Vegetation

Posted by Webby on 6/5/2002 8:48:04 AM | Views: 81465 |

Vegetation

Vegetation, simply defined, is the plant cover of the earth consisting of assemblages of plants. Together with physiography, it constitutes the most observable element of the landscape. Vegetation expresses and reflects environmental conditions, particularly climate.

Broadly speaking, the national vegetation over a geographical area is essentially a response to the climate in that area. Nigeria's vegetation belts reflect this very close link between vegetation and climate. Hence, the similarity in the west-to-east zonation of both climate and vegetation. With the south to north progressive decline in total rainfall and length of wet season, vegetation belts are demarcated on west-to-east zonation pattern characterised by transitional zones from one belt to another.

Nigeria has two broad belts of vegetation types, namely, the forest and savannah types. There is, however, also the mountain vegetation of the isolated high plateau regions in the central and far eastern parts of the country.

Forests: Forests are vegetation types or plant formations in which trees are the dominant species. Nigeria has a heavily forested coastal south where humid tropical conditions favour tree growth. Three forest zones can be sub-divided, from the coast inland, viz:

  1. Saline water swamp
  2. Freshwater swamp
  3. Tropical evergreen rainforest.

Saline Water Swamp: This vegetation type is restricted to the coastal strip, which varies in width from less than 1.5km in the Badagry and Lekki peninsula areas to overSOkm in the Sapele area. It is pronounced where the fresh water from the rivers meets and mixes with the salt water from the sea, forming brackish swamps. The low-lying nature of the Nigerian coastal zone allows for the influx of saline water through tidal movements into the lagoons, creeks and extensive brackish wetlands. This has encouraged the growth of different species of mangrove vegetation, typical in the wetlands of the backshore areas.

The mangrove vegetation is a hydromorphic forest type characterised by an entangled dense growth of stems and aerial roots behind the stretch of coconut palms overlooking the Atlantic Ocean. A good example is the Lekki Peninsula area east of Lagos.

Freshwater Swamp Forest: This vegetation belt, on freshwater wetlands, occurs further inland beyond the reach of tidal waters. Here, there is an enormous supply of freshwater from the inland rivers and run-off from abundant rainfall in the area.

The major drainage systems, from west to east, are the Ogun, Benin, Imo, Niger Delta and Cross River, which 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 a.s.l, thus, it facilitates the development of freshwater swamps along the Niger Delta, drowned estuaries, lagoons and creeks.

The intricate network of creeks and lagoons results in inaccessible swamps of forest vegetation in the southern parts. In the northern part are floodplains of sandy accumula- tions, colonised by bush thickets and by tall grasses in the cultivated areas. The most common specie of this vegetation type is the raffia palm (Raffia Hookers) which dominates the swamps. The better-drained areas support oil palm trees {Eleais guineenais) and big trees like Iroko {Chlorophora Exceisa).

The Tropical Evergreen Rainforest: This is a belt of tall trees with dense undergrowth of shorter species dominated by climbing plants. The dominant species of the climbing plants is the lianas which are clustered and entangled in nature, making accessibility and exploitation of big trees very difficult. The prolonged rainy season, resulting in high annual rainfall above 2000mm in this area, ensures adequate supply of water and promotes perennial tree growth. This luxuriant vegetation belt stretches from the western border of Nigeria with Benin Republic, through a narrow stretch on the Niger-Benue river system into the extensive area in the south-east of the country. The narrow stretch on the Niger-Benue river system is due to the northward stretch and influence of the freshwater swamp forest north of the depositional environment of the Niger Delta.

The tropical evergreen rainforest accounts for a great number of plant species classified by their layering structure into three, namely: lower, middle and top layers.

The Lower Layer: This forms the undergrowth where the vegetation is most dense, describing an abundance of herbs, shrubs and some grasses. They are hardly above 10m high as they are con- stantly subjected to destruction through clearing for cultivation. Apart from their climbing nature their development is also stifled by the taller and more luxuriant trees of the middle layer.

