Sahel
From Wikipedia, the free encyclopedia
- See also Sahel, Tunisia, a region of eastern Tunisia.
The Sahel or Sahel Belt (from Arabic ساحل, sahil, shore, border or coast of the Sahara desert) is a semi-arid tropical savanna ecoregion in Africa, which forms the transition between the Sahara desert to the north and the more fertile region to the south, known as the Sudan (not to be confused with the country of the same name).
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[edit] Geography
The Sahel runs 2,400 miles from the Atlantic Ocean in the west to the Red Sea in the east, in a belt that varies from several hundred to a thousand kilometers in width, covering an area of 3,053,200 square kilometers (1,178,800 square miles). It is a transitional ecoregion of semi-arid grasslands, savannas, and thorn shrublands lying between the wooded Sudanian savanna to the south and the Sahara desert to the north.[1] The countries of the Sahel today include Senegal, Mauritania, Mali, Burkina Faso, Niger, Nigeria, Chad, Sudan, and Eritrea.
The topography of the Sahel is mainly flat, and the region mostly lies between 200 and 400 meters elevation. Several isolated plateaus and mountain ranges rise from the Sahel, but are designated as separate ecoregions because their flora and fauna are distinct from the surrounding lowlands.[2]
Over the history of Africa the region has been home to some of the most advanced kingdoms benefiting from trade across the desert. Collectively these states are known as the Sahelian kingdoms.
[edit] Climate
The climate of the Sahel is arid and hot, with strong seasonal variations in rainfall and temperature. The Sahel receives about 200–600 mm (6–20 in) of rainfall a year, which falls mostly in the May to September monsoon season. Rainfall is generally higher in the south, declining rapidly as one reaches the northern edge of the Sahel. The rainfall is characterized by great variation from year to year and from decade to decade, determined by the movements of the Intertropical Convergence Zone (ITCZ).[3] There is a strong correlation between rainfall in the Sahel region and intense hurricane activity in the Atlantic[4]. Monthly mean temperatures vary from a maximum of 33° to 36°C to a minimum of 18° to 21°C. During the winter, hot, dry Harmattan winds off the Sahara can bring sand and dust storms.[5]
[edit] Climate history and soils
Over 12,000 years ago, the Sahel was a part of the Sahara desert, and was covered in sand dunes which have shaped the landscape that we see today. Huge lakes, of which Lake Chad and the Niger Inland Delta are remnants, formed during the humid period following the end of the ice age, but the lakes in the region have been shrinking for the last few thousand years, leaving large areas of lake bed soils. The most important limitations to land productivity in the Sahel are water and soil fertility.
Soils in the Sahel are mostly sandy and acidic (which results in aluminum toxicity to plants), and are very low in nitrogen and phosphate. Soils are generally very porous, and rainfall quickly drains away, leaving few permanent watercourses. [6]
[edit] Flora and fauna
The Sahel is mostly covered in grassland and savanna, with areas of woodland and shrubland. Grass cover is fairly continuous across the region, dominated by annual grass species such as Cenchrus biflorus, Schoenefeldia gracilis, and Aristida stipoides. Species of Acacia are the dominant trees, with Acacia tortilis the most common, along with Acacia senegal and Acacia laeta. Other tree species include Commiphora africana, Balanites aegyptiaca, Faidherbia albida, and Boscia senegalensis. In the northern part of the Sahel, areas of desert shrub, including Panicum turgidum and Aristida sieberana, alternate with areas of grassland and savanna. During the long dry season, many trees lose their leaves, and the predominantly annual grasses die.
The Sahel was formerly home to large populations of grazing mammals, including the Scimitar-horned Oryx (Oryx dammah), Dama Gazelle (Gazella dama), Dorcas Gazelle (Gazella dorcas) and Red-fronted Gazelle (Gazella rufifrons), and Bubal Hartebeest (Alcelaphus busephalus buselaphus), along with large predators like the African Wild Dog (Lycaon pictus), Cheetah (Acinonyx jubatus), and Lion (Panthera leo). The larger species have been greatly reduced in number by over-hunting and competition with livestock, and several species are vulnerable (Dorcas Gazelle and Red-fronted Gazelle), endangered (Dama Gazelle, African Wild Dog, cheetah, lion), or extinct (the Scimitar-horned Oryx is probably extinct in the wild, and the Bubal Hartebeest is extinct).
