THERE ARE MANY types of grasslands worldwide, especially in the continental interiors of temperate to subtropical regions. That of South America is known as the pampas, a Spanish term. The pampas occupies some 270,270 square mi (700,000 square km) in the countries of Argentina, Uruguay, and southeast Brazil. Here, there are extensive plains with isolated low hills in a region where the major drainage system is the River Plata. Climatically, average temperatures range from 43 to 79 degrees F (6 to 26 degrees C); only mild and short-lived frosts occur during the winter months (July and August), but summer temperatures can reach 104 degrees F (40 degrees C). The pattern of precipitation mirrors that of other continental grasslands such as the North American prairies with an east-to-west gradient that ranges from 47 in (120 cm) to 17.5 in (45 cm). Although rain occurs year-round in the east, there is a concentration in the winter. Toward the mountains that border the pampas in the west, there is a concentration in the spring that is advantageous for new growth in an areas that is semiarid. The geology comprises loess, a wind-blown sediment derived from volcanic ash, that gives rise to gray, brown, or black soils with high organic and nutrient content. They are fine-grained to the east and coarse-grained or sandy to the west, rich in calcium carbonate, and generally neutral or alkaline. The vegetation communities are dominated by tall grass in the east, which benefits from a higher rainfall than the more arid west, where there are mainly medium- and short-grass communities. Herbs are present throughout, and where there is a local abundance of water, small woodlands occur. There are also wetlands of international importance. Species of the grass Stipa are common throughout the region and the composition of the grass-herb-shrub communities depends on moisture and nutrient availability and topography. Most of the grasses form tussocks or clumps between which herbs and sedges grow. In the drier, western region, many of the species are xeromorphic (that is, tolerant of drought), and biodiversity is reduced compared with the eastern pampas. Here the grasslands merge with semidesert communities that occupy the rainshadow area created by the Andes. Locally, where soils are salt-rich, there are salt-tolerant species, that is, halophytes.