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Chapter V. Special forest plantations
2. Windbreaks and shelterbelts
3. Sand dune stabilization
4. Canal-side plantation
5. River-bank plantations
6. Amenity plantations
Of particular interest in many arid regions of the world are forest plantations that are established as windbreaks and shelterbelts, for sand dune stabilization, canal side and riverside plantations, and as amenity plantations. A discussion of these special forest plantations is presented below.
2. Windbreaks and shelterbelts
In arid zones, the harsh conditions of climate and the shortage of water are intensified by the strong winds. Living conditions and agricultural production can often be improved by planting trees and shrubs in protective windbreaks and shelterbelts which reduce wind velocity and provide shade. Windbreaks and shelterbelts, which are considered synonymous in this manual, are barriers of trees or shrubs that are planted to reduce wind velocities and, as a result, reduce evapotranspiration and prevent wind erosion; they frequently provide direct benefits to agricultural crops, resulting in higher yields, and provide shelter to livestock, grazing lands, and farms.
A main objective of windbreaks and shelterbelts is to protect the agricultural crops from physical damage by wind. Other benefits include:
- Preventing, or at least reducing, wind erosion;
- Reducing evaporation from the soil;
- Reducing transpiration from plants;
- Moderating extreme temperatures.
Quite often, protection can be combined with production by choosing tree and shrub species that, apart from furnishing the desired sheltering effect, yield needed wood products.
2.1 Design of windbreaks and shelterbelts
When considering windbreak or shelterbelt planting, three zones can be recognized: the windward zone (from which the wind blows); the leeward zone (on the side where the wind passes); and the protected zone (that in which the effect of the windbreak or shelterbelt is felt) (Figure 5.1).
Figure 5.1 Functioning of a windbreak
The effectiveness of the windbreak or shelterbelt is influenced by its permeability. If it is dense, like a solid wall (Figure 5.2), the airflow will pass over the top of it and cause turbulence on the leeward side due to the lower pressure on that side; this gives a comparatively limited zone of effective shelter on the leeward side compared to the zone that a moderately permeable shelter creates. Optimum permeability is 40 to 50 percent of open space, corresponding to a density of 50 to 60 percent in vegetation. Gaps in the barriers should be avoided. Permeability of dense shelterbelt can be improved by pruning lower branches at 0.50-0.8 m from the soil level (Figure 5.3).
It is generally accepted that a windbreak or shelterbelt protects an area over a distance up to its own height on the windward side and up to 20 times its height on the leeward side, depending on the strength of the wind. In reducing wind speeds, narrow barriers can be as effective as wide ones. Furthermore, a narrow shelterbelt has the advantage of occupying less land.
The shape of the cross-section of a windbreak or shelterbelt determines, to a great extent, the sheltering effect. To a large extent, the choice of tree or shrub species to plant, along with their planting arrangement, dictates the cross-sectional shape. In general, an inclined slope facing the wind should be avoided, as it only deflects the windflow upward. Barriers with a clear vertical side provide best wingspread reduction.
When designing a windbreak or shelterbelt, the direction of the wind must be considered. A barrier should be established perpendicular to the direction of the prevailing wind for maximum effect. To protect large areas, a number of separate barriers can be created as parts of an overall system. When the prevailing winds are mainly in one direction, a series of parallel shelterbelts perpendicular to that direction should be established; a checkerboard pattern is required when the winds originate from different directions. Before establishing windbreaks or shelterbelts, it is important to make a thorough study of the local winds and to plot on a map the direction and strength of the winds.
2.2 Selection of tree and shrub species
In the selection of tree or shrub species for windbreaks or shelterbelts, the following characteristics should be sought:
- Rapid growth;
- Straight stems;
- Wind firmness;
- Good crown formation;
- Deep root system, which does not spread into nearby fields;
- Resistance to drought;
- Desired phonological characteristics (leaves all year long or only part of the year).
