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UPPER MEZQUITAL RIVER REGION,
|Location: Western Sierra Madre mountains in the south of Durango State, between
latitudes 22°50'-23°40'N and longitudes 104°05'-105°30'W.
Area: c. 4600 km².
Altitude: 800-3350 m.
Vegetation: Principally conifer, pine-oak and oak forests; tropical dry forests, and patches of tropical subdeciduous forest.
Flora: High diversity - c. 2900 species of vascular plants; relatively high endemism.
Useful plants: More than 450 wild species used for medicinal, food and other purposes by local people; many timber species extracted.
Other values: Water resources, archaeological sites, refuge of Tepehuan Amerindians, germplasm reserve for timber species and wild relatives of cultivated plants, habitats for wild animals, threatened species.
Threats: Logging, erosion due to overgrazing and road construction, loss of traditional knowledge.
Conservation: La Michilía Biosphere Reserve (700 km², 70 km² as core). Part of temperate forests in area of the Tepehuan could be declared special reserve.
Map 19: CPD Site MA8
The Upper Mezquital River region of the Sierra Madre Occidental morphotectonic province is in north-central Mexico's State of Durango in the southern part, south of the city of Durango (Map 19). The Tropic of Cancer crosses the region's northern third. This region has a complex physiography (Ferrusquía-Villafranca 1993; Gómez-Pompa et al. 1994); the highest peak is Cerro Gordo. The western flank of the sierra is extremely steep; its rivers form deep gorges, in some sites with walls more than 2000 m high and up to 10 km in extent. Most prominent is the gorge of the Mezquital River, the only river which originates on the eastern side and crosses the sierra - it becomes the San Pedro River and flows into the Pacific Ocean in Nayarit State.
The region has a wide range of climates, from subhumid warm and semi-warm with a pronounced winter dry season at lower elevations on the western slope of the sierra, to various temperate and semi-cold climates at higher elevations. The annual precipitation ranges between 800 mm and 1400 mm.
The formation of the Sierra Madre Occidental began in the Eocene, but its upper layers are more recent, derived from a period of intense volcanic activity during the Pliocene and Pleistocene. Volcanic rocks 34 to 27 million years old predominate in the region, principally ignimbrites, rhyolites and rhyodacites as well as tuffs; very small outcrops of basalt also occur. The upper layer of the Sierra Madre Occidental is the Earth's most continuous ignimbritic layer.
The region is a mosaic comprised of a great variety of ecosystems because of its transitional location between the Holarctic and Neotropical realms, and the complexity of its physiography and climates. In the higher areas, temperate pine and/or oak forests predominate, with small enclaves of fir and pine-fir forests. In the gorges toward the west, tropical dry and subdeciduous forests are found (Rzedowski 1978; S. González 1983; Gómez-Pompa et al. 1994; González, González and Cortés, in press).
Fir and pine-fir forests
The fir and pine-fir forests occupy humid ravines and hillsides with northern exposure at altitudes above 2700 m. In the most extensive of these coniferous forests Pseudotsuga menziesii (Douglas fir) predominates. There are smaller areas of Abies durangensis and Picea chihuahuana - which is restricted to just a few sites in the Sierra Madre Occidental.
At elevations above 2500 m on hillsides and in broad ravines occur diverse combinations of pine species, principally Pinus durangensis, P. teocote and P. arizonica var. stormiae, and in more humid sites P. leiophylla and P. ayacahuite.
Pinus cooperi alone is dominant in broad valleys with deep soils, as well as on slight hillsides at 2400-2650 m. On the western slope of the sierra at 1350-1700 m, there are hillside communities dominated by Pinus douglasiana.
The pine-oak forests occur in very diverse associations, depending upon the altitude and physiography:
Above 2500 m, Pinus durangensis and Quercus sideroxyla associate with P. teocote, P. cooperi, Q. crassifolia, Q. rugosa and Arbutus madrensis, which in disturbed areas mingle with Alnus, Arbutus arizonica and A. tessellata. This mixture is probably the most representative of the forests of the Sierra Madre Occidental in Durango. In sites with more humidity, Pinus leiophylla becomes dominant.
