3.12 Land restoration and revegetation

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by
A. ELHOURI AHMED
Director, Forestry Research Centre Khartoum, Sudan

1. INTRODUCTION

Land degradation is due to misuse of the resource (soil and vegetation) beyond the recuperative resilience of the ecosystem. The causes of this misuse are population pressures that have resulted in overgrazing, wrong cultivation practices, and excessive deforestation for cultivation, grazing and woodfuel.

Corrective measures deemed necessary and justifiable can succeed only if they are formulated and then carried out within the socio-economic set-up to restore the ecological balance.

This paper deals with the objectives of land restoration and revegetation, the strategies and practices carried out to realize the objectives within the context of the constraints, and arrives at recommendations of lines of action to deal with the problem.

2. OBJECTIVES

In the absence of feasible alternatives such as migration and continuous support of the populations in the affected areas because of political realities and the dignity of man, the broad objectives of land restoration and revegetation are:

a) to restore the land and vegetation for increased food production, not only to support the present population, but also to provide for high population growth rates (2-3.6% per annum). Since present per capita income is low, ($ 200 per person per year in the Sudano-Sahelian zone, Thomas (1980)) the need is not only to maintain the status quo as populations increase, but also to try to improve on these minimum standards. Better social services including health, housing, education and other aspects ought to be realized;

b) to enhance food production and also to generate income and to improve the quality of life through resource conservation and development.

The specific goals are:

1. Protection of the soil from wind and water erosion and maintenance of its fertility.
2. Protection of catchment areas and perennial and seasonal water courses to assure regulated flow of water both in quantity and quality. Also efficient and wise use of the scarce resource of water.
3. Enhancement of the productive role of the vegetation to realize maximum production of fodder, wood, fibers, medicinal products, tannins, perfumes, gums and other products.

3. METHODS USED IN LAND RESTORATION AND REVEGETATION

Land restoration and revegetation is carried out through execution of corrective measures on land where the degradation has occurred. The current measures usually executed are summarized below:

a) On cultivated land:

1. Agro-silvicultural methods

These are practiced to restore the soil fertility. The traditional system throughout Africa, under rainfed conditions, is to restore the lost fertility through bush fallow system. Vegetation, mainly bushes, colonise the area naturally. After a resting period of 10-15 years in dry savanna land, the bushes are cleared and cropping is continued until the soil loses its fertility. The fallows are not protected and animals graze on them and they receive added fertilization from the dung of animals.

In the Sudan, Acacia senegal is sown with the last agricultural crop or occurs from the coppice old trees which are not cut back in the last cropping. The successful seedlings are protected from browsing animals until the seedlings are out of the reach of animals. Then animals graze in these fallows making use of the grass and the pods of the gum trees and add to the soil fertility by their droppings. The gum trees continue for 15-20 years before they are cleared for cultivation of crops.

This is the system, as used to be practiced in the past. However, rotations for agricultural crops tended to be longer than the soil can support and the rotations of the bushes or gum trees tended to be shorter.

The technology used remained unchanged, save for the change from seeding to the use of seedlings, as the conditions in the drier parts of the savanna would not allow for seeding or coppicing of the older stock of trees. The farmer does the work and recently the Forest Service has been providing the seedlings free of charge, and the technical advice.

2. Shelterbelts

These are used to protect both irrigated and rainfed farms. Their main function, at present, is to protect valuable agricultural land and irrigation canals from creeping sands. This occurs in North Africa and

Northern Sudan. Shelterbelts reduce wind velocity, improve the microclimate and increase livestock yields. Field investigations in dry areas show that crop production may be increased by as much as 300% while the increase in average years is often 30 to 50%. By ameliorating the microclimate, shelterbelts commonly improve fruit production by a third or more, and in windy years, the fruit value may be increased by over 75%. By protecting livestock from strong winds and heat, shelterbelts have a great influence on the production of flocks. Under protection, the lambing percentage is greater by about 30% (FAO, 1978). Excellent examples of shelterbelts have been established in Libya, Egypt, Syria and Iran during recent years. The farmers at large are, however, reluctant to sacrifice cultivated areas for them. The species used are mostly Eucalyptus, Casuarina sp., populus and prosopis. Seedlings are used for establishment.

