A savanna is a dry grassland with scattered trees. Most ecologists agree that a characteristic savanna has an open or sparse canopy with 10–25% tree cover, a dominant ground cover of annual and perennial grasses, and less than 20 in (50 cm) of rainfall per year. Greatly varied environments, from open deciduous forests and parklands, to dry, thorny scrub, to nearly pure grasslands, can be considered savannas. At their margins these communities merge, more or less gradually, with drier prairies or with denser, taller forests. Savannas occur at both tropical and temperate latitudes and on all continents except Antarctica. Most often savannas occupy relatively level, or sometimes rolling, terrain. Characteristic savanna soils are dry, well-developed ultisols, oxisols, and alfisols, usually basic and sometimes lateritic. These soils develop under savannas' strongly seasonal rainfall regimes, with extended dry periods that can last up to 10 months. Under natural conditions an abundance of insect, bird, reptile, and mammal species populate the land.
Savanna shrubs and trees have leathery, sometimes thorny, and often small leaves that resist drought, heat, and intense sunshine normally found in this environment. Savanna grasses are likewise thin and tough, frequently growing in clumps, with seasonal stalks rising from longerlived underground roots. Many of these plants have oily or resinous leaves that burn intensely and quickly in a fire. Extensive root systems allow most savanna plants to exploit moisture and nutrients in a large volume of soil. Most savanna trees stand less than 32 ft (10 m) tall; some have a wide spreading canopy while others have a narrower, more vertical shape. Characteristic trees of African and South American savannas include acacias and miombo (Brachystegia spp.). Australian savannas share the African baobab, but are dominated by eucalyptus species. Oaks characterize many European savannas, while in North America oaks, pines, and aspens are common savanna trees.
Because of their extensive and often nutritious grass cover, savannas support extensive populations of large herbivores. Giraffes, zebras, impalas, kudus, and other charismatic residents of African savannas are especially well-known. Savanna herbivores in other regions include North American bison and elk and Australian kangaroos and wallabies. Carnivores—lions, cheetahs, and jackals in Africa, tigers in Asia, wolves and pumas in the Americas—historically preyed upon these huge herds of grazers. Large running birds, such as the African ostrich and the Australian emu, inhabit savanna environments, as do a plethora of smaller animal species. In the past century or two many of the world's native savanna species, especially the large carnivores, have disappeared with the expansion of human settlement. Today ranchers and their livestock take the place of many native grazers and carnivores.
Savannas owe their existence to a great variety of convergent environmental conditions, including temperature and precipitation regimes, soil conditions, fire frequency, and fauna. Grazing and browsing activity can influence the balance of trees to grasses. Fires, common and useful for some savannas but rare and harmful in others, are an influential factor in these dry environments. Precipitation must be sufficient to allow some tree growth, but where rainfall is high some other factors, such as grazing, fire, or soil drainage needs to limit tree growth. Human activity also influences the occurrence of these lightly-treed grasslands. In some regions recent expansion of ranches, villages, or agriculture have visibly extended savanna conditions. Elsewhere centuries or millennia of human occupation make natural and anthropogenic conditions difficult to distinguish. Because savannas are well suited to human needs, people have occupied some savannas for tens of thousands of years. In such cases people appear to be an environmental factor, along with climate, soils, and grazing animals, that help savannas persist.
