Sierra Madre de Oaxaca pine-oak forests
The Sierra Madre de Oaxaca pine-oak forests ecoregion of northern Oaxaca, Mexico exhibits a large number of endangered species, so that their value for conservation is outstanding in terms of the uniqueness of the habitat that supports these species. The Sierra Madre de Oaxaca pine-oak forests is within the Tropical and Subtropical Conifer Forests biome, and is the ecoregion is known for elevated plant endemism, especially within the Sierra de Juarez montane forests. Indigenous peoples have long used the land extensively for agriculture and cattle farming.
Location and general depiction
This ecoregion is located in northern Oaxaca State, and is delineated by the Sierra Norte de Oaxaca Mountains, which have characteristically abrupt and rugged topography. Its tallest peak is Zempoaltepetl (3400 metres), and most of the terrain in this area is above 1000 metres. Three mountain chains or sierras constitute the Sierra Madre de Oaxaca: Juarez, Aloapaneca and Zempoaltepec. The climate is temperate and humid with annual temperatures ranging from 16°C to 20°C. The annual mean precipitation varies greatly from 700 millimetres (mm) to as great as 4000 mm. There are numerous special status organisms found in the Sierra Madre de Oaxaca pine-oak forests ecoregion, denoted variously as Near Threatened (NT), Vulnerable (VU), Endangered (EN), or Critically Endangered (CR).
This ecoregion presents a mosaic of vegetatative associations, due to the varied climate and topography. These formations include tropical evergreen forest, montane cloud forest, pine forest, pine-oak forest, and oak forest. The pine forests, at elevations between 1600 and 2600 m, include trees that are 25 to 40 m tall. Dominant pine species are Mexican White Pine (Pinus ayacahuite); Lawson's Pine (P. lawsonii), a Mexican endemic; Chiapas White Pine (P. strobus var. chiapensischiapensis); Michoacan Pine (P. devoniana LR/LC) and Smooth-barked Mexican Pine (P. pseudostrobus). These pine forests have a robust understory and an herbacious layer dominated by numerous species of the Ericaceae family.
Pine-oak forests are located at elevations ranging from 2000 to 2800 m. The dominant species at this elevation include Pinus hartwegii var. rudis, P. devoniana, P. lawsonii, Montezuma Pine (P. montezumae), Quercus depressa and Netleaf Oak (Q. rugosa). For the higher elevations, dominant species include P. rudis, P. ayacahuite, and Jelecote Pine (P. patula), and can be found in association with the cloud forests in northern Oaxaca.
Oak forests, found between elevations 2000 and 2500 m, include Quercus splendens, Q. castanea, Q. crassipes, Q. rugosa and Q. laurina. At the lower elevations of around 2000 m, composition of the forest transforms, and dominant tree taxa are: Encino Amarillo (Quercus magnoliifolia) and Cucharillo Oak (Q. urbanii), with an admixture of Lawson's Pine (Pinus lawsonnii), P. pseudostrobus var. apulcensis and Chihuahua Pine (P. leiophylla). The oak forests are intermingled with Engelhardtia mexicana at 1800 to 2050 m, forming a different association of pine-oak forest with a considerable biogeographical significance.
Some portions of this region have a more humid environment than commonly associated with pine-oak forest communities. These humid areas contain cloud forests and are characterised by the presence of abundant epiphytes (e.g. Odontoglossum spp. and the Mexican endemic Tillandsia prodigiosa). In addition, scrub taxa are apparent such as Thoroughwort (Eupatorium spp.) and Ternstroemia cuneifolia; moreover, a number of herbs (e.g. Smilax moranensis, Spigelia longiflora and Salvia spp.) are also found here. These forests constitute a mix of vegetation with neotropical and holoarctic elements; some of the dominant species are: Oreomunnea mexicana, Weinmannia pinnata and American Sweetgum (Liquidambar styraciflua).
The Sierra de Juarez, included in the Sierra Madre de Oaxaca, is one richest plant zones in Oaxaca. This area constitutes one of the five centres of endemism for the Leguminosae family. Most of the endemic species restricted to this mountain range are within the Rubiaceae and Monimiaceae families. Some other endemic plant species in other families include Siparuna stellulata, Carica cnidoscoloides, Renistipula galeottii, Anthurium cerropelonense, A. subovatum, A. yetlense, and Synognium sagittatum. The Sierra de Juarez is a refugium of two relict species: Gibasoides laxiflora and Matudanthus nanus.
The pine-oak forests of north Oaxaca contain a high number of endangered species; thus their conservation value is outstanding in terms of the uniqueness of the habitat that supports these species. The 45 species of Quercus present in the north Oaxaca pine-oak forests exceeds that of other temperate ecoregions in Mexico. Two special status and rare species of trees occur in these forests: Hickel's Fir (Abies hickelii VU) and Cupressus lusitanica var. lusitanica , as well as many species of endemic ferns and water lilies. Considered as one of the areas with notable masto-zoological content, these forests contain nearly forty percent of the endemic vertebrates of Mesoamerica. Comisión nacional para el conocimiento y uso de la biodiversidad (CONABIO) has identified a number of terrestrial priority areas which share part of their territories with this ecoregion, including the Sierras del Norte de Oaxaca-Mixe and Valle de Tehuacán-Cuicatlán. A number of Important Bird Areas also exist within this ecoregion, and include Sierra Norte, Cerro de Oro, Valle de Tehuacán, and Sierra de Zongolica..
