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The Gran Chaco constitutes the phytogeographic Chaquenian province, and occupies 500,000 km² in northern Argentina, 350,000 km² in north-western Paraguay, a few km² around Porto Murtinho in south-western Brazil (Mato Grosso do Sul State) and 160,000 km² in south-eastern Bolivia (in Tarija, Chuquisaca and Santa Cruz departments) (cf. Prado 1993). The landscape is almost flat, sloping gradually eastward (0.04% gradient). Geologically, the Chaco plain is a tectonic depression filled with 3000 m of sediments from the Palaeozoic, Mesozoic and Tertiary covered by fine unconsolidated Quaternary deposits (FAO and UNESCO 1971; FAO and UNEP 1985). Several interruptions of older rocks are elevated approximately 400 m to 2400 m above the plain, including the Pampean mountains in Argentina, Cerro León in Paraguay and the Chiquitos in Bolivia.
Soils are usually neutral or slightly alkaline, with a high base level of saturation (90-100%). In the west the soils may be acidic and are more open to sandy and with good drainage; in the east they are mainly clayish and with poor drainage. Because of the semi-arid climate, primary minerals and soluble salts are abundant and result in areas of saline soils (FAO and UNESCO 1971).
The topography is influenced by water and wind. Eighty percent of the region is in La Plata River watershed; its major tributaries crossing the Gran Chaco are the Pilcomayo, Bermejo (Teuco) and Juramento-Salado rivers. Significant confined watersheds are Sali-Dulce River and Salinas Grandes in Argentina, Salinas de San Miguel and San José in Bolivia and Timané (Lagerenza) River in Paraguay. In dry areas of the Gran Chaco, the watercourses are ephemeral, often changing locally from year to year; in wet areas, rivers persist. Winds blowing across the plain sometimes form undulating sand-dunes, which are best represented on the frontier between Paraguay and Bolivia.
Intense, long-lasting sheet lightning occurs in the Gran Chaco. These powerful discharges are considered to cause many of the large fires in the region (Schwerdtfeger 1976). The summers are hot and humid, the winters mild but with possible frost and drier - with a strong dry season to the west. Precipitation generally decreases westward, with the least rainfall just east of the sierras on the western plains (Prado 1993). During the irregular rainy season from October to April, inundations cover vast areas of the land - up to 15% and for several months (FAO and UNESCO 1971). Based on climatic conditions, three broad zones of the Gran Chaco can be recognized (Vargas 1988):
1. Humid-subhumid zone (Eastern Chaco), annually receiving from 1250 mm of rainfall in the east to 700 mm towards the west. Median temperatures vary from 23°C in the north to 19.5°C in the south, with extremes of 43°C and -2.5°C.
2. Semi-arid to arid zone (Central and Western Chaco), with 650-350 mm of rainfall from its east to west. Median temperatures are 28°C in the north to 12°C in the south. The isotherm of absolute maximum above 47°C passes through the Argentine provinces of Salta, Formosa and Chaco and surrounds Santiago del Estero Province, which has had lows to -7.2°C (Prado 1993).
3. Montane or highland zone (Sierran Chaco), with an average of 500-900 mm of rainfall and some sites receiving up to 1100 mm or more. Temperatures of these areas are very variable; the annual mean is 17°C or less.
The predominating vegetation is xerophytic deciduous forest with species in 3-4 strata: canopy and subcanopy trees, shrubby understorey and herbaceous grasses, with cacti and bromeliads. Localized edaphic and climatic conditions determine the vegetation in some parts of the region. Other types of vegetation include non-flooding and annually flooded riverine forests, wetlands, palm woodlands, savannas, grasslands, halophytic shrubby steppes, and cactus stands (Prado 1993; Herzog 1923; Spichiger et al. 1991; Ramella and Spichiger 1989; Cabrera 1976). The vegetation types are described below, based on the three climatic zones.
1. Humid-subhumid Chaco
On heavy-textured, sodium-ion-rich soils usually waterlogged during the rainy season, the stabilized forest community is dominated by Schinopsis balansae. Lianas are scarce, but epiphytes (especially Tillandsia spp.) are frequent. Other common canopy trees are Aspidosperma quebracho-blanco and Caesalpinia paraguariensis; the second storey includes Prosopis nigra, Acacia praecox, A. aroma, Geoffroea decorticans, Ziziphus mistol and Sideroxylon obtusifolium (Prado 1993). The common understorey species are Schinus sp., Castella coccinea, Opuntia retrorsa and Cereus sp. The common herbaceous species are Dyckia ferox, Aechmea distachantha and grasses. At the eastern beginning of the subhumid zone appear Schinopsis lorentzii and S. heterophylla.