3. Plantations on seriously degraded irrigated or rainfed crop land

Salinisation is of common occurrence in irrigated lands in some countries of the Middle East region. In Iraq, plantations of poplars and eucalyptus were established in some of these degraded lands to bring them back to production. The product is to be used to establish wood-based industries.

In the Gezira of the Sudan degraded lands, either because of high salinity or because of being in depressions and therefore liable to waterlogging, and those lands used as dumping ground for surplus water, are cut out of the cropping rotations. Eucalyptus microtheca plantations are established on such sites.

Also in the central clay plain of the Sudan where rainfed mechanized crop production is practiced, some of the areas that lost their fertility and could no longer be used for crop production were given to the Forest Department. Here, plantations of Acacia senegal were established where sowing and weeding were done by mechanical means. The objectives set for stocked ares are to restore fertility and to be kept as reserves of strategic value to stabilize the production of gum should the need arise.

b) On rangeland

Degradation of rangeland is mainly due to overgrazing. Le Houérou (1981) summarized the actions of overgrazing as follows:

1. to reduce plant cover and biomass, mainly of perennials;
2. to reduce the number of annuals in the long term through lack of seed production;
3. to reduce the number of palatable species and individuals;
4. to replace palatable species by unpalatable ones;
5. to strongly reduce primary production and still more drastically secondary production;
6. to increase erosion as a result of a reduced plant cover.

Overgrazing is of common occurrence around watering points, such as wells, streams and rivers. Another cause of degradation is incidental or deliberate fires, particularly in the drier parts of the savanna in Africa (300-800 mm of annual rainfall). Fires cause direct loss of fodder through burning and reduce its quality.

Corrective measures carried out are:

• total protection by fencing and thus restoring the vegetation after a few years;
• seeding of palatable indigenous species, mostly of grass. Seedlings of various bushes and trees are planted to restore the range. These species are: Atriplex, Prosopis, Acacia and Salsola;
• establishment of fire-lines in the dry savanna zone of Africa.

c) On forests and woodlands

Here again, fires are considered a cause of degradation in dry savanna zones and fire-lines are established along the boundaries of Forest Reserves. Enrichment planting through establishment of plantations blocks are carried out. The objectives of such plantations are generally the supply of fuelwood and poles. In the wetter savanna formations, the objectives of these enrichment plantings change to timber production. Species used are Euclyptus and Conifers, mainly: Cupressus lusitanica, Pinus patula and P. raddiata, P. halepensis, and other pines.

d) On bare land: sand and sand-dune fixation

When erosion reaches an acute level, and where sand starts to move and threatens habitations, establishments, roads and agricultural land, sand-dune fixation is carried out. This occurs along the coasts of seas and oceans or in continental areas. The conventional method used is the establishment of hedges of stalks of dry grass and/or bushes to restrain temporarily sand movement until the dunes are planted with trees, shrubs and grasses. Supplementary irrigation is sometimes used. Eucalypts, pines, tamarisks and acacias are used in the higher latitudes along the Mediterranean in North Africa and the Middle East. In Sudan, Somalia and Mauritania prosopis and acacias are used.

4. CASE STUDIES

The case studies mentioned below illustrate what happens when vegetation is destroyed by imbalanced use of land.

1. Millet cultivation in Sudano-Sahelian zone of the Sudan

Ibrahim (1984) studied millet cultivation in this zone. The following is a summary of this work.

A. Settlements

The most decisive precondition for establishing a settlement is to secure supply of permanent drinking water. In the 1960's a great number of bore holes were dug in the North and East where the aquiferous Nubian sandstone occurs. These bore holes were the centre of desertification rings seen on the satellite images. Millet cultivators often spend the wet season scattered on sandy Goz (stabilized sand dunes). After the harvest they return to spend the dry season in the central settlements with permanent water supply.

