UN UNIVERSITY LECTURES: 12

Man-Induced
Desertification?

Introduction

1. desertification can be considered as a set of biological, chemical and physical processes which converge to create desert-like conditions (Rozanov 1990);
2. desertification is a social problem, involving people at all stages, as a cause and as victims, also one of lower agricultural return and increasing poverty (Spooner 1987, 1989);
3. desertification occurs at the moment when land becomes irreversibly sterile in human time terms and with regard to reasonable economic limitations (Mainguet 1994);
4. desertification is the diminution or the loss of the potential for sustainable use (Warren and Agnew 1987).

The simplest and most useful definition of desertification, proven by field observations, aerial photographs and satellite imagery analysis, rejects the perception of growing deserts but refers to transformation of vegetated productive land into bare and unproductive land and, more precisely, to the appearance of desert-like landscapes and surface dynamics in semi-arid and dry subhumid ecosystems.

As shown by Dregne (1984), the difficulty of seed germination is another fundamental criterion of desertification, to which should be added characteristics of degrading soil:

- loss of soil aggregation, a key indicator for resilience of a dry ecosystem;
- decrease of general topsoil infiltration capacity;
- decrease of soil water storage;
- loss of resistance against mechanical disturbance (splash erosion);
- surface redistribution and profile redistribution of water;
- new threshold of runoff initiation.

In fact, seed germination, seedling establishment, production and reproduction potential of plant cover are hampered by all these changing soil characteristics, which all lead to the general expression of loss of resilience.

The first part of this article aims to determine whether and how man's behaviour has a destructive impact on the environment. The second part examines in which way desertification can really be apprehended by objective indicators (what do we see?). The third part questions the existence of new useful strategies.

1. Is It Possible to Talk about Man-induced Desertification? Analysis of Desertification in Terms of Causes

1.1.A Shift of the Blame from Colonization to Climate and Finally to the Traditional Land Use Systems

In the 1960s, after the complex processes of decolonization, the trend was to attribute land degradation to sectorial development during colonization. In a second phase, during the 1970s, essentially because of efforts by the United Nations Environment Programme (UNEP), middle-term climate changes and short-term droughts were taken in account as causes of desertification. Currently, a third phase has been reached with "the consensus view of the last decades that desertification is primarily human-induced" (Hulme 1989). This trend is well illustrated by a UNSO-UNDP survey, which also attempts to assess the degree to which local people perceive the present status of their environment and habitat, and also their degree of consciousness of the rate at which the environment is changing, and the risk of occurrence of desertification in the future.

The two last questions cannot be answered by the farmers nor even by scientists. "It has become fashionable these days to speak of poverty as the cause and consequence of environmental degradation and thereby to focus efforts at combating environmental degradation on the poor" (Masse Lo 1994). In other words, to shift "the burden of the drought to subsistence-oriented peasants, pastoralists and rural workers," as Barraclough states (1994).

Speaking of man-induced desertification means to attribute to human activities the responsibility of irreversible land degradation and to assess that socio-economically driven environmental degradation has a greater impact than climatically driven environmental change.

The concept of desertification should be approached through the notion of irreversibility, meaning that one human generation is not able to rehabilitate the land degraded during the same time-scale: twenty-five years, superposed on to age-old degradation which has reached an unbearable threshold.

1.2. Distinction of Droughts (Quality and Time-scale)

There is general agreement that many environmental changes result more from short-term natural climatic changes (droughts) than from aridity, a long-term climatic factor. It is more a question of recurrent droughts which are annual, biennial or multi-annual phenomena, like the discontinuous and not simultaneous drought of 1968-1985 in the seven Sahelian countries in Africa south of the Sahara. However, it is difficult to diagnose the real drought effects, there being several types of drought which may be differentiated:

1. meteorological droughts, which occur when rainfall is deficient;
2. hydrologial droughts, which correspond to the decrease of runoff in rivers and decrease of lake levels, modified by the rill wash/infiltration ratio and by evaporation, and which are mainly indirectly caused by meteorological droughts. These two types of drought result from short-term climate changes which are normal for dry ecosystems.
3. edaphic droughts, when the infiltration rate of the soil decreases, as does the capacity of seed germination by sealing;
4. agricultural droughts, which are defined by the deficit of water versus the needs of agriculture. The last two are non-climatic droughts, caused by other than climatic factors.