Along rivers, diverse forest vegetation develops, for example woodlands of Tessaria integrifolia and Salix humboldtiana. There are subtropical semi-evergreen gallery forests which are flooded annually, and taller (to 30 m) riverine forests on non-flooding higher ground; both types are rich in lianas and vascular epiphytes (Spichiger et al. 1991; Prado 1993).
On alkaline soils that are seasonally flooded, palm savannas of Copernicia alba occur, with dispersed individuals of Prosopis alba, P. algarobilla, Celtis tala, etc. The herbaceous species are primarily grasses - Spartina, Sporobolus and Paspalum.
Several kinds of bunch grasses grow in both flooded and dry areas of the Gran Chaco. Important species are Sorghastrum agrostoides and Paspalum intermedium. On saline soils that are periodically flooded, "esparto" zones are dominated by savanna of Elionurus muticus, generally with other grasses including Bothriochloa, Chloris and Schizachyrium.
2. Semi-arid to arid Chaco
Other types of vegetation in this zone include: on limey-clayish soils, forests of Bulnesia sarmientoi and Tabebuia nodosa; along riverbanks, forests dominated by Enterolobium contortisiliquum, Acacia caven or Calycophyllum multiflorum; in sandy areas, grassy savannas with Schinopsis heterophylla, as well as Jacaranda mimosifolia and Schinopsis quebracho-colorado; on somewhat saline soils, the giant cactus Stetsonia coryne in association with shrubs of Bulnesia, Maytenus and Suaeda; and on very saline soils, shrubby steppes of Heterostachys ritteriana, Allenrolfea patagonica, Atriplex and Prosopis.
3. Highland Chaco
In the Paraguayan uplands (cerros León and Cabrera), three zones of vegetation are recognized as well. On slopes with sufficient water there is deciduous forest dominated by Anadenanthera colubrina, with Pterogyne nitens, Amburana cearensis and Aspidosperma pyriformis; on hilltops with shallow soils is shrubby cerrado-like savanna with Pseudobombax campestre; and on higher elevation tablelands with shallow soils occurs species-rich grass savanna of Chloris, Digitaria and Stipa along with Tabebuia aurea.
The floristic inventory of the Gran Chaco is incomplete (Zellweger et al. 1990), but the region is estimated to have 1000-1200 species. The most important families and genera are Leguminosae (Prosopis, Acacia, Caesalpinia, Cercidium, Geoffroea); Anacardiaceae (Schinopsis, Lithrea); Apocynaceae (Aspidosperma); Palmae (Copernicia, Trithrinax); Zygophyllaceae (Bulnesia); Rhamnaceae (Ziziphus); Santalaceae (Jodina, Acanthosyris); Cactaceae (Opuntia, Cereus, Stetsonia, Pereskia, Quiabentia); Bromeliaceae (Dyckia, Bromelia, Puya); and Gramineae (e.g. Elionurus, Paspalum, Chloris, Trichloris) (Cabido, Acosta and Díaz 1990).
The region has floristic affinities with the neighbouring phytogeographic provinces - espinal, monte, prepuna, caatinga and campos. Certain associations, such as those in savannas or sandy or upland areas, share floristic elements with the caatinga and cerrado (cf. Prado 1993) (see CPD Sites SA19 and SA21).
Among the rare and/or endemic plants are Stetsonia, Lophocarpinia, Mimozyganthus, Stenodrepanum, Trachypteris pinnata, Berberis hieronymi, Arenaria achalensis, Cnicothamnus azafran, Soliva triniifolia and Jatropha matacensis (Prado 1993; Cabido, Acosta and Díaz 1990).
Large areas of the Gran Chaco have been intensively utilized. Many species that are commercially exploited may be at risk; appropriate multiple-use management and conservation efforts are needed. Chemical compounds that are useful for medicines, dyes and other industrial purposes are produced by many of the region's native species.
The genus Schinopsis ("quebracho"), represented by six species (out of a total of nine), is extensively exploited for timber and as an international source of tannin. Twenty of the 44 species of Prosopis ("algarrobo") are native to the region, and are used in furniture-making, as forage, and for industrial and medicinal purposes. The wood of Bulnesia sarmientoi ("palo santo"), from the Central Chaco, is the source of oil of guaiac, a fragrance used in soaps. Several species of Bromeliaceae are used by indigenous people as a source of fibre (not in the market economy). Trithrinax campestris leaves are processed industrially for fibre. Trunks of Copernicia alba ("caranday") are utilized for telephone and electric poles and in construction, and carnuba wax is extracted from the palm's leaves. Epiphytic Bromeliaceae and Orchidaceae are important in the horticultural trade, including the export market. Many species of bromeliads and orchids in the Gran Chaco may be scientifically undescribed. Promising agroforestry species include Caesalpinia paraguariensis, Prosopis alba var. panta and P. chilensis which are used as fodder; wild species of Ananas (pineapple) and Oryza (rice) are among the germplasm resources.
