2.2 Production systems

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1. FORESTRY SYSTEMS

Authorities concerned with land use and particularly forest land use are united in their recognition of the environmental fragility of the ecosystems in arid and semi-arid zones. In practice, generally all land within forestry production systems is normally wholly assigned to forestry use; excluding clearing and agriculture. In certain countries (Australia, Morocco) woodlands may be managed as part of a grazing system and in many other areas grazing, whether intended or illegal, often occurs in natural forests.

In certain Mediterranean countries (Morocco, for instance) thinning of holm oak (Quercus ilex, Q. coccifera) stands combined with pasture improvement and strict grazing control can improve the health and productivity of tree stands while providing fuelwood and fodder from ground cover and coppice shoots. In the Sudano-Sahelian zone, savanna woodlands not only provide feed for livestock but also fuel, building material, fencing, tools, hand-crafts, and raw material for drugs and medicines. The bark of some species is used for making rope, roots are used for fuel, charcoals well-lining and so on.

The range of products from natural forestry production systems varies from region to region according to the type and structure of the woody vegetation. In the Mediterranean Quercus suber is associated with Erica arborea, Myrtus communis, Phyllaria spp. Products obtained include cork, fuelwood, charcoal, fodder, honey and essential oils from aromatic shrubs and plants. In Northern Mexico, on the other hand, natural plant communities dominated by Agave lecheguilla are managed for fibre. Mali and Niger are undertaking pioneer work on agroforestry development for the supply of fuelwood and timber under Sahelian conditions.

Essentially, dry-zone forestry systems, either natural or man-made, are implemented for the production of primary or certain secondary wood products such as fuelwood, posts, poles, round timber for construction, charcoal, and sawnwood as well as non-wood products: extractives, fruits, gums, honey, oils and resins. Managed forests, particularly natural forests, can therefore contribute significantly to the well-being of rural populations in arid regions. However, in addition to the above functions, forestry systems also play an important role in reducing soil erosion, assisting in water harvesting and in modifying micro-climates for improved and sustained agricultural crop production and animal husbandry.

Management of dry-land woodlands is generally not intensive, even in forest reserves and does not involve much more than early burning and patrolling to control grazing, fuelwood harvesting, charcoal burning and prevention of agricultural encroachment. Very little revenue is produced while degradation continues through a number of conflicting demands from pastoralism, wood (primarily fuelwood) harvesting and agriculture. If ecological degradation is to be halted and reversed, management of dry-land resources is essential: the objective of management being to ensure multiple use of the vegetation resource rather than to maximize the return on any one product and thereby risk its degradation and destruction.

Examples of intensive management in Morocco and Tunisia show that integrated action of improving fodder resources both outside and within the forest, genetic improvement of livestock (Syria), control of grazing animals and the application of silvicultural principles to avoid overstocking and ensure regeneration can benefit forest resources and improve productivity. The application of silviculture and forest management to dry-zone forest presupposes an intimate knowledge of their extent and composition. Inventories and ecological studies to this end have been conducted in North America and are now being conducted in countries such as Morocco, and the need for them is appreciated in Australia and most Sahelian countries. The application of measures to ensure conservation of woody vegetation involves a recognition of the value of forestry in linkage with other natural and agricultural resources:, the cooperation of rural populations and a reduction in the communications gap between government agricultural and forestry institutions as well as foresters and agriculturalists.

In dry zones the main effort towards re-establishing forest resources has been directed to the introduction of artificial forestry production systems which, in lower rainfall areas, are frequently associated with water harvesting and soil and water conservation works. These man-made forests and tree stands assume many forms, ranging from large-scale rain-fed plantations, woodlots, greenbelts, and shelterbelts through the planting of trees at pre-determined or optional wide spacings on agricultural land to intensively-managed and closely-planted irrigated plantations.

Exotic tree species such as Eucalyptus, drought-resistant Australian Acacias, Prosopis spp. and Casuarina have often played a prominent part in such plantings. However in many Sahelian countries, the Mediterranean region and in the Indian sub-continent, considerable use has also been made of faster-growing indigenous or regionally-indigenous species, both coniferous and broad-leaved.