At 2300-2500 m, Pinus oocarpa subsp. trifoliata is the dominant tree on mesas and hillsides that are not very steep, and frequently mixes with P. lumholtzii, Quercus urbanii and some P. engelmannii, or in areas of more tropical influence with more P. engelmannii and P. douglasiana, Q. viminea and Q. fulva.
In semi-dry temperate areas, diverse species of Quercus may combine with Pinus engelmannii, P. chihuahuana and madrones - principally Arbutus arizonica and A. tessellata. In the drier forests, Pinus cembroides and Quercus grisea or Q. eduardii predominate (Passini 1985).
In areas between 1800-1900 m, Quercus coccolobifolia mixes with Pinus chihuahuana, Q. resinosa, Q. crassifolia and Q. viminea. On the western slope of the sierra at 1350-1700 m are communities dominated by Pinus herrerae, P. douglasiana and/or P. oocarpa along with Quercus and Arbutus.
In areas of shallow soil or strong outcrops of very unsheltered bedrock, forests of Pinus lumholtzii ("pino triste") mixed with oaks (Quercus urbanii and Q. crassifolia) are prominent. Isolated shrub-like elements include manzanita (Arctostaphylos pungens) and Juniperus durangensis.
Oak forests are common on the eastern slope of the Sierra Madre Occidental at its lower elevations (1900-2400 m), where the following associations are predominant:
In areas with a semi-dry temperate climate there are low open forests, in which the dominants are white oaks such as Quercus grisea, Q. arizonica and Q. laeta, red oaks such as Q. eduardii and Q. durifolia, and other oaks (e.g. Q. crassifolia).
Where there is a semi-humid climate, forests of Quercus rugosa occur on mesas and hillsides, sometimes up to 2800 m. In areas with deeper soil, Q. sideroxyla can also be found.
Tropical dry forest
The dry hillsides with a subtropical climate between 800-1900 m have open forests, in which the dominant species lose their leaves during the dry season. Among them are Acacia pennatula, Bursera spp., Ipomoea murucoides, Lysiloma divaricata and Plumeria rubra. Arborescent cacti are also dominant.
Tropical subdeciduous forest
This is restricted to sites with greater humidity, especially in gorges and along permanent streams; many of the principal species have foliage throughout the year. Common species are Pithecellobium dulce, Ceiba acuminata, Ficus spp., Brosimum alicastrum, Bursera spp. and Enterolobium cyclocarpum.
Of the 3800 phanerogamous species known to occur in Durango (González, González and Herrera 1991), c. 2900 are estimated to be present in the Upper Mezquital River region. This floristic diversity is high considering the latitude, and is enriched by the confluence of elements from the Holarctic and Neotropical realms and three floristic provinces: Sierra Madre Occidental, Altiplano and Pacific Coast (Rzedowski 1978, 1991), as well as the region's physiographic and climatic complexities.
The great gorge of the Mezquital River functions as a biological corridor that provides entrance of tropical elements to the eastern side of the Sierra Madre Occidental, but also functions as the major north-south geographic barrier to various plant species of the sierra. The region's location makes it an important zone of confluence for boreal and equatorial genera, whereas at the species level it is an area where Mexican endemics predominate. Probably because of its physiography and location, it is a zone favourable to speciation - many plant genera are in active evolution. The zone is consequently a refuge for many endemic species of western Mexico and at least sixteen local endemics e.g. Eleocharis svensoniana, Muhlenbergia durangensis, M. michisiensis, Axiniphyllum durangense, Sabazia gonzalezae, Senecio gesneriifolius, S. gonzalezae, Tridax durangensis, Verbesina durangensis and Pavonia durangensis.
Floristic inventories have been carried out through the Flora of Durango project of the Centro Interdisciplinario de Investigación para el Desarrollo Integral Regional, Unidad Durango - Instituto Politécnico Nacional (CIIDIR-IPN). Endemic species have been found and described in the last ten years but a good part of the region is still unexplored. It is highly probable that there are many taxa not yet discovered, and others that have been lost forever because of the intense exploitation in some areas.