What is more, no satisfactory model to explain drought has yet been provided. "The climate system is complex. It is governed not only by what happens in the atmosphere, but in the oceans, the cryosphere (glaciers, sea ice, and continental ice caps), the geosphere (the earth's solid surface) and the biosphere (living organisms in the oceans and on land). The interactions between these various 'spheres' are difficult to predict, not least because their respective processes occur on widely differing time scales. The typical equilibrium response times of the climate system's various elements range from a single day to a few centuries." (UNEP-GEMS-IUCC Fact Sheet 3, no date).

Another subdivision of dryness based on time-scale should be taken into account:

5. long-term dryness: aridity;
6. middle- to short-term dryness: annual, biennial or multi-annual droughts;
7. short- to middle-term dryness that is often human-induced and induced by land degradation.

1.3. Distinction of the Effects of Traditional Indigenous Land Management and of Imported Exogenous Land Management

- the environmental impact of traditional models of land utilization. However, these may have become anachronic in view of the ever growing population pressure. This fits into the hypothesis of "accelerated slow mining": rapid and recent socio-economic mutations might just accelerate the ongoing process of land degradation. - the environmental impact of exogenous (imported) techniques and models of management through bilateral or international development projects. These techniques and projects have been implemented in wetter ecosystems: one of the most convincing examples is given by the former USSR, when, in the 1960s and the 1970s, Moscow exported to the Aral Sea basin all kinds of irrigation methods and canals, responsible for the Aral ecological tragedy (Létolle and Mainguet 1993).

In the example of the Gulf War, where human settlements were systematically destroyed, oilfields burned (in 1991, Wafra was the first oil well to be burned), and telephone and electricity lines in Kuwait cut, it is easy to determine degradation of soils resulting from the digging of trenches and erection of elevation pits, walls and berms, the explosion of bombs and mines, trampling of troops, and transport of heavy vehicles. But, a short time after the war, it is difficult to differentiate the part of soil degradation directly resulting from the war effects, and land degradation which occurred after the two droughts preceding the war (one from 1960 to 1970 - except 1961 and 1967 - and the other from 1983 to 1987), exacerbating the overuse of land by grazing and the building of human settlements (Dumay 1993). In addition we might suggest the idea of human accelerated mining of the environment and land degradation.

The Gulf War resulted in a man-made environmental disaster, aggravating the previous human-induced land degradation mainly by increasing the load of mobile sand for aeolian deflation and soil loss. In semi-arid Tunisia, twentieth-century human activities led to the same reactivation and remobilization of the previously fixed sand-dunes.

1.4. Conclusion of the First Part

1. It is probably not possible to separate climate-induced short-term environmental changes from land degradation induced by human activities. That is the reason why it is unwise to propose two terms, desertification and desertization, the first referring to climate-driven natural change, the second to man-induced change. The latter was fortunately rejected by international institutions. 2. It is not possible to attribute the whole responsibility of land degradation to human activities, precisely because we are not able to distinguish the part that is due to man from the part due to short-term climate changes. On the whole, we follow Thomas and Middleton (1994) when they write "in the degradation debate, ultimately it is not possible to divorce environmental and economic aspects of the problem."

If in drylands the effects on the environment of short-term climatic changes are difficult to distinguish from the effects of human activities, how can desertification be defined in an objective manner?