Figure 5.2 A dense shelterbelt made of Acacia cysnophylla and Opuntesi pinus indica
Figure 5.3 Permeability of a dense shelterbelt of Parlinsonia oculeata and Azadiracheta indica improved by pruning lower branches.
2.3 Planting techniques
Planting techniques for windbreaks and shelterbelts are identical to those in other tree and shrub planting programmes. However, as windbreaks and shelterbelts require a high plant survival rate, as well as uniform and rapid growth, supplementary irrigation may be required during the establishment phase. Gaps cannot be tolerated and, when plants are lost, replacement must be prompt.
Although in theory, one-row barriers should suffice, experience has shown that the most effective windbreaks and shelterbelts are those consisting of several rows of trees. Quite often, initial spacing is 3 meters between the rows, with trees 2 meters apart in the rows. Where trees or shrubs have long roots that could extend into agricultural fields, vertical root pruning may be recommended; this can be done with special equipment or by digging trenches. A triangular arrangement of plants is frequently prescribed.
2.4 Management practices
Once established, the effectiveness and longevity of a windbreak or shelterbelt depends on its maintenance. As the trees and shrubs mature, they change in shape and appearance, which necessitates some level of maintenance to ensure a continuing shelter effect. Pruning may be required to stimulate height growth, while thinning can boost diameter growth. To keep a barrier at the desired density and permeability, occasional pruning or removal of plants may be necessary. If trees or shrubs are damaged by wind or pest attacks, a control is also needed. In all of these cases, the management practices depend on the desired composition of the barrier and the species used. Since these management practices can involve the removal of woody parts, the use of tree or shrub species that make fuelwood or fodder available on a continuous basis is desirable.
A windbreak or shelterbelt has a life that is dependent on the trees or shrubs of which it is composed. Therefore, to be able to furnish permanent shelter, a renewal plan should be adopted. To renew a barrier consisting of many rows, felling the rows on the leeward side and then replanting them is often recommended. If the windbreak or shelterbelt consists of one row, a new row may be planted parallel to the the old one; when the new row has matured, the old one is removed. To renew narrow windbreaks or shelterbelts arranged into a system, new belts can be planted midway between the existing barriers which, in turn, are to be removed when the new ones become effective.
When windbreaks or shelterbelts are established on grasslands or other areas where animals are allowed to graze, special attention must be paid to the protection of the barrier; this can be done by planting thorny vegetation or by using a barbed-wire fence along the edges of the barrier.
3. Sand dune stabilization
Sand dunes result from wind erosion. They are formed in many arid lands when winds regularly blow over poorly-vegetated areas. Sand dunes that are not covered with vegetation (because of overcropping or overgrazing) move in the direction of the wind at a speed which can approach 10 meters a year, endangering agricultural crops, forest plantations, irrigation canals, and roads (Figure 5.4). To prevent this encroachment, the sand dunes must be stabilized; one method of sand dune stabilization is to establish a vegetative cover.
In general, two types of sand dunes are recognized: coastal dunes and inland dunes. Techniques of stabilizing these two types of sand dunes through the establishment of a vegetative cover are discussed below.
3.1 Stabilization of coastal dunes
Coastal dunes originate from sand thrown up onto the shore by waves. At low tide, the sand dries and is blown away by the wind. When protective vegetation beyond the beaches is destroyed, coastal dunes move inland. To stop the advancement of coastal dunes, an artificial foredune should be constructed about 50 meters from the floodline. Normally, this initial barrier is built one year before a planting programme begins.
One method of building a foredune is by mechanical fixation of the sand by fences or palisades, 0.5 to 1 meter high. The materials used for the fences or palisades may include twigs from trees or shrubs, brushwood, grass sheaves, reeds, bushes, palm leaves, old railroad ties, used oil drums, and earth. When the prevailing wind has a prevailing direction, parallel lines of palisading are sufficient; however, a checkerboard system is advisable where fluctuating winds are common (Figure 5.5). Sand piles up behind the palisade and, when the artificial dune that is formed reaches a height of 0.5 to 0.75 meter, a second palisade is built on top of it. Sometimes, the original barriers can be raised, when necessary, instead of building a new palisade. Once the foredune is established, it is possible to stabilize the sand behind it by seeding or planting a vegetative species that provides good ground cover and is able to withstand (at least partially) covering by sand.
