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The Tehuacán-Cuicatlán region begins c. 150 km south-east of Mexico City approximately between the cities of Puebla and Orizaba in south-eastern Puebla State, and extends into northern Oaxaca. The region is dominated by a rift valley, La Cañada or Tehuacán-Cuicatlán-Quiotepec depression (Map 15), in the Oaxaca-Puebla Uplands subprovince of the Sierra Madre del Sur morphotectonic province (Ferrusquía-Villafranca 1993). Tamayo-L. (1962) considered the region within a Mixteca-Oaxaqueña physiographic province.
The predominating Tehuacán Valley opens to the north-west with low borders c. 15 km apart and declines gradually over some 100 km to the south-east, where it narrows to Tomellín Canyon. The north-eastern to eastern boundary is formed by a part of the Sierra Madre de Oaxaca known as the Sierra de Zongólica, which reaches 2500-3000 m or more (its northern portion is the Sierra de Tecamachalco, its south-eastern portion the Sierra Mazateca). The valley is bounded to the south and west by the Sierra de Zapotitlán, a part of the Sierras Mixtecas (to 2500 m high). A major valley adjoining from the west is Zapotitlán Valley.
Topography in the Tehuacán Valley is varied. Over much of its upper expanse, wide plains slope gradually to the south. There are many gently rolling hills, outliers from the parallel mountain borders. The northern portion across the valley floor lies at an elevation of 1500-1700 m and is drained southward, forming the Salado River. Permanent surface water begins more or less near Coxcatlán where several watercourses converge, including the Zapotitlán River, which is the valley's major seasonal source of water from rainfall in the bordering highlands. The more narrow southern portion of the valley is drained northward by the Grande River. Altitude in the Tehuacán Valley declines in gradual steps to the confluence of these rivers at an elevation of less than 600 m.
Early in the Quaternary, the Tehuacán-Cuicatlán region was transformed from a closed basin with a large lake at 1700 m into the dry eroding valleys - which are still hydrostatically draining their Quaternary groundwater reserves - by the upstream erosion of the Santo Domingo River with creation of Tecomavaca Canyon. The region thus forms part of the watershed of the Papaloapan River, which empties eastward into the Gulf of Mexico.
López-Ramos (1981) considered the region part of the Tlaxiaco geologic province. The Tehuacán Valley has a mosaic of outcrops of different geologic ages and composition. In general the area presents Early Tertiary sedimentation, followed by some volcanic activity in the Pliocene and Pleistocene. Centrally, near the regional capital Tehuacán (which is at 1676 m) are limestones, sandstones and gypsum outcrops from the Late and Early Cretaceous. From Tehuacán south-east to Teotitlán del Camino are Precambrian outcrops with undifferentiated metamorphic rock, as well as Early Jurassic mudstones and sandstones. Part of the Sierra de Zongólica has outcrops of metamorphic rocks from the Palaeozoic. In the lower parts are outcrops of sediments from the Tertiary and sandstones and conglomerates from the Quaternary.
The mean annual temperature in the Tehuacán Valley varies between 18°-22°C, rising to 24.5°C at Cuicatlán; the mean January temperature in the city Tehuacán is 15°C. The arid climate is largely controlled by the Sierra de Zongólica, which is between the valley and the Gulf of Mexico. These mountains intercept much of the moisture from trade winds blowing westward off the Caribbean Sea, resulting in a rain-shadow enclave. The average annual rainfall in the valley region varies from 250-500 mm, mainly occurring from May to October, with more precipitation in June and September (Smith 1965a, 1965b; Enge and Whiteford 1989).
During the Pleistocene the climate had less differentiated seasons, might have been slightly more moist and was cooler with regular winter frosts. Probably there was extensive grassland, much less thorn forest, and many of the cacti, Agave and other desert plants could not have lived in the valley. The transition to the Recent climate occurred between 7800-7400 BC (MacNeish, Peterson and Neely 1972). The generally grey soils are either intrazonal, ranging from halomorphic alkaline and saline to calcimorphic rendzina soils, or zonal sierozem, characteristic of arid and semi-arid regions.