2. Can Desertification Be Observed?

1. The observation of rings of degradation on the first Landsat imagery and on aerial photographs of Niger, and of deflation areas visible on satellite imagery of Sudan;
2. The already 40-year-old aeolian degradation in the sand-sea of Aoukar, in Central Mauritania;
3. Parallel to the shift of vegetation, a general shift of desert locust activity towards the south in the Sahel;
4. Progress of desertification in Burkina Faso.

2.1. Increase of the Figures and Indicators of Deflation (Wind Erosion)

Deflation areas and grooves in Kordofan (Sudan) have developed since 1973: satellite image No. 10 91-07532 is particularly representative, with a corridor of 30 km, where all the aeolian exportation patterns are exacerbated and becoming active, showing severe soil deflation. Increase of the precision as well as the width of the corrasion grooves and of the figures of aeolian soil erosion can be observed.

These images correspond with an impoverishment of the sandy soil by winnowing of organic matter and of the fine particles which normally give to the soil its structure and a coarser soil texture. In the Sahel, in the fixed red dune system, the mode varies between 250 and 360 micrometres. In the winnowed topsoil (20 cm), the mode can reach 800 micrometres. The soil has lost its structure, its organic matter and its capacity of water retention, resulting in a non-reversible sterility.

2.2. The Surprising Case of Severe Land Degradation Observed First in the mid-1950s in Aoukar (Mauritania)

In aerial photographs of Mauritania NE 29-XV No. 79-80, 1:50,000 scale, of December 1956 to January 1957, the first part of the dry season, the sand deposits around the Taokest were organized in a slightly fixed dune field of transverse dunes of 400 km2 (from 17°30 to 18° 30 N and 8° to 12° 30 W). This indicates a positive sediment balance.

After the phase of accumulation responsible for the Taokest dune field - the main wind direction being 19° 5' ENE - the dunes were vegetated, indicating a relative stability of the whole field (the south of which has a lower reflectance) because of a slightly denser vegetation cover. The interdunal depressions also have a lower reflectance. To the north of the photographs (18° 12 N), the dunes are already active with some vegetation at their foot. To the south of the same photographs, the whole barchanic edifices are vegetated with some bushes in the interdunal spaces.

Even if the dune field in 1956 seems fixed by an open vegetation cover, symptoms of land degradation can already be observed indicating prodromes of desertification:

- omnipresence of grooves of deflation, responsible for the transverse dunes of the first longitudinal patterns;
- elongation of the dextral barchanic wings in the majority;
- appearance of small depressions on the flat back of the barchanic edifices initiating a parabolic pattern;
- north of the photographs, the crest of the barchanic dunes is abraded, reactivated and rounded. The reactivation proceeds up from the foot of the back of the dune to the summit, which is reached by shallow furrows;
- appearance of a new family of small barchans at the crest of the primary ones;
- genesis of seifs (linear dunes) on the dextral wing of the original barchans.

These figures are all the preambles of longitudinal patterns, indicating an export of sand and the transition from a positive sediment balance to a negative sediment balance. Finally, at the end of 1956, indicators of erosion, sand movement and severe land degradation are already visible, and this is precisely 12 years before the discontinuous drought of 1968-1985. This important result shows that just after independence, signs of land degradation can be observed, probably human induced because the 1950s were a decade of good rainfall, and demonstrating that drought alone does not trigger land degradation but has probably reinforced pre-existing land degradation. This reorganization of dune fields from a positive sediment balance (a general trend in the Sahel) to a negative sediment balance is an objective indicator of land degradation by aeolian escape of soils, which occurred before the 1968-1985 drought. It is a clear example of long-term land degradation by traditional land management, probably accelerated by the socio-economic changes brought about by colonial rule and exacerbated by droughts.

2.3. The Notion of the Shift of the Vegetation Belts South of the Sahara

1. The north, where perennial short grasses are dominant: the Saharan steppe.
2. The middle, where annual grasses are dominant: the Sahelian steppe. The only explanation of the presence of this belt of annual grasses, interposed between the northern and southern Sahelian perennial grass belts, is anthropic and probably the result of several millennia of land occupation and human activities - principally grazing. This is probably the most impressive example of slow mining over several thousand years.
3. The southern belt, moving into the savannah, where high perennial grasses with inflorescences growing one metre from the soil surface are dominant.

