Sierra Madre Oriental pine-oak forests
Estanzuela Park, Monterrey, Mexico. Source: Colby Loucks
The Sierra Madre Oriental pine-oak forests contains a very diverse community of endemic and specialized species of plants, mammals, reptiles and amphibians. This ecoregion is classified as an element of the Tropical and Subtropical Coniferous Forests biome. These high mountains run north to south, beginning in the USA and ending in Mexico. The Sierra Madre Oriental pine-oak forests are a highly disjunctive ecoregion, owing to the fact that they are present only at higher elevations, within a region with considerable expanses of lower elevation desert floor.
The variation accumulated in this distance, between its ends and elevation ranges, increases the diversity of habitats and species present. For example, habitats in the north, near Big Bend, Texas are arid or desert-like with the influences of the Chihuahuan Desert; in the south, rainfall increases greatly, creating tropical forest communities in the higher elevations. The orientation of this mountain chain with the trans-Mexican volcanic belt, connecting it to the Sierra Madre Occidental, has allowed these ecoregions to share species, yet they are slightly different as they adapt to better fit their unique abiotic features.
Location and general depiction
Image Source: World Wildlife Fund
This ecoregion lies within the Sierra Madre Oriental mountain range of eastern Mexico and southern Texas, and is dominated by pine-oak forests growing at altitudes between 1000 and 3500 meters (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 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 highest of the peaks are Potosí (3625 m) and Peña Nevada (3480 m).
The climate is temperate humid on the northeastern slope, and temperate sub-humid on the western slope and highest portions of the mountain range. Pine-oak forest habitat covers most of the region, even though most of the primary forest has been destroyed or degraded. 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. 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 mainstem of the Sierra Madre Oriental. Average annual rainfall ranges from 250 to 300 millimeters (mm) in the north near Big Bend, Texas, USA and from 900 to 1500 mm in the southern portions of the ecoregion near Nuevo León, Mexico.
Dominant tree species include the pines: Nelson's Pine (Pinus nelsonii), Mexican Pinyon (P. cembroides), Smooth-bark Mexican Pine (P. pseudostrobus), and Arizona Pine (P. arizonica'); and the oaks Quercus castanea and Q. affinis. In mesic environments, the most common species are P. cembroides, and Alligator Juniper (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 to be near endangered in status. Nelson pine (P. nelsoni) is endemic to this ecoregion and considered near to endangered in status, while Gregg's Pine (P. greggii) and Jelecote Pine (P. patula) are endemic, but not threatened. 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.
Biodiversity characteristics
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. For example, there are a total of 648 vertebrate species recorded within the Sierra Madre Oriental pine-oak forests. 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, and is also recognized as the area of highest diversity for the genus Agave.
Maroon-fronted parrot (Rhynchopsitta terrisi), Sierra Madre Oriental, Mexico (Photograph by Colby Loucks) The forests are considered a zone of remarkable zoological biodiversity. The intermontane valleys and plains constitute biological corridors between the biota of the Chihuahuan and Tamaulipas Deserts. This has contributed to the diversity of taxa of both deserts. The Bravo River also represents a biological corridor that connects the biota of temperate and dry environments between Mexico and the USA, as well as a trail used by the American Black Bear (Ursus americanus) that move from northeast Coahuila state to Chisos, in Big Bend National Park, USA. 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 Mexican trans-volcanic belt. McDonald 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
Cliff Chipmunk. Source: Katja Schulz /EoL Many mammalian species wander these rugged hills. Mule Deer (Odocoileus hemionus), Puma (Puma concolor), Cliff Chipmunk (Tamias dorsalis), Collared Peccary (Tayassu tajacu), Coati (Nasua narica), Jaguar (Panthera onca) and Coyote (Canis latrans) are a few of the many diverse mammals that inhabit this ecoregion. Some threatened mammals found in the ecoregion are: Bolaños Woodrat (Neotoma palatina VU); Diminutive Woodrat (Nelsonia neotomodon NT), known chiefly from the western versant of the Sierra Madre; Chihuahuan Mouse (Peromyscus polius NT); and Mexican Long-nosed Bat (Leptonycteris nivalis EN).
Reptiles
A considerable number of reptilian taxa are found in the Sierra Madre Oriental pine-oak forests, including three endemic snakes: Ridgenose Rattlesnake (Crotalus willardi); Fox´s Mountain Meadow Snake (Adelophis foxi); and the Longtail Rattlesnake (Crotalus stejnegeri VU), restricted to the central Sierra Madre. An endemic skink occurring in the ecoregion is the Fair-headed Skink (Plestiodon callicephalus). The Striped Plateau Lizard (Sceloporus virgatus) is endemic to the ecoregion. The Sonoran Mud Turtle (Kinosternon sonoriense VU) is found in the ecoregion and ranges from southwestern New Mexico south to northwestern Chihuahua.