Techniques of nursery propagation and establishment of rainfed plantations appear to be remarkably similar overall. Nurseries generally use containerized stock in black or clear polythene planting bags of from 10cm to 7cm diameter and from 30cm to 22 cm in length; of about 0.05 to 0.08mm gauge of polythene, gusseted and with numerous drainage holes punched in the lower 1/3 of the pot. Tree seed is generally sown direct in the pots and grown for 5 months to 1 or more years depending on the species and climatic conditions. When seed is delicate, in short supply and/or expensive, seedlings may be grown in boxes or beds and pricked into pots. In India, and particularly in Pakistan, certain species, notably Dalbergia Sissoo, Albizzia lebbek, Morus alba, Melia azaderach and Salmalia malabarica are raised in beds and planted out as stumps. Shade is often, but certainly not always used. When used it is generally of natural materials: palm fronds, branches or stems of reeds such as Arundo donax.

Weed growth is frequently discouraged by the laying of sheets of black polythene under pot-storage beds supported by brick sidewalls, wire strung on pickets or often in rectangular hollows excavated in the ground.

Potting soils comprise various mixtures of soil, sand and composted farmyard manure; fertilizer is infrequently applied. In certain areas sun-cured earthen blocks or mottes are pressed out of selected clays mixed with chopped straw and used as plant containers (Morocco).

Watering is achieved either by channel irrigation, watering cans, hand-held hoses or (infrequently) by spray irrigation systems and in the case of sprays is applied at the rate of about 1.5 to 2.0 litres per m² per day of pot-storage bed.

However, in view of the high cost of plantation establishment, research is being conducted into direct sowing: P. halepensis and P. brutia (Syria and Pakistan). In the last instance a cheap successful sowing was achieved on a flood plain directly after extensive flooding. However, this was difficult to repeat and aerial sowing without intensive site preparation and careful timing appears doomed to failure. Under irrigated conditions, however, spot or drill sowing of pre-treated tree seed (Acacia and Prosopis) is carried out routinely along the banks of channels into which irrigation water is diverted.

Planting in the field is almost always preceded by the digging of large holes from 30x30x30 cm to 50x50 x50 cm, associated with contour ridging, formation of gradonis or water-retaining chevron-shaped or half-moon ridges. If the site is prepared by machines, deep ripping is carried out in lines or along prepared terraces and planting holes prepared on the ripped line. Tree spacings vary considerably but are generally fairly wide: from 3m or more upwards providing from about 1100 to less than 100 trees per ha (when association with soil and water conservation works).

Follow-up weeding is extremely important for the promotion of survival and tree growth, particularly under rainfed conditions, and can absorb about 17-18% of the costs (Morocco).

Large-scale irigated plantations require costly earthmoving: levelling and channelling for the efficient distribution of water. The presence of abundant water (normally applied 6 times during the first season, 15 April to 15 October - Pakistan) allows direct sowing of seeds, planting of stumps (Mows spp., Melia spp., etc. - Pakistan) and cuttings or sets (Populus spp. Syria) as well as the use of the usual containerized seedlings: (E. microtheca -Sudan). At the present time, irrigated plantations in Syria cover some 4 000 ha, in Pakistan about 80 000 ha and the Sudan some 39000 ha. Raw sewage has also been used for irrigation in some cases (Sudan, Iraq) or has been used in a diluted form (Egypt, Syria, Iraq). Brackish water has also been used for this purpose but only on a smaller scale, particularly in the Gulf States. Meanwhile work is being conducted in Australia to identify and propagate particularly salt-tolerant provenances of E. camaldulensis and other Eucalyptus.

With the exception of irrigated plantations and a few examples of rainfed plantations, the yield of artificial forestry production systems under dry conditions has been generally very low, not exceeding 5m³/ha/year. Since the majority of these artificial systems are generally monocultures, a wide range of secondary products (food, forage, medicines, etc.) are lost to the local users, and the management of natural woody vegetation should be considered a viable lower-cost alternative to man-made forest establishment. Steps should be taken to enlist the collaboration of local land-users for this purpose. Little appears to have been done so far in this direction in arid and semi-arid lands.

2. SYLVO-PASTORAL SYSTEMS

Pastoralism is the dominant land use in arid zones unsuitable for agricultural food cropping. Livestock in these regions thrive on browse during the dry seasons of the drier tropical and sub-tropical areas. In Senegal, for instance, 25% of the local cattle intake is browse increasing to 45% by the end of the dry season. In drier Brazilian savannas 60% of the forage is derived from leguminous trees and shrubs.

