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Chapter II. Function and place of trees and shrubs in arid zones
1. Introduction
2. Function of trees and shrubs
3. Place of trees and shrubs in rural landscapes
4. Combined production systems
5. Highlights of section
Trees and shrubs play a vital role in maintaining an ecological balance and improving the livelihood of people in the arid regions. If this role is to be developed and expanded, the function and place of trees and shrubs in the rural landscape must be analysed and understood.
2. Function of trees and shrubs
Trees and shrubs in arid zones perform several functions, some of which are described below:
- They can act as a soil stabilizer and prevent water and soil erosion. Woody vegetation protects the soil better and lasts longer than annual plants. Its roots deepen and improve the soil, and the shade it provides facilitates ecosystem metabolism. These functions are essential for ensuring the soil stability and the continuity of agricultural activities.
- They are an important source of forage for livestock and wildlife at a time when herbaceous fodder is not available. A number of multi-purpose trees and shrubs are ideal for protecting and improving the soil, while providing a high fodder yield in the dry months without impairing agricultural production in the rainy season.
- They are a source of wood products, including fuelwood, poles, and lumber. Fuelwood is almost the only domestic fuel, not only in the rural areas but in some urbanized areas as well. Wood is also used as a construction material.
- They are a source of foodstuff for the population. Many fruits, leaves, young shoots, and roots provide valuable food in the dry season and, therefore, comprise an important reserve for emergencies.
- They are a source of non-woody products. Many trees and shrubs yield products which are important for everyday use by the inhabitants, for industry, and at times, for export. For example, a variety of tree and shrub species are characterized by a high content of tannin (utilized by the leather industry) in their bark or fruit. Other trees and shrubs yield fibers, dyes, and pharmaceuticals. The pollen of many trees and shrubs is used for honey production (beekeeping).
Due to unrestricted cutting of wood of vegetation, overgrazing by livestock, and cultivation of unsuitable lands, many arid zones have an inadequate forest cover and, therefore, inadequate timber, fuelwood, and fodder resources.
Development programmes in arid regions should include a forestry component. This component should not be seen separately but must be integrated with agriculture and animal husbandry to optimize land use. The elements for such integration are explained in the following sections.
2.1 Fuelwood
Forest plantations in arid zones are often proposed for the production of fuelwood. The production of fuelwood can be crucial to people, because over 50 percent of the wood removed from the world's forests is used for fuel and 90 percent of the inhabitants of developing countries rely on it for their domestic needs; these people simply cannot afford other sources of energy.
Fuelwood is a marketable commodity that is transported over long distances (Figure 2.1). Demands for fuelwood and charcoal are increasing, and wood is likely to continue to be an important source of domestic fuel and fuel for small-scale industrial use in rural and urban areas (figure 2.2). Most of the fuelwood still comes from natural forests and woodlands that are being cut down and destroyed at alarming rates. However, fuelwood can also be grown on an intensive and sustained scale in forest plantations.
Scarcity of fuelwood can create further problems. People frequently turn to the next available fuel, such as agricultural residues and dung, instead of using these materials to maintain the soil fertility of agricultural land. Furthermore, a change in availability of fuelwood will often affect the health and nutrition of a whole family, which will use more fast-cooking foods and have less money for food as fuel prices increase. Also, fuelwood scarcities affect several aspects of family life, as more time must be spent in fuel gathering at the expense of more productive work.
To meet the increasing demand for fuelwood, land must be set aside to secure a production base, either as well-managed natural stands or in forest plantations. Large-scale fuelwood plantations may be required to supply urban areas, including industrial operations. For rural areas, which are under less population pressure, small-scale forestry activities may be sufficient to meet the demand for fuelwood. It is this latter type of plantation which is usually required in arid zones.
Figure 2.1 Fuelwood from Prosopei spp.
Figure 2.2 Wood prepared for charcoal making.
2.2 Fodder
A significant role of woody vegetation in arid zones is its contribution to a pastoral economy by providing arboreal fodder. The protein from ligneous vegetation during the dry season constitutes an essential element in the animal diet. Among the various sources of feed (concentrates, cereals, and annual fodder crops), woody vegetation is generally the cheapest and the one on which the majority of the livestock rely (figure 2.3).
The role of woody vegetation in fodder production can be examined in three situations:
- Normal scarcity situations - During the dry season (when grass and forte vegetation is not available), only trees and shrubs can provide the necessary feed for livestock; this is a traditional use of the woody vegetation in arid regions. When such vegetation is not available, the production of livestock can be seriously affected, as people do not normally have the resources to acquire other types of feed for their animals. The creation of fodder resources for scarcity situations is, therefore, a vital activity for maintaining the production of animals. Overall fodder resources can be enhanced by managing existing woody vegetation for increased fodder production or by creating additional fodder resources through tree and shrub plantations.