Amphibians
Madrean Treefrog. Source: Ron Savage /EoL The following anuran taxa occur in the Sierra Madre Oriental pine-oak forests: Red-spotted Toad (Anaxyrus punctatus); Cane Toad (Rhinella marina); Elegant
Narrow-mouthed Toad (Gastrophryne elegans); New Mexico Spadefoot Toad (Spea multiplicata); Sinaloa Toad (Incilius mazatlanensis); Pine Toad (Incilius occidentalis); Southwestern Toad (Anaxyrus microscaphus); Woodhouse's Toad (Anaxyrus woodhousii); Great Plains Narrowmouth Toad (Gastrophryne olivacea); Great Plains Toad (Anaxyrus cognatus); Plateau Toad (Anaxyrus compactilis); Texas Toad (Anaxyrus speciosus); Sonoran Desert Toad (Incilius alvarius), found only at lower ecoregion elevations here; Rana-ladrona Silbadora (Eleutherodactylus teretistes); Sabinal Frog (Leptodactylus melanonotus); Mexican Leaf Frog (Pachymedusa dacnicolor); Montezuma Leopard Frog (Lithobates montezumae); Yavapai Leopard Frog (Lithobates yavapaiensis); Northwest Mexico Leopard Frog (Lithobates magnaocularis); Bigfoot Leopard Frog (Lithobates megapoda), who generally breeds in permanent surface water bodies; Mexican Cascade Frog (Lithobates pustulosus); Tarahumara Frog (Lithobates tarahumarae VU); Western Barking Frog (Craugastor augusti); Lowland Burrowing Frog (Smilisca fodiens); Taylor's Barking Frog (Craugastor occidentalis); Blunt-toed Chirping Frog (Eleutherodactylus modestus VU), found only at the very lowest elevations of the ecoregion; Shiny Peeping Frog (Eleutherodactylus nitidus); California Chorus Frog (Pseudacris cadaverina); Rio Grande Frog (Lithobates berlandieri); Madrean Treefrog (Hyla eximia); Mexican Treefrog (Smilisca baudinii); Dwarf Mexican Treefrog (Tlalocohyla smithii); Canyon Treefrog (Hyla arenicolor); Northern Sheep Frog (Hypopachus variolosus); Chiricahua Leopard Frog (Lithobates chiricahuensis). There are three salamanders found in the ecoregion: the endemic Sacramento Mountains Salamander (Aneides hardii), found only in very high montane reaches above 2400 meters; Tiger Salamander (Ambystoma tigrinum); and the Tarahumara Salamander (Ambystoma rosaceum).
Avian species
Stattersfield et al. also recognize this ecoregion as a significant endemic bird area. The Maroon-fronted Parrot (Rhynchopsitta terrisi EN) and the Colima warbler (Vermivora crissalis NT) are endemic to the Sierra Madre Oriental ecoregion. The Colima warbler, however, does winter outside of the ecoregion on the Pacific versant of western Mexico. Wild Turkey (Meleagris gallapavo), Peregrine Falcon (Falco peregrinus) and Golden Eagle (Aquila chrsaetos) also inhabit this mountainous habitat..
Ecological status
Foothills of the Sierra Madres, Mexico (Photograph by Colby Loucks)
Centuries of logging and agricultural conversion have almost completely eliminated the prehistoric native pine-oak forests of the Sierra Madre Oriental. In general, the temperate forests of Mexico are considered a habitat in danger of eradication, largely due to intensive human exploitation by indigenous peoples. Toledo et al. conclude that, while sixty percent of the temperate forests in Mexico remain intact, thirty-seven percent 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 further areas connected to the existing ones; however, the ecoregion lacks enforcement of laws prohibiting activities such as wildlife extraction and logging, particularly in the Mexican portion of the region.
The Cumbres de Monterrey National Park is one of the largest in Mexico, covering 2465 km2. However according to Stattersfield, this park is inadequately 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. The destruction of the Maroon-fronted Parrot habitat has caused the decline of this population. El Cielo Biosphere Reserve at 2400 meters elevation supports some endemic species of the Sierra Madre Oriental, including Quercus germana,Xalapa Oak (Q. xalapensis) and Ternstroemia cuneifolia. In the northern-most element of the ecoregion is Big Bend National Park in Texas with an area of 2866 km2.
Ecological threat profile
At the present, chief threats include deforestation, resin extraction, and agricultural land uses. Selective logging for income of local residents in the area is occurring throughout much of the region. Drug trafficking and utilization of the ecoregion as an illegal immigration corridor are taking some toll, although of a lesser impact than that to the surrounding desert floor. 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 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. Linework was reviewed by experts during ecoregional priority setting workshops in Mexico.
References
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