Windreaks are arriers used to reduce and redirect wind - yimgcom

By Connie Morris,2014-12-26 04:19
8 views 0
Windreaks are arriers used to reduce and redirect wind - yimgcomWindre



     Windbreaks or shelterbelts are linear plantings of single or multiple rows of trees or shrubs that may also include perennial or annual crops and grasses acting as barriers to slow down the velocity of the wind and redirect it up off the land. Windbreaks are ―environmental buffers‖ that are planted in a variety of settings, such as on cropland, agroforest, pasture, livestock farm, rangeland, along roads, farmsteads, and in urban areas.


    ; Provide shield for structures, crops, livestock and people. ; Supply tree fruits, nuts, fuelwood, timber, handicraft materials and specialty woods, medicinals and botanicals, fodder, green manure, and other useful secondary tree and shrub materials and products for personal use and for sale thereby enhancing revenue flow. ; Modify the environmental conditions or microclimate inside the sheltered zone which result in improved livestock production and plant growth.

    ; Contribute to global warming mitigation by sequestering carbon dioxide gases in the air and store them as carbon in biomass and soils. ; Improve air quality by reducing and intercepting air borne particulate matter (including plant pathogens), chemicals, and odors. ; Reduce soil erosion from wind.

    ; Conserve soil moisture.

    ; Protect plants from wind related damage.

    ; Enhance biodiversity from microscopic biota such as fungi and soil microbes to invertebrates such as beneficial insects and vertebrates such as birds, mammals and other wildlife and various beneficial flora and fauna.

    ; Provide noise screens.

    ; Provide visual screens.

    ; Delineate property and field boundaries.

    ; Improve irrigation efficiency.

    ; Add aesthetic beauty to the landscape.

    ; Increase the value of the property.

     While windbreak reduces area for crop production, the various economic and environmental benefits derived from it far outweigh the area reduction since the windbreak itself is diversified and productive not to mention that it results in a net increase in total crop yield and crop quality of the sheltered area. Moreover, while the windbreak trees and shrubs are still young and not yet productive, the area can be planted to crops and when the trees start to cast shade, it can still be used to plant shade tolerant plants and culture mushrooms. Taking advantage of these many benefits, the net economic return will be higher, input costs will be reduced and environmental conditions will be significantly improved.


     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)

     As wind blows against a windbreak, air pressure builds up on the windward side (the side toward the wind), and decreases on the leeward side (the side away from the wind). Some of the approaching wind flows through the windbreak, some goes around the ends, but most of it is forced up and over the top of the windbreak. Windbreak structure---height, density, number of rows, species composition, length, orientation and continuity--- determines which path the wind will take, and as a result, determines how effective the windbreak will be in reducing wind speed and changing the microclimate. The efficiency of a windbreak is determined by factors such as height, continuity and length of windbreak. Windbreak height (H) is the most important factor determining the distance downwind protected by a windbreak. Although the height of the windbreak determines the extent of the protected areas, the length times the extent determines the area receiving protection. For maximum efficiency, the uninterrupted length of the windbreak should be at least 10 times its height.

     The continuity of a windbreak also influences its efficiency. Gaps in a windbreak become funnels that accelerate wind flow, creating areas on the downwind side of the gap in which wind speeds often exceed open field wind speeds. Where gaps occur, the effectiveness of the windbreak is diminished. Lanes or field access should be located at the end of a windbreak. If a lane must go through a windbreak, it should be located such that the opening is at an angle to the problem winds.

     On the windward side of a windbreak, wind speed reductions are measurable upwind for a distance of 2 to 5 times the height of the windbreak (2H to 5H). On the leeward side, wind speed reductions occur up to 20H downwind of the barrier. For example, in a windbreak where the height of the tallest tree row is 20 feet, lower wind speeds are measurable for 40 to 100 feet on the windward side and up to 400 feet on the leeward side.