The number of inhabitants in villages does not increase substantially, but the multiplication happens very often through establishment of new satellite settlements at a distance of 3-5 km from the mother village.

What is particularly remarkable is the increase of settlements during the drought disaster in 1970-73. As cultivation is carried out near the settlements, the circles of desertification around settlements or due to cultivation gradually expand to cover the whole tract.

B. Subsistence millet production and desertification

It has been proved that excessive rainfed cultivation is one of the major causes of desertification in the semi-arid zone. The reasons are varied and the processes are rather complex. However, in all cases, three factors are involved: high rainfall variability, high soil vulnerability and unsuitable land use methods. The latter is, in fact, the immediate cause, as the soil and rainfall conditions described are constant components of the natural ecosystems of the arid and semi-arid zones. Man can either use these available natural resources in an adapted manner, which secures their productivity in the long run, or misuse them by unadapted methods to realize high returns in a short time, thereby recklessly destroying the ability of the natural ecosystem to regenerate. Of course, there are social and political pressures as well as financial and technical constraints which force agricultural practices in directions which are far from suitable to the respective natural resources and limits. Here, compromises have to be made, but not at the cost of the next generation. The present generation is in fact following a policy of agricultural strip mining, exhausting all the productivity of soil, while doing nothing for its improvement or even conservation.

The prevalent methods of millet farming enhance desertification in northern Darfur. In order to prepare a field for millet growing the peasants begin by cutting all the trees and clearing all shrubs, herbs and grasses Though trees neither impede the growth of millet nor compete with it in water demand, peasants assert that trees are gathering places for birds and locusts which eat their millet. Peasants send their children onto the millet fields, especially during the ripening phase, to scare birds and locusts by drumming on empty tins. This work would be much more strenuous if trees were left on the fields to shelter those birds and insects. Preparation for cultivation, therefore, means complete clearance of the natural vegetation until the soil becomes completely bare and vulnerable to erosion. This is usually done several weeks before millet is sown and it takes several weeks more until the plants can render the soil any effective protection. During the growth period of millet, the peasants weed twice. Weeding is absolutely necessary as the indigenous plants are much more adapted to the climatic conditions of the Sahelian zone than millet is. Their roots form a thick net, which has the ability to absorb every drop of rain which seeps through the soil. On the other hand, millet has short roots occupying a relatively small area compared with the considerable height of the plant ranging between 150 and 300 cm. In order to test the different abilities of millet and the indigenous weeds as to their ability to profit from the available soil moisture, we dug holes into the sandy soil of a millet field in northern Darfur (14° 15'N, 25° 45'E) and similar holes on a neighbouring fallow field of the same kind of soil but having a cover of sparse vegetation with dominance of low creeping weeds. On the millet field, soil moisture was detected only 10 cm deep, while it occured at a depth of 100 cm on the fallow field sparsely covered with herbs. This was during the rainy season, and the last rainfall before the test had taken place a week before. Unlike millet, the natural herbs were able to absorb all the soil moisture provided by rain. It is to be noted that the soil moisture at a depth of 100 cm is common all over that area, even in the dry season.

Aware of this rivalry between weeds and millet for moisture, the peasants spare no effort to clear their fields of these undesirable herbs and grasses. The process of weeding is done by scratching and loosening the top-soil with a hoe called garraya. This practice, repeated regularly in rain-cultivation areas, results ultimately in the irreversible extinction of the natural vegetation species which, of course, are much more adapted than crop plants to the arid climatic conditions with the typical recurrence of drought periods. Moreover, the sandy soil, now deprived of its natural plant cover, is exposed to strong deflation, especially during the long dry season of 8 to 10 months. About 200 million tons p.a. of fertile top-soil are reckoned to be blown away from the Sahelian zone into the atmosphere, Junge (1977). The amount of atmospheric dust increases every year due to the expansion of cultivation, a fact which can be easily proved if we study the annual frequency of poor visibility under 1,000 m. The erosion of the fertile top-soil has led to a considerable deterioration of land productivity. That the decrease of the amount of precipitation is only to a limited extent responsible is shown by Table 1. It is quite apparent that the decrease in production per hectare is not directly proportionate to the variability of precipitation (Table 1).

