Many grassy landscapes and mixed communities of trees, shrubs, and grasses were described as savanna before the middle of the 19th century, when the concept of a tropical savanna climate became established. The Köppen climate classification system was strongly influenced by effects of temperature and precipitation upon tree growth, and his oversimplified assumptions resulted in a tropical savanna classification concept which resulted in it being considered as a “climatic climax” formation. The common usage meaning to describe vegetation now conflicts with a simplified yet widespread climatic concept meaning. The divergence has sometimes caused areas such as extensive savannas north and south of the Congo and Amazon Rivers to be excluded from mapped savanna categories.
“Barrens” has been used almost interchangeably with savanna in different parts of North America. Sometimes midwestern savanna were described as “grassland with trees”. Different authors have defined the lower limits of savanna tree coverage as 5–10% and upper limits range as 25–80% of an area. Two factors common to all savanna environments are rainfall variations from year to year, and dry season wildfires. In the Americas, e.g. in Belize, Central America, savanna vegetation is similar from Mexico to South America and to the Caribbean.
Types of Savannah
Tropical and subtropical savannas are classified with tropical and subtropical grasslands and shrublands as the tropical and subtropical grasslands, savannas, and shrublands biome. The savannas of Africa, including the Serengeti, famous for its wildlife, are typical of this type. The Brazilian savanna (Cerrado) is also included in this category, known for its exotic and varied flora. Temperate savannas are mid-latitude savannas with wetter summers and drier winters. They are classified with temperate savannas and shrublands as the temperate grasslands, savannas, and shrublands biome, that for example cover much of the Great Plains of the United States. (See areas such as the Central forest-grasslands transition).
Mediterranean savannas are mid-latitude savannas in Mediterranean climate regions, with mild, rainy winters and hot, dry summers, part of the Mediterranean forests, woodlands, and scrub biome. The oak tree savannas of California, part of the California chaparral and woodlands ecoregion, fall into this category. Flooded savannas are savannas that are flooded seasonally or year-round. They are classified with flooded savannas as the flooded grasslands and savannasbiome, which occurs mostly in the tropics and subtropics.
Montane savannas are mid- to high-altitude savannas, located in a few spots around the world’s high mountain regions, part of the montane grasslands and shrublands biome. The Bogotá savanna, located at an average altitude of 2,550 metres (8,370 ft) on the Altiplano Cundiboyacense, Eastern Ranges of the Andes, is an example of a montane savanna. The savannas of the Angolan Scarp savanna and woodlands ecoregion are a lower altitude example, up to 1,000 metres (3,300 ft).
Threats to Savannahs
This threat to a savanna ecosystem include effects caused by climate change, farming practices, overgrazing, aggressive agricultural irrigation, which lowers the level of the water table away from plant roots, deforestation and erosion. Each year, over 46,000 square kilometers of African savanna becomes desert.
Adaptations in Savannahs
In trees, most savanna adaptations are to drought–long tap roots to reach the deep water table, thick bark for resistance to annual fires, deciduousness to avoid moisture loss during the dry season, and use of the trunk as a water-storage organ (as in baobab). In grasses, most adaptations are against grazing–siliceous spicules to deter herbivores, growth from base of the plant rather than its tip to avoid damage to growing tissue, and vegetative reproduction in many types to overgrow competing forbs. Many plants have vegetative storage organs–bulbs and corms, for example–to make it through the dry (nongrowing) season.
Many animals have effective locomotion for long-distance migrations to coincide with the seasonal flush of growth–primarily mammals in Africa and birds in Australia. Many forms burrow to avoid predation and desiccation (during drought), and many others use these burrows. Savannas are perfect for birds of prey, with wide open spaces for hunting with their long-range vision and trees for perches and nest sites.
Termite mounds are significant features, supporting a surprising diversity of termite specialists–aardvark and aardwolf in Africa and giant anteater in South America. Ratite birds have ecological equivalents in open country on each tropical continent–ostrich in Africa, rheas in South America (as much in grassland as savanna), and emu in Australia.
There are substantial niche separations in African ungulates, even in this fairly simple environment. The primary dichotomy is between browsers and grazers, but it is not a simple one, as many species do both in different proportions. Within grazers, some species are generalists, others specialists.
The proportion of grasses and forbs in the diet varies among species, as does the parts of the plant eaten, down to distinct differences in which species eat leaves, sheaths, or stems of various grasses. Finally, some species are migratory, others resident, which is correlated with diet, social system, and defense adaptations. The abundant but patchy food and the ease of keeping in contact have promoted a high degree of sociality in savanna mammals (ungulates, baboons, lions, and others). Birds are the same, also perhaps social because of the scarcity of arboreal nest site.
Soil of Savannahs
Savannah grasslands are much richer in humus than the equatorial forests. The C4 grasses are found in majority in Savannah.
Animals in Savannahs
The species of animals in a savanna depends upon the geographic location of the biome. The African savannah, the savannah with which most people are familiar, is home to a wide variety of animals. A short list of some of those animals includes wildebeest, warthogs, elephants, zebras, rhinos, gazelles, hyenas, cheetahs, lions, leopards, ostrich, mousebirds, starlings, and weavers.
During the rainy season, birds, insects, and both large and small mammals thrive in the savannah, but the rainy season only lasts 6 to 8 months. During the dry season, surface water from the rain is quickly absorbed into the ground because the soil is extremely porous. Competition for water during the dry season is intense. Consequently, most birds and many of the large mammals migrate during the dry season in search of water.
Because drought conditions are sometimes localized, the migration may be just to another area within the savannah. When drought conditions exist for a long time and over a wide area, the animals may migrate to another biome until the rainy season begins again.