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SIERRA DE JUAREZ, OAXACA
In north-eastern Oaxaca between Tuxtepec and Ixtlán de Juárez (Map 14), the Sierra de Juárez is separated from the Sierra de Zongólica to its north by the Santo Domingo River with Tecomavaca Canyon, and extends south-eastward to the Cajones River and Sierra de Villa Alta, which connects to the Sierra de Los Mixes (Paray 1951; Lorence and García-Mendoza 1989). These mountain chains are part of the Sierra Madre de Oaxaca in the Oaxaca-Puebla Uplands subprovince of the Sierra Madre del Sur morphotectonic province (cf. Ferrusquía-Villafranca 1993).
North of the Trans-Mexican Volcanic Belt is the Sierra Madre Oriental, and just south of the belt begins the Sierra Madre de Oaxaca. Mountains thus front the Gulf of Mexico lowlands from the belt's Pico de Orizaba (Veracruz) to the Isthmus of Tehuantepec, where they meet the Sierra Madre del Sur.
The region is characterized by large deep ravines (barrancas). The Sierra de Juárez geological subprovince is composed of folded sedimentary rocks with series of younger granitic intrusions dating from the Palaeozoic to Cenozoic, with the majority being Mesozoic, but the region is complex and not well known (Lorence and García-Mendoza 1989; Ferrusquía-Villafranca 1993). Various watercourses (e.g. the Valle Nacional River) have their origin in the Sierra de Juárez and eventually form the Papaloapan River which empties into the Gulf of Mexico.
The climate ranges from subtropical to mostly temperate and subhumid - above 1000 m. The average temperature varies between 16°-20°C, with regular frost in the high mountains. Average annual precipitation varies locally from 700 mm to 2000-4000 mm or more, often as moisture removed from trade winds coming off the Caribbean Sea - which in some years reach gale force.
The Sierra de Juárez (picture) has a great variety of habitats due to variations in topography, altitude, geological substrates and climate. These environmental characteristics determine a mosaic of formations and communities that always include some evergreen vegetation. Although there are many factors that affect seasonality within these vegetation types, humidity seems to limit distribution in the mountain areas in comparison to the vegetation that extends to the lower, warmer areas (Lorence and García-Mendoza 1989).
Five general formations are distinguished in the Sierra de Juárez, of which broadleaf montane cloud forest is predominant (while extensive on these mountains, it covers less than 1% of Mexico):
1. Tropical evergreen forest (800-1000 m) is dominated by evergreen trees 30-40 m tall. There are abundant lianas and epiphytes of tropical affinity. The characteristic or dominant trees are Acosmium panamense, Andira sp., Brosimum alicastrum var. alicastrum, Calophyllum brasiliense var. rekoi, Dialium guianense, Dussia mexicana, Ormosia isthmensis and Robinsonella sp. (cf. Wendt 1993).
2. Montane cloud forest (bosque mesófilo de montaña Rzedowski 1978) forms a band between (1000-) 1400-2250 m along the northern and eastern slopes of the Sierra de Juárez and Sierra de Los Mixes. The climate is cool (20°-14°C) and humid, with a mean annual precipitation exceeding 2000 mm and probably reaching 6000 mm in places (e.g. at Vista Hermosa) (Rzedowski and Palacios- Chávez 1977). Dominant trees average 20-30 m tall. Evergreen and deciduous species bearing many epiphytes occur together with palms, tree ferns, Ericaceae shrubs, vines, and moisture-loving herbs (Paray 1951, Lorence and García-Mendoza 1989; Martin and de Avila-B. 1990).
Floristically this formation is a mixture of both neotropical and holarctic elements, including affinities with South America and Asia. Rzedowski and Palacios-Chávez (1977) studied a community at Vista Hermosa, where they reported Oreomunnea mexicana, Weinmannia pinnata and Liquidambar styraciflua to be dominant. Additional characteristic species were Magnolia schiedeana, Brunellia mexicana, Nyssa sylvatica and species of Alnus, Carpinus, Cedrela, Clethra, Ilex, Ocotea, Phoebe and Podocarpus. This vegetation type in the region remains little known.