The Tehuacán-Cuicatlán region has a great variety of habitats related to the variations in topography, altitude, geological substrates and climate. There are many vegetation formations and associations (Miranda 1948; Rzedowski 1978; Zavala-H. 1982; Dávila-Aranda 1983; Jaramillo-Luque and González-Medrano 1983). Some are edaphically determined, such as communities of gypsophytes and halophytes. Communities are strongly affected by the marked seasonality of precipitation with six essentially dry months, as well as frequent droughts; some types of vegetation lose their leaves very rapidly. Six vegetation formations are recognized, along a descending altitudinal gradient:
1. Sclerophyllous scrub or chaparral is located above 1900 m on limestone. It is a dense community 1-2.5 m high, composed of evergreen or briefly deciduous shrubs with leathery leaves. Common genera include Ceanothus, Cercocarpus, Condalia, Ephedra, Quercus, Rhus, Sophora and Vauquelinia.
2. Spiny rosette scrub is located at 1500-1800 m on limestone. It is characterized by rosette-formed shrubs and plants with somewhat thick, spiny leaves. The dominant shrubs are 50-60 cm tall, and 1-3 m tall plants also occur. Typical genera are Agave, Dasylirion, Nolina, Beaucarnea and Yucca all of which have the majority of their species and perhaps their origins in Mexico.
3. High, scarcely thorny scrub is a complex, variable formation with great species diversity. It is dominated by shrubs over 2 m tall, some with thorns or spines but most unarmed or nearly so. Low trees are sometimes interspersed. Some species are deciduous during the dry season. Frequent genera are Acacia, Forestiera, Fraxinus, Gochnatia, Leucaena, Ptelea and Schaefferia.
4. High thorn scrub is characteristic of alluvial soils in the lower portion of valleys. Thorny shrubs 2-3 m tall are dominant, and trees 5-6 m tall are frequently interspersed. Common genera are Acacia, Agonandra, Bumelia, Celtis, Cercidium, Lysiloma, Myrtillocactus, Opuntia, Stenocereus, Parkinsonia, Prosopis and Ximenia.
5. High, spiny thick-stemmed scrub is dominated by cacti which characterize the Tehuacán region (Meyrán-García 1980). Species may be unbranched, or slightly to strongly branched. Outstanding is Neobuxbaumia tetetzo, a few-branched columnar cactus 10-15 m tall, which sometimes densely dominates in communities ("tetecheras"). Other associated species are Cephalocereus columna-trajani and Neobuxbaumia mezcalaensis, and sometimes Beaucarnea gracilis - a notable rosette plant with distinctive thick stems. A strikingly dense stand of arborescent cacti is near Calipan, south-east of the city of Tehuacán. This type of scrub grows on latosol soils and often on mudstone. Other important genera are Escontria, Pachycereus, Pilosocereus, Polaskia, Stenocereus, Fouquieria, Hesperothamnus and Lasiocarpus.
6. Low early deciduous forest is very rich in number of species, and notable for the mostly quickly deciduous trees 8-10 m tall with exfoliating bark. This type of vegetation is characteristic of the warmer and moderately more humid areas in the lower portion of valleys and canyons, on varied soils ranging from latosols and conglomerate to mudstone. Often represented are cacti in Cephalocereus and Neobuxbaumia, as well as species of Beaucarnea. Also important are Amphipterygium, Bursera, Ceiba, Cercidium, Cyrtocarpa, Lasiocarpus, Lysiloma, Pseudobombax and Pseudosmodingium.