Among the species with economic value, predominant are Pinus durangensis, P. cooperi, P. teocote and P. ayacahuite, which are extracted for their wood. Diverse species of Quercus have economic value for charcoal, and lumber potential as well. In addition to using various species for timber, there is a great potential for utilization of the region's flora for non-timber products.
Over 450 wild plant species are used by the indigenous and mestizo populations in the region and elsewhere in Durango for medicinal, food, construction, forage and handicraft purposes. Some of these species have possibilities at the industrial level. A few species, such as Laelia speciosa, are collected to sell in nearby cities as ornamentals, whereas others such as Senecio sessilifolius ("peyotillo") and S. albo-lutescens ("matarique") are sold for medicinal purposes in local markets (cf. M. González 1984).
The northern part of the region is home to the mestizo population, whereas the south is inhabited by the Tepehuan indigenous people. The ethnobotany of the region's mestizos and Tepehuans has been studied by M. González (1984, 1991) and González and Galván (1992), and several additional works on the useful plants and agro-ecological practices are in preparation.
Archaeological sites are found in the region, including the core area (Cerro Blanco) of the Biosphere Reserve (Gómez-Pompa et al. 1994). Because of its inaccessibility, the region has been a refuge for indigenous peoples, especially Tepehuans, who were displaced from the lower lands that they originally had occupied. The main Tepehuan community, Santa María Ocotán y Xoconoste, is the principal political and religious centre. Its 13,500 inhabitants are distributed in four towns and 22 dependencies. Other important communities are Taxicaringa and San Bernardino de Milpillas. In lower areas, including within Nayarit State, the Mezquital watershed also sustains the indigenous towns of Coras and Huicholes.
The region's water resources are utilized for agricultural areas of the lower lands of Durango and in the coastal plain of Nayarit. The watershed's vegetation protects the soil from erosion and helps avoid flooding at lower elevations - thus its conservation is of vital importance for the equilibrium of a diversity of ecosystems.
The populations of timber species and wild relatives of cultivated plants are reserves for germplasm. Examples of such crop relatives are found in Phaseolus and Solanum. The region also helps to sustain a diverse wild fauna (Halffter 1978), including the threatened thick-billed parrot (Rhynchopsitta pachyrhyncha) and perhaps the imperial woodpecker (Campephilus imperialis) - if it is not already extinct. The Instituto de Ecología has a research station (Piedra Herrada) in the Cerro Blanco area.
During the last 25 years the region has been exploited intensively by a private timber company. Currently, a complex socio-economic problem has allowed a profound deterioration of the forests to take place in the southern area inhabited by the Tepehuan. Considering their low standard of living and that these forests constitute not only their patrimony but their source of sustenance and protection for their culture, it has become urgent to reach a solution to the deteriorating situation (González and González 1992).
Many populations of the plants and animals have been affected by destruction of their natural habitats. The main problem is deforestation resulting from the intense exploitation of timber trees, which is being carried out without any conservation or management programmes. Along with extraction of the pines, it is common that the oaks are girdled or eliminated in other ways to comply with guidelines of SARH (Mexico's Secretaría de Agricultura y Recursos Hidráulicos), which has stated that a volume of oaks must be extracted equal to half that of the pines extracted.
Furthermore, overgrazing and the introduction of sheep and goats, seasonal utilization of steep hillsides as agricultural areas and the construction of a road south-westward to link the cities of Durango and Tepic have caused erosion and reduced the regeneration of plants (Carrillo-S. 1982) and the recuperation of the forests.
The unregulated exploitation of the resources and road construction are affecting the cultural identity of the indigenous people. Symptoms of social maladjustment have become clearly manifest, and their traditional knowledge is in danger of disappearing.
