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Nigeria is found in the Tropics, where the climate is seasonally damp and very humid. The natural vegetative zones that exist in the country are governed by the combined effects of temperature, humidity, rainfall and particularly, the variations that occur in the rainfall. This forms a major influence on the type of indigenous plants that grows successfully in different parts of the country.
The humid tropical forest zone of the South that has longer rains is capable of supporting a number of plantation crops such as cocoa, oil palm, rubber, coffee, cotton and staple crops like, yam, cassava, cocoyam, sweet potatoes, melon, groundnut, rice maize and cowpeas. However, in some parts of the East and many areas near the coast, the high rainfall has led to badly leached soils and severe erosion in some places.
The Northern part of the country representing about 80% of the vegetative zones experiences lower rainfall and shorter rainy season and they make up the Savannah land. The Savannah land forms an excellent natural habitat for a large number of grazing livestock such as cattle, goats, horses, sheep, camels, and donkeys.
The natural vegetative zones resulted from the interaction of climate, humidity, rainfall and soils. These factors have been modified by human activities and man’s pattern of land use. Based on the above, Nigeria’s agro-ecological zones can be classified into: -
(i) The Mangrove forest and coastal vegetation
(ii) The Freshwater swamp forest
(iii) The tropical high forest zone
(iv) The derived Guinea Savannah
(v) The Guinea Savannah zone
(vi) The Sudan savannah (Short grass savanna)
(vii) The Sahel savannah (Marginal Savanna)
(viii) The Montane vegetation
THE MANGROVE FOREST AND COASTAL VEGETATION
This is found in places near the coast that is under the influence of brackish water commonly found in the Niger Delta. It is also found also in low lying swamp land associated with rivers and Lagoon near the coast and under the influence of the sea. Soil in the mangrove area is poorly aerated with water logged mud and is high in salt content due to the constant flooding by the sea.
The coastal swamp area is not widely cultivated except for swamp rice in places where they are stabilized and non-saline.
THE FRESHWATER SWAMP FOREST
This area lies immediately inland of the mangrove swamp but on a slightly higher ground. This vegetation belt, on freshwater wetlands occur further inland, beyond the reach of tidal waters. The lagoons or the rivers that overflow their banks in the wet season supply it with fresh water because the area is low lying, therefore it is flooded with rain water and lies under rain for sometimes, eight or nine months of the year.
The area of the country under this agro ecological zone, are Ogun, Benin, Imo, Niger Delta and Cross River. The high influx of water deposit vast quantities of silt, mud and sandy materials into this area. It is a low-lying region, with hardly any part rising over 30m above sea level, thus, it facilitates the development of freshwater swamps along the Niger Delta, drowned estuaries, lagoons and creeks. This zone consists of a mixture of trees. Important among the vegetation of this zone are the various Palm and Fibre plants such as Raphia spp., Raphia vinifera, the Wine Palm and Raphia hookeri, the Roof-mat Palm. They are used for thatching mats and for providing rafter, poles and stiff piassava fibre for the production of brooms. The better-drained areas support Oil Palm trees (Eleais guineenais) and big trees like Iroko (Chlorophora exceisa). Fishing and fibre-making are the important products of the fresh-water swamp communities.
THE TROPICAL HIGH FOREST ZONE
This area is characterized with a prolonged rainy season, resulting in high annual rainfall above 2000mm, thereby ensuring an adequate supply of water and promoting perennial tree growth. This luxuriant vegetation belt stretches from the western border of Nigeria to Benin Republic, through a narrow stretch on the Niger-Benue river system into the extensive area in the South-East of the country. This zone is the major source of timber for the large construction and funiture making industry. Of all the zones it contains the most valuable species of vegetation. However due to human activities, this one-time highly forested area has been drastically reduced. Bush fallows, villages and farms are found scattered throughout the zone. Presently the drier end of its inland side is becoming reduced to derived Guinea Savannah because of felling and clearings. In the humid rain forest are found economic cash crops such as Oil Palm, (Elaeis guineensis), Cocoa (Theobroma cacao), Rubber (Hevea brasiliensis) Banana/Plantain (Musa spp.) and Cola nut (Cola nitida). Also found are some principal staple food crops such as Yam, Cocoyams, Sweet Potato, Maize, Rice, Groundnut, Cowpeas and Beans as well as a number of fruits. A number of timber trees such as the African Mahogany, the scented Sapele wood (Entandrophragma cylindricum) and Iroko (Chlorophora excelsa) to mention but three are found in this zone. This zone therefore is very important in terms of food production and timber for construction and cabinet making[i].
THE DERIVED GUINEA SAVANNAH
This zone is found immediately after the tropical rainforest zone. It is the transition between the tropical rainforest and guinea savannah zones. The average annual rainfall and temperature are 1314mm and 26.5ºC respectively. Due to bush burning, overgrazing, cultivation and hunting activities over a long period in the zone, the high forest trees were destroyed and the forest that used to exist is now replaced with a mixture of grasses and scattered trees. The zone is covered with scattered trees and tall grasses. Maize, Cassava, Yam and Rice are the major crops grown in this zone. The savannah in general has an enormous potential for food production in the country. Bush burning and erosion as a result of over grazing by animal especially cattle constitute a major problem to agricultural production in the zone.
