ramírez et al. 1990). The intermontane valleys and plains constitute bridges between the biotas of the Chihuahuan and Tamaulipas deserts (SEMARNAP 1997). This has contributed to the diversity of taxa of both deserts. The Bravo River also represents a biological corridor that connects the biotas of temperate and dry environments between Mexico and the U.S., as well as a trail used by black bears (Ursus americanus) that move from northeast Coahuila state to Chisos, in Big Bend National Park, U.S. In addition, some portions of the ecoregion in the northern part of the state of Coahuila are very important for the monarch butterfly (Danaus plexippus) migration. Another factor responsible for the biodiversity of this region is the fact that it is connected with the Sierra Madre Occidental pine-oak forests through the Trans-volcanic Mexican belt. McDonald (1993) has proposed that this connection allowed an important radiation and diversification of taxa of temperate environments in Mexico after they became separated from their North American counterparts.
Mammals of all types wander these rugged hills. Mule deer (Odocoileus hemionus), puma (Puma concolor), cliff chipmunk (Tamias dorsalis), collared peccari (Tayassu tajacu), coati (Nasua narica), jaguar (Panthera onca) and coyote (Canis latrans) are a few of the many diverse mammals that inhabit this ecoregion. Stattersfield et al.(1998) also recognize this ecoregion as an endemic bird area. The maroon-fronted parrot (Rhynchopsitta terrisi) (VU) and the Colima warbler (Vermivora crissalis) (nt) are endemic to the Sierra Madre Oriental ecoregion (Stattersfield 1998). The Colima warbler, however, does winter outside of the ecoregion on the Pacific slope of western Mexico (Stattersfield 1998). Wild turkeys (Meleagris gallapavo), peregrine falcons (Falco peregrinus) and golden eagles (Aquila chrsaetos) also inhabit this mountainous habitat.
Centuries of logging and cultivation have almost totally eliminated native pine-oak forests of the Sierra Madre Oriental. In general, the temperate forests of Mexico are considered a habitat in danger of extinction (Cárdenas-Hernández et al. 1994), mostly due to intensive human exploitation. Toledo et al. (1989) specify that while 60% of the temperate forests in Mexico remain intact, 37% of the pine-oak forests have been logged and converted to agricultural lands.
At least thirteen protected areas have been established in the ecoregion, and there have been proposals to create new areas connected to the existing ones. Overall, however, the region lacks enforcement of laws prohibiting activities such as wildlife extraction and logging. The Cumbres de Monterrey National Park is one of the largest in Mexico, covering 2,465 km2. However according to Stattersfield (1998) it is also poorly administered. The El Taray Sanctuary covers 3.6 km2 of the largest cliff nesting area of the maroon-fronted parrot, which protects about a quarter of the total breeding population (Stattersfield 1998). The destruction of the maroon-fronted parrot’s habitat has caused the decline of this population. El Cielo Biosphere Reserve at 2,400 m contains some endemic species of the Sierra Madre Oriental, including Quercus germana, Q. xalapensis and Ternstroemia sylvatica (Sosa et al. 1997). In the northern part of the ecoregion is Big Bend National Park in Texas with an area of 2,866 km2.
Types and Severity of Threats
Currently, principal threats include logging, resin extraction, and agricultural activities. Selective logging for income of residents in the area is taking place throughout the region. Cattle farming, hunting and road building are additional threats.
Justification of Ecoregion Delineation
These montane pine and oak forests of the Sierra Madre Oriental occur along ridge tops, high valleys, and isolated peaks and slopes in a patchwork distribution from the southern United States (Texas) to central Mexico (near Mexico D.F.) 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 Oriental 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