This zonation and millenary slow mining are the long-term time and spatial scale answers to aridity and human activities. Embodied in this long-term scale are the middle- and short-term twentieth-century droughts and land use.

Deterioration in natural plant cover, characterized by a decrease in quality and quantity, is often considered as the prime indicator of land degradation. We think that the vegetative indicator alone is not a good one because the status of vegetation does not reflect a general trend, but rather seasonal or annual variability of rainfall. With regard to the studies of Dregne and Tucker (1988) - which compared biomass in 1984 (drought year) and 1985 (normal to wet year) in Sudan - and of Tucker and Choudhury (1987) - who highlight changes which occurred between 1981 and 1986 - and of Tucker et al. (1991) - investigating the Saharan vegetation changes in the decade 1980-1990 - the same conclusion appears: biomass fluctuations are seasonal and annual, in response to rainfall. Hellden (1994) notes that the responses of vegetation do not display a front-like movement but more short-term and spatial patterns in response to patchy rainfall distribution rather than to man's mismanagement of the natural resources. In 1991, he wrote that no observation confirms "the hypothesis of a secular mainly human-made trend towards desert-like conditions." Maybe the time-scale did not suffice.

By comparing aerial photographs from the 1950s to the 1980s, we have observed other important aspects of vegetation changes: the increase of the bushy vegetation in the Gourma (Mali) and the increase of the density of vegetation in the wadis in southern Mali. Vegetation degradation increases water erosion in the water catchments and concentration of colluvial material in the valleys, which is indeed profitable to vegetation but only in concentrated locations. The same trends are confirmed in the dry areas of Tunisia. This increasing concentration of vegetation in the valleys where the water catchment is bare with desert-like aspects in non-desert climatic conditions exists also in savannah and dry woodland conditions of 1 m?yr rainfall.

After the end of the drought in the Sahel in 1985, healing occurred naturally in the graminaceous grazing lands, but to a lesser extent. Nevertheless, nomads did not return, even after the reappearance of grass, because of the disappearance of trees and the lack of shade for the cattle. That is why the vegetation cover could rehabilitate. The trees have in fact disappeared, as is the case in Mauritania, because of the combination of droughts, overuse as forage or firewood, and bush fires.

2.4. Facets of Desertification Observed in North Burkina Faso

The bare surfaces, despite increased rainfall since 1985, did not recover, indicating a loss of resilience and real desertification. The development of the bare surfaces is, according to the two scientists, the result of decreased rainfall in a 20-year period combined with grazing pressure, fuel wood consumption and other human activities.

2.5. Ecological Migration

Some 250,000 Mexicans emigrate each year to the USA, according to Masse Lo (1994), who also estimates that 40 per cent of the active population of the upper and the middle regions of the Senegal valley (particularly Bakel) have emigrated to France and Italy. As a result, there is a shortage of men of working age: there are only 60-80 men per 100 women in the 15-50 age group.

Nevertheless, can we maintain that the collapse of the agricultural systems in the Sahel or in Central America, for example, is the main cause of this emigration, or should we also suggest that the different levels of development between Africa and Europe on the one side, Mexico and the USA on the other, are among the other causes of migration?

2.6. Conclusion

On Landsat satellite imagery, we detect an increase in the figures of deflation in Niger as early as the 1950s, and wind erosion in Sudan, showing that it is not new or recent. In Mauritania, in aerial photographs, a reorganization of the topsoil of the Aouker dune field from positive to negative sediment balance was observed, during years of above average rainfall and before the droughts between 1968 and 1985. This demonstrates pre-existing human-made land degradation exacerbated by droughts.