THE GUINEA SAVANNAH
The Guinea Savannah, located in the middle of the country, is the most extensive ecological zone in Nigeria, covering near half of the country. Guinea savannah zone has a unimodal rainfall distribution with the average annual temperature and rainfall of 27.3ºC and 1051.7mm respectively where the wet season lasts for 6–8 months[ii]. This zone consists of the larger part of the savannah zone and is sometimes divided into the Southern Guinea Savannah and Northern Guinea Savannah. It is the broadest vegetation zone in the country and it occupies almost half of its area. It extends from Ondo, Edo, Anambra and Enugu States in the South, through Oyo State to beyond Zaria in Kaduna State. It is a belt of mixture of trees and tall grasses in the South, with shorter grasses and less trees in the North. The Guinea Savannah, with its typically short trees and tall grasses, is the most luxuriant of the Savannah vegetation belts in Nigeria. The zone is characterized by low rainfall and long dry period, which call for alternative water supply (irrigation) to enhance full utilization of the zone’s potential in agricultural production.
The Guinea savannah is characterized by grasses such as Pennisetum, Andropogon, Panicum, Chloris, Hyparrhenia, Paspalum and Melinis. These tall grasses are characteristic of the Guinea Savannah proper.
In the Northern Guinea Savannah species such as Isoberlinia doka and I. tomentosa form the bulk of the scattered woodland. Also found are Locust Bean trees (Parkia filicoidea), Shea Butter trees (Butyrospermum parkii) and Mangoes (Mangifera indica). Comparatively, there are fewer trees in the Northern Guinea Savannah than in the Southern Guinea Savannah and the trees are not as tall as those found in the Southern Guinea Savannah. Most of the tall grasses found in the derived Guinea Savannah, are also found in the Guinea Savannah, however, they are less luxuriant. The appearance of this zone differs from season to season. During the rainy season, the whole zone is green and covered with tall grasses that grow and reach maturity rapidly and thus become fibrous and tough. In the dry season they tend to die and disappear and one can see for kilometers without obstruction. This clearing is due to several periodical bush-burning that occurs during the dry season between November and April, carried out to either assist in farm clearance or hunting.
THE SUDAN SAVANNAH (Short grass savannah)
The Sudan Savannah zone is found in the Northwest stretching from the Sokoto plains in the West, through the Northern sections of the Central highland. It spans almost the entire Northern States bordering the Niger Republic and covers over one quarter of Nigeria's total area.
Map showing the Sudan Savanna region of Nigeria
Courtesy: ojo tolulope Samuel
The low average annual rainfall of 657.3mm and the prolonged dry season (6-9 months)[iii] sustain fewer trees and shorter grasses than the Guinea Savannah. It is characterized by abundant short grasses of 1.5 - 2m and few stunted trees hardly above 15m. It is by far the most densely human populated zone of Northern Nigeria. Thus, the vegetation has undergone severe destruction in the process of clearing land for the cultivation of important economic crops such as Cotton, Groundnut, Sorghum, Millet, Maize and Wheat. The grass vegetation is interspersed with farms and thick bush trees such as Shea Butter tree (Butyrospermum parkii) and Acacia albida. Also found in the zone are Locust Bean trees (Parkia filicoidea), Tamarind tree (Tamarindus indica) and Mango (Mangifera indica). A large portion of this zone falls within the Tsetse Fly free belt of West Africa and it is excellent for the rearing and breeding of ruminant Livestock (Cattle, Goats, Sheep, Donkeys, Horses and Camels). The nomadic Fulani roam about this zone in search of fodder and water for their Livestock.
THE SAHEL SAVANNAH (Marginal savannah)
This is the last ecological zoological zone with proximity to the fringes of the fast- encroaching Sahara desert. Occupies about 18 130 km2 of the extreme Northeast corner of Nigeria and is the last vegetation zone in the extreme northern part of the country, close to Lake Chad, where the dry season lasts for up to 9 months and the total average annual rainfall is hardly up to 700mm. Here the vegetation is not only sparse but the grasses are very short. As a rule this zone is not cultivated without irrigation. The people found in this zone are the nomadic herdsmen, and they are careful not to burn the grass found because sparse as it is it provides the only pasture available for their grazing Livestock. It is characterized by either very short grasses of not more than one meter high located in –between sand dunes. The area is dominated by several varieties of the Acacia and Date – palms. The Lake Chad basin, with its seasonally flooded undulating plains, supports a few tall trees. At the same time, the drainage system of rivers and streams into the Lake Chad basin has favored irrigation, without which cultivation would be virtually impossible. The increasing aridity in the area accounts for the progressive drying up of the Lake Chad.
The Montane zone is located in the high altitude areas of the country like Jos Plateau, Mandara, Adamawa Mountain and Obudu Plateau. The zone is characterized by low average annual temperature (21.5ºC). The average annual rainfall is 1450mm. The Montane zone vegetation is covered with grass at the top and base, while forests cover the slopes, favored by moisture-laden wind. The zone has a great potential for the cultivation of Maize, Wheat, Carrot, Cabbage and other exotic vegetables but the mountainous nature of the zone prevents commercial farming. The Fulani who live in great numbers in the area turn the available fields into good pasture for their grazing animals.
The main constraints on feed resources in all the zones are the destruction of perennial tree cover for firewood, bush fires caused by hunters; livestock rearing and overgrazing. These man-made constraints often lead to serious degradation of the pastoral resources and in some cases to an irreversible process of desertification, especially in the Sahel zone.
[i] Oyenuga, V.A. (1967). Agriculture in Nigeria. Food and Agriculture Organization of the United Nations). FAO, Rome, Italy. 308 pp.
[iii] F.A. Sowunmi and J. O. A kintola (2010) Effect of Climatic Variability on Maize Production in Nigeria. Research Journal of Environmental and Earth Sciences 2(1): 19-30,