A major regional community (*biome
) in which grasses are the dominant vegetation. Usually the rainfall is insufficient for trees to grow (about 250-500 mm annually in temperate regions and 750-1500 mm in the tropics). Grasslands are common in continental interiors where the rain falls mainly in spring and summer. Most grasses are perennials but some are annuals or biennials. There are the low-growing closely packed turf-forming grasses of temperate regions and the tufted or tussock grasses, which grow in separate clumps and occur widely in temperate, tropical, and tundra regions. Temperate grasslands do contain other herbaceous plants though trees are found only along rivers and streams. They include the North American prairies, the steppes of southwest Russia, the grasslands of Manchuria and Mongolia, and the South American pampas. Tropical grasslands, known as savanna, have tall coarse tufted grasses and there are often scattered trees. In Africa baobab (Adansonia digitata) and Acacia and Euphorbia species are common. The climate alternates between cool dry winters and hot summers with heavy rains. Savanna is found in large areas of South America, East and South Africa, Southeast Asia, and northern Australia.