Another reason for the deterioration of soil productivity is the abandonment of the old rotational system of shifting cultivation. Owing to population pressure in the last few decades the same land is being cultivated year after year without fallow years in between. Fallow occurs mostly in dry years, when precipitation is not enough for the growth of millet. Thus, leaving the land fallow is compulsory and not part of a soil regeneration farming system.

Cultivation - fallow rotation in northern Darfur (354 respondents)

It may sound paradoxical in a vast country like the Sudan to speak of lack of land. One admits that the population density in the Sahelian zone of the Republic of the Sudan is only 3-6 inhabitants per square kilometre, but the distribution of population is strongly restricted by the availability of drinking water all the year round. The population density in the surroundings of El Fasher and El Obeid, for instance, reaches about 100 inhabitants per km². The land is intensively cultivated, though the annual precipitation is less than 300 mm in the Fasher region and less than 400 mm in that of El Obeid.

In this way, the sound, traditional system of shifting cultivation turned into land misuse, and a chain of processes of deterioration of land productivity was begun: population increase led to excessive cultivation, which, in turn, led to increased soil erosion and soil impoverishment. This resulted in the decrease of millet yields per hectare in the Sudan by half in the last 15 years. To make amends for this, the population, which is constantly increasing at an annual rate of 2.5%, had to increase the area cultivated with millet from 392,000 ha in 1960 to 1,055,000 ha in 1975. This expansion of cultivation meant a fresh wave of desertification for the reasons mentioned above.

The average sorghum yield in the Sudan in 1973 was 770 kg per ha (by comparison, the average yields of sorghum in the USA were 3,690 kg. and in Egypt 4,170 in the same year). In the northern Sahelian zone, however, where precipitation is less than 300 mm in the annual mean, the yields are much lower. Our study in northern Darfur revealed that 50% of the years are to be considered dry ones, in which the peasants reap either nothing at all or up to only 100 kg/ha. At the same time, the average consumption of millet of a six-membered family amounts to 1,500 kg p.a. This means that the area cultivated for millet per family has to be no less than 15 ha (i.e. about 36 feddans or 29 mokhammas), if subsistence is to be guaranteed. The actual area cultivated by an average family is, however, much smaller and, therefore, the population suffers from chronic undernourishment. It is also characteristic of this arid zone that, especially in dry years, millet cultivation areas are greatly expanded so as to meet the demand of the population. This results in large-scale destruction of the ecosystem, just in the very years in which it is most vulnerable and its regeneration capability is immensely reduced by the dryness.

Table 1: Decrease of millet yields in the Sudan from 1960 to 1975

2. Desertification in Mediterranean arid lands of North Africa

Grainger (1982) reports the following case of Oglat Merteba, Tunisia:

Oglat Merteba, Tunisia:

This 200 km region in southern Tunisia on the fringes of the Sahara receives only 100-200 mm of rain in an average year. Transhumant grazing of sheep and goats on communal pastures has long been the dominant form of land use.

Cultivated land more than tripled in Oglat Merteba between 1975 and 1984, and the use of factors and disc ploughs led to wind erosion of the dry, sandy soils.

Livestock numbers did not drop despite the decrease in the grazing area. Sedentarisation (the settling of nomads) meant that the herds were concentrated around settlements and water-holes, increasing the pressures on both land and water.

The region could lose five centimetres of topsoil by the end of this century. Degraded pasture lands by 1975 covered 25% of the total area, and decertified croplands covered 12%.

3. Desertification in subtropical arid zones of India: Rajasthan

In 1972, almost two-thirds of India's 320,000 km of arid land lay in the state of Rajasthan. This accounted for 60% of the total land area of the state.

