Forest patches in the Brazilian cerrado.

Neotropical grasslands support a wide variety of plants and animals, and cover a range of habitat types and regions.

ECOLOGICAL AND REGIONAL DEFINITIONS

We use the terms "Neotropical" and "grasslands" in a broad sense.

Neotropical – In the strictest sense, the area in the New World between the tropical latitudes (23.5 degrees). Here, we define Neotropical as the biogeographic region of Central and South America.

Neotropical Grasslands – Cover a range of habitat types and regions. From permanently flooded wetlands to cyclically dry and flooded savannas, these regions support a wide variety of plants and animals.

Savannas – Open areas where grasses dominate and where seasonal droughts and frequent fires are normal ecological factors. Savannas may include trees and shrubs, but not in a continuous cover.

Wetlands – Similar to savannas but no fire or dry season; dominated by grasses other than families Graminae and Cyperaceae.


NGC FUNDED ECOSYSTEMS


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Savannas cover some 23 million square kilometers or about 45% of South America. American savannas have distinctive climate-soil environments.

Caatinga – Areas of dry shrub lands in northeast Brazil, extend over 800,000 square kilometers. They have virtually no grass substrate and more dry season moisture availability. Do not qualify as cerrado as it has virtually no grass substrate.

Cerrado – Brazilian savannas. There is a wide range in the density of trees over the savannas, especially in the well-drained savannas; these range from pure grasslands to virtual forests. In Brazil, savannas are referred to as:

campo limpo (clean grass fields)

campo sujo (grassland with some shrubs)

campo cerrado (open savanna)

cerradão (closed savanna approaching a forest)

Click here for more information on cerrados.

Chaco – Thorn scrub, over 900,000 square kilometers, extends from the northern Argentina to southern Bolivia and across to Paraguay.

Espinal – Dry woodlands bordering open grasslands.

Gallery Forests – Vegetation bordering streams and rivers. Comparable to riparian habitat in North America.

Llanos – Seasonally inundated grasslands and wetlands in Venezuela, Colombia, and Bolivia.  Comparable to the Pantanal.

Monte – Shrub steppes and light woodlands.

Pampas – Temperate grasslands in Argentina, Uruguay, and Paraguay covering some 777,000 square kilometers.

Pantanal – Seasonally inundated grasslands and wetlands in Brazil and along the Parana-Paraguay river system. These areas are comparable to llanos.

Patagonia – Semi-arid, temperate grasslands.

Puna – High elevation grasslands and moors of the Andes.

Regional Savannas and Wetlands – Small patches of savanna woodlands within the Amazon basin provide "stepping stones" between the Pantanal and the llanos.


WHAT IS A TROPICAL SAVANNA?

The term savanna is derived from sabana of Amerindian origin. In pre-Columbian times it was used in Haiti and Cuba to designate plains devoid of trees but with a tall herbaceous cover. Today it is used in ordinary Spanish to refer to a flat, grassy landscape that may or may not have some shrubs or trees. It is used in Venezuela in opposition to mata (an isolated forest grove or island within an open landscape), or montaña (forest in popular usage).

In Brazil, campo is used rather than savanna to refer to open grassland formations (campos cerrados in the broad sense); both campo and savanna are used in Argentina.

• Despite their great diversity, Neotropical savannas are found exclusively in warm and humid tropical areas. Their essential characteristic is an herbaceous cover of grasses and sedges, often supporting an open tree stratum. Fire plays an important role that favors fire resistant vegetation.
• All savannas are intrinsically seasonal systems. There is always a period of reduced activity of the herbaceous cover, coinciding with a drought season (i.e., xeropause). This xeropause, common to all savannas, may be accompanied by the opposite stress produced by an excess of water.
• Most tropical ecosystems rely on economy of essential nutrients; however, tropical forests occupy soils from rich to poor, while grasslands only exist on poor soils. Forests tend to rely on locking the nutrients in the vegetation, while grasslands, with faster nutrient cycling that excludes the possibility of maintaining large biomass reserves, rely on the soil as the principal nutrient reservoir.
• The nutrient poverty of savanna soils is largely independent of the vegetation, since it is related in great measure to climate and soil formations. However, once a forest is converted to savanna, fire and low biomass stock tend to reinforce nutrient impoverishment. In contrast, the cycling of organic nitrogen is largely held in slowly decomposing biomass, so that there is little nitrogen that is not immediately incorporated into the biomass of producers or decomposers.

In terms of land use and production potential, savannas separate into two broad categories: poorly drained and well-drained.

Poorly Drained Savannas – Extensive tracts of savannas with a predominance of poorly drained soils or seasonally inundated soils cover the Pantanal of Mato Grosso in southwest Brazil, the Bolivian Llanos de Moxos, the Araguaia Pantanal in southwest Amazonia, the Colombian-Venezuelan Llanos de Casanare and Apure, and a large part of the island of Marajó at the mouth of the Amazon river. They also include the Humaita Savannas in southern Amazonia, the Rupununi Savannas of Guyana, the coastal savannas of the Guyanas and the savannas of Belize.

Most of these have Alfisol and Ultisol soils, which are characterized by clay textured, relatively impermeable soils. Due to flat topography, run-off is slow and soils become flooded in the wet season.