The shift of the vegetation belts south of the Sahara is not proved, but the replacement in the Sahel of the perennial grass belt by an annual grass belt can suggest a long-term degradation resulting from prehistorical and historical human activities. Nevertheless, we are not able to determine the part played by climate effects. No downward trend is detectable, but superimposed short-term changes in biomass of the Saharo-Sahelian vegetation in response to rainfall variability are detectable. It seems that rehabilitation after drought occurs, at a lesser level (without trees), and is possible because of lesser exploitation.

Finally, the vegetation is more and more concentrated in privileged sites (valleys, depressions) while the interfluves become desert-like landscape. However, not one of the observed consequences has a univocal explanation and can lead to the certainty that human activities are the unique cause of land degradation. In north Burkina Faso, the loss of recovery potential of the bare soils in spite of increased rainfall shows the irreversibility, and thus the human factors, in the observed land degradation.

Ecological emigration finally is a very clear human sign of the loss of the production potential of land. This loss of production potential is generally in all the semi-arid desert margins, as well as north and south of the Sahara around the Chalbi desert and in the Kalahari up to 600-700 mm/yr rainfall. This fact is recognized by managers and has led to the attempt at new strategies of development, which we shall try to discuss.

3. Does the Experience of Severe Land Degradation Lead to Efficient New Strategies?

3.1. What Should Be Avoided?

3.1.1. Symptomatic of mismanagement is the example of the Algerian steppe, where in areas of lower rainfall, a strategy of cultivating the grazing land was adopted in the 1960s and 1970s.

North of the Sahara, the diversified topography explains the patchy rainfall distribution followed by the same patchy distribution of droughts. Therefore it is difficult to accept the general belief that there is a general decreasing trend in rainfall in the Algerian steppe and in the dry areas of Algeria. Even for the region of Djelfa, the first part of the decade of the 1990s had higher rainfall than average, with a better distribution. The observation of rainfall measures from 1930 to 1984 of more than 80 stations reveals four droughts of four to five years below the average annual rainfall.

Table 1: Droughts in Algeria 1930-1984 (Source: verbal communication from El Hadi Oldache)

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1937-1941 	moderate drought
1944-1949 	more severe drought
1959-1961 	moderate to severe drought
1979-1983 	drought often very severe
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At the beginning of the 1960s, sheep numbered 3.5 million; in the 1990s the number reached 16 million. During each drought, many animals died, thus avoiding the overgrazing phenomenon after the dry period when the grazing areas were in their phase of restoration. With the advice and financial help of the World Bank, the government changed its strategy, abandoning the traditional nomadism and bringing industrial fodder for the animals with water and possibilities to be moved by truck. This prevented flock decimation but resulted in a tragic overgrazing after the drought when the vegetation was supposed to recover. The same applies to Tunisia and has led to Visser's (1994) statement: "government subsidized desertification."

Worse is the national and international programme of substitution in the grazing areas of cereal cultivation and the encouragement of accession to land ownership. Thanks to national and international financial help, even areas beside sebkhas with halophytes were ploughed. After two years these areas were irreversibly salinized.

These observations go against the belief that in northern Africa worsening climatic conditions have accelerated man-made land degradation and forced people to migrate towards the more crowded northern coastal areas, where fertile land for agriculture is occupied by urban settlements. Therefore, in Algeria currently 50 per cent of the food needs are satisfied by imports. This percentage is likely to grow to as much as 80 per cent over the next 50 years.

3.1.2. Another sad example consists in closing the grazing areas so as to replace them by nature reserves. That is the case of the territory of the Raikas (or Rebaris) - traditional camel and sheep breeders of Rajasthan and Gujarat in India, once caretakers of the camel herds owned by the maharajahs. The Raikas are landless or owners of very small plots of land. Their only capital is in their animals. Their summer grazing area in the Aravalli Hills was transformed into a nature reserve. The reserve was closed to grazing, and no alternative grazing was provided to the Raikas.