Social and environmental values
Population density decreases westward and northward. In Argentina, indigenous Mataco inhabit Salta and Tobas in the provinces of Formosa and Chaco. In Paraguay, 13 ethnic groups inhabit the Central Chaco, including several which until recently had little involvement with Western civilization (Williams 1982; FAO and UNEP 1985).
The highland areas are the source of water for many of the streams that supply the arid areas of the Gran Chaco. Adequate management of these watersheds is important to sustain the people, plants and animals of the more arid areas and for hydroelectric power.
The diversity of fauna is high, and subsistence and sport hunting are popular. The word "chaco" comes from the Quechua language and means hunting land. As recently as 1975, a new species of peccary was recognized, the taguá or giant peccary. Other native mammals include giant anteater, ocelot, Geoffroy's cat, pampas cat, jaguarundi, puma and jaguar. Further research on the ecology of the Gran Chaco and adaptations of species to the environmental stresses are needed. Some plants have evolved special adaptations to cope with the rigorous environmental conditions and other factors. The northern areas (in Bolivia and Paraguay) generally require greater scientific research.
Tourism is growing in the region, including ecotourism. Around Cerro Colorado in Córdoba, Argentina, 35,000 ancient rock paintings have been discovered.
The ecosystems of the Gran Chaco are relatively fragile and susceptible to erosion by wind and water. In some areas erosion has caused severe damage, for example north-west of Tarija in Bolivia.
Logging combined with livestock grazing is reducing natural diversity, decreasing populations and the genetic potential of species. For example, over a period of 60 years, the exploitation of Schinopsis balansae in Argentina decimated 104,000 km² (Bunstorf 1971). The massive felling of trees together with grazing create shrubland, which has much thorny Prosopis ruscifolia in humid areas and Acacia furcatispina in arid areas. This process can lead to desertification. People who depend on the forests for subsistence are losing a source of food (e.g. fruit, meat, honey), shelter and other amenities (Morello et al. 1978). Land clearing and over-hunting have become problems along the paved Trans-Chaco highway in Paraguay (Taber 1989).
Road construction for oil and gas exploration in Paraguay and Bolivia and for selective logging in Argentina affects the fauna and vegetation. Stones used in road construction are quarried from the scarce lower hills on the Chaco plain, causing further habitat loss.
Some environmental education is carried out in the region, and just over 1% of the Gran Chaco is represented in protected areas (see Map 58). In Argentina, where there was the greatest portion of the Chaco, c. 720 km² are in three protected areas: in the humid zone, Chaco National Park (c. 150 km²), with Schinopsis balansae; in the subhumid zone, Río Pilcomayo NP (c. 470 km² near Asunción), with Copernicia alba; and in the subhumid and semi-arid zones, Formosa National Nature Reserve (100 km² of south-western Formosa Province).
In Paraguay, Chaco ecosystems are better represented in protected areas, with four National Parks covering c. 12,000 km² (DPNVS 1993): in the humid zone, Ypoá NP (1000 km²), and Tinfunqué NP (c. 2800 km²) with Copernicia alba; in the subhumid and semi-arid zones, Defensores del Chaco NP (7800 km²), which includes Cerro León (c. 700 km²) and has Schinopsis balansae and Aspidosperma quebracho-blanco; and in the semi-arid zone, Teniente Agripino Enciso NP (400 km²), with Aspidosperma quebracho-blanco and Chorisia insignis. Bolivia has no protected areas within its portion of the Chaco (WCMC 1992).
New protected areas for the Gran Chaco have been recommended in all three countries (FAO and UNEP 1985) (Map 58). In northern Argentina, three recommended National Nature Reserves would encompass 2950-3150 km²: Pirámide El Triunfo, Pozo del Cimarrón and El Pintado. In Paraguay there are recommendations (DPNVS 1993) for five National Parks totalling 14,600 km², two ecological reserves (3500 km²) and a scientific reserve (1500 km²), and in the west of Nueva Asunción Department, a sand-dune area (Las Dunas) also has been recommended for conservation. In Bolivia, 300 km² have been recommended as a Nature Reserve in the Villamontes-Sachapera area of Tarija Department, which would protect highland and semi-arid zones. In Santa Cruz Department, several conservation units have been recommended (see Data Sheet for CPD Site SA23).
The ecosystems of the sand dunes on the frontier of Bolivia and Paraguay warrant more attention. Several sites within the Gran Chaco merit attention that have not yet been recommended for protected status. Forests of Prosopis (five species and hybrids) in floodplain areas of the Pilcomayo River are at risk because of the recent popularity of algarrobo wood to manufacture furniture for use locally and export. In the arid zone in north-western Córdoba Province (Argentina), a Nature Reserve of 20,000 km² would protect a very high diversity of Prosopis species. Pampa de Achala, an extensive high plateau in Córdoba Province, has tremendous plant diversity and endemics. Cerros San Miguel (839 m high, c. 10 km²), Cabrera (720 m high, c. 7 km²) and Caimán in Bolivia and north-western Paraguay deserve consideration for their plant endemism and status as ancient refugia and for the maintenance of watersheds.