A well-designed windbreak The prevailing wind is slowed down at ground level, but wind is still able to pass through the trees. Stronger air currents stay high above the trees. Source: Rocheleau et al., 1988

     A poorly-designed windbreak If wind is blocked completely, it will cause strong air currents over the land that should be protected. These can damage crops and promote erosion. If there are gaps in rows of trees, the wind is funnelled through them at higher speeds, resulting in more soil erosion. Source: Rocheleau et al., 1988

     The magnitude of wind reduction at any location in the protected zone is determined by the structure of the windbreak. Windbreak structure is made up of 2 components: the internal structurethe

    amount and arrangement of the solid elements and open spaces; and the external structurethe cross-sectional shape of the windbreak.

     The internal structure is simply described in terms of density. Windbreak density is the ratio of the solid portion of the barrier to the total area of the barrier. As wind flows through a windbreak, the trunk, branches and leaves (the solid portion) absorb some of the momentum of the wind and wind speed is reduced. In addition, as wind flows over the tree surfaces, it is slowed by the roughness of the surface and wind space is reduced. Together, these two processes help determine the amount of wind speed reduction that occurs.

     By adjusting windbreak density, different wind flow patterns and areas of protection can be established. Around very dense windbreaks, air pressure builds up on the windward side and a zone of low pressure develops on the leeward side. The windward air pressure pushes air through and over the windbreak, while the leeward low pressure area behind the windbreak pulls air coming over the windbreak downward, creating turbulence and reducing protection downwind. As density decreases, the amount of air passing through the windbreak increases, moderating the pressure differences between the windward and the leeward sides and reducing the level of turbulence created by the dense windbreak. As a result, the extent of the down-wind protected area increases. While the extent of this protected area is larger, the wind speed reductions are not as great as those leeward of the more dense windbreak.

     The species used and their arrangement, the number of rows and the distance between rows, and the distance between trees are the main factors controlling windbreak density. Increasing the number of windbreak rows and decreasing the distance between trees increases density and provides a more solid barrier to the wind.

     The interaction of height and density determines the degree of wind speed reduction and, ultimately, the extent of the protected area. A windbreak density of 40% to 60% provides the greatest downwind area of protection. If density is below 20%, the windbreak does not provide useful wind reduction. If density is above 80%, excessive leeward turbulence may reduce the windbreak effectiveness.

     The external structure or cross-sectional shape of a windbreak is determined by the width, height and arrangement of the individual tree and/or shrub rows within the windbreak. The cross-sectional shape of windbreaks with similar internal structures has minimal influence on wind speed reductions within 10H of the barrier. Beyond 10H, windbreak with a vertical windward side tend to provide slightly more protection than those with slanted windward side, because more wind passes through the barrier reducing turbulence and extending the protected area farther to the lee.


    The size and shape of the protected zone are controlled by windbreak design and placement. Factors that should be considered when designing a windbreak include:

    1. DENSITYWindbreaks that are dense let very little wind through. This results in a low pressure zone on the downwind side of the windbreak. This low pressure causes the wind going over the top of the windbreak to come down to the ground sooner. The result is a protected zone that is fairly calm but that does not extend very far downwind. Windbreaks that are more porous let some air through, reducing the low pressure zone. The result is a larger protected zone that is not quite as calm as with a dense windbreak. Very porous windbreaks let too much wind through to be effective. Some porosity can be achieved by planting fewer rows, spacing trees farther apart within rows, and using tree species with less dense crown. 2. WIND ORIENTATIONWindbreaks protect the widest area when

    placed at right angles or perpendicular to the prevailing wind direction. Wind tends to curl in around the ends of a windbreak, so windbreaks should extend beyond the area or object to be protected. 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. 3. WINDBREAK PROFILEThe cross-sectional shape of a

    windbreak affects how wind is directed up and over and thus affects the protected area. Windbreaks that start short and become tall (going in the downwind direction) act like dense windbreaks. Most wind is deflected up and over and very little gets through to reduce the low-pressure zone on the downwind side. Wind comes back to ground level fairly soon and the protected area is short. Windbreaks that rise abruptly on the upwind side tend to force some air through, resulting in a larger protected zone.