In the arid zones of Northern Africa, the production of chamaephyte shrubs such as Artemisia herba alba, Rhanterium suaveolens, and Helianthemum lippii represent the greater part of fodder production, i.e. 60-80% or 50-90% expressed in kg of DM/ha/year. Total production of steppes in North Africa varies from 5 to 9.5 kg of DM/ha/year for each mm of rainfall, the average being 2 kg of edible DM/ha for each mm of rainfall. In North America, shrubs of the Great Basin desert rangelands provide 50 to 70% of the diet of sheep and 40% of cattle that graze these lands during the winter. In the Sahelian Zone, more than a dozen species of Acacia provide fodder and more than 30 other species supplement this. These shrubs and trees are extremely valuable; in fact, stock raising would probably not be possible without them in many locales. During the dry season, they provide green feed (leaves, flowers, and fruits), often rich in proteins, vitamins and valuable mineral elements. Wherever these shrubs and trees are absent, animals have only poor straw from native annual grasses. The classic method of management for the above systems is still free pastoralism dominated by the use of fire, or by lopping trees and shrubs at head height.

The above examples demonstrate that pure pastoralism in arid regions is modified by a dependence on green matter and fruit derived from woody vegetation which is often managed to some extent. When land is used both for forestry and animal husbandry the combined system can be correctly described as sylvo-pastoral.

Artificial silvo-pastoral systems include the plantation of ligneous species suitable for browse in pure stands or in association with a grass or a forte layer. Among the most frequently cultivated species is Opuntia ficus indica. Spineless cacti plantations for browse cover over 300 000 ha in North-Eastern Brazil; nearly 200 000 ha in Northern Africa; 100 000 ha in Sicily, South-Western Madagascar, Mexico and Texas. The carob tree in the Mediterranean area should also be noted as well as various Australian acacias in various countries especially around the Mediterranean, Atriplex species in several arid Mediterranean, North and South American areas and Haloxylon species Prosopis cineraria, Zizyphus mauritiana and Calligonum species in arid areas of Asia. Prosopis and Parkinsonia species in South America (Chile), India and finally the African acacias especially Faidherbia albida, A. senegal, A tortilis, A. nilotica.

The management of these plantations depends on many factors, amongst which the species, the nature of production (fruits or leaves), the nature of utilization (picking or gathering by man or direct browsing by animals or combined solutions such as trimming, polling and lopping, etc.), and of course whether the production system is semi-sedentary, extensive or intensive. Species such as F. albida are subject to mixed utilization involving fruits, leaves and wood. Most species, (Atriplex, Acacia) are browsed directly. Other species are subject to mixed utilization by trimming and stall feeding as in the case of spineless cacti. The intensity of these practices and the intervals at which they are carried out have not been studied systematically with a view to optimizing management. Similarly, the best combination of trees, shrubs and grass, under various ecological zones has not been investigated. Knowledge is also inadequate as regards the genetic variability of the main species, their biology and their reaction to various management methods such as trimming, lopping, pruning and cutting back.

The use of fire as a management tool appears more common in the less densely populated semi-arid zones of Brazil (Cerrado) the South West USA and Australia. Prescribed burning is used to reduce woody vegetation cover, renew grass growth and reduce pests (snakes, ticks, insects). Its use for the improvement of the composition of herbaceous ground cover is said to be debatable. In lightly populated semi-arid woodlands (Australia) wildfire can occur from natural causes (lightning strike) and its effects can be damaging both on vegetation and livestock alike. Such occurrences in the more heavily grazed lands of developing countries are uncommon and fire is nearly always anthropogenic and damaging in effect as it increases the drought stress on vegetation when it occurs during the driest periods of the year. Protection is sought against wildfire and Sahelian countries have identified the need to institute a fire danger rating system to assist in determining periods for prescribed burning as well as for the mobilization of fire-suppression personnel and equipment. Fires are also a serious problem under semi-arid Mediterranean conditions.

Africa and the Near East have established networks of firebreaks and roads in forest areas as a protective measure. The need for intensified forest fire management and all it entails (fire prevention, detection and suppression) is essential in many arid and semi-arid areas.

3. AGRO-SILVICULTURE

Where land is used to produce agricultural crops and forest products and amenities, frequently, simultaneously or sequentially, the combined production system is termed agro-silvicultural. In many respects, agro-silviculture in arid zones can represent an improvement over traditional systems of shifting cultivation, in which farmers cut the tree crop, remove and/or burn it, and raise agricultural crops for one or more years before moving onto another site to repeat the process.

Agro-silviculture is thus essentially the art of growing and managing trees to yield forest products and benefits in an agricultural environment and on agricultural land.

