INTRODUCTION Forests perform a broad range of environmental functions, including the ground water recharging. It provides habitats for plants and animals generate economic, aesthetic, industrial and religious benefits to man. However, economic development pressures often lead to the conversion of forest ecosystem without consideration for both the long-term economic costs and the implications of the immediate loss of biodiversity, ecosystem structure and function. Presently, Nigerias rainforests harbour about 4000 (Gbile et al, 1981) different species of plants, including those that with medicinal values. There are also animals, including birds that cannot be found elsewhere, except Nigeria. These include the Ibadan Malimbe, the Anambra waxbill, the Jos indigo bird, the white-throated monkey, the Niger Delta pigmy hippo and the Niger Delta colobus monkey (Orimoogunje, 2000). Forests are valuable and important to the livelihood systems of rural communities built around them. Loss of ecosystem structures has far-reaching adverse consequences for these communities. At the present, the tropical rainforest is under attack, both from within and from without. This is due to socio-economic development in every sphere and the need of meeting the ever - increasing demands for forest products as population increases (the two parts of the sentence mean the same thing). For instances between 1981 and 1994 Nigeria lost 3.7 million hectares of forests (Orimoogunje, 2000). At the present, less than 4 per cent of Nigerias untouched rainforest cover is left (Orimoogunje, 2005). More frightening is the fact that the loss is continuing at the rate of more than 3.5 per cent annually (Orimoogunje, 2005). This implies a colossal loss of biodiversity. The greatest threats to the forests in Nigeria have been bush burning and illegal logging, which have been rife in the study area (yet to define the study area) over the years. The degradation process in the forest area of Nigeria has been found to be significant in terms of the disappearance of the three crowns layers of trees characteristic of the region (Ekanade, 1991). The focus of this paper is, therefore, to investigate how human activities have influenced and altered tree cover; and to suggest the need for conservation and sustainable management of forest resources.
STUDY AREA The study area is Oluwa Forest Reserve in the Southwestern part of Nigeria. It lies approximately between latitudes 60371 and 70201 North and longitudes 40271 and 50051 East, the area is part of the western plains and ranges of Nigeria with much of it lying approximately between 300 and 600 metre above the sea level (Iloeje, 1965). Most rivers and streams draining this area rise from the southern part of the study area. Notable among the rivers are Oni, Oluwa, Ominla and Owena. The study area is under the influence of Koppens Af humid tropical rain forest climate. Mean annual rainfall ranges from 1,200mm to 1450mm and temperatures are high throughout the year with a mean of about 270C with annual range of 30C (Nigeria Meteorological Observation, 1962). The natural vegetation of the area is tropical rainforest characterized by emergent with multiple canopies and lianas. Some of the most commonly found trees in the area include Melicia excelsa, Afzelia bipindensis, Antiaris africana, Brachystegia nigerica, Lophira alata, Lovoa trichiliodes, Terminalia ivorensis, T. superba, and Triplochiton scleroxylon. However, the natural vegetation of the area with the exception of the areas devoted to forest reserve has now been reduced to secondary regrowth forest thickets and fallow regrowth at ranging stages of development or replaced by perennial and annual crops (Osunade, 1991). These perennial crops include cocoa, kola and citrus. Most of the rural settlements in the study area came into existence between 1920 and 1950 and by 1970 human colonization of the area had been completed or tending towards completion (Adejuwon, 1971).
METHODOLOGY A map of the study area was compiled from appropriate topographical map Sheet and satellites imageries of that part of Nigeria. The study area was demarcated on the map and grided into 5km x 5 km, which gives 25 square grids out of which 10 square grids were selected using the table of random numbers. Each grid was again grided into 1km x 1 km out of which 10 were again picked randomly. Ten quadrats 40m x 25m in dimension were demarcated from each of the selected 1km x 1km grid for vegetation analysis. This quadrat falls within the range of quadrat sizes suggested by Weshoff and Maarel (1978), which has been used by Aweto (1978), Ekanade (1985), Adejuwon and Adesina (1988) and Salami (1995) for vegetation sampling in the tropics. In all, a hundred quadrats were selected altogether. All tree species occurring in each quadrat were identified and recorded. Digital image processing was carried out on Landsat MSS 1972, Landsat TM 1991 and 2000, using Multipscope software package. Land uses were identified and classified from the Landsat imageries based on colour, texture, shape and size using the Integrated Land and Water Information Systems (ILWIS) 3.1 software. Training parcels were sampled based on ground survey after the contrast enhancement. The classes of training sites include: the arable crop cultivation, tree crop cultivation, exotic tree plantation, dense forest and settlement/open space. The maximum probability algorithm was used for final classification. The mathematics of the maximum likelihood decision rule, which was applied, has been explained by Tatsuoka (1971). The result of the classification in 2000 was cross-checked with ground truthing of the land cover in the study area. The precision was above 90%. This shows that the classification method was reliable.
