Nigeria is located in western Africa and lies between latitudes 4

INTRODUCTION

Nigeria is located in western Africa and lies between latitudes 4° and 14° N and longitudes 2° and 15° E and has a total area of 923,768 km². It shares borders in the continental interiors with Benin, Niger, Chad, and Cameroon. Furthermore it has an extended coastline of at least 853 kilometers in the gulf of guinea. Its position in the tropics coupled with the coastal boarder lead to possession of two distinct seasons resulted from the action of Trade winds.

Movement of tropical maritime air mass (MT) which originates from the South Atlantic Ocean, from the coastal area to the continental interiors creates a rainfall gradient. This humid and warm air mass starts to move southward in February and takes four months to invade the whole country. The wind prevails due to the northward retreat of tropical continental air mass (CT) resulted from shifting of sun from south to northern hemisphere. The domination of the atmosphere with this unstable trade wind leads to the formation of conventional current that easily triggers the formation of heavy rainfall in the area. Although African easterly waves also contributes to the formation of rainfall in Nigeria.

Therefore rainy season starts from March to October and is progressively reducing from the southern part to the northern where it lasts for only four months, from June to September with The low average annual rainfall of 657.3mm and the prolonged dry season (6-9 months (Sowunmi and Akintola, 2010) . while in the central Nigeria like in Abuja it goes from April to October. On the other hand, the tropical continental airmass originates from North Africa and blows southward across the Sahara desert into Nigeria. The tropical continental airmass has low humidity and picks up dust as it crosses the desert which renders the atmosphere hazy. Consequently, the sun rays reaching the earth are reduced and in turn reduce the atmospheric temperatures considerably to about 9^oc in some part of the North. This condition may range from three months in the north to few weeks in the southern part of Nigeria .This condition is significant for drying of farm products and for good yield of some crops such as the wheat.

FACTORS AFFECTING PLANT DISTRIBUTION IN NIGERIA

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  CLIMATIC FACTORS;
  

Rainfall is one of the key factors affecting the distribution of plants in Nigeria. The annual average precipitation in the coastal areas exceeds 3,000 mm, remains over 2,000mm in the south, between 1,000mm and 1,500mm in the central regions and below 500mm in the far north-eastern region. Temperature and humidity are relatively constant or variable in the south and north respectively. On the coast the mean monthly maximum temperatures remaining about 90 °F (32 °C) at Lagos and about 91 °F (33 °C) at port Harcourt. In general, mean maximum temperatures are higher in the north, while mean minimum temperatures are lower. In the northeastern city of Maiduguri, for example, the mean monthly maximum temperature may exceed 100 °F (38 °C) during the hot months of April and May, while in the same season frosts may occur at night. The humidity generally is high in the north, but it falls during the Harmattan (the hot, dry northeast trade wind), which blows for more than three months in the north but rarely for more than two weeks along the coast.

Few mountainous areas exist with relative different climatic conditions. For instance in Jos plateau central Nigeria, the presence of highland up to 1200m height resulted in semi temperate weather conditions. Annual average temperatures remain within the range of 16°C

to 25°C throughout the year, however the average annual rainfall may reach up to 1500mm.

The highest mountainous areas are also found on the Nigeria Cameroon border with the highest peak elevation about 2419m above sea level. The weather conditions prevailing in these regions are the temperate type.
Vegetation zones in Nigeria

The kind of vegetation is usually dependant on the variations in climatic condition of the area, on that basis Nigerian vegetation can be seen to composed of the following belts (figure 1)

Biodiversity and rainfall

We used fairly large grid cells to describe regional

patterns in biodiversity. Local, regional, and landscape

diversity contribute to the species richness of the grid-cell.

To understand which factors govern variation in regional

diversity, we should look at environmental factors that

operate at this spatial scale (O’Brien 2000).

In Upper Guinea there is a strong rainfall gradient

that varies from 1000 mm at the forest-savanna boundary

to 4000 mm at the coast. Species richness increases along

this rainfall gradient, showing an optimum around 2500

mm, whereafter it may level-off or decline. An increase in

tree species richness with rainfall has also been found in

other studies (Hall & Swaine 1976, O’Brien 1993,

Clinebell et al. 1995). Two mechanisms may give rise to

this pattern. Over the first part of the rainfall gradient, an

increase in forest height and structural complexity provides

more niches, thus allowing more species to coexist.