Important and frequently encountered agro- silvicultural systems in arid and semi-arid zones include the use of trees on crop land, linear plantations such as windbreaks and shelterbelts, bush-fallow with acacia spp. The most important systems are summarized in Table 5.

TABLE 5

Combined Systems

Combined production systems or systems commonly known as agroforestry, are defined as systems in which agriculture, animal husbandry and/or forestry are practiced on the same piece of land in rotation, simultaneously or spatially. At best, agroforestry is a name for a collection of land use systems and technologies where two or more of the above production systems are deliberately implemented on the same land. Of course, agroforestry is a new word, not a new concept, for land use in the dry regions of the world.

Agroforestry systems have been classified in a number of different ways, according to their physical structure, temporal arrangement, relative importance and role of components, production outputs, and social and economic features. The divisions between categories of agroforestry in arid regions are not very clear cut. Rather, they represent a continuum of combined production systems.

Given that pastoralism is the dominant land use in arid zones, it is logical that most agroforestry systems would include animal husbandry with greater emphasis on this aspect in arid and a lesser emphasis in semi-arid zones where some rainfed agriculture could be practiced in combination with techniques for water harvesting and spreading.

In lightly populated arid and semi-arid zones, pastoralism or silvo-pastoralism is dominant. However, in more densely populated arid zones (Sahelien Africa) wet-seasonal subsistence farming is commonly practiced giving rise to a number of rainfed agroforestry systems which include grazing: examples of such systems are given in Table 2.

In developed countries, agro-silvo-pastoralism scarcely exists apart from the use of browse species plus pasture improvement through reseeding of ground fodder species. The main activities in these arid lands are control of grazing and reduction of tree cover to promote better herbaceous fodder production, the selective removal of non-palatable woody species and the establishment of reserves of fodder-bearing shrubs. This work is generally carried out on privately-owned land or as demonstrations on government land for the benefit of private land-holders. High costs of planting trees and shrubs has placed emphasis on the feasibility of effectively establishing them through research into various direct seeding methods (Australia).

In other countries such as Pakistan, Syria and Sudan, greater emphasis appears to be given to wood production under irrigated conditions than in Sahelian Africa where a tradition of integrated harvesting of tree and agricultural products still exists.

Under the heavy population pressure exerted on dry lands in India (said to be the "most thickly populated deserts in the world", with an average of 61 (1971) persons per km²) antidesertification measures have concentrated on grazing control and improvement of animal husbandry which is the major occupation of inhabitants of these arid regions. Progress has been made through the creation of village-level institutions such as cooperatives and producers' unions as well as in the field of improved strains of livestock. Meanwhile, other institutions such as the Drought Prone Area Programme (DPAP) and Department of Desert Afforestation and Pasture Development (DDAPD) are developing range management programmes. The Central Arid Zone Research Institute (CAZRI) is also selecting and testing multi-purpose tree species for degraded land reclamation and popular use such as Zizyphus mauritiana for rainfed orchards in desert areas.

Table 5: Examples of combined systems

FAO/SIDA (1981) India and Sri Lanka Agroforestry. GCP/INT/347, FLCD programme Rome.Advisory Committee on the Sahel et al (1983) Agroforestry in the West African Sahel. National Academy of Science, Washington D.C.
Widkens G.E., Goodwin J.R, Field D.V. (1984), Plants for Arid Lands. Proceedings Kew International Conference on Economic Plants for Arid Lands,
Kew, U.K. 23-27/7/84 George Allen + UNWIN London

On many village lands traditional sylvo-pastoral systems and combined systems use Zizyphus nummularia for the protection and improvement of ground fodder and Prosopis cineraria as a managed fodder supplement, fuelwood producer and part-time nurse-crop for agricultural crops.

In general, agroforestry systems attempt to "optimize" ecological and economic considerations among their various components (e.g. trees and shrubs/agricultural crops and livestock) to obtain a higher, more diversified and more sustainable total production than is possible with a single land use. Collectively, typical characteristics of agroforestry systems are two or more species of plants (or plants and animals) at least one of which is a woody perennial, two or more outputs, and a production cycle of more than one year.

In brief, agroforestry or combined production systems can provide many goods and services. Depending upon the particular arid zone situation they can:

• increase and improve the quality of food production yields;
• produce fuelwood plus a variety of other raw materials from trees and shrubs for a farmer's subsistence, for local sale in the rural community and sometimes for export;
• protect and improve the soil's productive potential;
• improve socio-economic conditions in rural areas by creating employment and income and reducing risks through the diversification of crop production.