RESULTS Table 1 summarises the trend of land use and land cover changes in the study area between 1972 and 2000 (see Figures 2 and 3). The table shows that the area covered by exotic plantation cover class increased from 49,034.4 hectares in 1972 to 55,683 hectares in 1991, but decreased to 22,276.hectares in 2000. This indicates a decrease of 60% between 1991 and 2000 with a degradation rate of 6.7% per year.
Table 1: Areal extent of land use types in the study area between 1972 and 2000 Land use Type
1972
1991
2000
Areal Extent Ha % of Total Areal Extent (Ha) % of Total Areal Extent (Ha) % of Total Dense forest 130,774.00 70.79 110,826 59.99 19,383 10.49 Exotic trees plantation 49,034.40 26.54 55,683 30.14 22,276 12.06 Arable crop cultivation - - 2,671 1.45 56,229 30.44 Tree crops cultivation - - 5,505 2.97 55,943 30.28 Settlement/ open space 4,932.60 2.67 10,056 5.45 30,911 16.73 Total 184,741 100.00 184,741 100.00 184,741 100.00 Source: Landsat imagery classification
The areal coverage of forest cover class has declined from 1972 to 2000. By 1972, 130,774 hectares of this unit cover had declined to 110,826 hectares in 1991. This decreased by 19,948 hectares i.e. 15% in 1991 compared with situation in 1972. Its degradation rate is 0.8% per year. By 1991, this unit cover has declined from 110,826 hectares to 19,382 hectares; its rate of degradation was 9.17% per year. These results show that the degradation of forest cover class have been existing in Oluwa Forest Reserve before 1991 but after 1991 its degradation rate started to increase because the demand for both minor and major products of the forest reserve was on the increase.
The area of coverage by tree crop cover class had an increased trend. Between 1991 and 2000, this cover class has increased by 49,438 hectares, which was 85.39% of the area covered in 1991. Its increasing rate was 9.49% per year. The result showed that the tree crop cover class was taking over or encroaching into the forest reserve land. If this trend should continue, the forest reserve may be totally taken over by this class cover. The area of coverage by settlement cover class has increased. The areal extent of this cover unit from 1972 to 1991 increased by 5,123.4 hectares (i.e. 50.95%) of the area covered in 1991. And between 1991 and 2000, it has increased by 20,855 hectares, i.e. about 67.47% of the area covered in 1991. The result shows that settlement cover class was in the increasing trend. This shows that more and more people are settling within the forest reserve enclosure, which is supposed not to be so. According to the field survey most of these settlers are illegal farmers and poachers. The trend shown by this analysis indicates that there is serious encroachment into the forest reserve area. If the trend should be allowed to continue at the rate shown by this analysis, it is estimated that the forest reserve would have gone totally by the year 2010. It was observed during the field study in 2005 that the pocket of natural forest still existing in the area is due to inaccessibility or rugged nature of the terrain existing in the study area.
