Location and General Description
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 (3,400 m), and most of the terrain in this area is above 1,000 m. 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 oC to 20 oC. The annual mean precipitation varies greatly from 700 mm to 2000-4000 mm (Davila et al. 1997).
This ecoregion has a mosaic of vegetation formations 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 1,600 and 2,600 m, include trees that are 25-40 m tall. Dominant species are Pinus ayacahuite, P cornuta, P. lawsonii, P. chiapensis, P. devoniana, and P. pseudostrobus var. oaxacana. The pine forests have a thick understory and herbacious layer composed of many species of the Ericaceae family (Rzedowski 1977).
Pine-oak forests are located at elevations ranging from 2,000 to 2,800 m. The dominant species at this elevation include Pinus rudis, P. devoniana, P. lawsonii P. montezumae, Quercus laurina and Q. rugosa (Davila et al. 1997). In the higher elevations, dominant species include P. rudis, P. ayacahuite, and P. patula, and can be found in association with the cloud forests of north Oaxaca (Rzedowski 1978).
Oak forests, found at 2,000-2,500 m, include Quercus crassifolia, Q. castanea, Q. crassipes, Q. rugosa and Q. laurina (Davila et al. 1997). At lower elevations of 2,000 m, the composition of the forest transforms and the dominant species are Quercus magniolifolia and Q. urbanii, with Pinus lawsonnii, P. oaxacana and P. leiophylla (Rzedowski 1978). The oak forests mix with Engelhardtia mexicana at 1,800-2,050 m, forming another association of pine-oak forest with an enormous biogeographical importance.
Some portions of this region have a more humid environment than what is common for pine-oak forest associations. These humid areas contain cloud forests and are characterized by the presence of abundant epiphytes (e.g. Odontoglossum sp., Tillandsia prodigiosa, Peperomia galioides and others), scrub (e.g. Eupatorium sp. and Ternstroemia pringlei) and herbs (e.g. Smilax moranensis, Spigelia longiflora and Salvia spp.). According to Davila et al. (1997) these forests constitute a mix of vegetation with netropical and holoarctic elements; some of the dominant species include Oreomunnea mexicana, Weinmannia pinnata and Liquidambar styraciflua.
The Sierra de Juarez, included in the Sierra Madre de Oaxaca, is one richest plant areas in Oaxaca. This area constitutes one of the five centers of endemism for the Leguminosae family. Most of the endemic species restricted to this Sierra are in the Rubiaceae and Monimiaceae families. Some other endemics in other families include Siparuna scadens, Carica cnidoscoloides, Rondeletia ginettei, Anthurium cerroplonenese, A. subovatum, A. yetlense, and Synognium sagittatum. The Sierra de Juarez is a refugium of two relict species Gibasoides laxiflora and Matudanthus nanus (Davila et al. 1997).
The pine-oak forests of north Oaxaca contain a high number of endangered species; thus their value for conservation is outstanding in terms of the uniqueness of the habitat that supports these species. The number of species of Quercus (45) present in the north Oaxaca pine-oak forests exceeds that of other temperate areas in Mexico (González-Rivera 1993). Two highly endangered species of trees, Abies hickelii and Cupressus bethamii var. lindleyi grow in these forests, as well as many species of endemic ferns and water lilies (Challenger 1998). Considered as one of the areas with notorious mastozoological content (proposed by Gaona-Ramírez et al. 1990), these forests contain nearly 40% of the endemic vertebrates of Mesoamerica (Sánchez-Cordero et al. 1990). 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 (Arriaga et al., 2000). Several important bird areas have also been located within this ecoregion, and include Sierra Norte, Cerro de Oro, Valle de Tehuacán, and Sierra de Zongolica (Benitez et al. 1999).
The forests also house a high diversity of amphibians and reptiles in all of Mexico including the canyon treefrog (Hyla arenicolor) and Mexican salamander (Ambystoma mexicanum), Ptychohyla erythroma, Hyla pentheter, Hyla sabrina, Nototriton adelos, Pseudoeurycea juarezi, and Thorius macdougalli (Flores-Villela 1993). Stattersfield et al. (1998) 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.
Perhaps 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 main human activities that threaten the habitat.
The outstanding value of Oaxaca pine-oak forests lies in its 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 the state. National parks however, have been proposed and established to promote the knowledge and appreciation of this ecosystem and the invaluable resources they possess.
Types and Severity of Threats
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 leads to weakening of the pine trees, which in turn become prone to disease and die (Styles 1993). 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 will inevitably result in their status changing to "seriously 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 INEGI (1996) 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 (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, and Tom Allnutt
Reviewed by: In Process