This arid region has been recognized as the Tehuacán-Cuicatlán Floristic Province (Rzedowski 1978; cf. Delgadillo-M. and Zander 1984). The flora is very rich (Dávila-Aranda 1983), with c. 910 genera of seed plants, including 8 gymnosperm genera; there are c. 2700 species of vascular plants. Endemism is estimated to be 30%. The southern Tehuacán Valley is considered one of three Mexican centres rich in Agave taxa. Of the c. 250 genera endemic to Mexico, c. 10% occur in this region (Jaramillo-Luque and González-Medrano 1983; Rzedowski 1978, 1993). Local endemics include several plants recognized near or at the generic level, such as Gypsacanthus, Mammillaria pectinifera (Solisia), Oaxacania, Pringleochloa, Salvia sect. Conzattiana (S. aspera) - which may be relictual (Ramamoorthy and Lorence 1987), and Trichostema sect. Rhodanthum (T. purpusii). Rare endemics include Agave lurida, A. peacockii, A. titanota (perhaps), Mammillaria pectinifera (Solisia pectinata) and Salvia aspera.
The surrounding mountains are part of an interrupted chain from the U.S.A. into Central America. From the north, the Sierra Madre Oriental separates Puebla and Veracruz states and ends at the Trans-Mexican Volcanic Belt, which extends to the Atlantic coast; south of the belt is the Sierra de Zongólica, and then the Sierra de Juárez (CPD Site MA3, see Data Sheet). These extended mountains have been a bridge for the dispersal of plants and animals for millennia. South of the Tehuacán Valley are other dry valleys and mountains that may serve as a pathway for many plants that require xeric habitats (Smith 1965b; López-Ramos 1981).
Phytogeographically, the region's flora is related to the floras of other arid and semi-arid regions, such as the Balsas Depession nearby to the west; to the north-west in Hidalgo State, a small desert enclave - the only other locale for Lepechinia mexicana (Sphacele mexicana); and farther away (250 km) to the north, the Chihuahuan Desert and as well southern Texas. The valley has floristic affinities not only with the arid regions; there also is a marked influence of tropical elements, especially those characteristic of Mexico's Pacific slope (Smith 1965b; Rzedowski 1978; Villaseñor, Dávila and Chiang 1990). The Tehuacán-Cuicatlán region is geologically ancient, and probably was more climatically isolated in the past - which helps to explain the high endemism (Smith 1965b).
Native species are used traditionally for example as medicinals, food (e.g. cactus fruits), fibre (e.g. basket-making), fuelwood, living fences and in ceremonials; some are sold in local markets (Meyrán-García 1980). Various species of considerable horticultural value have attracted national and international markets, and some are being readily produced in cultivation elsewhere. For example among hobbyists and landscapers, interest is strong for some cacti, Nolina, Beaucarnea, Dasylirion, Agave, Hechtia and Tillandsia (Dávila-Aranda 1983).
Social and environmental values
The Tehuacán-Cuicatlán region has localities (e.g. San Juan Raya and near Tepeji de Rodríguez) with important Mesozoic fossils. The region is one of five truly desert biotic provinces in Mexico. The Tehuacán-Cuicatlán Valley is within the arid intermontane areas of the Balsas drainage and interior Oaxaca, which is recognized as an Endemic Bird Area (EBA A27). A number of restricted-range species occur, including dusky hummingbird (Cynanthus sordidus), grey-breasted woodpecker (Melanerpes hypopolius), pileated flycatcher (Xenotriccus mexicanus), Boucard's wren (Campylorhynchus jocosus), bridled sparrow (Aimophila mystacalis) and white-throated towhee (Pipilo albicollis).
Archaeological research in the Tehuacán Valley has contributed greatly to understanding processes on the origin and spread of plant cultivation and domestication (MacNeish 1972; Bye 1993; Hernández-Xolocotzi 1993). Comprehensive research by the Tehuacan Archaeological-Botanical Project has reconstructed the 11,500-year prehistoric chronology of the inhabitants' patterns of subsistence and development of agriculture, including the domestication of some plants (Byers 1967; MacNeish, Peterson and Neely 1972). Remains include up to 15 races of maize (Zea mays subsp. mays) - the earliest dated 5600 years ago (Long et al. 1989); several kinds of squashes and gourds (Cucurbita spp., Lagenaria siceraria, Crescentia cujete, Apodanthera sp.); species and races of beans (Phaseolus spp., Canavalia sp.); amaranths (Amaranthus spp.); cassava (Manihot esculenta); chili pepper (Capsicum annuum); avocado (Persea americana); guava (Psidium guajava); and tempesquite (Mastichodendron).