The seasonal wetlands of the Sahel are important for migratory birds moving within Africa and on the African-Eurasian flyways. [7]
[edit] Early agriculture
The first instances of domestication of plants for agricultural purposes in Africa occurred in the Sahel region circa 5000 BCE, when sorghum and African Rice (Oryza glaberrima) began to be cultivated. Around this time, and in the same region, the small Guineafowl were domesticated.
Around 4000 BCE the climate of the Sahara and the Sahel started to become drier at an exceedingly fast pace. This climate change caused lakes and rivers to shrink rather significantly and caused increasing desertification. This, in turn, decreased the amount of land conducive to settlements and helped to cause migrations of farming communities to the more humid climate of West Africa.[8]
[edit] Transhumance
Traditionally, most of the people in the Sahel have been semi-nomads, farming and raising livestock in a system of transhumance, which is probably the most sustainable way of utilizing the Sahel. The difference between the dry north with higher levels of soil-nutrients and the wetter south is utilized so that the herds graze on high quality feed in the North during the wet season, and trek several hundred kilometers down to the south, to graze on more abundant, but less nutritious feed during the dry period. Increased permanent settlement and pastoralism in fertile areas has been the source of conflicts with traditional nomadic herders.
[edit] Sahelian kingdoms
The Sahelian kingdoms were a series of empires, based in the Sahel, which had many similarities. The wealth of the states came from controlling the Trans-Saharan trade routes across the desert. Their power came from having large pack animals like camels and horses that were fast enough to keep a large empire under central control and were also useful in battle. All of these empires were also quite decentralized with member cities having a great deal of autonomy. The first large Sahelian kingdoms emerged after 750, and supported several large trading cities in the Niger Bend region, including Timbuktu, Gao, and Djenné.
The Sahel states were limited from expanding south into the forest zone of the Ashanti and Yoruba as mounted warriors were all but useless in the forests and the horses and camels could not survive the heat and diseases of the region.
[edit] 20th-century droughts
- Further information: Sahel drought
There was a major drought in the Sahel in 1914, caused by annual rains far below average, that caused a large-scale famine. The 1960's saw a large increase in rainfall in the region, making the Northern drier region more accessible. There was a push, supported by governments, for people to move northwards, and as the long drought-period from 1968 through 1974 kicked in, the grazing quickly became unsustainable, and large-scale denuding of the terrain followed. Like the drought in 1914, this led to a large-scale famine, but this time it was somewhat tempered by international visibility and an outpouring of aid. This catastrophe led to the founding of the International Fund for Agricultural Development.
[edit] See also
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[edit] References
- ^ "Sahelian Acacia savanna" WWF Scientific Report [1]. Accessed December 29, 2007.
- ^ "Sahelian Acacia savanna" WWF Scientific Report [2]. Accessed December 29, 2007.
- ^ "Sahelian Acacia savanna" WWF Scientific Report [3]. Accessed December 29, 2007.
- ^ Landsea, C., and Gray, n. The Strong Association between Western Sahel Monsoon Rainfall and Intense Atlantic Hurricanes. Journal Of Climate, Vol. 5, No. 5, May 1992.
- ^ "Sahelian Acacia savanna" WWF Scientific Report [4]. Accessed December 29, 2007.
- ^ "Sahelian Acacia savanna" WWF Scientific Report [5]. Accessed December 29, 2007.
- ^ "Sahelian Acacia savanna" WWF Scientific Report [6]. Accessed December 29, 2007.
- ^ O'Brien, Patrick K. (General Editor). Oxford Atlas of World History. New York: Oxford University Press, 2005. pp.22-23
- Azam (ed.), Conflict and Growth in Africa: The Sahel, Organisation for Economic Co-operation and Development (1999), ISBN 9264171010.