An area reaching from 2000-2985 m on the eastern flank of the Sierra Madre Occidental in 1979 was officially declared a Biosphere Reserve (La Michilía) and a Zone of Forest Protection, but within the sierra's expansive range through Durango there is no protected area for its ecosystems. La Michilía BR is located between 23°15'-23°30'N and 104°05'-104°20'W (Halffter 1978; González, González and Cortés, in press); it is administered by the federal government's Instituto de Ecología. The reserve's core (Cerro Blanco) includes 70 km², and a buffer zone increases the area to (350) 700 km². This large buffer zone has public grazing pastures and private properties in which grazing and occasional extraction of wood take place.
To achieve an optimal and sustained management of the natural resources of the Tepehuan area in the region's south, it is essential to address the socio-economic problems of the indigenous community and the problem of logging.
Among technical recommendations for management of the region are the following: Exclude from utilization the steep hillsides, and strips 100 m wide along permanent streams, as well as isolated areas of 1 km² and all areas with Picea and Abies. Control grazing of goats and sheep, which damage regeneration of the forests. Revegetate with native species those areas that are severely damaged.
Carrillo-S., A. (1982). Producción primaria neta aérea del estrato herbáceo y efecto del ganado sobre su composición florística en la Reserva de la Biósfera "La Michilía", Dgo. Professional Thesis. Facultad de Ciencias, UNAM, Mexico, D.F. 187 pp.
Ferrusquía-Villafranca, I. (1993). Geology of Mexico: a synopsis. In Ramamoorthy, T.P., Bye, R., Lot, A. and Fa, J.E. (eds), Biological diversity of Mexico: origins and distribution. Oxford University Press, New York. Pp. 3-107.
Gómez-Pompa, A. and Dirzo, R. with Kaus, A., Noguerón- Chang, C.R. and Ordoñez, M. de J. (1994). Las áreas naturales protegidas de México de la Secretaría de Desarrollo Social. SEDESOL, Mexico, D.F. 331 pp. Unpublished.
González, M. (1984). Las plantas medicinales de Durango. Centro Interdisciplinario de Investigación para el Desarrollo Integral Regional, Instituto Politécnico Nacional (CIIDIR-IPN), Durango, Mexico. Cuad. Inv. Tecnol. (Durango) 1(2): 1-117.
González, M. (1991). Ethnobotany of the Southern Tepehuan of Durango, Mexico: I. Edible mushrooms. J. Ethnobiol. 11: 165-173.
González, M. and Galván, R. (1992). El maguey (Agave spp.) y los Tepehuanes de Durango. Cact. Suc. Mex. 37: 3-11.
González, M. and González, S. (1992). The plight of Tepehuan forest land. Abstracts of the III International Congress of Ethnobiology, Mexico, D.F.
González, M., González, S. and Herrera, Y. (1991). Listados florísticos de México. IX. Flora de Durango. Instituto de Biología, Universidad Nacional Autónoma de México, Mexico, D.F. 167 pp.
González, S. (1983). La vegetación de Durango. CIIDIR-IPN, Durango, Mexico. Cuad. Inv. Tecnol. (Durango) 1(1): 1-114.
González, S., González, M. and Cortés, A. (in press). Vegetación de la Reserva de la Biósfera La Michilía. Acta Bot. Mex. [140 pp.]
Halffter, G. (ed.) (1978). Reservas de la Biósfera en el estado de Durango. Instituto de Ecología, Mexico, D.F. 198 pp.
Passini, M.F. (1985). Les fôrets de Pinus cembroides Zucc. de la Sierra de Urica. Rèserve de la Biosphère "La Michilía" (Estado de Durango, Mexique). Bull. Ecol. 16: 161-166.
Rzedowski, J. (1978). Vegetación de México. Editorial Limusa, Mexico, D.F. 432 pp.
Rzedowski, J. (1991). Diversidad y orígenes de la flora fanerogámica de México. Acta Bot. Mex. 14: 3-21.
This Data Sheet was written by Dra. Socorro
González-Elizondo (CIIDIR and COFAA, Instituto Politécnico Nacional, Apdo. Postal 738,
Durango, Dgo., Mexico).
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