Although only 20% of Rajasthan's arid land is considered by FAO to be suitable for rainfed cropping, the area under cultivation virtually doubled from 30% in 1951 to nearly 60% in 1971, mainly at the expense of grazing lands and long fallows.

Both overall production and average yield per hectare (acre) fell between 1954 and 1970 for three out of the four important crops: jowar, sesame, and kharif. This is despite the fact that the area planted with sesame and kharif regularly increased over the period.

A third of the area of the Luni Development Block, 1,989 km of land in the state of Rajasthan's arid zone, was in 1976 classified as being under "very high" desertification hazard. The remainder is subject to "high" desertification hazard.

What do these terms mean? In 1972, 88% of the area was cultivated, the human population had trebled since the start of the century so that there were now 48 people per km², very high for this type of area. Natural vegetation only survived on part of the area occupied by common grazing lands; tree and shrub cover was sparse; and most of the area was covered by a sand sheet of 0.2-2 metres.

Things are not improving. Sand coverage was only a quarter of its present level in 1958. By 1984 existing dunes had grown in height by up to five metres at their crests. Wells were yielding less water, and a growing number were becoming saline. Grainger (1984).

5. STRATEGIES AND CONSTRAINTS

1. Strategies

In reviewing strategies the following elements must be recognized:

1. The magnitude of land and vegetation degradation is great and need not be repeated here..
2. This degradation is caused by people pursuing food production through cultivation and animal raising practices that are not compatible with ecological conditions. Similarly, the people are deforesting large areas for various purposes, including fuelwood.
3. Water, a powerful tool for production and development, is sometimes misused and leads to destruction of land and vegetation.
4. Corrective measures for land restoration and revegetation cannot realize rapid pay-offs. Given the conditions above, corrective measures may have two directions:

(i) Control measures

These are meant to stop harmful practices by prohibition and other means such as closing watering points or manipulating them, and the provision of alternatives. These are detailed below:

a. Prohibition of rainfed cultivation on marginal dry lands where such practices lead to large-scale degradation, and will lead to greater degradation if continued.

b. Regulating the densities of animals using lands and in some cases prohibiting grazing altogether to give to the vegetation a chance to recuperate. Also prohibition of the incineration of fires especially in marginal zones.

c. Prohibition of felling trees and bushes on marginal lands for fuelwood and other purposes and on catchment areas.

The premises of the above measures rest on the conception that under natural conditions and without destructive interference of man and his animals, ecological systems may maintain their balance. Reduction of plant growth in drier years may be recovered in wetter years. Perennial and woody plants survive by virtue of their morphological and physiological attributes throughout the dry season and drier years, and provide skeletal plant cover for the ground. Natural processes are capable of eventually correcting the imbalance so and that the habitat will regain its natural physiogonomy. Kassas (1977), Le Houérou (1981), McKell and Norton (1981), Man (1981).

(ii) Active intervention methods

These require the actions of man to restore the degraded land and vegetation. The options for these actions decrease with increasing aridity, FAO Near East Mission (1976), Ben Salem and Eren (1982). These measures are detailed below:

a. Sand and sand-dune fixation through artificial sowing of plants including tree, shrub and grass species.

b. Establishment of shelterbelts on irrigated and rainfed cultivated land, and for protection of habitations and infrastructures.

c. Establishment of plantations on degraded lands, especially on irrigated degraded lands (salinity and waterlogging). These offer a great potential for generation of wood-based industries in dry lands.

d. Establishment of tree, shrub and grass plantations of value to restore the fertility of degraded lands where rainfed cultivation is practiced (use of Acacia and leguminous plants to restore soil fertility).

e. Establishment of fire-lines and live green hedges for protection of savanna woodlands.

f. Seeding and planting catchment areas and banks of permanent and seasonal water-courses for regulation of water-flows and erosion control.

g. Finally, in the execution of all these activities on degraded lands as well as on good lands, the means to realize the optimum product ion from the resource should be mobilized. Among these are the use of improved species and varieties, efficient tending operations and efficient management systems.