The Tehuacán Valley has been irrigated for over two millennia. Local communities have transformed areas by means of an irrigation system that has grown elaborate and very extensive. Construction between 1944-1969 greatly expanded the system and has resulted in irrigation of 166 km², mostly throughout the year. The water-capture technology includes not only use of surface water in a lengthy network of canals, but also tunnels from the New World's only well-known development of qanats - laterally hand-dug tunnels ("galerías") that tap distant aquifers, creating gradual flow of their groundwater into wells (Enge and Whiteford 1989).
Amerindian bands apparently developed into loosely organized agricultural communities that were chiefdoms, independent small villages or city-states prior to conquest by the Aztecs at the beginning of the 16th century (Flannery 1983). The present regional inhabitants tend to be ethnically associated with local communities and speak dialects of Náhuatl and/or Spanish. Population density in the productive valley is c. 100 persons per km².
Agriculture in the Tehuacán Valley is of two main types: upland farming on good areas above 1800 m in the oak-pine zone of the surrounding mountains, and intensive valley farming. About 410 km² of the valley were in cultivation in 1981: on the limited alluvial areas of canyons and watercourses, c. 40% of the total as irrigated croplands on the valley floor and gentle hillsides, and dry farming on the marginal land of hillsides. Crops are diverse, e.g. maize, beans, squash, tomatillo (Physalis philadelphica), tomato, garlic, dry peppers, peas, sugarcane, saffron, alfalfa and sorghum. Between 1973 and 1981, many of the region's farmers converted from mainly subsistence cultivation to almost completely cash-crop agriculture.
The Tehuacán-Cuicatlán region connects the city of Oaxaca with Mexico's heavily populated Central Plateau. The Mexico City-Oaxaca railroad (constructed in 1892) and a major highway pass through the valley. Another main highway links the cities of Tehuacán and Orizaba (67 km away) and continues east to important markets in the Veracruz lowlands.
The surrounding mountains have been heavily deforested, with the inevitable results of decreased water infiltration for aquifer recharge, and rapid runoff with erosion and flash floods. The groundwater level is decreasing in the Tehuacán Valley's upper aquifer stratum (which is between 30-50 m subsurface). Natural springs around the city Tehuacán decreased c. 12% in flow rate from 1976 to 1983; galería systems measured from 1968 or 1976 to 1983 decreased c. 38%. Demand is intensifying to drill more deep wells to obtain fossil water from a separate stratum at 114-195 m - an essentially non-renewable supply already also showing decline. Yet increasing the use of water might expand agriculture.
The valley's flora is severely threatened by agricultural and pastoral activities, especially overgrazing by goats. About 500 km² in the Tehuacán Valley are used for grazing. In the broad lower part of the valley, the vegetation is affected most by the agricultural activities. Road improvements cause additional losses of natural vegetation. Also, some of the native species and populations are threatened by trade and commercialization of wild specimens, especially those with ornamental value such as some of the cacti and other succulent or thick-stemmed (pachycaul) species.
Although there is general interest in protection of representative areas within the region, there is no official plan to establish nature reserves. So far, just 1 km² near Zapotitlán de las Salinas has been set aside, as the Helia Bravo Hollis Botanical Garden. The necessity for conservation is clear from the biological standpoint, and because the region's hydrology is critically important to support an increasing human population (Enge and Whiteford 1989).
Map 15. Tehuacán-Cuicatlán Region, Mexico (CPD Site MA4)
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Byers, D.S. (ed.) (1967). The prehistory of the Tehuacan Valley, Vol. 1: Environment and subsistence. University of Texas Press, Austin. 331 pp.