- Emergency situations - Rainfall in arid zones is not only variable during the year, but there is considerable annual variation and, at times, extended periods of drought. Under this situation, trees and shrubs assume greater importance in the form of emergency fodder reserves for livestock, since ligneous vegetation is better able to survive extended periods of drought than annual plants.
- Contribution to the feed budget - The most intensive method of fodder production may be the creation of year-long forage plantations on convenient sites to improve animal production. Forage species can be grown in pure stands, harvested in a controlled way, and then fed to livestock. Where grasses are grown, livestock could be moved between the different areas of production to enable optimal use of both types of forage. Another possibility is to establish two-storied pastures, with suitable browse species over an understorey of grasses or fortes and legumes (figure 2.4).
Figure 2.3 Fodder from the forest
Figure 2.4 Eucalyptus Camalanlensis and Acacia cyanaphylla The ground was covered with grass.
2.3 Improvement in Agricultural Production
The productivity of agricultural land in arid zones is inherently low and the risk of failure is high.
This is due not only to the minimal and unreliable rainfall, but to the effect of wind and water erosion and low soil fertility, as described below:
- Wind erosion - In most of the arid zones, wind erosion is a serious problem. The destruction of the vegetative cover exposes the soil to the desiccative effects of hot, dry wind, resulting in dust storms, the formation of sand dunes, and other forms of severe wind erosion. Winds are not only responsible for the transport of soil particles, but through their desiccating effect, they prevent the growth and development of food and animal production. In irrigated agriculture, wind, by increasing evaporation, facilitates the upward movement of salts and their subsequent concentration in the rooting zones of agricultural crops. Particles of dust and sand carried by wind can be deposited in irrigation channels and drainage ditches, increasing the maintenance costs of irrigation. Such damage can be diminished by the establishment of windbreaks and shelterbelts.
- Water erosion - Erosion by water is an important phenomenon in most of the arid zones; this type of erosion is the result of the susceptibility of the soil to high rainfall intensities, and the frequent destruction of the vegetative cover. When these conditions occur, considerable amounts of soil are washed down from catchment areas. Roads are damaged, lowlands are flooded, and streams and wadis are filled with muddy water. Some of this sediment-laden water accumulates in reservoirs or is transported to lakes or the sea. The loss of water through runoff and the ensuing soil erosion can be controlled by adopting preventive soil conservation measures. The role of vegetation in reducing siltation of dams, regulating stream flow and preventing floods and soil erosion can be invaluable (Figure 2.5).
- Soil fertility - Agricultural production in arid zones is frequently hindered by poor soil fertility. However, the importance of soil fertility is often overlooked; water shortage is considered the principal constraint. Whereas the conventional method to improve soil fertility commonly consists of repeated application of mineral fertilizers, this problem may also be solved through the systematic use of soil-improving species.
3. Place of trees and shrubs in rural landscapes
To grow trees or shrubs (in any form) is a forestry practice; forestry, in turn, is a land use exercise. Pressure on land for agriculture is high in arid zones, so high that land unsuitable for agriculture is sometimes used in a desperate effort to grow agricultural crops. As a result, forestry can be relegated to lands which are too poor for plant growth.
Figure 2.5 A watershed protected against erosion by a dense vegetation cover.
There is a generally-held misconception that forestry is best suited to poor sites. However, it must be realized that forestry, like agriculture, places demands on the land to reach satisfying production levels. Two basic requirements are needed: trees and shrubs should not be confined to areas designated as "marginal", and forestry should be integrated in the overall land use (Figure 2.6).
There are several ways to plant trees and shrubs in the rural landscape, including:
- Trees in rows (windbreaks and shelterbelts) to protect crops and pastures against wind and desiccation.
- Trees intermingled with agricultural crops to protect the crops and to reconstitute and enrich the soil.
- Trees and shrubs grown during the fallow period to enrich the soil, and to provide fuel, fodder, and secondary forest products.
- Linear plantations along roads and waterways to protect infrastructures and adjacent fields, and to provide shade and contribute to the production of fuelwood, fodder and non-woody products.
- Woodlots established under rain-fed or irrigated conditions to make the best use of unused land, and to contribute to needed wood supplies.
- Intensive management of natural forests and woodlands to maintain a stable environment and yield essential products traditionally used by the local population.
- Areas threatened by sand dune encroachment can be stabilized by making use of trees and shrubs.
Within the above, it is possible to select the most appropriate combination of land uses to:
- Improve agricultural and livestock production.
- Stabilize and enrich the environment.
- Meet essential needs for fuelwood, farm timber and non-wood products for the rural population.
In general, the introduction of trees and shrubs into rural landscapes can improve the living conditions and the rural economy of the arid zone inhabitants, and contribute to rural development.
Figure 2.6 Place of trees and shrubs in the management of a mountain and a plain.
4. Combined production systems
In combined production systems, agriculture, livestock production, forestry, and combinations thereof are practiced on the same piece of land, either in rotation, simultaneously, or spatially. Such combinations, also called "agroforestry", can involve agricultural crop production or animal husbandry, within which trees or shrubs play a significant role. The basic aim of agroforestry is to attain ecological stability and, at the same time, to provide maximum short-term and long-term benefits to the user of the land.