2. Constraints

A. Biophysical constraints

Rainfall variability, both in time and place, is a feature of the arid and semi-arid zones and, like desertification, restoration and revegetation of land, is there a slow process and rapid pay-offs cannot be expected. Also, because of the variability of techniques of selecting the right species and varieties to suit the objectives set, methods of planting or tending, soil working, tending operations and methods of utilization need special skills. These are often lacking.

Also the integration of methods of land reclamation and revegetation into the general agricultural activities and the general developmental aspects of the countries need highly specialized skills.

B. Socio-economic constraints

On the social front, the situation is a difficult one. Restoration and revegetation of land call for a policy of restriction and coercion. Cultivation of annual crops on marginal lands (less than 300 mm) must be prohibited. Grazing on these lands, if not prohibited, should be regulated to a degree compatible with the ability of plants to recuperate. These are imposed on populations to whom the right to pasture is unquestionable, as land is often communally owned, and to whom the right to cultivate is only checked by traditional rights of other individuals and not by other consequences.

Also the lack of integration of different activities of development agencies affects the core of the problem. Wells, for instance, are dug without provision of measures to feed the animals and to secure fuelwood for the population in the area. Politically, it is more expedient to provide for health and education services than to provide pasture, wood and protection measures.

On the economic front, the tendency is to finance projects of so-called quick returns. This is the case at national and indeed international level. Finance is directed to annual crop production with not even a small portion set aside for pasture, wood and environment amelioration oriented projects. The World Bank, for instance, financed large-scale irrigated or rainfed annual crop production projects which involved large-scale tree-clearance without catering for pasture, wood or environment.

The National Agencies of Forests, Pastures and Natural Resources at large are poorly staffed and poorly financed at all levels. They cannot carry out their tasks of planning, research, extension and management. The very incidence of desertification is a witness to failure of policies due to constraints.

6. GAPS IN KNOWLEDGE

1. Prediction of rainfall

Rainfall variability both in time and place is well researched, but no method as yet exists to predict the rainfall. It is hoped, with the tremendous development of space science, that predictions may become possible.

2. Mapping and quantitative data of stream flows and catchment areas

Water is the most important tool of potential development in arid zones. Yet detailed maps of stream and seasonal water-course flows and their catchment areas are rare. The information, to be useful, must be systematically collected over long periods. Yet a start must be made to establish a network of equipment to collect the necessary information. This may transcend the boundaries of countries, and regional efforts are essential in this case.

Spate irrigation, supplementary irrigation and other forms of irrigation to restore the vegetation and the land are potentially valuable tools. If water is available, considerable improvements can be achieved, e.g. partial irrigation of food crops to augment the rainfall and evade their failures; pasture improvement including trees and bushes. Such practices are envisaged to release the pressure on the natural pastures.

3. Manipulation of location of water points to restore vegetation

Information is lacking on how to manipulate the location of water-points with a view to land reclamation and revegetation and to avoid the accompanying desertification. This is not an easy question to answer as it involves many disciplines, yet it is of vital importance and knowledge should be generated in this aspect.

4. Studies of indigenous vegetation

The broad zones of vegetation in arid lands are largely known and considerable information exists on taxonomic aspects of existing species. The most important factors affecting vegetation such as rainfall and temperature are largely known. Investigations carried out on the ecology of arid lands have shown that the distribution of vegetation in arid lands changes into a more and more uneven distribution with increasing aridity. This is closely related to the habitat type which is closely related to landforms units. Surface drainage systems control collection and redistribution of run off water. Transportation and deposits of aeolian sediments are influenced by topographical features. Soil physical features, especially depth, are effective factors in soil/plant relationships because of their influence on soil moisture regimes. A thin soil may be moistened during the rainy season, a deep soil may allow for storage of moisture in deeper layers, Kassas (1953), Walter (1971) and Ayyad (19813. Yet maps delimiting these land-forms units are lacking. These are essential for realizing the potentials of these lands on a rational basis without endangering the ecosystem.

Animals are an important component of these systems and losses due to insects are encountered, yet quantitative information on this aspect is lacking.