Dávila-Aranda, P.D. (1983). Flora genérica del Valle de Tehuacán-Cuicatlán. M.S. thesis. Universidad Nacional Autónoma de México, Mexico, D.F. 694 pp.
Delgadillo-M., C. and Zander, R.H. (1984). The mosses of the Tehuacan Valley, Mexico, and notes on their distribution. Bryologist 87: 319-322.
Enge, K.I. and Whiteford, S. (1989). The keepers of water and earth: Mexican rural social organization and irrigation. University of Texas Press, Austin. 222 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.
Flannery, K.V. (1983). Pre-Columbian farming in the valleys of Oaxaca, Nochixtlán, Tehuacán, and Cuicatlán: a comparative study. In Flannery, K.V. and Marcus, J. (eds), The cloud people: divergent evolution of the Zapotec and Mixtec civilizations. Academic Press, New York.
Hernández-Xolocotzi, E. (1993). Aspects of plant domestication in Mexico: a personal view. 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. 733-753.
Jaramillo-Luque, V. and González-Medrano, F. (1983). Análisis de la vegetación arbórea en la provincia florística de Tehuacán-Cuicatlán. Bol. Soc. Bot. Méx. 45: 49-64.
Long, A., Benz, B.F., Donahue, D.J., Jull, A.T. and Toolin, L.J. (1989). First direct AMS dates on early maize from Tehuacán, Mexico. Radiocarbon 31: 1035-1040.
López-Ramos, E. (1981). Geología de México, Tomo III. Publ. particular autorizada, Mexico, D.F. 446 pp.
MacNeish, R.S. (1972). Summary of the cultural sequence and its implications in the Tehuacan Valley. In MacNeish, R.S. et al., The prehistory of the Tehuacan Valley, Vol. 5: Excavations and reconnaissance. University of Texas Press, Austin. Pp. 496-504.
MacNeish, R.S., Peterson, F.A. and Neely, J.A. (1972). The archaeological reconnaissance: [introduction]. In MacNeish, R.S. et al., The prehistory of the Tehuacan Valley, Vol. 5: Excavations and reconnaissance. University of Texas Press, Austin. Pp. 341-360.
Meyrán-García, J. (1980). Guía botánica de cactáceas y otras suculentas del Valle de Tehuacán, 2nd edition. Soc. Mexicana de Cactología, Mexico, D.F. 52 pp.
Miranda, F. (1948). Datos sobre la vegetación en la cuenca alta del Papaloapan. Anales Inst. Biol. Univ. Nac. Méx. 19: 333-364.
Ramamoorthy, T.P. and Lorence, D.H. (1987). Species vicariance in the Mexican flora and description of a new species of Salvia (Lamiaceae). Adansonia 2: 167-175.
Rzedowski, J. (1978). Vegetación de México. Editorial Limusa, Mexico, D.F. 432 pp.
Rzedowski, J. (1993). Diversity and origins of the phanerogamic flora 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. 129-144.
Smith, C.E., Jr. (1965a). Agriculture, Tehuacan Valley. Fieldiana, Bot. 31: 49-100.
Smith, C.E., Jr. (1965b). Flora, Tehuacán Valley. Fieldiana, Bot. 31: 101-143.
Tamayo-L., J. (1962). Geografía general de México, 2nd edition, Vol. 1. Inst. Mex. Invest. Econ., Mexico, D.F. 562 pp.
Villaseñor, J.L., Dávila, P. and Chiang, F. (1990). Fitogeografía del Valle de Tehuacán-Cuicatlán. Bol. Soc. Bot. Méx. 50: 135-149.
Zavala-H., J.A. (1982). Estudios ecológicos en el valle semiárido de Zapotitlán, Pue. I. Clasificación numérica de la vegetación basada en atributos binarios de presencia o ausencia de las especies. Biótica 7: 99-120.
This Data Sheet was written by Dra. Patricia D. Dávila [Universidad Nacional Autónoma
de México (UNAM), Instituto de Biología, Departamento de Botánica, Apdo. 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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