Friday, January 20, 2012

BIOMES: Savannah grassland

The savannah biome is located further away from the equator than the tropical rainforest biome in the central part of Africa and in South America. It is dry, but not as dry as desert areas.

Savannahs - also known as tropical grasslands - are found to the north and south of tropical rainforest biomes. The largest expanses of savannah are in Africa, where much of the central part of the continent, for example Kenya and Tanzania, consists of tropical grassland. Savannah grasslands can also be found in Brazil in South America. Savannah regions have two distinct seasons - a wet season and a dry season. There is very little rain in the dry season. In the wet season vegetation grows, including lush green grasses and wooded areas. As you move further away from the equator and its heavy rainfall, the grassland becomes drier and drier - particularly in the dry season. Savannah vegetation includes scrub, grasses and occasional trees, which grow near water holes, seasonal rivers or aquifers.
Plants and animals have to adapt to the long dry periods. Many plants are xerophytic - for example, the acacia tree with its small, waxy leaves and thorns. Plants may also store water, for example the baobab tree) or have long roots that reach down to the water table. Animals may migrate great distances in search of food and water. The graph below shows average monthly temperatures and rainfall levels in the savannah region of Mali. Notice how the temperature and rainfall patterns relate to each other - the hottest temperatures come just before heavy rainfall, and the coolest time of the year comes just after the rains. This pattern is typical of savannah climates.

Temperature and rainfall in Mali

Savannah grassland soils are not very fertile. The nutrients in the soil are found near the surface as they come from decayed organic matter (vegetation) from the previous growing season. This organic matter decays rapidly due to the high temperatures. Soils tend to be red in colour due to their high iron content. The diagram below shows the different horizons, or layers, of soil in savannah grassland. Plant roots cannot penetrate the hard 'pan' layer in the B-horizon, or subsoil. This restricts vegetation growth.

An umbrella acacia tree in Africa

Savannah soil profile

The Serengeti
The Serengeti plains of Tanzania are probably the best known African savannah. There are two main types of vegetation in the Serengeti. Grasses cover the vast open plains of the southeast while in the central region, acacia plants are more common. The Serengeti is rich in wildlife - including giraffes, zebras, elephants, lions and over 2 million wildebeest. Many of the animals found on the Serengeti can be found nowhere else in the world.

Courtesy: The BBC

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