Establishment of Timber Plantations: On modern process of land cover change is th replacement of mature forests by single specie timber plantations. The system, known as taungyc is adopted in upgrading the timber productivity ( some of the existing forest reserves (Adeyoj 1975). In its essential features, it is not differer from the cultivation of tree crops except tha instead of crops such as rubber, cocoa or oil paliT either Cassia sieberiana, Gmelina arborea, Tecton grandis, Terminalia superba, T. ivorensis or Tripk eft/ton scleronxylon is planted. Much of the matur forests of the reserves in Oyo State have been cor verted in this way. Also in Ogun, Akwa lbom an Kwara states, considerable areas have bee brought under Gmelina arborea to provide the feec stock for the already commissioned or propose paper mills.
Timber and Firewood Production: Another process of change results from the exploitation ol the wood resources of both woodlands and forests. This involves the selective removal of the larger ele- ments of certain species for timber or firewood. Especially in the rain forests, this process results in considerable modification to the structure and floris- tic composition of the natural vegetation. The closed foliage canopy of the rain forest becomes broken.
Full sunlight may reach the floor in patches to encourage the profuse growth and development of heliophytic species. With the removal of their usual hosts and support, the large woody climbers form thick tangles on the floor to make movement through the vegetation difficult.
The resulting vege- tation is usually described as disturbed mature forests. The important timber producing species in the forest zone include: Celtis spp, Chlorophora excelsa, Distemonanthus benthianum, Entandrophrag-rna spp, Quarea spp, Khaya grandHolia, K. lvorensis, Lovoa klaineana, Mansonia altissima, Mimusops spp, Piptadeniastrum africanum, Terminalia super- ba, Triplochiton scleroxylon. The common tree species in the savanna adjudged capable of yield- ing high quality woods include Anogeissus leiocar- pus, Daniellia oliveri, Diospiros melspiliformis, Isobertinia doka, 1. tomentosa, Khaya senegalensis, and Pterocarpus erinaceous.
QUANTITATIVE CHANGE IN VEGETATION
ERGO (1994) describes land use changes in Nigeria between 1976 and 1990. The data for 1976 were from the Side-Looking Air borne Imageries (SLAR) acquired by the Federal Department of Forestry earlier referred to, while the 1990 data were got from the low altitude aerial photographs acquired for the Nigerian National Livestock Survey (RIM 1992). In order to avoid presenting an unrealistic linear trend using the two data points provided by the 1976 and 1990, vegetation indices derived from 1987-1989 Advanced Very High Resolution Radiometer (AVHERR) were used as intermediate data point (ERGO 1994).
The results of this study are particularly rele- vant to our understanding of changes in land cover in recent times. For example, they show that between 1976 and 1990, there was an increase of 7.46 per cent in the area of land brought under cul- tivation. This is equivalent to an area of 68,632 km2. During this period, forest loss totalled 2.90 per cent (19,228 km2). By projection, the forest area is expected to decrease to 2.3 per cent by the year 2020, while a little more than half of the entire coun- try is expected to be under cultivation by the same year. Although possible interventions, such as adoption of agroforestry programmes and popula- tion control, may alter the projected figure, there is no doubt that more significant changes in cover characteristics are expected in the future.