The ecoregion exhibits a moderate to high level of species richness; for example 766 taxa of vertebrates have been recorded in the ecoregion. The ecoregion is particularly noted for its diverse amphibian and reptilian fauna, including many threatened taxa and numerous endemic salamanders and anurans.
The forests also exhibit a high diversity of amphibians in Mexico, including: the endemic Acultzingo Pigmy Salamander (Thorius dubitus EN), known only from the type locality near Puerto del Aire near Veracruz; the endemic Claw-toed False Brook Salamander (Pseudoeurycea unguidentis CR), known solely from Cerro San Felipe /Cerro San Luis in north-central Oaxaca; the endemic Lower Cerro Pygmy Salamander (Thorius pulmonaris EN), known only from Cerro San Felipe region, central Oaxaca; MacDougal's Pygmy Salamander (Thorius macdougalli VU); and the endemic Mexican Axolotl (Ambystoma mexicanum CR), found in Lakes Chalco and Xochimilco of the Valley of Mexico near Mexico City; the near-endemic Sierra Juarez Moss Salamander (Cryptotriton adelos EN); the endemic Schmidt's Pygmy Salamander (Thorius schmidti EN), known only from near the village of Zoquitlán in southern Puebla, Mexico; the endemic Mustache False Brook Salamander (Pseudoeurycea mystax EN); and the Critically Endangered endemic Pseudoeurycea juarezi, found only in Cerro Pelón and Vista Hermosa in the Sierra de Juarez, north-central Oaxaca. Also found here are the anuran taxa: Guerreran Stream Frog (Ptychohyla erythromma EN); Mourning Treefrog (Plectrohyla pentheter EN), found in cloud forests on the Pacific versant of the Sierra Madre del Sur in Guerrero and Oaxaca, and in central Guerrero and southwestern Oaxaca; Canyon Treefrog (Hyla arenicolor); and Plectrohyla sabrina CR.
The Sierra Juarez Spiny Lizard (Sceloporus cryptus) is endemic to the ecoregion, and limited in range to drier parts of the Sierra de Juarez, in northeastern Oaxaca. There are a number of threatened reptilian taxa in the ecoregion including the Ribbon Graceful Brown Snake (Rhadinaea fulvivittis VU), a limited distribution snake endemic to southern Mexico.
Stattersfield et al. list the dwarf jay (EN) (Cyanolyca nana), bearded wood-partridge (CR) (Dendrortyx barbatus), Tamaulipas pygmy-owl (Glaucidium sanchezi) and grey-barred wren (Campylorhynchus megalopterus) as restricted-range bird species, which includes this ecoregion. The Oaxaca sparrow (Aimophila notosticta), golden-cheeked warbler (Dendroica chrysoparia), russet nightingale-thrush (Catharus occidentalis), hooded yellowthroat (Geothlypis nelsoni), and collared towhee (Pipilo ocai) are also species which prefer the habitats offered by this mountainous ecoregion.
The extremely abrupt and rugged topography of the Sierra Madre of Oaxaca has allowed these and other forests to remain relatively intact despite human exploitation. In the highest montane areas of northern Oaxaca, agriculture and cattle farming are the principal human activities that threaten habitat.
The outstanding ecological value of Oaxaca pine-oak forests resides in the high content of endemic and endangered species, as well as in the uniqueness of its floral composition. Despite this, no federal protected areas exist in Oaxaca. National parks however, have been proposed to promote the knowledge and appreciation of this ecosystem and the invaluable biological resources here.
Ecological threat profile
Cattle farming and grazing are responsible for the deforestation in some portions of the area. This kind of land management facilitates erosion and soil loss, and thus hinders the ability of the forest to regenerate. The flora in the area is also threatened by collection of plants with ornamental value and resin extraction, which if performed in a controlled manner does not harm the trees. However, in this region and others in the temperate forests of Veracruz, Oaxaca and Puebla, resin extraction is undertaken carelessly. This practise leads to weakening of the pine trees, which in turn become prone to disease and die. Loss of habitat could severely impact the populations of many animals and plants that depend on pine-oak forests. In Oaxaca, these forests are surrounded either by fragments of perturbed (secondary) vegetation or, in the lowest elevations, by different vegetation associations. The inability of many species to move from gradually deforested pine-oak forests to other areas may result in their status changing to critically endangered species.
Justification of ecoregion delineation
These montane pine and oak forests of the southern Sierra Madre Oriental occur along ridge tops, valleys and slopes in a patchwork distribution and are host to a number of endemic species. Linework for this ecoregion follows the Instituto Nacional de Estadística Geografía e Informática (INEGI) current landcover maps, encompassing all "pine-oak forests", "oak with pine forests", and "pine forest" classifications within the Sierra de la Laguna 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.
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