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CENTRAL REGION OF BAJA
In the Baja California Peninsula morphotectonic province (1300 km ×30-240 km), this mid-peninsular region includes portions of the Pacific Coastal and the Peninsular Ranges subprovinces (Ferrusquía-Villafranca 1993). Topography and climatic and edaphic conditions largely determine the distribution of the vegetation formations and plant communities.
Characteristic in the western and central portions of this region are lower elevations, nearly constant onshore wind, strong solar radiation with high day and low night temperatures, and annual precipitation usually less than 80 mm - predominantly in winter. The median annual temperature ranges between 18°-22°C. The cold marine current from California, U.S.A. influences the climate of this area, bringing cooling temperatures and moisture (regular fog).
Summer rains in the mid-peninsular region are often torrential, due to tropical depressions or cyclones ("chubascos"), which sometimes penetrate the Gulf of California. In contrast, winter rains are usually milder, resulting from the meeting of cold dry air masses with warm humid air masses (Salinas-Zavala, Coria and Díaz 1991). Sometimes storms from both north and south bypass this central region, which consequently can have irregular and patchy intensive droughts.
This region's central portion is within a closed watershed more extensive to its south-west, into which drain many watercourses from the bordering mountains. As well, several watercourses drain westward into the lagoon complex (Ojo de Liebre) near Guerrero Negro, and others flow southward into San Ignacio Lagoon. Few drainages directly reach the coast. Evaporation has caused high concentrations of several salts to deposit through much of the south-western portion of the region. The incessant transverse wind has contributed to the formation of extensive, commonly parallel dunes. The presence of salt-tolerant vegetation in large low areas is considered evidence for shallow seas having occurred there during rather recent marine intrusions (Durham and Allison 1960; Ferrusquía-Villafranca 1993).
This region's eastern portion is more mountainous; it forms a crest (mostly not over 1600 m) that is nearer to the peninsula's coast on the Gulf of California, and it slopes more abruptly into the Sea of Cortes. These mountains are composed principally of extrusive volcanic and various sedimentary rocks (Ferrusquía-Villafranca 1993). This area's basic climate is fairly similar, but characterized by colder winters occasionally with light snows that may linger on the peaks. The precipitation is very variable in amount and location, ranging from 0 mm (for 1-4 years) to a few hundred mm during the year, mostly in the summer.
The region is characterized by xerophilous scrubland, brush or thickets (Rzedowski 1978), and is almost entirely encompassed in two subdivisions of the Sonoran Desert, with a very small portion in the extreme north-east in a third subdivision (Shreve and Wiggins 1964; Wiggins 1980; Turner and Brown 1982). The vegetation occurs in four principal natural systems - three lowland deserts and highlands.
Shreve called this biotic zone the Sarcophyllous Desert (Wiggins 1969, 1980), referring to the physiognomic dominance of species with succulent or thick leaves. Agave and Ambrosia (Franseria) are characteristic, and Tillandsia recurvata ("gallitos") and various lichens (e.g. Rocella) are abundant as epiphytes in the fog belt. Common taxa include Agave shawii (picture) subsp. goldmaniana, A. sobria subsp. sobria, Yucca valida ("datilillo"), Fouquieria columnaris (Idria columnaris) (picture), Pachycormus discolor (picture) (elephant tree or "copalquín"), Stenocereus gummosus ("pitaya agria"), Ambrosia chenopodiifolia (Franseria chenopodiifolia), A. bryantii (F. bryantii), Atriplex magdalenae, A. polycarpa, Lycium spp. and Frankenia palmeri.
Within this biotic zone is a brushy plant community on inland sand dunes, which are relatively unstable; the diversity of species is rather low. Among the characteristic species are Chaenactis lacera, Nicolletia trifida and Dalea mollis. These thickets provide refuge for the animals that inhabit this community and are sufficiently long-lasting for trophic relationships to have developed.
