E+-+Savannah+Grasslands



__**Climate:**__

In tropical parts of the world where the climate alternates between a long wet season and a long dry season, grasses tend to dominate the landscape and there tends to be just a scattering of trees. The savannah grasslands are generally found between 5 and 15 degrees latitude, both north and south of the equator. This belt of grassland is squeezed between the tropical forests and the deserts. Savannah grassland is found in Venezuela and north-eastern parts of Brazil in South America, northern Australia, and across Africa between the equator and the Sahara Desert.


 * Wet season** - in the northern hemisphere this is between May and October and 90% of the annual rainfall can fall during this period, often in heavy downpours. Temperatures are around 25 degrees celsuis. August can receive as much as 240mm


 * Dry season** - in the northern hemisphere, the dry season is from November to April. There's little if any rain during this time. Temperatures can be over 30 degrees cesluis. This creates a harsh environment for plants and animals.


 * Soil conditions** - savannah soils tend to be very porous. This means water drain easily and quickly through them. The nutrients available to plants are mainly found in a thin layer on the surface, where vegetation has decomposed from the previous year.

__**Special plants:**__

Savannah plants have developed special ways of surviving the long dry season. They are xerophytic features - which make them drought-resistant. Different specioes have developed ways of both storing and preserving their moisture supplies - which is vital when there's no rain and temperatures are high.


 * Elephant grass[[image:Elephant_grass.jpg width="535" height="308" align="right"]]**

This type of grass grows very tall - anything from 3 to 5 metres in height. The individual blades of grass curve away from strong sunlight to preserve moisture. At the end of the wet season the grass turns yellow and dies back, putting nutrients back into the soil. The seeds lie dormant on the surface ready to grow immediately when the next rains arrive.


 * Baobab tree[[image:baobab.jpg width="326" height="250" align="right"]]**

This giant tree can grow up to 25 metres tall and an incredible 10 metres wide (one tree in Botswana has a bar in it!), it is sometimes called the 'upside tree' because the trunk can be wider than the crown. Water is stored in the trunk during the drought (dry season), protected by very thick bark. It has small waxy leaves and thorns which reduce the loss of water through transpiration.


 * Acacia tree[[image:acaica.jpg width="508" height="272" align="right"]]**

With its flat top or crown, the acacia is a very recognisable savannah tree. It can grow up to 20 metres in height. It has a very long tap-root - up to 35 metres - reaching down to the underground water supplies. It has small waxy leaves and thorns to help reduce the loss of water through transpiration.

__**Human impact in the savannah grasslands**__

In the Sahel a part of Africa man has had a severe impact on the savannah grassland ecosystem:
 * fire - some fires are started by lightning strikes, but many are started deliberately - the ash makes the sooil more fertile. The problem is that they can get out of control
 * hunting - people hunt for food - especially species such as zebra and wildebeest
 * agriculture - more and more land is being used for growing crops and grazing cattle and goats
 * deforestation - population growth has resulted in increased demand for wood - it's used for cooking and heating, so trees are cut down
 * tourism - tourist come to see the wildlife - especially animals like lions, cheetahs, giraffes and rhinos


 * Desertification**

This is essentially where once productive agricultural land id turned into desert through the land being damaged.

The causes of desertification are complex and interlinked. They are listed below:
 * overcultivation - population growth has put pressure on the farmland to produce more and more food. This more intensive use of the land exhausts it of nutrients - which means crops can no longer grow and soil is lost through wind and water erosion
 * overgrazing - permanent and nomadic pastoral farmers are having to let their livestock graze on marginal land closesr to the desert. Once the protective vegetation cover is eaten away, the soil becomes vulnerable to erosion
 * deforestation - across the Sahel, 80% of domestic energy comes from burning firewood. Increased population means increased demand for wood, and so the land is cleared of trees. Again, the soil become vulnerable to erosion
 * climate change - the climate of the Sahel has become much drier over the last forty years. Less rainfall means poorer grazing and lower crop production. Underground water reserves have been used up. Population pressure makes the problems worse

__**Sustainable land management**__
 * tethering cattle and goats to limit the area they can graze. This has the added advantage of creating manure in one place that can spread as a fertiliser. Fodder (animal food) is taken to the animals
 * farmers diversifying into a more varied range of crops and trees. This helps bind the soil together and prevents erosion and the vegetation matter will then put a continual supply of nutrients back into the soil
 * harvesting fuel-wood from branches - rather than taking the whole tree - is more sustainable as the branches will grow back. Thid prevents deforestation.
 * building stone walls on levelled pieces of land to trap soil and water when the rains arrive. This prevents erosion and can help increase crop production.
 * controlled buring of grasses to prevent the spread of devastating wildfires