Quantitative aspects of production of biomass, whether for forage or wood, are lacking and at best these are often informed guesses. No rational systems of utilization can be formulated without this information. Also data on the ecology of regeneration of the species are often not available.

Foresters for instance know much more about Eucalypts than Acacias, particularly indigenous acacias. The ecology of grasses and herbs and their methods of regeneration and tending are largely unknown. Relations between animals and flora are largely unknown, e.g. animals eat the fruits of tree species and are bound to distribute some of them (Acacias, Prosopis) and protect them from predators as they are pelleted and enhance their germination and establishment capabilities, yet they eat the young seedlings. Lopping trees and bushes for fodder is practiced by herdsmen, yet this had not been quantified nor has the plant reaction to lopping. Systems of rational efficient management can be evolved that are in line with conservation measures once these basic relationships are studied.

To summarize, the following aspects deserve serious consideration:

a. Site classification to evolve appropriate land use.

b. Intensive studies on indigenous vegetation for better utilization.

c. Studies on plant/animal relationships to evolve efficient management practices. These may include exotic as well as indigenous species.

5. Socio-economic studies

No effort on land restoration and revegetation can achieve its objectives without the active participation of the people. This cannot be done without motivation. That coercion through laws alone has largely failed needs no illustration. There is a need for studies to evolve methods of motivation, coercion and others that can lead to sound practices of efficient utilization and conservation.

Also, coordination of activities is often called for but how to do it remains unanswered. There is a need to evolve methods of coordination at all levels.

7. CONCLUSIONS AND RECOMMENDATIONS

Land and vegetation degradation is essentially a land use problem and the key to success for restoration and revegetation centres on:

1. Integration of land use within ecological context.
2. Active peoples' participation, through various means, as no government can cope with these problems.
3. Building and strengthening the staff base at all levels to execute these programmes, and filling the present gaps in knowledge.
4. Integration of all available knowledge into working practical models that can solve the problems.

8. REFERENCES

Ayyad, M.A. Soil - vegetation - atmosphere interactions. In arid-land (1981) ecosystems Vol. 2, pp. 9-31, Cambridge University Press. Cambridge, U.R.

Ben Salem, 8. and Eren T.M. Forestry in A Sandy World. (1982) Unasylva, Vol. 34 No. 135, pp. 8-12, FAO, Rome.

Grainger, A. Desertification. Earthscan - International Institute for (1984) Environment and Development - IIED, London. Second edition.

Ibrahim, F.N. Ecological Imbalance in the Sudan - with reference to (1984) desertification in Darfur. Bayreuth - Bayreuther Geowissenschaftliche Arbeiten.

Junge, C. The importance of mineral dust as an atmospheric (1977) constituent. Reprint from the Workshop on Saharian dust, Gothemburg.

Kassas, M. Land forms and plant cover in the Egyptian desert. Bull. (1953) Soc. Geog. d'Egypte, 26, pp. 193-205.

Kassas, M. Ecology of desertification. Paper presented to the (1977) Veterinary Conference held in Khartoum, April 1977 - Veterinary Association, Khartoum, Sudan.

Le Houérou Long-term dynamics in arid-land vegetation and ecosystems of (1981) North Africa. In Arid-land Ecosystems Vol. 2, pp. 357- 384. Cambridge University Press, Cambridge, U.K.

Man, H.S. Management of arid-land resources for dry land and irrigated (1981) crops. In Arid-land Ecosystems, vol. 2, pp. 479-493. Cambridge University Press, Cambridge, U.K.

McKell, C.M. and Norton, B.E. Management of arid-land resources for (1981) domestic livestock forage. In Arid-land Ecosystems, Vol. 2, pp. 455-478. Cambridge University Press, Cambridge, U.K.

Thomas, G.W. The Sahelian Zones of Africa, Profile of a fragile (1980) environment. A report to the Rockfeller Foundation.

Walter, H. Ecology of tropical and sub-tropical vegetation. Oliver and (1971) Boyd. Edinburgh, U.K.

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