Another reason is that in addition to the drought tolerant

species, also more drought intolerant life forms and species

are able to persist at higher rainfall levels (Huston 1994).

The shape of the biodiversity-rainfall curve depends

on the section of the rainfall gradient regarded. In southern

Africa the species richness of dry vegetation formations

shows a strong, linear increase with rainfall between 200 to

1000 mm (O’Brien 1993). For the closed forest formations

in the Neotropics this linear relationship is maintained

between 1000 and 2000 mm, above which it disappears

(ter Steege et al. 2000b). One may argue that under

perhumid conditions water availability is not a limiting

factor for species richness anymore. However, it is very

likely that the biodiversity-rainfall relationship is to a large

extent scale-dependent. At a macro scale the potential

species richness is determined by the climate (Whittaker et

al. 2001), at a meso scale on the configuration of different

vegetation types within the landscape, whereas at a micro

scale, the realised species richness depends on local site

conditions. Ter Steege et al. (2000b) evaluated the diversity

of 1-ha forest plots in the Neotropics. The maximum

species richness at a given amount of rainfall indeed shows

a linear increase with rainfall up to 3500 mm. Adverse

local site conditions (such as swamps, flooding, extreme

soil conditions) may prevent the forest from attaining its

climatic potential, leading to a lower diversity and a

specific flora. Biodiversity-rainfall relationships are also

likely to vary with the life forms considered. Tree species

diversity increases with rainfall, and levels-off at higher

rainfall levels, while the diversity of epiphytes, mosses and

understorey herbs still increases (Gentry 1988). Our

species list includes a diverse array of life forms (trees,

shrubs, lianas, herbs, epiphytes) but has some emphasis on

woody plants. It is therefore likely that it shows a

saturating response to rainfall.

Our study is the first to show a potential decline of

biodiversity at very high rainfall levels (>3000 mm). It is

not clear to what extent this is a spurious result. Many of

these extremely high rainfall sites are situated along the

coast of Liberia, Sierra Leone and Guinea. The coastal grid

cells cover only part of the land surface, so fewer habitats

and less landscape diversity might be sampled, resulting in

lower diversity values for such cells. This is supported by

the fact that coastal areas are in general less rich than

adjacent continental areas (Figure 6.7). The coastal zone in

Guinea and Sierra Leone also receives a high amount of

rainfall which is unevenly distributed over the year. The

length and severity of the dry season is likely to be a

stronger determinant of humidity and species richness than

rainfall per se.

If the decrease in diversity with rainfall is not

spurious, this might pinpoint to other mechanisms. First,

some species might be intolerant to waterlogged

conditions. This hypothesis is supported by the fact that a

high rainfall, in combination with a high water holding

capacity leads to a reduced diversity (Table 6.3). Second,

very humid areas, with an annual rainfall larger than 4500

mm do not exist in West Africa. Species that require very

wet conditions (i.e. >4000 mm) are likely to have

populations that are too small to persist, or even to have

gone extinct during the dry glacial per



Figure :1 The map of Nigeria showing vegetation belts.

2.4.1 Rainforest

Tropical forest are among the richest and most complex terrestrial ecosystem that support variety of life forms not less than half of all species on earth (Philips,1996) .

High-mountainous multi-tiered tropical forests have been preserved mainly along the right bank of the lower reaches of the River Niger and in the valley of the River Cross (figure 2). Trees with a height of 40-45 m form the first upper tier.

The trees of the second and third tiers are especially densely strewn with epiphytes, intertwined with lianas that rush towards the sun. The rays of the sun almost do not penetrate through the dense green canopy formed by woody crowns. Some the plants inhabit the are Khaya seneglensis,Elaeis guinensis,Theobroma cacao, Anthocleista djalonensis, Aidia genipiflora ,Antidesma laciniatum , Afzelia Africana, Aningeria robusta Albizia feruginea, Albizia zygia ,Antiaris toxicaria, Annimidium mannii ,Anthonatha aubryanum , Alstonia boonei , Alstonia congensis ,Anthonatha Bridelia micrantha ,Berlinia confuse, Bombax buonopozense ,Baphia nitida and many others(Ojo , 2004)

figure 2: Rainforest