The Middle Layer: The tropical evergreen rain 5 forest derives its name from the nature of this layer. 3 The middle layer consists of heavily branched tall trees ranging between 15-30m with well-developed and deep green foliage. The layer's continued exposure to solar energy and prolonged humid conditions account for the hundreds of evergreen plant species. The luxuriant nature of this layer is typified by the interlocking of the tree branches to form an extensive canopy of evergreen foliage. The Top Layer: When viewed from the air, the extensive canopy of the middle layer is broken by very tall trees in a scattered manner, rather than the closely packed nature of the lower and middle layers. Trees of the top layer have tall straight stems of 50-60m with leaves growing on a few branches at the top of the trees. They thus possess very if striking stems developed over highly buttressed roots.

The top layer accounts for valuable econom ic trees such as the Mahogany, Iroko, Obeche, Sapele Wood and Walnut. They are very widely scattered making exploitation expensive. The prop roots are known to rise some four metres above n ground level in most cases, making felling difficult. The tropical evergreen rainforest belt of Nigeria is characterised by very high human population densities, with agriculture as the primary occupation of the people. The great demand for farmland has led to the destruction of extensive areas of the rainforest. The eastern zone of this belt has virtually been replaced by the oil palm plantation which produces oil, kernel and palm wine for economic purposes, as well as yam, cassava and vegetables, for subsistence. Some of the high rainforest are however still retained in pockets as reserves by the Federal and State governments, or as community bushes.

Savannah: The word 'Savannah' is an Indian American name which means a grassland area with no forest cover. It has its most extensive belt over the high interior plateau of the African continent. Thus, the term savannah is commonly used to describe areas within the tropics under grass or grass with scattered trees. Savannah vegetation in Nigeria, as in other parts of West Africa, consists of three major belts, from south to north, viz:

  • Guinea Savannah
  • Sudan Savannah; and
  • Sahel Savannah.

One major characteristic of savannah vegetation is that trees vary in size and density from the Guinea, through the Sudan, to the Sahel Savannah.

Guinea Savannah: The Guinea Savannah, located in the middle of the country, is the most extensive vegetation belt in Nigeria, covering near half of the country. 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.

This is occasioned by the local climatic conditions of low rainfall and long dry periods. This is in addi- tion to the devastation caused by man through bush clearing for agriculture. This devastation has been observed in the southern part of the Guinea savannah where population density and demand for farm- land are very high. The trees, which are taller and bigger in this area than in the northern part of the Guinea savannah, are easily exploited due to accessibility over the grassland terrain. The term derived savannah is given to its southern portion, which today marks the transition between the two broad groups of vegetation types in Nigeria: the forest in the south and the true savannah in the north. The Guinea savannah, with its typically short trees and tall grasses, is the most luxuriant of the savan- nah vegetation belts in Nigeria.

Sudan Savannah: This vegetation belt is found in the north-west stretching from the Sokoto plains in the west, through the northern sections of the central highland. It spans almost the entire north- ern states bordering the Niger Republic and covers over one quarter of Nigeria's total area. The low annual rainfall of usually less than 1000 mm and the prolonged dry season (6-9 months) sustain fewer trees and shorter grasses than the Guinea savannah. It is characterised 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, millet, maize and wheat. This is in addition to devastation due to animal husbandry, especially cattle rearing, which is greatly favoured in this belt because the area is relatively free from tse-tse fly. The trees of the Sudan savannah include the acacia, the shea-butter, baobab and the silk cotton.

Sahel Savannah: This is the last vegetational belt to the north of Nigeria with proximity to the fringes of the fast-encroaching Sahara desert. It is located in the extreme north-eastern part of the country, close to Lake Chad, where the dry season lasts for up to 9 months and the total annual rainfall is hardly up to 700mm. It is characterised by very short grasses of not more than one metre 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 favoured irri- gation, without which cultivation would be virtually impossible. The increasing aridity in the area accounts for the progressive drying up of the Lake Chad.

The Mountain Vegetation of IsolatedPlateaus:

The mountain vegetation of the isolated high mountains and plateaus of the central and eastern part of Nigeria is not well developed because of the great influence and interference by man and animals. For instance, the Jos plateau, which is one of the highest points in Nigeria, is in a grassland zone, but its vegetation depicts grassland at the top and base of the Plateau, while the slopes, favoured by moisture-laden wind, are covered by forests. These are also true of the Mandara and Adamawa mountains and the Obudu plateau.