Gulf Coast Desert
Shreve called this biotic zone the Sarcocaulescent Desert (Wiggins 1980); it is physiognomically dominated by species with succulent or similarly thick (pachycaul) trunks or stems, such as Cercidium microphyllum ("dipúa"), Bursera hindsiana ("copal"), B. microphylla (picture) ("torote"), Jatropha cinerea ("lomboy"), J. cuneata, Pachycereus pringlei ("cardón"), Ferocactus spp., Opuntia cholla, O. molesta and O. bigelovii. Also notable are Fouquieria splendens (ocotillo), Lysiloma candida and Errazurizia megacarpa.
San Felipe Desert
These highlands have mesas, hillsides with pronounced gradients, ravines, arroyos and canyons (Wiggins 1969, 1980). Xerophytic species and species more common to less arid climates occur. Characteristic taxa include Fouquieria columnaris (Idria columnaris), Rhamnus crocea, Prunus ilicifolia, Prosopis glandulosa var. torreyana (honey mesquite), Yucca whipplei ("lecheguilla"), Xylococcus bicolor (Arctostaphylos bicolor), Ferocactus emoryi var. rectispinus, Croton ciliato-glanduliferum, Quercus oblongifolia, Sabal uresana, Cordia curassavica and Aralia scopulorum.
Central Baja California has linkages with various nearby floristic regions (Wiggins 1960, 1980; Bowers, Delgadillo-M. and Sharp 1976), and many species reach their phytogeographic limit in approximately mid-peninsula (28°N). The mountains of southern California, U.S.A. have widely distributed genera shared with mountainous areas of Baja California, including Rhus, Prunus, Penstemon, Rhamnus and Salvia subgenus Audibertia (Nelson 1921). Affiliations with the arid south-western U.S.A.'s Mojave Desert or Great Basin include the following genera well represented in drier regions: Astragalus, Phacelia, Lotus, Cryptantha, and to a lesser degree Atriplex, Ephedra, Sphaeralcea, Chaenactis, Frankenia, Haplopappus, Chorizanthe and Eschscholzia (Hastings, Turner and Warren 1972).
The Vizcaíno Desert and the other subdivisions of the Sonoran Desert share many species that are salt-tolerant, or are widely distributed - especially those that are subtropical, e.g. in Bursera, Lycium, Stenocereus, Opuntia and Proboscidea, as well as Larrea tridentata (Shreve and Wiggins 1964; Wiggins 1980).
The Baja California Peninsula is recognized as an area of moderate endemism (Rzedowski 1993), with approximately 10 genera and 20% of the species, and many of these peninsular endemics are present in the central region (Wiggins 1980). In Vizcaíno Desert Biosphere Reserve, excluding the flora of coastal lagoons, 496 species of vascular plants have been documented, 39 of which are endemic locally (León de la Luz, Cancino and Arriaga 1991). Eriogonum (Polygonaceae) is represented by 10 species, 8 of which are endemic. The families with the greatest species diversity are Compositae, Leguminosae, Gramineae, Cactaceae and Euphorbiaceae.
This central region of Baja California is thus a rich transitional zone (cf. Wiggins 1960), considering the lowland and highland habitats, the many species distributed on the peninsula mostly to the north or south and/or including species in northern North American and/or on the Mexican mainland, and the low proportion of local endemics (7.9%) but higher number of peninsular endemics.
The region was occupied by indigenous people, who are now extinct. There is only very limited indirect evidence to indicate how they utilized the natural resources. Nevertheless, ethnobotanical information on many species in this region is known from elsewhere (cf. Coyle and Roberts 1975), and a good many plants represent potential genetic reserves and aesthetic and commercial resources. For example, species such as Fouquieria columnaris (Idria columnaris) (boojum tree or "cirio") and Pachycereus schottii (Lophocereus schottii) forma monstrosus and many other succulents, pachycauls and dryland species are attractive for hobbyists, landscapers and others, and are in cultivation elsewhere.
Social and environmental values
In the mountainous areas ancient rock paintings are found, which are culturally significant and possibly among the earliest human documentations on the North American continent. The mountains are the principal renewable sources for water that supplies the towns and agricultural areas of the plains. Wild bighorn sheep (Ovis canadensis) inhabit the mountain slopes, and constitute a resource that when well managed for hunting, is important to obtain foreign currency for the state and federal governments.