4. WILDLIFE SYSTEMS

Limited availability of water and wildlife-borne diseases of domestic livestock were the factors which initially controlled man's impact on arid and semi-arid zones where wildlife was an important resource.

Improvements in public health, animal husbandry and control of animal diseases have permitted human occupation of arid and semi-arid lands on a permanent basis, often resulting in the elimination or reduction of game as a measure to reduce competition for fodder as well as the occurrence of disease. In many arid ecosystems, wildlife production systems are as important as, or at least supplementary to, livestock production. Hunting is considered vital to many people, such as the scattered population of bushmen in the Kalahari for supplies of meat, hides and skins. For pastoralists and marginal cultivators in the dry zones, wildlife has often represented an emergency food resource in time of drought. In more favourable years, wild animal meat may supplement and add variety to staple diets.

Wildlife, in fact, has the potential of being one of the most important renewable resources of arid regions, as wildlife populations are capable of varied and flexible spatial, temporal and dieting utilization of vegetation resources combined with a relatively low demand for water. This gives wildlife superiority over cattle, in addition to its resistance to disease, inherent productivity and adaptation to the physical environment.

The institution of wildlife utilization as a component of sylvo-pastoral systems (in view of its wide use of herbaceous and woody vegetation) has a considerable appeal firstly because under arid conditions certain forms of wildlife can thrive under conditions and on vegetation unsuitable for cattle and secondly because it provides an opportunity to conserve wildlife under socio-economic conditions which normally strongly favour purely pastoral livestock land use.

Combined use of forage resources by cattle and wildlife would require careful control and rationalization of stocking of domestic animals, restriction of fuelwood cutting and the fostering of local approval and support for the idea. Special methods of harvesting the highly mobile wild animals would also have to be developed as well as quality control of products and development of markets for these. At the present time such measures appear more appropriate to large areas of privately-owned rather than communally-owned land.

Under conditions where extensive rangeland could be shared by both cattle and wildlife, there should be excellent opportunities under fine, dry climatic conditions to develop wildlife management for the tourist industry.

The development of fish resources in rivers and dams occuring in arid lands is another important adjunct to the supply of protein to rural populations engaged in agriculture, whether rainfed or irrigated.

The potential production of freshwater fish populations is considerable in arid and semi-arid lands. The fishing industry along the Niger river in Mali is an example of such activity which also requires in support the extensive harvesting of fuelwood for drying and smoking fish. Dams created for flood control, irrigation and hydro-electric power, canals for transfer of irrigation water, associated irrigated areas, lakes and permanent streams are resources which are not seriously affected during drought and which provide opportunities for significant production if managed, particularly with the use of fast-growing fish species.

5. GAPS AND SOME ISSUES

5.1 General

The development potentials of dry areas are limited if analyzed and examined in terms of individual land use possibilities. For instance, agricultural crop production. However, in the absence of water for irrigation has sub-marginal possibilities. If crop production however, is integrated with livestock production, the prospects become less marginal. With further integration of forestry with emphasis on production, it is possible to further improve development potentials. When the development includes the possibilities offered by wildlife management and associated incomes and benefits then prospects are more likely to move above the "marginality" base and into profitability. The problem is therefore how to integrate and translate at local, regional and national level the various production systems in coherent land-use management plans and programmes.

Certain basic concepts of forest management - be it for forestry production, silvo-pastoral or agro-silvo-pastoral systems - such as the principle of "Sustained" yield, the fact that management is inseparable from its technical substructure, that the aim of management is increased and diversified production, and that the economic and social prosperity of the people are paramount, are pertinent concepts in the management of production systems in dry zones.

Forest management in these areas, however, needs to be considered as a specialized endeavour often having little resemblance to its "counterparts" in temperate regions. The evolution of appropriate management techniques for the dry areas meets with a number of gaps which are listed below:

5.2 Forestry production systems

The natural vegetation of dry zones constitutes what these lands could best provide under the prevailing environmental conditions. Plants in dry areas have evolved for survival rather than high productivity. But their total production when analyzed is certainly very high and much more could be done to improve the yield of the vegetation, for fodder, fuelwood, fruit and non-wood forest products through studies and programmes on:

• Statistical information on woody resources: such information is a prerequisite for the development of land management systems; often such information is not only lacking but also difficult to obtain. Apart from total area, site quality and other static and dynamic conditions, more detailed information is also needed on all the factors influencing the development of the vegetation cover. Specific surveying and inventory need therefore to be developed and applied.
• Physiology and auto-ecology: to determine characteristics and requirements of various multi-purpose species such as: seed germination, seedling establishment, survival, growth, seed production, optimum and tolerable range of moisture, temperature, nutrients, light, resistance to fire, defoliation, cutting, grazing and other impact of use.
• Regeneration: ability of species to fruit and regenerate naturally and to withstand competition from other plants.
• Windbreaks: quantitative data on benefits derived in increased productivity of crops and livestock as well as from products harvested from the shelterbelts themselves.

