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Windbreaks (also termed shelterbelts) are barriers to reduce wind velocity and create a relatively sheltered area.
- 1 Article Scope
- 2 Terminology & Background
- 3 Types
- 4 Aerodynamic Modelling of Windbreaks
- 5 Microclimate: The Quiet Zone
- 6 Advantages
- 7 Disadvantages
- 8 Design considerations
- 9 Species Selection
- 10 Secondary Uses
- 11 Large Scale Examples
- 12 References
- 13 Further reading
- 14 See also
This article discusses considerations in windbreak design. The main focus is windbreaks to create a favorable microclimate for a temperate climate forest garden. This situation is one of the main reasons windbreaks are relevant to sustainable agriculture in the West, particularly in view of climate change. However windbreaks have a wider relevance in industrial agriculture settings such as agroforestry and animal husbandry, and the technique can be easily applied to other climates.
Terminology & Background
- Wind - the movement of atmospheric gases on a large scale. Generally air moves from an area of high atmospheric pressure to an area of low pressure. Differences in air pressure are caused by air rising creating low pressure or air sinking causing high pressure.
- Upwind (Windward) - the direction the wind is blowing from.
- Downwind (Leeward) - the direction the wind is blowing to.
- Winds are usually named according to their direction of origin. For example wind blowing from east to west is termed "Easterly".
- Prevailing wind - the direction which wind tends to originate over a period of time for a particular location (e.g. the mean annual wind direction).
- Laminar flow - the smooth flow of air. The opposite of laminar flow is turbulance or turbulant flow which is a rougher, more chaotic movement with eddies (swirls).
Prevailing Wind: Global Influences
Global wind patterns are created by 2 main factors: 1. the uneven heating of the surface of the Earth (Tropical regions are heated more than polar regions, caused by the curvature of the Earth), and 2. the rotation of the Earth. Air at the equator is heated, becoming less dense. Air would tend to rise in the equator and move towards the poles, then cooling, becoming more dense and sinking again and moving back towards the equator. This convection system is termed a cell. However, due to uneven distribution of land and ocean and the rotation of the earth, there exist 3 such cells per hemisphere (Polar, Ferrel and Hadley cells). The polar and Hadley cells are temperature driven cells, whereas the Ferrel or mid-latitude cell is caused by an interaction of the polar and tropical cells. The earth's surface moves faster at the equator than near the poles (the axis of rotation). Therefore as air moves away from the equator it is also under the influence of the Earth's anticlockwise rotation, so it does not move in a straight line but in a curved direction to the right in the northern hemisphere and to the left in the southern hemisphere. This is termed the Coriolis effect, and gives rise to the "westerlies" and the "trade winds". Air rising and falling within these cells also hugely influence the climates of the world by the creation of semi-permanent areas of high and low pressure at particular latitudes, dictating the degree of precipitation and therefore the distribution of deserts and rainforests.
Take the example of the British Isles, which are within the northern hemisphere's Ferrel cell. The suface wind would be southerly, but due to the Coriolis effect, typical prevailing winds are from the south-westerly or westerly.
Prevailing Wind: Local Influences
There is a variation in mean annual wind direction on a regional level, due to large scale topography such as coasts and mountainous regions. The simplest example is the sea breeze. The sea/ocean takes longer to warm compared to land. This leads to a difference in how much the surface air temperature between sea and land. As the air over the land is heated, it becomes less dense and rises, creating low pressure. The surface air over the sea is relativley cooler and denser, and there is movement of air from the high pressure area to the low pressure area. The direction of the sea breeze is reversed at night. Breezes over lakes or seas are approximately twice as strong as land breezes. The direction of these are reversed at night. Another example is the Foehn effect. Air approaching an elevated area is forced upwards, where is cools and becomes less dense due to lower atmospheric pressure at higher altitutes. Smaller scale topography is also very important on local wind conditions.
Again taking the example of the British Isles, high ground over Wales, northern England and Scotland leads to Smaller topographical features have an even greater effect on prevailing wind. However, the prevailing wind is also greatly influenced by seasonal variation. North-easterly winds are as common than southwesterly winds in spring (in some years considerably more common).
Effects of Wind on Plants
Generally speaking, gentle wind is beneficial for plants, but as wind speed increases this quickly can cause detrimental changes.
Temperature - Wind cools plants down, both by removing heat from the leaf directly and by removing warm air surrounding the leaf. The greater the windspeed, the more the boundary layer of still air is thinned, and the more exposed to air temperature the leaf becomes, and the more the leaf temperature adjusts to the ambient temperature. The lemperature of a leaf determines the rate of physiological processes, most significantly the rate of water loss (see transpiration).