Peninsular endemism in various groups of fauna is considerable (Ramamoorthy et al. 1993). Notably, the central region includes portions of two of the five mammalian primary areas on the peninsula, as determined by an index of faunistic change.
Baja California is one of five truly desert biotic provinces in Mexico, and still offers wilderness as well as rugged landscapes. The transverse dunes of the Vizcaíno Desert, resembling furrows in a mammoth ploughed field, occur in only a few places in the world.
Tourism contributes to the economy of the region. Tourists (mainly from the U.S.A.) visit the lagoons around Guerrero Negro where grey whales (Eschrichtius robustus) come to reproduce. The lagoons also support hundreds of migratory birds from northern North America. The ancient rock paintings attract tourists to the sierras de Santa Martha and San Francisco.
Approximately 42,000 people live in central Baja California, mostly in rural areas. Santa Rosalía, San Ignacio and Guerrero Negro are the main towns, with a total population of c. 19,000. Only c. 8000 of the entire population are employed (BCS 1993). The main income-generating occupations in the general area are mining of salt and gypsum, harvesting of clams, and agriculture (Castellanos and Mendoza 1991).
The salt mines of Guerrero Negro are considered among the largest anywhere, with an annual production of 5.4 million tonnes. Gypsum mining on San Marcos Island (6 km south-east of Santa Rosalía) annually produces nearly 2.7 million tonnes. In the Vizcaíno plains, oil and gas reserves and deposits of magnesite, diatomite, chromite and asbestos have been found.
Agriculture is concentrated around the town of El Vizcaíno, which has c. 65 km² of irrigated land. Many of the field workers come from southern Mexico. However, too many demands are being placed on the limited groundwater aquifer, and it is estimated that fairly soon water problems will develop.
In the mountainous areas there are c. 24,000 goats, which cause significant damage to the native vegetation. At lower elevations, the cattle population is estimated to be c. 19,000, but they have no significant general impact on the native vegetation.
The ranchers do not control the grazing of their goats on the mountain slopes, which has led to serious plant damage and soil erosion. In some areas, overgrazing is so significant that only plants that are toxic to the goats can grow - e.g. Astragalus francisquitensis and A. prorifer.
On the western plains, roads have been built to bring in large equipment used for natural gas exploration by Petroleos Mexicanos. Reserves of natural gas have been found, but extraction is not profitable at present. On the western plains in Baja California Sur, six public grazing areas ("ejidos") encompassing 9500 km² have been set aside for 383 families. However, the pumped water supply is inadequate to support both agriculture and the livestock for these public grazing lands. Within a decade problems are expected to develop from brackish water intrusion into the groundwater.
The company Exportadora de Sal, S.A. manages the world's most productive salt works. Expansion of its activities has generated various impacts on the region's terrestrial and marine habitats. Terrestrial areas are inundated by saltwater, which for example affects the distribution of the peninsular pronghorn antelope (Antilocapra americana subsp. peninsularis), a deer-like mammal in danger of extinction.
Approximately 42% (15,000 km²) of the central region of Baja California is protected in the Vizcaíno Desert Biosphere Reserve, which was declared in November 1988 and covers 25,500 km² (Ortega and Arriaga 1991) (see Map 23). Arid lands constitute nearly 40% of Mexico's territory, and c. 8% of these arid-land ecosystems are in conserved areas - c. 40% of the protected total is in this reserve. More research is needed on the region's biodiversity, including scientific studies to understand ecological interactions and adaptations of the species to this harsh environment.
Since goats are destroying extensive areas of native vegetation, special efforts are urgently needed to work with the ranchers to contain their animals and to convey the importance of maintaining naturally vegetated highlands. On the public-grazing lowlands, management plans for sustainable utilization of natural resources need to be provided. The personnel of the Centro de Investigaciones Biológicas de Baja California Sur in La Paz are familiar with these concerns, and although with limited support, they are attempting to address them.
