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Savannah is an ecological system with a tropical or subtropical type of vegetation that consists of a closed herb layer and a rather open wood layer.
Savannah is usually located on both sides the equator next to rain forest regions, and it forms the transition to the deserts. Savannahs cover approximately 15 % of the mainland surface of the earth.
Depending on the duration of dry seasons and the average yearly precipitation, we distinguish between wet savannah, dry savannah and thorn savannah.
Semideciduous plants prevail in areas close to rainforests, with two rain seasons per year. Deciduous plants prevail in savannahs located in summer rain areas with just one annual rain season. The falling of the leaves results in a decrease of the animals’ food reserves and therefore makes migrations necessary.
Savannahs are very common in Africa. A savannah belt stretches in a semi-circular form over the whole the continent from the Sahel of West Africa over the East African savannahs up to the South African Veld.
Photo: Jürgen Baisch
Photo: Jürgen Baisch
A large part of the wood formations as well as expanded grass savannah formations are man-made, often by slash-and-burn to obtain farmland. Too short rest periods for the soil lead to a degradation of the vegetation and to the expansion of grass formations. This is aggravated by a strong population growth in rural areas as well as in the periphery of cities in almost all parts of Africa.
Development of the Savannah
Three conditions are necessary for the development of savannahs: the appropriate ground conditions, amounts of precipitation and naturally ocurring bush fires.
The amount of precipitation determines grass growth, while the ground conditions determine which plants can grow.
Natural selection of the savannah vegetation happens through bush fires. They make room for new plants and at the same time fertilize the soil with ashes. Seeds and roots of the grasses remain undamaged underground. Trees protect themselves with their bark, only sick trees burn down.
Savannahs can also develop due to human intervention, e.g., by the destruction of forests by slash-and-burn.