Figure 5.4 Dunes invading agricultural land (YAR)
Figure 5.5 A checkboard system of palisading.
Sand dune fixation also can be done by mechanical mulching; that is, the spreading of solid material on the surface of the sand. Chemical fixation can also be employed. Chemical fixation consists of stabilizing the sand surface by covering it with a continuous crust of sprayed chemical substances, such as petroleum derivatives or latex mixtures. Vegetative establishment is usually a follow-up or a concurrent operation. Chemical fixation is advisable when the cost of labour is high and the chemicals are readily available.
Sand dune stabilization with plant species is more permanent than mechanical mulching and chemical fixation techniques which are, in most cases, only temporary measures.
3.2 Stabilization of inland dunes
Inland dunes originate from sand produced by the weathering of rocks, mainly sandstone. The fine fraction can be blown far away, while the heavier fraction is blown short distances and forms dunes. Such dunes can pose serious stabilization problems, especially when the dunes are large and active. One way to combat this problem is by creating an artificial dune at the windward end of the dune. The method followed is similar to the one used to create the foredune in the stabilization of coastal sand dunes. The stabilization of inland dunes also follows the same general lines as mentioned for coastal dunes.
When a specific area of value (for example, an oasis) is threatened, protective work is initiated as close as possible to the area of concern, with the work gradually progressing toward the sand source area.
3.3 Planting techniques
As mentioned above, planting of vegetation is the best and most permanent method of coastal and inland sand dune stabilization; both direct and indirect benefits can be realized, including:
- Protection (of roads, canals, agricultural lands, and industrial areas);
- Wood production (fuel, lumber, etc.);
- Protection of watershed areas and water supplies;
- Livestock grazing benefits (including fodder);
- Wildlife benefits, recreation, and other amenities;
- Public works to combat unemployment.
The choice of vegetative species for planting should be based on studies of the natural vegetation in the area and on the environmental conditions. As planting of vegetation on sand dunes frequently consists of afforestation practices, it is recommended that species trials be included in the planting programmes to permit an evaluation of tree and shrub species for long-term use.
In practice, it is often necessary to plant relatively large containerized plants close together (1 x 1 meter) on the windward side, but they can be planted further apart (2 x 2 meters) on the sheltered side. Irrigation for initial establishment may be required to help the plants survive until they have sufficiently deep root systems. If water is not available in adequate quantities for irrigation to take place on a long term basis, it is advisable to irrigate (at least) during the first two or three months after planting, at weekly intervals.
Concerning maintenance, hand weeding is preferred to avoid problems of machinery traction in the sand. As a rule, all livestock movement and other traffic should be eliminated on the sand dunes; when necessary, delimited and protected passages for livestock can be established.
4. Canal-side plantation
In may arid countries, wherever rivers are available, efforts have been made to utilize the water for irrigation purposes through the construction of dams or using lift irrigation for the agricultural needs. Several thousands of kilometers of irrigation canals have been laid. The banks of such canals are available for planting purposes and constitute a considerable area for production of timber and firewood for the rural population. Full advantage is being taken of this in many countries like China, Egypt, India and Pakistan. A few rows of trees, varying from 4 to 6, are generally planted on each bank of the canal with an espacement depending on the characteristics of the species and the type of produce desired (Figures 5.6A and 5.6B).
When designing a canal plantation, the requirement may be the same as for the design of irrigated plantations with respect to climatic and soil conditions and to supply and quality of water. However, it should be remembered that the only water supply available to the trees is seepage from the canal into the root zone. In some places, it is cheaper to grow trees and thus utilize the seepage water rather than prevent seepage by canal linings of concrete, asphalt or other material.