Map 23. Central Region of Baja California Peninsula, Mexico (CPD Site MA12)
BCS (1993). Datos básicos 1992. Gobierno Constitucional del Estado de Baja California Sur (BCS), La Paz, B.C.S., Mexico.
Bowers, F.D., Delgadillo-M., C. and Sharp, A.J. (1976). The mosses of Baja California. J. Hattori Bot. Lab. 40: 397-410.
Castellanos, A. and Mendoza, R. (1991). Aspectos socioeconómicos. In Ortega, R. and Arriaga, L. (eds), La Reserva de la Biósfera El Vizcaíno en la Península de Baja California. Publicación No. 4 del Centro de Investigaciones Biológicas de Baja California Sur (CIB-BCS), La Paz, B.C.S., Mexico. Pp. 33-52.
Coyle, J. and Roberts, N.C. (1975). A field guide to the common and interesting plants of Baja California. Natural History Publishing Co., La Jolla, Calif., U.S.A. 206 pp.
Durham, J.W. and Allison, E.C. (1960). The geologic history of Baja California and its marine fauna. In Symposium: The biogeography of Baja California and adjacent seas. Syst. Zool. 9: 47-91.
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.
Hastings, J.R., Turner, R.M. and Warren, D.K. (1972). An atlas of some plant distributions in the Sonoran Desert. University of Arizona, Institute of Atmospheric Physics, Technical Reports on Meteorology and Climatology of Arid Regions No. 21. 255 pp.
León de la Luz, J.L., Cancino, J. and Arriaga, L. (1991). Asociaciones fisonómico-florísticas y flora. In Ortega, R. and Arriaga, L. (eds), La Reserva de la Biósfera El Vizcaíno en la Península de Baja California. Public. No. 4 del CIB-BCS, La Paz, B.C.S., Mexico. Pp. 123-154.
Nelson, E.W. (1921). Lower California and its natural resources. Mem. Natl. Acad. Sci. 16: 1-194.
Ortega, R. and Arriaga, L. (eds) (1991). La Reserva de la Biósfera El Vizcaíno en la Península de Baja California. Public. No. 4 del CIB-BCS, La Paz, B.C.S., Mexico. 317 pp.
Ramamoorthy, T.P., Bye, R., Lot, A. and Fa, J.E. (eds) (1993). Biological diversity of Mexico: origins and distribution. Oxford University Press, New York. 812 pp.
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.
Salinas-Zavala, C., Coria, R. and Díaz, E. (1991). Climatología y meteorología. In Ortega, R. and Arriaga, L. (eds), La Reserva de la Biósfera El Vizcaíno en la Península de Baja California. Public. No. 4 del CIB-BCS, La Paz, B.C.S., Mexico. Pp. 95-115.
Shreve, F. and Wiggins, I.L. (1964). Vegetation and Flora of the Sonoran Desert. Stanford University Press, Stanford, Calif., U.S.A. 2 vols., 1740 pp.
Turner, R.M. and Brown, D.E. (1982). Sonoran desert scrub. In Brown, D.E. (ed.), Biotic communities of the American Southwest United States and Mexico. Desert Plants 4. Pp. 181-221.
Wiggins, I.L. (1960). The origins and relationships of the land flora. In Symposium: The biogeography of Baja California and adjacent seas. Part III, Terrestrial and fresh-water biotas. Syst. Zool. 9: 148-165.
Wiggins, I.L. (1969). Observations on the Vizcaíno Desert and its biota. Proc. Calif. Acad. Sci., ser. 4, 36: 317-346.
Wiggins, I.L. (1980). Flora of Baja California. Stanford University Press, Stanford, Calif., U.S.A. 1025 pp.
This Data Sheet was written by José Luis León de la Luz (Centro de Investigaciones
Biológicas de Baja California Sur, A.C., División de Biología Terrestre, Apdo. Postal
128, La Paz, Baja California Sur 23000, Mexico) and Olga Herrera-MacBryde (Smithsonian
Institution, SI/MAB Program, S. Dillon Ripley Center, Suite 3123, Washington, DC
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