- INTRODUCTION
In recent years, deforestation has become increasingly widespread within tropical regions especially where acute population pressures are experienced. While clearance for cultivation and collection for firewood are the major causes of deforestation in many areas the commercial exploitation of trees has also taken its toll on the tropical woodlands and forests. The environmental and socio-economic impact of deforestation depends on a variety of factors including the extent of the destruction of the woody vegetation, the physical characteristics of the site that is cleared and the subsequent use of the land. Clearance of the trees and shrubs is sometimes a necessary and costly precursor to the more productive use of land for cropping or livestock rearing.
However, there are extensive areas where the removal of woody vegetation has given rise to two major problems. The initiation of land degradation and shortages of forest products. In sub-Saharan Africa, it has been estimated that nearly 40% of the forests have been destroyed (Phillips, 1974) and that large areas of savannah woodlands have been devastated. As a result, there has been a lowering of the productivity of the land with the onset of desert-like conditions in certain areas and subsequent problems of malnutrition and starvation for both humans and livestock. In this respect, deforestation forms only part of a broader spectrum of problems concerned with the mismanagement of and pressures on natural resources. In the fact of population increases it is imperative that effective solutions should be implemented as a matter of urgency to prevent further deterioration of theseresources.
Whilst the deliberate clearance of woodlands for farming operations possibly dates from the arrival of Iron Age hunters and shifting cultivators several centuries ago, it is only since the mid-twentieth century that deforestation has reached such proportions as to constitute a major ecological and economic problem. This is in part due to the massive increase in the human population, which resulted in the widespread clearance of land for cultivation purposes in both the commercial and peasant farming areas, and the increasing demands for firewood from rural and urban populations.
2. INTER-RELATIONSHIP BETWEEN VEGETATION AND LANDUSE
Deforestation can be defined as the destruction of woody plant species or woody vegetation types. The natural or climax vegetation in Central Africa resembles a mosaic of woody plant communities grading one into another in response to changing climatic and soil conditions. The most extensive woody vegetation types are the miombo woodlands and the dry savannahs. The miombo woodlands are dominated by the msasa and mnondo. These woodland areas tend to coincide with some of the more densely settled areas, therefore extensive areas of woodland have been modified.
For example, initial clearance of woodlands is normally followed by a period of cropping the abandonment of the cropland; secondary regrowth will eventually lead back to the climax woodland. This sequence would be typical of shifting cultivation systems operating under low population densities and such a system can only be maintained if there is sufficient land to allow long periods of fallow to restore soil fertility through the regeneration of the woody plant cover. However, there are few areas in Africa where a shifting cultivation strategy can still operate today. More commonly one finds that with the increasing population pressure in the tribal lands, areas of secondary regrowth are cleared for cropping and then are subjected to excessive erosion. These situations result ultimately in the development of degraded lands.
Figure 1: The grazing of livestock often forms an important part of the changes shown in the following table
Various cycles can be seen in the table above, for example initial crop clearance of woodland is normally followed by a period of cropping. On abandonment of the cropland secondary regrowth will eventually lead back to the climax woodland. This is typical of shifting cultivation systems operating under low population densities. Grazing of livestock forms an important part of the changes shown in Figure 1. The abandoned croplands are often used for grazing purposes and this may inhibit the regeneration of woody species, especially where this is combined withperiodic fires.
3. ENVIRONMENTAL CONSEQUENCES OF DEFORESTATION
There are four main consequences of deforestation:
- Changes in themicroclimate;
- Rapid mineralisation of organic matter and the breakdown of soilstructure;
- Increased soil erosion;and
- Decreased waterretention.
Firstly, the environmental changes initiated after the clearance of the woody vegetation tend to be closely inter-related and reinforcing, e.g. the exposure of the soil results in higher rates of runoff; thedestructionofthesoilinhibitstheplantgrowthand‘asaresult’thepoorprotectionaffordedby
the sparse vegetation-cover increases the incidence of erosion even further.
Secondly, the regeneration of savannah woodland takes place readily, primarily through a combination of suckering and coppicing. However, natural regeneration is extremely slow and may take several decades for secondary regrowth to reach a stage resembling the original woodland prior to clearance.