3. Pine forest, on basaltic substrate at 1600-2600 m. The evergreen trees are 25-40 m tall. Dominant are Pinus ayacahuite, P. cornuta, P. lawsonii, P. chiapensis, P. devoniana (P. michoacana) and P. pseudostrobus var. oaxacana. Grasses dominate the lower stratum.
4. Pine-oak forest (2000-2800 m). Predominant are Pinus rudis, P. devoniana (P. michoacana), P. lawsonii and P. montezumae together with Quercus laurina and Q. rugosa. Some individuals of the rare Abies guatemalensis and also A. oaxacana are associated with the Pinus, mainly in ravines and above 2700 m. Plecosorus speciosissimus and Dryopteris wallichiana are exceptional among the terrestrial ferns (at 2800 m) (Riba 1993).
5. Oak forest (2000-2500 m). Precipitation is relatively low, with a summer dry season. The dominants are Quercus crassifolia, Q. castanea, Q. crassipes, Q. rugosa and Q. laurina. This formation occurs westward (inland) to the Grande River Basin, where it changes to matorral and/or low forest.
Located in the Mixteca-Oaxaqueña Floristic Province, the Sierra de Juárez is one of Oaxaca State's wettest and foristically richest areas, with probably 2000 of the 8000 or more species in Oaxaca. The northernmost limit of many Mesoamerican taxa occurs in the Sierra de Juárez (Lorence and García-Mendoza 1989), and higher areas in Oaxaca may be ancient centres of diversity (Hunt 1993). The Instituto de Biología of the Universidad Nacional Autónoma de México (UNAM) is conducting floristic inventories in the region. Although knowledge of the sierra's flora is limited, the number of endemics is expected to be significant.
For example, the Sierra de Juárez is one of five Oaxacan centres of endemism for the Leguminosae, and 6-7% of Mexican legume species are endemic to Oaxaca (Lorence and García-Mendoza 1989). Estimates of species endemic to Oaxaca for several other families occurring in the Sierra de Juárez are Rubiaceae (18%) and Monimiaceae (40%). Most of these endemics seem to be restricted to the sierra or its vicinity. As in some other families, Oaxaca seems to be a refugium for relict species of Commelinaceae e.g. in temperate forest, the generically monotypic endemics Gibasoides laxiflora and Matudanthus nanus (Hunt 1993). Oaxaca also has many endemic Labiatae (Ramamoorthy and Elliott 1993) and Gramineae (31%) (Valdés-Reyna and Cabral-Cordero 1993). Some endemics in other families are: Siparuna scandens, Carica cnidoscoloides, Rondeletia ginettei, Anthurium cerroplonense, A. subovatum, A. yetlense and Syngonium sagittatum.
The probable high endemism in the Sierra de Juárez supports Rzedowski's (1991) finding that 60% of the species occurring in the montane cloud forests of MegaMexico (south- western United States, Mexico and northern Central America) are endemic species.
In the Sierra Norte of Oaxaca, which includes the Sierra de Juárez, several indigenous peoples (Chinantec (picture), Mixe and Mixtec) have extensive knowledge of and uses for the flora, which have been receiving thorough ethnobotanical study (Martin and de Avila-B. 1990; Martin 1992). In the 1970s there were large-scale collections of Dioscorea tubers, used in the synthesis of birth-control pills. The Sierra de Juárez contains rich timber resources such as Abies, Pinus, Liquidambar and Quercus. Among the region's various ornamental species are tree ferns, cycads, pipers, aroids, bromeliads and orchids (Paray 1951).
Social and environmental values
A major highway from Tuxtepec south-westward to the city of Oaxaca crosses the Sierra de Juárez north of Cerro Pelón (3250 m), and unpaved roads provide access to many of the indigenous communities and forests in this rugged region.