Well-Drained Savannas – Approximately 80% of tropical America's savannas are predominantly well drained. The major single expanse covers central Brazil (100-1200 meters) and is locally called cerrado. Lesser extensions are found in eastern Bolivia on the Pre-Cambrian Shield formation (400-800 meters), near sea level in northeast Bolivia, and in the Colombian Llanos mainly south of the Meta river. They are also found in the Venezuelan Llanos north of the Orinoco river, as a part of the Boa Vista savannas in northern Amazonia, and the Cerrado of Amapá on the northern lip of the mouth of the Amazon River in many lesser areas. A large portion of these lands have Oxisol soils, including some of the most nutrient poor soils in the tropics.

Grasses predominate and belong exclusively to Panicoidea. In central Brazil the dominant genus is Trachypogon, followed by Axonopus, but Agenuium, Aristidia, Echinolaena, Elyonurus, Mesosetum, Panicum, Paspalum, and Schyzachyrium, are also found. Legume species are common, especially in the cooler savannas, including Stylosanthes, Desmodium, Zornia, Centrosema, Aeschynomene, and Arachis.

Shrubs and trees provide up to 60% of dry season cover in well-drained areas. Traditional dry season burning is often used to obtain fresh growth and to restore calcium to the soil that had built up in the dry grasses.

Approximately 750,000 square kilometers of well-drained savanna is suitable as arable land or suitable for cattle production. These extend over large areas of the Brazilian cerrados, the Venezuelan, and to a lesser extent Colombian, llanos and the Bolivian pampas. In contrast, another 800,000 square kilometers of well-drained savanna with micro-topography impractical to cultivation, and another 450,000 square kilometers in poorly drained savanna are unsuitable altogether.

Currently, the savannas of tropical America support an estimated 65 million head of cattle, which includes about 46 million in the Brazilian cerrados (IBGE 1983). This population is expected to reach 250 million by 2003.

Savannas constitute approximately a third of the surface of Venezuela. The llanos between Colombia and Venezuela cover a surface of approximately half a million square kilometers, constituting the largest uninterrupted surface of Neotropical savanna north of the equator.

The Precambrian area of the Guayana Shield also has extensive regions of savannas with isolated fingerlike extensions to the Atlantic coast forming an arc between Guyana and the mouth of the Amazon. Central America and the islands of the Antilles contain savanna in various dimensions.


SOURCES AND FURTHER READING:

Bourlière, F. (ed) (1982) Tropical savannas. (Ecosystems of the World 13). Amsterdam: Elsevier.

Bullock, S.H., Mooney, H.A. and Medina, E. (eds) (1995) Seasonally dry tropical forests. Cambridge: Cambridge University Press.

Chandler, C., Cheney, P, Thomas, P., Trabaud, L. & Williams, D. (1983)Fire in forestry. Vols I & II. New York: Wiley-Interscience.

Cole, M.M. (1986) The savannas: biogeography and geobotany. London: Academic Press.

Crutzen, P.J & Goldammer, J.G. (eds) (1993) Fire and the environment: the ecological, atmospheric, and climatic importance of vegetation fires. Chichester: John Wiley.

Furley, P.A., Proctor, J. and Ratter, J.A. (eds) (1992) Nature and dynamics of forest-savanna boundaries. London: Chapman & Hall.

Goldammer, J.G. (ed) (1990) Fire in the tropical biota: ecosystem processes and global challenges. Berlin: Springer-Verlag.

Huntley, B.J. and Walker, B. H. (eds) (1982) Ecology of tropical savannas. Berlin: Springer-Verlag.

Koslowski, T.T. & Ahlgren, C.E. (eds) (1974) Fire and ecosystems. New York: Academic Press.

Sarmiento, G. (1984) The ecology of Neotropical savannas. Cambridge, Mass.: Harvard University Press.

Solbrig, O. (ed) (1991) "Savanna modelling for global change." Biology International (Special Issue) 24. Paris: IUBS.

Stott, P. (1991) "Recent trends in the ecology and management of the world’s savanna formations." Progress in Physical Geography 15: 18-28.

Stott, P. (1994) "Savanna landscapes and global environmental change." In Global environmental change, edited by Neil Roberts. Cambridge, Mass.: Blackwell, pp. 287 - 303.

Sullivan, S. (1996) "Towards a non-equilibrium ecology: perspectives from an arid land." Journal of Biogeography 23: 1-5.

Tothill, J.C. and Mott, J.J. (eds) (1984) Ecology and management of the World’s savannas. Australian Academy of Science: Canberra.

Walker, B.H. and Ménaut, J.C. (eds) (1988) Research procedure and experimental design for savanna ecology and management. (RSSD Australia Publication 1). CSIRO for IUBS/Unesco MAB: Melbourne.

Whelan, R.J. (1995) The ecology of fire. (Cambridge Studies in Ecology). Cambridge: Cambridge University Press.

Young, M.D. and Solbrig, O.T. (1992) Savanna management for ecological sustainability, economic profit and social equity. (MAB Digest 13). Paris: UNESCO.

Young, M.D. and Solbrig, O.T. (eds) (1993) The world’s savannas: economic driving forces, ecological constraints and policy options for sustainable land use. (MAB 12). Paris and Carnforth: UNESCO & Parthenon Press.