Grasslands are defined as areas where grasses predominate over large shrubs or trees. Mountains rose in western North America during the Miocene and Pliocene Epochs, which lasted about 25 million years and created a continental climate favourable to grasslands. Ancient forests dwindled, and grasslands proliferated. Following the Pleistocene Ice Ages, grasslands spread across the globe as hotter and drier climates prevailed.
Grasslands are classified into two types:
Savannas or Tropical grasslands
Example of tropical grassland include the hot savannas of sub-Saharan Africa and northern Australia.
Savanna Tropical Grassland
Savanna is grassland with a few scattered trees. Savannas of one kind or another cover nearly half of Africa's surface (about five million square miles, mostly in central Africa) as well as large areas of Australia, South America, and India.
Tropical Grassland Climate
The most important factor in the formation of a savanna is the climate.
Savannas are always found in warm or hot climates with annual rainfall ranging from 50.8 to 127 cm (20-50 inches).
It is critical that rainfall be concentrated in six to eight months of the year, followed by a long period of drought during which fires can occur. Many of these areas would become tropical forests if rain fell evenly throughout the year.
Climatic savannas are savannas formed as a result of climatic conditions.
Edaphic savannas are savannas that are caused by soil conditions and are not entirely maintained by fire. These can occur on steep hills or ridges with shallow soil, or in valleys with clay soils that become waterlogged in wet weather.
People clearing forest land for cultivation creates a third type of savanna, known as derived savanna.
Farmers cut down a forest, burn the dead trees, and plant crops in the ashes for as long as the soil is fertile. The field is then abandoned, and, while forest trees may recolonize, grass (succession) takes over on the bare ground, becoming luxuriant enough to burn within a year or so.
In Africa, elephants have created a savanna by eating leaves and twigs and breaking off branches, smashing trunks, and stripping the bark of trees in protected parkland. Elephants can quickly transform dense woodland into open grassland. Annual fires keep the area as a savanna.
The average annual rainfall in savannas is 76.2-101.6 cm (30-40 inches).
The savanna has both a dry and wet season. Seasonal fires are critical to the biodiversity of the savanna.
The start of the dry season is signalled in October by a series of violent thunderstorms followed by a strong drying wind.
During the dry season, fires are common around January. Poachers who want to clear away dead grass to make it easier to see their prey often start fires in savannas. The fires do not completely destroy the community. The majority of the animals killed in the fires are insects with short lives.
Some animals, such as birds, come to fire sites to eat grasshoppers, stick insects, beetles, mice, and lizards that are killed or driven away.
Small creatures can find refuge in underground holes and crevices.
Larger animals can usually run fast enough to avoid the fire. Although fire consumes the dry stems and leaves of grasses, the grasses' deep roots are unharmed. When the soil becomes moisture, these roots, with all of their starch reserves, are ready to send up new growth.
The scattered shrubs can also survive on food reserves stored in their roots until the time comes for them to rise above the soil again.
Trees, unlike grasses and shrubs, can withstand fire by retaining moisture in all of their above-ground parts throughout the dry season.
Tropical Grasslands Location
Tropical grasslands of the world near the equator produce plants that can withstand hot weather for the majority of the year, as well as drought and fires.
The African savannas are probably the most well-known, but tropical grasslands can also be found in South America, India, and Australia.
There are llanos in Colombia and Venezuela, Campos in the Brazilian highlands, Pantanals in Upper Paraguay, plains in Australia, and India's Deccan Plateau.
Even though they are all hot, the annual rainfall varies. The Australian plains may only receive 18 inches (45.72 centimetres) of rain per year, but African savannas receive more than 50 inches (127 centimetres).
South America's llanos and Pantanal are frequently flooded during a portion of the year.
Tropical Grassland Animals
Although it may appear that animal life is scarce on the savanna, it is actually thriving. Elephants, zebras, wildebeest, giraffes, and other browsers eat the grasses on Africa's savannas, which are then eaten by cheetahs, lions, and other predators.
Emus and other foragers in Australia rely on hot grasslands. Insects, on the other hand, constitute the majority of animal life in the savannas. There are billions of locusts, termites, and flies here.
Zebras prefer fibrous grass, whereas hartebeest eat plant stalks left by previous foragers. Giraffes and elephants eat the trees, and carnivorous animals hide in the tall grasses before pouncing.
Tropical Grassland Vegetation
The natural vegetation of tropical grasslands
The savanna soil is porous, allowing water to drain quickly.
It only has a thin layer of humus (the organic portion of the soil formed by the partial decomposition of plant or animal matter), which provides nutrients to vegetation. Savannas are sometimes referred to as forests.
Grass and forbs are the most common types of vegetation (small broad-leaved plants that grow with grasses). Because of differences in rainfall and soil conditions, different savannas support different grasses.
Because the savanna supports so many species competing for living space, only one or a few types of grass are usually more successful than the others in a given area.
For example, in drier savannas such as the Serengeti plains or Kenya's Laikipia plateau, the dominant grasses on well-drained soils are Rhodes grass and red oat grass; star grasses are dominant throughout East African savannas, and lemon grasses are common in many western Uganda savannas. The open landscape is dotted with deciduous trees and shrubs.
One type of savanna, known as grouped-tree grassland, is found in southwestern Kenya, Tanzania, and Uganda, and features trees that grow only on termite mounds, with the intervening soil being too thin or poorly drained to support tree growth at all.
Frequent fires and large grazing mammals kill seedlings, keeping tree and shrub density low.