The lack of pasture reserves is the most severe difficulty met by pastoralists all over the planet. No remedy has yet been found for shrinking pasture resources and for the identification of new economic options.

3.1.3. In the northern pre-Saharan and southern semi-arid zones, the ever growing population is less and less dependent on local and renewable resources, and more and more on foreign income and on massive food imports (often subsidized). Furthermore, the limited water resources are increasingly exhausted. The livestock have needs which exceed the local depleted range land resources and survive thanks only to complementary fodder and government subsidies. The growing densities of population and cattle should be considered as the primary cause of land degradation.

In response to the restructuring programme imposed by the World Bank to pay the foreign debt, the government of Tunisia concentrates on investments in export-oriented agriculture: olive oil, dates, out-of-season fruits and vegetables and flowers. This requires an ever increasing import of staple foods: cereals, milk, meat, sugar - sold to its population at subsidized prices. The dependence of Tunisia on fossil oil, tourism (a highly unreliable and unsustainable source of national income) and on foreign income sent home by emigrants, mainly in Europe, is in contradiction to the large part of these foreign currencies needed to pay food imports.

The age-old pastoral survival strategy was to diversify activities, since herding alone could not provide staple food, nor basic utensils. The post-war emergence of a salaried workforce (local or abroad) amongst the agro-pastoralists has created a new form of the multi-activity of extended families: foreign income feeds their subsistence agro-pastoralism. This way of coping with the limited local natural resources has not been seriously studied, but should be taken into account when trying to combat land degradation.

This evolution is responsible for the fragmentation of the steppe, for soil and biodiversity erosion, and leads (Visser 1994) to a set of questions:

1. How, by whom and for whom should the vegetation cover be rehabilitated?
2. What are the wishes and the suggestions of the double economy (agro-pastoralist or rural and oasis-based or urban) which has replaced the ancient pastoral society?
3. Are the economic objectives of subsidized fodder during the droughts and substitution of cultivated land for grazing land compatible with the fight against land degradation?
4. Is it possible, at present, to say whether such very recent mutations are advantageous or not for the growing local population? In both the semi-arid belts, north and south of the Sahara, is somebody now able to propose a new agro-pastoral strategy which combats the erratic droughts and the vulnerability (a-vis the erosion of the soils more efficiently than the traditional agro-pastoral activities?

3.2. What Can Be Done?

As we have seen, countering the causes of desertification, natural or anthropozoic in cause, is still more or less impossible. We are in fact not able to combat the supervening of the droughts, nor even to understand their mechanisms. We are not able in one generation to combat the anthropozoic causes of desertification; it requires two or three generations. In fact, we do not know how many generations are required. We must nevertheless undertake the battle immediately against the physical effects of land degradation and begin to improve human resources, mainly through education, which is a huge task.

As a basic rule, in all development programmes, factors which must be taken into account include aridity of the climate, recurrent droughts, scarcity of surface water, population pressure, the percentage of the population that can live from local pastoral and agricultural resources and how many people need foreign resources. The more arid a climate is the less the ecosystem can bear high demographic pressure. The more severe the demographic pressure, the easier it becomes for a pastoral society, with its economy based on extensive use of pasture land and maximum mobility, to evolve towards a peasant society with agriculture, fragmentation and clearing of the grazing areas and sedentarization. Population growth must be stopped even if benefits and effects will only be felt one generation later. For agriculture, which cultures are optimal in the face of the vulnerability of the landscape? For pastoralism, which animals? If the people are sedentary, how can pastoral exploitation be compatible with their needs and wishes? To determine actual and potential land capacity, sustainable land use is necessary, even though difficult to achieve.

So, what can be achieved? Which are the techniques that could upgrade productive capacities of currently degraded land? How can resilience be given back to these lands? How can these techniques be applied and assimilated by the local people? How can they be trained or educated? Are tourism and industrial development feasible, knowing that these require high quantities of water?

