WINDSBREAK of trees and/or shrubs can substantially improve productivity of farmlands which are exposed to strong winds. It reduces the speed of wind conserving soil moisture for growing crops and decreases the energy available to cause soil erosion.In hot climates, windbreaks may also provide some beneficial shading of field crops. When properly planned and managed, windbreaks produce large quantities of wood products and refuge for wildlife.
WINDSBREAK of trees and/or shrubs can substantially improve productivity of farmlands which are exposed to strong winds. It reduces the speed of wind conserving soil moisture for growing crops and decreases the energy available to cause soil erosion.
In hot climates, windbreaks may also provide some beneficial shading of field crops. When properly planned and managed, windbreaks produce large quantities of wood products and refuge for wildlife.A properly designed farm windbreak can take up as much as 50 per cent of land under cultivation without causing any net reduction in crop yields. This is because fields under the protection of windbreaks have higher per acre yield than unprotected fields. The kind of windbreak to be planted on any farm depends upon the geographic location of the farm with respect to the expected severity of winds, the kinds of the soils available, the kinds of trees and shrubs best suited or preferred by the farmer, how much space is available and whether the land is irrigated or is rain-fed.
Windbreaks disturb the normal pattern of wind movement because they are a vertical screen to the free movement of wind. The leeward side of a windbreak is expected to be the place where wind speed is reduced. But there is also reduction of velocity of wind on the upwind side of windbreaks. This is because the air is baffled and mixed as it hits the windbreaks, causing a disruptive turbulence patterns which spoils the energy and velocity of the wind. This mixing and churning continues as the pressure gradient is built by the windbreak and the air leaks through or rolls over the top of the screen.
Some of the air mass passes through the tops of the trees at increased speed providing a high pressure airfoil which forces the air upward over the tops of the trees. This air cushion continues to carry the air mass down wind until it gradually approaches original velocity. For traditional multiple row windbreaks with two or three rows of tall in the centre and rows of progressively shorter trees on both sides, this is a point down wind which is equal to 50 times (50H) height of the windbreak. In this situation, some reduction of ground level wind velocity can be detected upwind of the barrier for a distance to 10 times height.
The best windbreak approach is 100 per cent crown density in the low profile shrub, rows and 50 per cent density in the tall rows. In field application, it is common to attain this ideal crown density and texture of the trees. The downwind effect is normally expected to reach only about 12 H downwind and 2H upwind.
A windbreak with 50 per cent crown density is more effective than one of either 100 per cent or of 25 per cent density. The acceleration of air through the opening in the crown provides the high pressure cushion needed to support the air which tumbles over the top. When the barrier is 100 per cent density like a stone wall the up wind builds up with high pressure air boils over the top of the barrier, its volume and turbulence force it back to ground level with increased velocity. In the case of windbreaks with less than 50 per cent crown density, the effectiveness is reduced because too much of the air leaks through under pressure.
If crop yields are carefully measured at various points on a line perpendicular to the center of the windbreak, by calculation, it can be shown that the potential yield for most of the crops will be two to 10 per cent greater of the field.
Some trees and shrubs have shallow, wide spreading root systems which compete directly in the root zone of the field crops for water and nutrients. In irrigated fields, this is not a significant problem, but in barani fields it can be serious. A few years after the trees are established, the shallow rooted species can be prevented from causing root competition by digging a trench alongside the trees and to a depth of about 20 inches. To speed up this process, a standard single subsoil ripper mounted on a tractor, can adequately do the job of cutting these roots and preventing the sapping of fields. If the soil is hard, the roots large or the available power to pull the tool is limited; remember that it is not necessary to cut the full 20 inches depth in one pass.
Windbreaks are especially valuable for protection of high value orchard and vegetable crops. Wind reduction significantly improves the efficiency and safety of application of pesticides on such areas and can save substantial amounts of expensive chemicals.
After proper design and establishment of the windbreak, the farmer needs to pay particular attention to a few key management needs:
Grazing Protection: Domestic livestock, especially goats, need to be kept out of the windbreaks. They especially damage the trees and shrubs which are used in the low level protection layer of the windbreak.
Fire protection: Woody plants are easily damaged by fire. Even though the plants may not be killed, the usual effect of fire is to reduce the density of the low level foliage of plants. This will destroy the ground level wind control which is so essential to the continued function of the windbreak. Litter will build up rapidly under the windbreak and may need to be swept out occasionally to reduce the hazard of fire. These leaves and twigs may be used on the adjacent field as traditional organic matter for the soil or may be taken away for fuel.
Pruning: Limbs and branches of windbreak plants should never be pruned. Even dead branches have some beneficial effect in the reduction of wind.
Harvesting: The tall trees in windbreaks can be periodically harvested for wood and timber products, but only after the low profile trees in the outer row have attained enough height to maintain the windbreak effect while the tall tree rows are being replaced by replanting. At the time of harvest, be sure to take all the trees in one row at the same time to maintain the uniformity of the windbreak profile.
The low profile rows should not be harvested until the trees have reached such size and maturity that they have started to lose lower branches and become open at the bottom.
Insect and disease: Most windbreaks are relatively free of serious insect and disease pests. If the proper species are planted for the site and growing conditions, trees grown on field borders have the advantage of more free growing space and will be more vigorous and pest resistant than trees grown in closed stands. Another advantage especially in the case of windbreaks around fruit orchards is that the regular spraying schedule for orchard pest control usually provides additional protection to the windbreak trees.
Irrigation: Windbreak trees will perform best if they receive some irrigation at the same time the adjacent field crops are irrigated. Farmers should be advised that tree roots are especially damaging to the structure of irrigation ditches and that it is a risky practice to plant the trees in the bottom or on the bunds of the permanent irrigation channels.
Sericulture: In those cases where mulberry is used as the shade tolerant low profile row, it may be suitable to harvest leaves for sericulture. This harvesting should be limited to the leeward foliage of the tree and should not be done on the exposed windward side of the plants.
By Muhammad Ahmad, Ahmad Faraz and Erum Baloch