    4. THICKNESSWindbreaks with more rows tend to be denser. This leads to a very calm zone directly downwind but a short zone of protection.

    5. HEIGHTSince windbreaks protect for 10 to 20 times their height (H) downwind, taller windbreaks will protect a larger area. The zone of protection will extend farther as the trees grow. An area can be completely protected by short or tall windbreaks, but more of the short windbreaks will be needed since they protect for a smaller distance.

    6. GAPSGaps in windbreaks due to dead trees or for road access funnel wind and cause reduced protection. Replace dead trees and angle any planned openings through the windbreak at right angles to the prevailing wind direction.

     If the wind blows from east to

    west, then you should plant the

    windbreak from north to south,

    on the eastern side of the land.


     A successful windbreak requires several basic steps: (1) selection and ordering of plant species, (2) proper preparation of planting site, (3) suitable planting techniques, and (4) proper care after planting.


     Plants must be matched with local conditions, including soil types, specific site problems and climate. Select at least six and preferably eight or more tree and shrub species for each shelterbelt. Remember, a shelterbelt with an array of plant species will attract a greater variety of wildlife and have a better chance of surviving a wide range of environmental conditions.

     Tree or shrub species that must be selected for windbreaks should:

    ; tolerate harsh environments;

    ; have many uses;

    ; live long;

    ; grow quickly;

    ; have straight firm stems;

    ; be resistant to drought;

    ; have a bushy, deep crown but that still allows some wind


    ; keep lower limbs for a long time;

    ; have strong roots;

    ; tolerate pests and diseases;

    ; not harbour pests that affect nearby crops;

    ; have deep root systems that do not spread to nearby field to

    compete excessively with nearby crops for water and


     In ordering plant species from nursery, thorough inspection of the seedlings must be conducted. The seedlings must be healthy and do not harbour any disease. They must manifest vigorous growth in stem, branches, leaves and roots. Reject damaged and unhealthy seedlings. Seedlings planted in thin soft flexible containers such as the usual black plastic containers are preferable since they can be easily slit causing little damage to the root system when transplanted.


     After identifying the proper site for the windbreak, the preparation of the planting site comes next. Preliminary groundwork must be carried out to ensure the survival of the tree and shrub seedlings. Soil analysis of the land must be done to know the appropriate soil amendments that must be incorporated into the soil. The soil profile and soil structure are important soil properties that must be ascertained so that suitable plant species can be selected or appropriate measures can be taken to modify the soil properties.

     Clearing of the site and the surrounding areas is of utmost importance to prevent unwanted weeds and harmful insects from proliferating which will surely be detrimental to the initial growth of seedlings.

     When all essential factors for the proper establishment of the

    windbreak have been taken into account, it is now time to layout the dimensions of the windbreak, including the spacing within and between rows. The layout must be customized for the positions of structures inside the farm, the field boundaries, utility lines, irrigation canals, and other features of the site.


     As windbreaks require a high plant survival rate, as well as uniform and rapid growth, adequate water is critical during the establishment phase. Sources of water must be made certain. 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. 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

     Plant at least three rows to achieve good wind protection the

    greater the windbreak‘s density, the greater the reduction in wind velocity. Shrubs or shorter trees can comprise outer rows, while tall trees (the foundation of the windbreak) should make up at least one inside row. Do not use the same species of plant in every windbreak row; diversification of planting rows increases insect and disease resistance and enhances wildlife habitat. Different tree types should not be alternated or mixed within a row (to avoid plant overtopping by faster growing species).