The solutions to combat the physical and chemical processes of land degradation are the easiest to put into place: we know how to combat water erosion, approximately how to combat wind erosion and salinization. However, we are more devoid of solutions that can cope with the socio-economic facets of land degradation, mainly because we cannot discern environmental from socio-economic aspects and lack politically appropriate and sufficiently opportunistic means to counter the latter.

When proposing "Implementing Integrated Environmental Management for the United States," Cairns (1994) does not realize the extent to which his idea should be applied to combat desertification.

We propose to select some adaptations which we consider to be the most effective: improving camel husbandry and pastoralism in the driest areas; development of the arid zones north of the Sahara. Tunisia is the example chosen.

3.2.1. Improvement of Animal Husbandry and Mainly Camel Breeding in Arid Zones. Population pressure on the tropical and temperate deserts is threatening economic and cultural survival of most of the pastoral nomadic groups. They are also threatened by sedentarization projects - which are badly adapted to their ecological and social needs - and development policies.

Camel husbandry is a means of extracting resources even from arid environments receiving less than 200 mm?yr rainfall and, in this domain, the traditional pastoral societies have accumulated generations of expertise in managing desert environments. They have accumulated a rich indigenous knowledge in coping with vegetation and animal management. Above 300 mm/yr rainfall, camel husbandry is not common; agriculture can be envisaged but semi-arid land cannot always sustain intensive mechanized or irrigated agriculture (Mainguet 1994).

Pastoralists are the people who traditionally survive in the arid zones (deserts) and semi-arid zones (steppes) thanks to livestock grazing. They are familiar with range vegetation, the qualities and effects of forage plants on their animals and breeding, and still more with techniques for survival in arid zones. "This traditional body of knowledge should be regarded as a starting point and essential foundation of all development interventions" (Köhler-Rollefson 1994). The highly evolved way of life of pastoralists is a successful adaptation to the harsh and vulnerable environment. Their herds - sheep, goats, camels, cattle, yaks, llamas - convert poor xerophytic vegetation into food, energy (for transport), meat, milk, skin and kinship.

According to the degree of aridity, the species best adapted are cattle, sheep, goats, and camels. As a general rule, it should be considered that the areas receiving less than 300 mm/yr of rainfall can accommodate grazing and livestock raising and are economically uncultivable lands, except when irrigated.

If we consider that real pastoralism should be improved mainly in the driest lands, we should not forget that, so as to avoid overgrazing, it requires vast grazing areas. More attention needs to be brought to camel husbandry.

What is more, continued utilization of the arid lands of our planet depends to a large extent on viable pastoral systems. The level of knowledge about indigenous pastoralists and how pastoralism might be improved in project design is still rudimentary. In this domain, the main progress concerns the knowledge of the advantage of camel grazing.

Two sets of measures should be envisaged:

1. The first set of solutions based on improvement and rehabilitation of steppe vegetation covers should improve the local pastoralist approaches and strategies;
2. The second concentrates on soil and water conservation.

3.2.2. Improvements Proposed for the Arid Zone of Tunisia (Visser 1994). The southern part of Tunisia's arid zone is called pre-Saharan Tunisia (Floret and Pontanier 1982). The pristine open forest persisted until pre-Roman times, thanks to a climate which was continuously humid during the Quaternary as shown by the abundance of fluvial deposits in this area. This can also explain the richness of sand in the northern half of the Grand Erg Occidental and the Grand Erg Oriental (Mainguet 1985). The pristine vegetation was dominated by scattered trees and shrubs and tall grasses (Floret 1987). At the turn of this century, the pre-Saharan vegetation was devoid of trees: it consisted of grass steppes, dwarf shrub steppes, crassulaceous steppes on saline soil and tall shrub steppes in wadis and depressions with an attenuated edaphic aridity. During this century, these steppes have also been severely altered. Nevertheless, nothing proves that climatic changes have taken place in northern Africa since Roman times.

