SavannaSavanna is grassland
with scattered individual trees. Savannas of one sort or another cover almost
half the surface of Africa (about five million square miles, generally central
Africa) and large areas of Australia, South America, and India. Climate is the
most important factor in creating a savanna. Savannas are always found in warm
or hot climates where the annual rainfall is from about 50.8 to 127 cm (20-50
inches) per year. It is crucial that the rainfall is concentrated in six or
eight months of the year, followed by a long period of drought when fires can
occur. If the rain were well distributed throughout the year, many such areas
would become tropical forest. Savannas which result from climatic conditions are
called climatic savannas. Savannas that are caused by soil conditions and
that are not entirely maintained by fire are called edaphic savannas.
These can occur on hills or ridges where the soil is shallow, or in valleys
where clay soils become waterlogged in wet weather. A third type of savanna,
known as derived savanna, is the result of people clearing forest land
for cultivation. Farmers fell a tract of forest, burn the dead trees, and plant
crops in the ashes for as long as the soil remains fertile. Then, the field is
abandoned and, although forest trees may recolonize, grass takes over on the
bare ground (succession), becoming luxuriant enough to burn within a year or so.
In Africa, a heavy concentration of elephants in protected parkland have created
a savanna by eating leaves and twigs and breaking off the branches, smashing the
trunks and stripping the bark of trees. Elephants can convert a dense woodland
into an open grassland in a short period of time. Annual fires then maintain the
area as a savanna.