The above types of information would lead to the evolution of appropriate techniques for managing natural vegetation. Many of the cultural practices (weeding, cleaning, thinning, coppicing) employed by forestry deserve special investigation for adaptation and application in the management of natural vegetation with a view to raising productivity.

Further data is also required to promote the growing of trees for protective purposes either as individual trees or shelterbelts for the improvement of micro-climates and arid/semi-arid landscapes.

5.3 Silvo-pastoral systems

Fodder trees and shrubs have been successfully planted over several thousand hectares, especially in arid and mountainous areas. Such plantations could be expanded within the context of drought evasion strategies which aim at maintaining a balance between livestock requirements and feed supplies during extended dry periods. Further knowledge and experience are needed on the following:

• review, analysis and dissemination of available experience on silvo-pastoral systems in the arid zones;
• development of low-cost plantations and regeneration methods;
• techniques of fencing with live hedges;
• breeding and genetic improvement to prepare high yielding biomass material;
• methods to evaluate primary production and to monitor silvo-pastoral systems;
• determining non-destructive intensities and methods of tree and shrub lopping;
• establishment of fodder "reserves" in the form of fodder trees and shrubs for unforeseen prolonged periods of drought.

5.4 Agro-silvo-pastoral production systems

Technologies of improved agroforestry are evolving due largely to the high interest in this integrated form of land management, but the technologies need refinement and on-site validation before they can be incorporated into operational programmes.

Information is required on a range of topics, including alley cropping, improved fallow and multi-purpose species. Regarding species, the reviews suggested the following lines of research actions:

a) tree crops tolerant of initial wide spacing;

b) tree crops that are self-pruning or amenable to heavy pruning;

c) forest species that protect the soil from erosion forces yet do not reduce energy levels on the forest floor to such an extent that agricultural crop growth is inhibited;

d) forest species with a low crown/bole diameter ratio;

e) forest species that are tolerant of overhead shade or of side-shade in the early stages of their growth, i.e. in the sapling stage.

5.5 Wildlife production systems

Despite the considerable efforts made by some countries to develop wildlife resources, wildlife everywhere is threatened by nomadic herdsmen, over-grazing, shifting cultivation and bushfires. A number of activities are proposed to improve the management of wildlife resources:

• creation of national inter-agency working groups or commissions to identify and analyze high-priority wildland resource areas;
• planning, management, organization and administration of the various types of wildlands;
• selection and preparation of management and development plans to be integrated into national plans and budgets, tourism routes, employment and rural integration activities;
• preparation of feasibility studies and projects in relation to the development and management of wildlife, wildlands and national parks (including projects based on wildlife tourism);
• a coordinated regional (or inter-country) system of national parks and equivalent reserves;
• studies of game domestication, integrated cattle and game ranching and associated game cropping.

5.6 Social-Economic-Institutional

Desertification stems from complex socio-economic-environmental problems requiring integrated multi-disciplinary approaches and action programmes to solve them. A number of common constraints inhibit effective remedial action and need priority attention in the course of determining practical physical means to combat desertification. Major constraints are:

• lack of econometric models to analyse the efficiency of alternative production systems;
• insufficient knowledge of economic gains, direct benefits and costs, opportunity costs and externalities;
• under-exploitation, in many instances, of cash economies in rural communities;
• insufficient knowledge of marketing opportunities that are external to rural communities;
• lack of institutional arrangements by which production systems, and particularly agroforestry systems, can be planned, financed and administered;
• insufficient or partially complete planning;
• informing decision-makers and others that in many instances, long periods of protection are needed for noticeable yields and desired results; this is especially the case with agroforestry systems;
• communication and attitudes between technical administrators and rural people;
• legal conflicts, including land tenure;
• education and training, which too often is patterned on "Western models" not suited to arid zones.

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