Choice of species for canal side plantations should take into account both the particular character of the plantation and its purpose. The roots of the trees should strengthen the banks of the canal and the trees should keep the canal and its banks well shaded in order to suppress weed growth and reduce evaporation. Species that tend to increase water seepage through the sides and bottom of the canal should be avoided. Where canals have an intermittent flow, such as flood discharge canals, only trees able to adjust to varying water levels in the soil can be used.
Species that reproduce by suckers such as Robinia pseudo acacia should not be planted along canals. Plantation techniques should favor deep planting and roots should be planted in the moist layer.
5. River-bank plantations
There are many areas where river lengths are considerable. The ground on either side of the river is partly within the reach of the high level of water during the period the rivers are in flood. Beyond this level - and on the fringes of the agricultural land, strip plantation can be established to produce wood, fuelwood and fodder. Generally, the width of such strips is limited but does constitute a useful and productive linear plantation. Underground water is available at different levels. The species to be planted should be matched with this water level variation. Spacing within and between the rows depends on the characteristics of the species and the rotation planned for the crop. In the more arid areas, trees with xerophytic habit constitute the outermost rows while those close to the river bank are the ones with higher water requirement. In such locations, phreatophyte species such as Populus spp., Acacia nilotica, Dalbergio sisso, Prosopis spp. can be planted.
6. Amenity plantations
This type of plantation includes trees planted in gardens and parks, street planting, green belts around villages and cities, trees planted along roadsides to reduce noise and beautify the homestead or landscape. In arid zones "beautifying the landscape" usually means changing the countryside from its normal brown color to green, or "greening" of the landscape.
6.1 Tree planting in gardens
Tree planting in gardens is usually regarded as a beautification of the home environment but it also has a profound effect on man's psychological attitude toward life. A house located in a barren landscape without trees and shrubs lacks appeal and there is certainly a different psychological attitude towards it, compared with a home which has been beautified and protected by wisely selected trees and shrubs.
Tree planting in gardens also enhances self-esteem. The gardener identifies with his garden and builds a personal relationship with it.
Figure 5.6 A. Canal side planting
Planting along an irrigation canal
The garden becomes an extension of himself, a visible representation of his individuality. When it blooms, he has evidence of his success. He also becomes aware that a number of people he does not know pass by each day and enjoy his garden. He has given them an anonymous gift. All of this enhances his self image, helping to create self-esteem. The gardener, feeling better about himself, feels better about where he lives.
In selecting species for tree planting in gardens, the following should be considered:
- Trees have several functions to play in plantations around houses. The first point to be considered is therefore to decide on the purpose for which planting is to be done, that is: shelter, shade, ornament, hedges, scent and odors, as source of fruits or nesting sites for birds. Some plants will fill only one or two of these objectives while others may fill more.
- Trees selected should be suitable to local climate and soil conditions.
- Adaptability to pruning. The degree to which trees and shrubs will tolerate pruning and pollarding is an important consideration for planting around the homestead.
- Evergreen or deciduous habit: most arid-zone species are evergreen but many deciduous trees are also used for planting in gardens. In the Mediterranean arid region, deciduous trees have certain advantages and disadvantages when compared with evergreens. The most important advantage is their ability to provide shelter in summer but allow sunlight to penetrate in winter. The most important disadvantage is that the leaves need to be gathered during and after leaf fall.
6.2 Planting in parks
The first objective for planting in parks in arid zones is for shelter from the sun and dust. Where trees are not growing naturally, shelter may be provided by planting trees in favorable sites. A second objective is to enhance the beauty of the park by planting trees of different color, shape and size. Planting formal layout patterns of trees in long straight rows does not usually fit into the landscape pattern, and planting in small clumps or as individual dispersed trees is usually more suitable. Choice of species should not be dictated by the value of trees for wood, nor planting expenses weighed against the value of the wood produced. Costs for planting in parks for recreation should take into account aesthetic benefits and the intangible gains in the health and well-being of the people.