     In transplanting tree seedlings, the following guidelines and techniques must be followed to avoid transplanting shock and to ensure survival:

    1. Clear away the grass and weeds by uprooting them 1

    meter around the spot where the tree seedling is to be

    planted. Grass, weeds, and other unwanted plants will

    compete with the tree seedling for soil nutrients,

    water and sunlight. Insects and other pests will be

    discouraged and can easily be controlled in a cleared


    2. Do not allow seedling to dry out from nursery to

    planting. Ample moisture is the key factor in seedling


    3. Transport seedling carefully and avoid extreme

    temperature such as excessive heat to prevent damage

    to the root system and trauma to the whole seedling.

    4. Plant immediately. Minimize transport and storage


    5. Dig a hole 30 cm. deep and 45 cm. wide to loosen the

    soil below and around the spot to give ample space so

    that many roots will be kept intact especially and

    primarily the taproot: the comparatively long, thick

    root extending from the trunk. This root gives your

    seedling the strongest chance of survival.

    6. Be sure that the soil is moist but not too wet. Too wet

    soil will clog up the soil and rot the roots of the

    seedlings due to reduced air in the soil.

    7. When the hole is ready, water the seedling. If the

    seedling is in a container that will decay, just slit the

    sides of the container from top to bottom with a knife.

    Then plant the whole thing in the planting hole. If the

    container won't rot right away remove the seedling

    very carefully. Don't disturb the soil mass around

    the roots. And better do the transplanting in a

    cool, cloudy day or preferably in the late

    afternoon when the sun is beginning to set down.

    8. Carefully and gently place the seedling in the middle

    of the hole planting it not too deep or too shallow.

    Before backfilling, have someone view the tree

    seedling from a distance to see if the seedling is

    properly positioned. Backfill the hole with the

    excavated topsoil and compact the earth firmly but

    not too hard to hold the seedling in place. Put stakes

    to support the tree if needed.


     After transplanting the tree seedling, proper care and maintenance

     must be observed to ensure high survival rate for the seedling. The

    following important care and maintenance practices must be applied: ; The seedling must be watered regularly or as the need

    arises. Do not allow the soil to dry up for this will kill

    the young seedling. It is better to water heavily once

    or twice a week for the roots to penetrate deeply.

    Frequent light watering will only encourage roots to

    spread on surfaces of soil or slightly below it. ; Weeds, grasses and other unwanted plants around the

    seedling must always be uprooted since they will

    compete with seedling for water, nutrients, sunlight

    and space and if allowed to spread can kill the

    seedling. Uprooting weeds and unwanted plants

    could cut the time for seedling to reach mature height

    by 50%.

    ; To prevent weeds and other grasses from

    regenerating in the seedling area and to reduce

    frequent watering during hot season, a three to four

    inch mulch can be applied in a three foot diameter

    around the seedling but don't let the mulch touch the

    seedling trunk. Mulch is very effective in controlling

    weeds and conserving moisture. Moreover, the mulch

    encourages microbial activity in the soil and

    eventually adds organic matter and nutrients in the

    soil when it decays. It also gradually improves the

    structure of the soil. Fertilizer is not needed and

    should not be applied when transplanting the


    ; Encircle the seedling with tube wire mesh or fence it

    with bamboo sticks or other wood branch cuttings to

    prevent animals from harming the seedling or anyone

    from accidentally trampling on the seedling.

    ; Look for any sign of dying seedling such as wilting,

    yellowing and shedding of leaves and immediately

    replace it with a new seedling so that there will be a

    uniform growth of trees and no gap will occur in the


    ; 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 attack, 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.

     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 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.



     It is not enough that we plant any tree or shrub for windbreaks. We must choose the tree and shrub species that we must plant. The planting materials must have many uses and products to enhance the productivity and profitability of the farm. Because trees have long growing period, they must be construed as "savings account" of the farmers that will yield revenue and interest in the future. Once the trees become productive, the principal and interest will start to flow into the pockets of the farmers and their descendants or, in other words, the fruits of their labour will be

Report this document

For any questions or suggestions please email