A tribal society composed of nomadic herdsmen and mountain berbers or oasis sedentarians existed until some decennia ago. They specialized in mobility: trans-Saharan trade, guiding expeditions through their territories, seasonal labour and emigration for grain harvests to the north. They could afford this "multi-activity" without harm to the true pastoral activity because they operated as clans or extended families (Duvigneau 1995, verbal communication; Nasr 1994).

With the French protectorate of Tunisia and the increase of population, nomadism was abandoned. The nomads settled down and replaced their herding activities by a farmer's life and cereal cropping. Population pressure and sedentarization triggered a "land rush," steppe clearing, social conflicts and land degradation. But it must be noted that already before colonization the tribes of the steppes were too numerous for their extensive way of living to be "sustainable" and were probably mining the steppe resources already before colonial rule accelerated the trends. Three main ways in which the people of the steppes have degraded their lands can be distinguished: firewood cutting (the cause of the disappearance of trees), overgrazing and steppe clearing. Nowadays, steppe clearing is common and has become the most important cause of desertifying the landscape.

Technical improvements could be developed but they should be generated within the agro-pastoral communities themselves, and encouraged by the appropriate government policies (Visser 1994). The agro-pastoral communities need:

- to be convinced that pastoralism is not marginal and to invest in its improvement. To this end, governments need to increase the density, productivity and pasture value of the non-cleared patches by reseeding or overseeding. These methods, adapted in American and Australian ranches, have required considerable investments in sophisticated agricultural equipment, but can be developed more modestly by local people;
- to give back a pastoral value to abandoned good land without knowing if the pastoralists will return to this land;
- to allow agricultural land to rest by introducing legumes on cultivated fallow under the cereals;
- to allow grazing in the olive tree plantations, to minimize sand mobilization and dune genesis by less ploughing;
- to insert on both cultivated and on grazing land autochthonous arborescent vegetation as a multiple resource so as to create natural nitrogen-input anti-erosive shelters and wood producing vegetation.

4. Conclusion

- after a period of belief that climate changes are the primary cause of land degradation, the meteorological studies in the last decades have introduced doubt concerning the existence of a long-term secular trend;
- the difficulty to separate effects of short-term climate changes (recurrent droughts) and effects of human activities in land degradation invites special caution;
- in the majority of the dry countries, it is impossible to say whether or not land degradation results exclusively from man's actions;
- to understand desertification, it is more and more necessary to combine the analysis of climatic fluctuations with land use transformations under political changes, the abolition of traditional land use rights and imported technological innovations.

Dryland ecosystems experience drastic changes in biomass (cover density, species diversity and disappearance) - first degree land degradation - in response to short-term natural climatic fluctuations. These can be easily confused with the effects of human actions. But, according to field observations, these changes, when not aggravated by human activities, are often reversible because of the resilience of the dryland ecosystems. If land degradation, aggravated by human activities, leads to desertification, resilience cannot play its natural role since desertification is irreversible. Second degree land degradation refers to alterations of dryland soils, and is more difficult to discern and curb.

Does the level of our present knowledge allow us to introduce new socio-economic strategies into dryland ecosystems to combat desertification, or should the improvement of traditional methods of land utilization not have priority over all the others (Visser 1991)?

It is obvious that local people recognize insidious desertification taking place in their land, but they are more concerned about daily subsistence and cannot take action when the first prodromes of land degradation appear. One of the most tragic observations in dry ecosystems and probably everywhere among mankind is that with the increase of man-created bad environmental conditions, "inevitably each individual or each organization is trying to optimize utilization of particular components of the environment, all too often to the detriment of others" (Cairns et al. 1994).

Many examples of adaptation have proved that traditional knowledge is able to provide new responses to droughts and land degradation. Unfortunately - and this is really crucial to be aware of - because of a lack of collective organization, traditional knowledge centres on individual or little groups, without an extension or sharing of these precious abilities.

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