DISCUSSION AND IMPLICATION The analysis shows that the mode of incursion into the forest reserve is mainly through tree and arable crops cultivation. This finding confirms the view of the World Bank (1991) that most of the tropical forests cleared each year are due to agricultural practices. It also lends credence to the hypothesis of Bilsborrow (1994) that deforestation is largely due to intensification of agriculture, involving clearing the land of trees to plant crops. Although taungya system, which was encouraged by the Forestry Department, has been noted to be similar in its appearance to forest ecosystem but its cover closure is obviously not the same as mature or high forest (see Adejuwon and Ekanade, 1988). The study area, which falls within the tropical rain forest, was once described by White (1983) as an area consisting essentially of a continuous stands of varied trees with canopies varying in height from 10 to 50 metres. He further reveals that the crowns of individual trees overlap each other and are often interlaced with lianas. This description contradicts the present state of the study area (see Plates 1 & 2). The high forest is highly variable with regard to species composition and stocking. Hall (1977) suggested that the variation could be associated with soil differences. Hall shows further that the use of ferralitic soil for intensive plantation establishment is not advisable unless adequate precautions are taken to compensate for substantial nutrient removal from the cycle during harvest. Also the inventory of faunas population compiled from the study area shows that monkey is absent on the list and monkeys are indicator of the presence of original forest in a particular place (Gbile, et al, 1981). This shows that the natural forest is dwindling in the study area. Buffalos and antelope are also at the point of extinction in the study area. This shows that the problem of forest destruction is directly related to loss of biodiversity. As a result of low yield and problems encountered in improving the productivity of the natural forest and increasing demand for wood and wood products, large area of Oluwa Forest has been converted to monoculture plantations of exotic and indigenous tree species. For instance, 30 hectares have been established in the Nirowi quarters through taungya system and Lisagbede area of the forest reserve which has been totally changed to Ondo State Afforestation Project. The following hectares of high forest in the study area have been cleared and replaced with pulpwood plantations within the last 27 years: * 1980 1996: 8,201 hectares were cleared and planted to Gmelina * 1989 1996: 6,546 hectares were cleared and planted to Gmelina * 1989 1996: 119 hectares were cleared and planted to Pines * 1987 1996: 197 hectares were cleared and planted to Cattapa and Cashew * 1987 1996: 1,100 hectares were cleared and planted to Tectona grandis * 1989 1996: 843 hectares were cleared and planted to Terminalia superba, Terminalia ivorensis and Melicia excelsa; 20 hectares were cleared and planted with Elaeis guineensis. It was observed that the indigenous tree species under this section of the reserve were cleared to give way to exotic ones in order to meet the demands of Iwopin Paper Mills located in Ogun State which was owned by the Federal Government of Nigeria. Adams (1978) and Chijioke (1980) reported that total biomass removal, which invariably is the practice in tree harvesting for industrial use accounts for a tremendous loss in the nutrient status of the soil environment. The area of farmland within the study area covered 56,229 ha for arable crops cultivation (Table 1). Farming encroachment accounted for 30.28% of the area in 2000. Indiscriminate burning of the natural forest in the area has led to depletion and destruction of natural forest in the area as it could be seen from the Plates. According to Ola-Adams (1981), the dangers of losing genetic diversity are greatest in the high forest ecosystems where there is great diversity of species and where forest destruction is more rapid. Ekanade (1991) reported that this might affect the ground surface albedo. Clayton (1958) demonstrated the possibility of a total degradation of the forest reverting to a grassland ecosystem as a result of tree crop cultivation, which might encourage weed infestation and appearance of grasses. It is not difficult to understand therefore that changes in vegetation components over Nigeria, as elsewhere, have definite and significant specific components. Already, many plant species known to be endemic in this part of the world have become endangered or in some cases completely extinct. This fact was supported by Federal Environmental Protection Agencys (FEPAs) (1992) finding that gave a list of 484 species of plants as endangered in Nigeria. This figure could be on the increase as a result of the intensification of human activities in the forest reserves. Even this study confirmed that from all the sampled plots Melicia excelsa was totally absent because it was in high demand by the commercial timber contractors while the forest reserve is now turning to a monocultural species plantation. Land degradation, through agricultural activities will drain the soil of its life sustaining nutrients. A trend observed here is that once the land has been exhausted it is abandoned and the cycle then continues as more forest is cleared to provide more productive agricultural land. This is the problem cycle between ecological sustainability and economic and social prosperity.
CONCLUSION The findings of this study have shown that the study area is under serious threat by human activities, the evidence of which include the loss of about 84% of the forest reserve to human activities especially agriculture, loss of biodiversity and genetic resources, loss of protection which plant gives to soil and increasing carbon dioxide which have complicated environmental warming crisis. Therefore, successful conservation programme in the study area that would depends upon ecologically sound rural development technique or method that lessens the pressures for the destruction of the remaining natural forest resources should be embarked upon. The government and other stakeholders should place emphasis on promoting, strengthening and sensitizing communities as a strategy to invigorate environmental conservation and management. This should go with implementation of sound strategies on poverty eradication as this is highly tied with unsustainable resources utilization and environmental degradation. There should be need to look into the possibility of combining in situ conservation with wildlife conservation, education, tourism and commercial forestry provided the aims and the objectives do not conflict. This practice can itself help to instill public confidence in the overall strategy of management and hence in the policy of strict protection of the core areas. Also public enlightenment campaigns and workshop for farmers should be intensified.
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