An alternative view of the environmental responses to deforestation is depicted as one concerned with the changes in geomorphological and hydrological conditions within a small drainage basin following the denudation of the protective woody vegetation cover. The drainage-basin system is relatively stable before clearance and levels of erosion are low. Rainfall over the catchment is intercepted by the tree canopy or, on infiltrating into the soil, is taken up by plant roots or slowly percolates downslope to the stream. The surface runoff and hence the soil erosion is minimal. After clearance of the protective vegetation cover the drainage-basin system responds in a number of ways. Interception of rainfall by vegetation is greatly reduced resulting in an increase in surface run- off. Secondly, deterioration of soil structure through raindrop impact on the bared surfaces and reduction of organic matter inputs from the vegetation tends to reduce infiltration of water into the soil and to foster more rapid rates of surface run-off, increased rates of erosion in the headwater region and subsequent deposition of sediment in the lower portion of the basin.
Hydrological changes are closely related to these geomorphological responses. The stream hydrographs in Table 3B indicate the rise and fall of a stream in response to the same rainstorm before and after the clearance of vegetation.
The magnitude of the environmental changes resulting from deforestation depends on a variety of factors, including the extent of destruction of the woody vegetation, the physical characteristics of the site that is cleared and the subsequent use of the land. Careful conservation practices must be observed to reduce the soil and other damage caused by careless destruction of the woodlands, especially on marginal agricultural lands under conditions of population pressure. Once they have been subjected to severe erosion recovery is likely to be a very slow process.
4. THE URBANPROBLEM
The destruction of woodlands in the peri-urban areas of the main towns and cities is regrettable but has served to draw attention to the very serious problems of energy supplies experienced by the low-income families in the urban townships. This group depends, like their rural counter-parts, on wood as a source of fuel for cooking and heating. Not surprisingly there is a strong incentive to fell the trees around the townships.
The denudation of woodlands around urban centres is likely to continue in the absence of alternative and cheaper forms of energy. Firewood is already an expensive commodity for many families and it may require as much as 20% of the earnings of a household simply to cook meals and provide warmth during the Winter months. This wood is usually obtained from sources as far as 200 kilometres away. Costs of such an operation increase with rises in the price of petrol and oil, increased scarcity of timber, and soforth.
5. POSSIBLE SOLUTIONS
The essence of the deforestation problem is simply one of increasing population pressures on dwindling woodland resources; resources which are normally regarded as renewable but under the present conditions are becoming increasingly scarce. Unfortunately, the solution to the destruction of woodlands and the provision of timber products is not simple. Since wood fuel is a basic commodity in daily use in the majority of rural African households we shall have to look at methods of overcoming shortages of firewood in both the rural and urbanareas.
There are various alternatives whereby future demands for wood fuel and other energy sources could be met. Since there are large concentrations of people in towns and cities, there is a fairly wide range of energy sources which could be supplied relatively early and economically to these urbandwellers.
In the rural areas, however, one is dealing primarily with a dispersed population which restricts the choice of viable energy sources. In addition within these areas the land use system may place constraints on the re-afforestation schemes. Here also, there is an inherent conservatism towards changing to unaccustomed energy sources.
The utilisation of wood fuel is likely to continue for many years in the urban centres because of the existing system of wood vending but primarily as a result of traditional preferences for wood. However, as wood becomes more costly and difficult to obtain and as urban dwellers become more affluent and sophisticated in their consumer behaviour there will be a pressing need for alternative energy sources. In general there are four main alternatives in the urban areas; these are coal, paraffin and gas, plantation timber and electricity. It would seem that electricity is the better long- term prospect.
Apart from local concentrations of population in the rural areas where imported fuel can be used there are only two real alternatives in the subsistence and commercial farming areas; these are plantation timber and indigenous woodlands. Where there is still enough natural woodland remaining, it is vital that areas be set aside now to provide timber needs for the present and future generations.
To relieve the pressure on the woodland which could result from population increases, it would be prudent to establish village woodlots as well as setting aside stands of indigenous woodland. Methods would have to be developed to manage such woodlands on a sustained-yield basis.
The main problems occur in the extensively denuded rural areas. The only viable alternative in such areas would seem to be exotic plantations, which might be operated in the form of large-scale commercial forests and/or smaller village woodlots. Plantations could also assist in the conservation of marginal lands not suited to agricultural production.
There is the problem of persuading the small-scale farmers to grow a tree crop which takes 8 to 10 years before producing timber suitable for construction and fuel. Such long-term planning is not readily acceptable to most farmers at this level.
Whilst the technical aspects of establishing and managing woodlots are relatively simple, there are many more complex socio-economic problems to be tackled and there remains the fact that as long as there are scattered remnants of indigenous woodland surviving, the rural population appears to
be quite content to spend time and effort in gathering the available indigenous firewood rather than plant exotic trees.
In conclusion, therefore, evaluating the various strategies to deal with the deforestation problem is not by any means an easy one and perhaps methods used elsewhere to overcome similar problems should be considered.
