One can recognise two climax vegetation types in Nigeria. One is the Rain Forests that reflect a regional climate characterised by perennial humid topicality. The other climax is the Deciduous Woodlands that are in a state of dynamic equilibrium with another climate type characterised by seasonal humid topicality. The Rain Forests consist essentially of a continuous stand of trees with canopies varying in height from 10 to 50m. The crowns of individual trees overlap and are often woven together by woody and succulent climbers.
Most botanists who have had field experiences in these forests describe them as consisting of several layers of trees apart from the usual layers of shrubs and herbs. The Deciduous Woodlands is the most wide- spread climax in Africa. It is especially characteristic of the Sudan region with its continental climate and moderate summer precipitation. The dominant type of woodland consists of an open stand of trees, the crowns of which form a canopy from 8 to 20m or more in height, and cover at least 40 per cent of the surface.
The crowns of adjacent trees are often in contact but are not densely interlocking. Most of the woodlands are deciduous or semi-deciduous but nearly all contain a few evergreen species. Because the canopy casts relatively light shade, there is usually a ground cover consisting principal- ly of herbaceous tussock grasses, the columns of which are up to 2m high. Occasionally, stands of woodlands have a closed canopy and, as a conse- ; quence, also a poorly developed grass layer.
THE PROCESSES OF VEGETATION CHANGE AND THE RESULTING DERIVATIVES
There are five main processes by which most of the present vegetation categories have been derived .from the original forests and woodlands. Three of these are related to agriculture while the other two are related to forestry practices.
Shifting Cultivation:Shifting cultivation and rotational bush fallow are the chief enemies of the natural forests and woodlands. These involve the clearing of the woody elements, in small patches, in preparation for the cultivation of field crops. After a harvest or two, the land is allowed to rest under fal- low vegetation. Under a 'normal' condition of low farming population, such patches are re-cultivated after a period of five to ten years (rotational bush fallow). It may however remain longer under fallow and have enough time to develop into secondary forest woodland (shifting cultivation).
But it is not common to have the original natural plant cover reestablished as mature vegetation. The resulting secondary (cultural) vegetation is a mosaic consist- ing of field crop plots and fallow vegetation at vari- ous stages of recovery. In the drier woodland zones, the fallow vegetation consists mainly of grass or savannah while in the forest zones, the vegetation consists mainly of woody secondary forests. There is, however, a transition zone where the original vegetation is forest but the man-made derivatives are savannah. The major elements in the resulting anthropogenic vegetation can be clas- sified into two broad groups of mosaics - forest and savannah.
Forest Mosaice1. Farmland- oil palm mosaic
2. Farmland- cocoa-kola-secondary forest mosaic
3. Farmland- secondary forest mosaic
4. Farmland- secondary forest - oil palm mosaic
5. Farmland -wooded shrub grassland mosaic
6. Farmland -rubber - secondary forest mosaic
7. Farmland -swamp forest mosaic
1. Farmland- open grassland mosaic
2. Farmland- shrub savannah mosaic
3. Farmland- park savannah mosaic
4. Farmland- low savannah woodland mosaic
5. Farmland- savannah wonriland mosaic
Permanent Cultivation: With a high density of population and relatively good soils, an area long used for shifting cultivation can adopt a system of permanent cultivation. This can lead to further veg- etation degradation. In the vicinity of the larger towns in northern Nigeria, continuous cultivation based on simple crop rotations and the use of manure and fertiliser on permanent and well defined holdings has replaced shifting cultivation. Because of the intensity of cultivation, the vegetation cannot be described as savanna since the surface is covered by one type of crop or the other during the rainy season and left bare during the dry season.
However, the landscape is dotted with scattered trees that appear to be protected. From these, food, medicinal and industrial products are derived. Among the best known of such trees are Acacia albida, A. arsbica, A. seyal, A. senega/ens/s, Adasonia digitata, Azadiracta indica, Butyrospermum parkii and Parkia filicoidea.
Tree Crop Cultivation: Tree crop cultivation results in the replacement of the main forests by permanent (tree) crops arnonq which the most widespread are the oil palm, rubber, cocoa an kolanut. Oil palm trees grow naturally in large num bers in the fallow vegetation created by shifting cultivation. Over extensive areas in the forest zone, < practice of protection creates oil palm groves tha once made Nigeria the world's leading producer o oil palm products. In many cases, the shade of th( foliage crowns of the palm groves are deep enougl to discourage the growth of other crops. In th< majority of cases, however, crops such as th(shade-loving varieties of Xanthosoma spp. (cocoy am) are planted in the shade of the oil palms am where gaps in the foliage canopy are large enough small plots of other food crops are established.
In Delta, Edo, Ondo and Ekiti States, within the forest zone, the forest is removed to make way fo the cultivation of food crops intercropped with rub her. When the food crops are harvested, the peas ant-farmer plots become rubber farms. Such rubber plots exist in a mosaic in which the food crops farmlands may be dominant or sub-dominant. Within the territorial limits of the tour states, there are also hundreds of thousands of hectares of land devote to rubber plantations on which major rubber processing plants are based.
Such plantations const tute spectacular features on the landscape whethe seen through aerial photographs or other remotel sensed imageries. Cocoa on peasant farmers' plot is established more or less in the same way as rut ber, that is, as the last crop in a rotation starting wit food crops. In south-western Nigeria, especially i Ogun, Oyo, Osun, Ondo and Ekiti States, cocoa, in association with kola and citrus, covers as much a 15 per cent of the land on the average, not considering the large areas underforest reserves. In some localities in Akure, lfe, Ondo and lbadan divisions more than 25 per cent of the land is cultivated t these crops. Blocks of land extending over thousands of hectares and covered exclusively by these crops, can be observed to the south east of lbada and between lle-ife and Ondo towns.