6.3 Street planting
Street plantings are often the responsibility of local municipalities and are made to beautify the cities, provide shade and control outdoor noise and traffic pollution (Figure 5.7A and B). In recent years, many of the towns and cities in the world have learned through experience that paving roadways, streets, and sidewalks does not complete the job. In most communities where there has been an increase in population and greater congestion of motorized traffic, it has become important that the city municipality assumes its responsibility of providing the amenities which add much pleasure to life in the city.
For use in street planting, trees should:
- be easy to establish, preferably with the ability to be transplanted as advanced nursery stock, and grow relatively quickly to the stage that they provide some amenity value;
- be healthy in the environment, relatively long-lived and not subject to wind-throw or the breakage of large limbs;
- be as maintenance-free as possible. Trees requiring permanent pruning and removal of fallen leaves will have a high maintenance cost.
- the form and height of the species must be suitable for the width of the street in which they are to be planted;
- whether to use a single variety or mix on the same street is a matter of taste. A variety might be used depending on the geographic section of the country and the width of the street.
6.4 Greenbelt planting
Several cities in arid zones have established in their municipal area green belts with a number of purposes:
- to enhance the beauty of the site;
- to provide a recreation area for the urban dwellers;
- to reduce the harmful effect of dry winds and dust storm and control sand encroachment.
There is a wide array of trees and shrubs for greenbelt planting. Any programme of this sort, however, should be well designed in advance, planned for a number of years and carefully implemented.
Figure 5.7 A. Planting in city.
Figure 5.7 B. Planting in industrial unit to reduce pollution.
6.5 Roadside plantations
Roadside plantations have several objectives:
- trees increase the comfort of travellers by providing shade and attractive surroundings;
- trees may protect the road itself against moving dunes or act as a windbreak for adjacent fields.
- trees may become an important factor by alleviating timber and fuelwood shortage. In fact, roadside trees are frequently considered a part of the national forest planting programme. Such trees may produce edible fruit, yield pods for feeding animals, furnish food and shelter for birds or, when in bloom, be valuable in beekeeping.
Species should be carefully chosen. Among the important factors which should be considered in planning the use of trees along highways are (Figures 5.8 A and 5.8 B):
- selection of the species for hardiness, longevity, freedom from windthrow and breakage, attractive appearance and minimal maintenance. In the arid zone there are a large number of native small trees and large shrubs which can be used, as well as some exotic species. Where the environment is suitable, consideration should be given to small patches of deciduous species with colorful foliage.
- suitability of the species to the climate, topography and soil.
- location of the trees in relation to road formation. Firstly consideration should be given to the existing road formation so that trees ar not planted close to the inside of curves or near road junctions where they could obscure vision and so create a driving hazard. Secondly, consideration should be given to the possibility of the future widening of roads, including the development of double traffic lanes.
6.6 Rail-side plantations
A trend toward rail-side planting for the provision of greenery, protection from dust and winds and creation of additional tree resources has developed in recent years in many countries. This trend is likely to spread to other countries due to the favourable results already achieved in certain countries. Three to six rows of trees on either side of the track are considered useful. The planting techniques are similar to those for roadside planting. Species vary and depend on the prevailing climatic conditions, mainly temperature, soil and rainfall.
Figure 5.8 A. Road side planting: Eucalyptus
Figure 5.8 B. Road side planting (one row of Populus nigra)
Under very arid conditions, the choice of species is rather limited. Where water is available, several species can be selected. Within an area each row can be given over to one or more species. Mixtures are thus created and are considered better as they yield different produce to meet local needs.
Species generally used for rail-side planting are:
- Arid areas: Tamarix spp, Eucalyptus spp, Acacia spp, Opuntia ficus indica, Prosopis spp;
- Semi-arid areas: Eucalyptus spp, Avaga spp, Acacia spp, Ficus spp, Sapium spp, Bauhinia spp.
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