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:
- Saline water swamp
- Freshwater swamp
- 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.