The sierra harbours various threatened mammals such as jaguar, ocelot and brocket deer. There are a number of species of birds from the Central Mexican Highlands Endemic Bird Area (EBA A11) and the Balsas Drainage and Interior Oaxaca EBA (A27). Among them is the threatened dwarf jay (Cyanloyca nana), which occurs only in central Oaxaca and western Veracruz in pine-oak forest. The Sierra de Juárez is the richest region in Mexico for butterflies and significant in endemic butterflies (Llorente- Bousquets and Luis-Martínez 1993).
The flora is threatened by logging (for wood, pulp and cellulose), agriculture and grazing (crops, cattle and sheep), colonization, commercial collecting of those species with ornamental value, and potentially threatened by hydroelectric dams. Over one third of the Pinus forest of the Sierra Norte may have been destroyed from 1956-1981, with much of the remainder being degraded (G.J. Martin, pers. comm.).
Abies guatemalensis is in Appendix I of CITES. There is general interest within academia for protection of some areas in the Sierra de Juárez, but there is no official plan to establish nature reserves. Several foundations have supported ethnobotanical research and conservation projects in collaboration with SERBO (the Society for the Study of Biotic Resources of Oaxaca / Sociedad para el Estudio de Recursos Bióticos de Oaxaca), which hopes to establish an outreach centre of information in the city of Oaxaca (Martin and de Avila-B. 1990). This sierra may harbor the best remaining occurrence of the highly threatened Mexican montane cloud forest; a substantial portion needs to be saved.
Map 14. Sierra de Juárez, Oaxaca, Mexico (CPD Site MA3)
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.
Hunt, D.R. (1993). The Commelinaceae of Mexico. 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. 421-437.
Llorente-Bousquets, J. and Luis-Martínez, A. (1993). Conservation-oriented analysis of Mexican butterflies: Papilionidae (Lepidoptera, Papilionoidea). 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. 147-177.
Lorence, D.H. and García-Mendoza, A. (1989). Oaxaca, Mexico. In Campbell, D.G. and Hammond, H.D. (eds), Floristic inventory of tropical countries: the status of plant systematics, collections, and vegetation, plus recommendations for the future. New York Botanical Garden, Bronx. Pp. 253-269.
Martin, G.J. (1992). Searching for plants in peasant marketplaces. In Plotkin, M. and Famolare, L. (eds), Sustainable harvest and marketing of rain forest products. Island Press, Washington, D.C. Pp. 212-223.
Martin, G.J. and de Avila-B., A. (1990). Exploring the cloud forests of Oaxaca, Mexico / Explorando el bosque nuboso de Oaxaca, México. WWF Reports (Oct.-Dec.): 8-11/11- 14.
Paray, L. (1951). Exploraciones en la Sierra de Juárez. Bol. Soc. Bot. Méx. 13: 4-10.
Ramamoorthy, T.P. and Elliott, M. (1993). Mexican Lamiaceae: diversity, distribution, endemism and evolution. 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. 513-539.
Riba, R. (1993). Mexican pteridophytes: distribution and endemism. 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. 379-395.
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.
Rzedowski, J. and Palacios-Chávez, R. (1977). El bosque de Engelhardtia (Oreomunnea) mexicana en la región de la Chinantla (Oaxaca, México) una reliquia del cenozoico. Bol. Soc. Bot. Méx. 36: 93-123.
Valdéz-Reyna, J. and Cabral-Cordero, I. (1993). Chorology of Mexican grasses. 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. 439-446.
Wendt, T. (1993). Composition, floristic affinities, and origins of the canopy tree flora of the Mexican Atlantic slope rain forests. 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. 595-680.
This Data Sheet was written by Dra. Patricia Dávila and Leticia Torres and Rafael
Torres [Universidad Nacional Autónoma de México (UNAM), Instituto de Biología,
Departamento de Botánica, Apartado Postal 70-367, Mexico 04510, D.F., Mexico] and Olga
Herrera-MacBryde (Smithsonian Institution, SI/MAB Program, S. Dillon Ripley Center, Suite
3123, Washington, DC 20560-0705, U.S.A.).
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