Agroforestry in rotation involves the alternation of agriculture, livestock production, and/or forestry practices through time on the same piece of land. When combinations of these land uses are implemented at the same time on the same piece of land, agroforestry is considered to be practiced simultaneously. Where the land use practices are placed side-by-side, as they are in the case of windbreaks and shelterbelts, agroforestry is spatially practiced. All three practices are legitimate types of agroforestry, and each should be followed where most appropriate.
Depending on the land use, three "types" of agroforestry can be distinguished in arid zones; namely:
- Agrisilviculture - where the land use is agriculture and forestry production.
- Silvipasture - when the land use is forestry and animal husbandry.
- Agrosilvipasture - where the land use is agriculture, forestry, and animal husbandry.
The elements for each type are outlined in the following sections.
4.1 Agrisilviculture
Agrisilviculture is a land use system where both agricultural crops and forest products are produced, simultaneously or sequentially. This form of land use represents an improvement on the traditional system of "shifting cultivation", a method of cyclical agricultural cultivation in which farmers cut some or all of the tree crop, burn it, and raise agricultural crops for one or more years before moving on to another site and repeating the process, as illustrated in Figure 2.7.
Agrisilviculture is ecologically sound, provided that the fallow period is long enough to allow the trees to restore soil fertility. To shorten the fallow, trees or shrubs can be planted or sown instead of allowing the forest to establish itself by natural regeneration when the shifting cultivator abandons the land.
4.2 Silvipasture
Silvipasture, practiced when the land use is primarily the growing of forest products and the raising of livestock through grazing, involves controlled grazing of forest vegetation. Arid zones are generally livestock raising areas, where silvipasture is a dominant land use system. The vegetative resources of these vast low-yield areas are frequently best utilized through grazing. Under this system, the main source of fodder for livestock consists of natural vegetation, including grasses and other forage plants, and trees and shrubs.
At times, silvipasture involves the controlled grazing of forest vegetation, but it must be realized that there is a limit to the number of cattle the land can support. Proper management of the vegetation resources to prevent overgrazing is vitally important. Sometimes, introducing trees and shrubs in natural grasslands can be feasible, since (quite often) a combination of trees, shrubs, and grasses offers optimal benefits. Individual trees on grazing lands offer the additional benefit of providing shade and shelter for the animals (Figure 2.8). Animals droppings that collect under these trees can be of further benefit in enriching soil fertility.
To avoid problems of overgrazing, an effort must be made to equate the number of grazing animals to the carrying capacity of the land. Efforts to improve the grazing land's capability should be introduced at the same time, although there is no point in undertaking improvements where livestock numbers cannot be controlled.
Because periods of drought are impossible to predict and will always occur, silvipastoral systems must have a "built-in" component to handle a drought, such as the establishment of fodder-producing trees and shrubs.
4.3 Agrisilvipasture
As the name indicates, this land use system is a combination of agricultural and silvipastoral practices. The land use can be a mixture of agriculture and livestock raising, relying heavily on fodder from tree and shrub species. Agrisilvipasture should be practiced in areas that can support agriculture. Quite often, agrisilvipasture can take place in a valley, where agriculture is practiced on the valley floor and silvipasture is employed on the forest-covered slopes around the valley. Agrisilvipasture also can be practiced on the same piece of land, but not always at the same time. In some cases, fields in which trees or shrubs are growing can be farmed only during certain periods of the year, and grazed during other periods.
Figure 2.7 Examples of agrisilviculture practices.
Trees and shrubs can maintain ecological balance and, in many instances, can improve the livelihood for the inhabitants of arid zones. Also, woody vegetation acts as a soil stabilizer and prevents water and soil erosion. Woody vegetation can be a source of forage for livestock and wildlife; fuelwood, poles, lumber, and food for humans; and non-wood products such as fibers, dyes, and pharmaceuticals. Over 50 percent of the wood from the world's forests is used as fuel. Scarcity of this resource can have detrimental effects on the well-being of some populations.
Woody vegetation is important as fodder for livestock during dry seasons and years when grasses and fortes are scarce. Creating forage plantations and two-storied pastures using trees and shrubs are ways to ensure a source of fodder. Also, trees and shrubs can improve agricultural production by reducing wind and water erosion and by improving soil fertility.
Agriculture, livestock production, and forestry can be practiced in rotation, simultaneously, or spatially on the same piece of land. Agroforestry aims to ensure ecological stability and to maximize benefits to the user of the land. Agroforestry can be agrosilviculture (where agricultural crops and forest products are grown simultaneously or sequentially), silvipasture (in which forest products and livestock are grown), and agrosilvipasture (where food is grown for humans, domestic livestock graze, and woody vegetation provides wood products for humans and fodder for animals).
Figure 2.8 Trees for shade for animals.