Map 58. Gran Chaco, Argentina, Paraguay, Brazil, and Bolivia (CPD Site SA22) (after FAO and UNEP 1985)
Bunstorf, J. (1971). Tanningewinnung und Landerschliessung in argentinischen Gran Chaco. Geogr. Zeitschr. 59: 117-204.
Cabido, M., Acosta, A. and Díaz, S. (1990). The vascular flora and vegetation of granitic outcrops in the upper Córdoba mountains, Argentina. Phytocoenologia 19: 267-281.
Cabrera, A.L. (1976). Regiones fitogeográficas argentinas. In Kugler, W.F. (ed.), Enciclopedia argentina de agricultura y jardinería, 2nd edition. Vol. 2(1). Editorial ACME, Buenos Aires. Pp. 1-85.
DPNVS (1993). SINASIP. Plan estratégico del sistema nacional de áreas silvestres protegidas. Dirección de Parques Nacionales y Vida Silvestre (DPNVS) and Fundación Moisés Bertoni para la Conservación de la Naturaleza, Asunción, Paraguay.
FAO and UNEP (1985). Un sistema de áreas silvestres protegidas para el Gran Chaco. Proyecto FAO y PNUMA FP 6105-85 - 01, Documento Técnico No. 1. FAO, Santiago, Chile. 159 pp.
FAO and UNESCO (1971). Soil map of the world 1:5,000,000. Vol. IV, South America. UNESCO, Paris. 193 pp.
Herzog, T. (1923). Die Pflanzenwelt der bolivischen Anden und ihres östlichen Vorlandes. In Engler, A. and Drude, O. (eds), Die Vegetation der Erde. Vol. 15. Engelmann, Leipzig. Pp. 84-105.
Morello, J. et al. (1978). Estudio de factibilidad para la creación de una reserva de ecosistemas en la Provincia del Chaco argentino. SISAGRO, Buenos Aires.
Prado, D.E. (1993). What is the Gran Chaco vegetation in South America? I. A review. Contribution to the study of flora and vegetation of the Chaco. V. Candollea 48: 145-172.
Ramella, L. and Spichiger, R. (1989). Interpretación preliminar del medio físico y de la vegetación del Chaco Boreal. Contribución al estudio de la flora y de la vegetación del Chaco. I. Candollea 44: 639-680.
Schwerdtfeger, W. (ed.) (1976). World survey of climatology. Vol. 12: Climates of Central and South America. Elsevier Scientific Publishing, Amsterdam, The Netherlands. 532 pp.
Spichiger, R., Ramella, L., Palese, R. and Mereles, F. (1991). Proposición de leyenda para la cartografía de las formaciones vegetales del Chaco paraguayo. Contribución al estudio de la flora y de la vegetación del Chaco. III. Candollea 46: 541-564.
Taber, A.B. (1989). Pig from green hell. Animal Kingdom 92(4): 20-27.
Vargas, G. (1988). Chaco sudamericano: regiones naturales. In Alessandría, E., Bernardón, A., Díaz, R., Elisetch, M. and Virasoro, J. (eds), X Reunión Grupo Campos y Chaco: memoria. FAO, UNESCO- MAB, INTA (Instituto Nacional de Tecnología Agropecuaria) and UNC (Universidad Nacional de Córdoba). Córdoba, Argentina. Pp. 16-20.
WCMC (1992). Protected areas of the world. A review of national systems. Vol. 4. Nearctic and Neotropical. IUCN, Gland, Switzerland and Cambridge, U.K. 459 pp.
Williams, J.H. (1982). Paraguay's unchanging Chaco. Américas 34(4): 14-19.
Zellweger, C., Palese, R., Perret, P., Ramella, L. and Spichiger, R. (1990). Concept and use of an integrated database system for the Chaco. Application to a preliminary checklist. Contribution to the study of the flora and vegetation of the Chaco. II. Candollea 45: 681-690.
This Data Sheet was written by Dra. Francisca M. Galera (Universidad Nacional de
Córdoba, Facultad de Ciencias Exactas, Físicas y Naturales, Centro de Ecología y
Recursos Naturales Renovables, Avda. Vélez Sarsfield 299, 5000 Córdoba, Argentina) and
Dr Lorenzo Ramella (Conservatoire et Jardin botaniques de la Ville de Genève, Case
postale 60, CH-1292 Chambésy-Geneva, Switzerland).
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