Photosynthesis - While photosynthesis occurs, carbon dioxide is being consumed in the leaf constantly. This creates a concentration gradient with low conentration of carbon dioxide inside the leaf relative to the air outside the leaf. Carbon dioxide then enters the leaf stomata by diffusion. In still air conditions, carbon dioxide is used up progressively further from the leaf surface, creating a shallow concentration gradient and a reduced rate of diffusion. In moving air conditions (e.g. ventillation) there is replenishment of carbon dioxide supplies for photosynthesis in the air around the leaf so the concentration gradient is steep, the rate of diffusion is greater. Therefore, the rate of photosynthesis is lowered in still air compared to when air is moving. At higher windspeeds, stomata close to prevent transpiration (see below), and therefore the rate of photosynthesis would then be reduced.
Respiration - Respiration increases as wind increases, and this may be related to the mechanical stimulation of the plant.
Transpiration - A boundary layer of still air surrounds each leaf. The lower the windspeed, the wider this layer will be. Water vapour diffuses from the high concentration inside the leaf to the lower concentration in the air moving beyond the boundary layer. The wider the boundary layer, the greater the distance the water vapor must move. Therefore transpiration increases as windspeed increases, until a thershold of high windspeed at which point the stomata close and transpiration stops.
Pollination & Seed dispersal - Some plants are wind pollinated, and some plants require wind to disperse their seeds. The activity of insect pollinators is hampered by high winds. European bees will not fly in windspeeds of 24 km/h, and flowers in sheletered location receive more bee visits than flowers in exposed sites. Without successful pollination, some plants will not fruit or produce a poor yield (e.g. fruit trees). Wind exposure seems to often be a strong factor in dermining whether a seed germinates and grows in a location.
Growth - Still air is detrminental to plant growth. Wind sway has been shown to result in thicker stems and trunks. Plants also tend to be slightly shorter under the influence of wind. Chronic exposure to strong winds causes unbalanced growth (e.g. see: "Krumholz"W).
Pests & diseases - Still air is known to promote many plant diseases. In still air, particularly in high humidity conditions, moisture can persist on leaf surfaces. This acts as a vector for fungal and bacterial diseases, as spores can stick more readily and linger long enough to infect the surface. Insect pests may be more able to locate plants in low wind conditions, while beneficial insect species that predate plant pests may be hampered by strong winds.
Mechanical Damage - If wind speed is too high, shoots and stems may be broken (e.g. see "lodging"W), and extreme wind speeds uproot trees or seriously damage their root systems (see "windthrow"W). Woody plants exposed to too much wind can get wind scorched, which is dessication of leaves and shoots caused by water loss under strong wind and high temperature conditions. In colder areas, if the water in the soil is frozen the plant cannot replace water losses in high wind conditions, and similar damage may occur.
Soil erosion - Land deforested for arable or pasture land may undergo soil erosion under the influence of wind exposure. This fundamentally alters the resulting ecosystem and is a large global problem.
Windbreaks may be considered according to their purpose as either Field windbreaks (for crops), Livestock windbreaks and living snowfences (create snow drifts in predetermined locations).
Aerodynamic Modelling of Windbreaks
Wind approaching a windbreak is termed the approach flow, it has a direction and a windspeed. As wind encounters a porous windbreak, some of the wind is lifted over it and some of it filters through it. The wind speed is reduced and the energy is partially transferred to kinectic energy of the movement of trees. In a non-permeable windbreak all of the wind is deflected over the top (see: Density). Distinct zones are described in the lee of a windbreak:
- Bleed Flow / Competition Zone - this zone extends from the base of windbreak and extending horizontally near ground level for a length approximately equal to 2 times the windbreak height (2h). In this zone there is competition for water, nutrients and light between the plants of windbreak with the crop, reducing yield. Shade from the windbreak offsets any temperature increase from the shelter it provides.
- Quiet Zone - the area downwind of a windbreak where shelter from wind is maximum. It is a triangular area from the top of the windbreak to the base of the windbreak, and extending horizontally near ground level for approximately 8h.
- Wake Zone - air that passes over the top of the windbreak forms a turbulant mixing layer. It is the area downwind of a windbreak where turbulance is greater than in the open. It is located above and downwind relative to the quiet zone. At ground level the wake zone begins at approximately 8h. After a distance, the air flow returns to speed of the approach flow, this is termed Re-equiliberation.
Microclimate: The Quiet Zone
In the lee of a shelterbelt, the following microclimate is produced:
- Windspeed and turbulance: reduced.
- Temperature: generally higher during day and cooler at night.
- Humidity: increased.
- Sensible heat & vapor transport: reduced due to reduced turbulance.
- Transpiration & evaporation: Lower after rainfall, overall higher due to increased water availability.
- Photosynthesis: increased.
- Snow storage: allowing increased water storage.
The microlimate that windbreaks create brings particular advantages depending on the use of land.
- Increase crop yields. Windbreaks provide plants with mechanical protection, but also reduce evotranspiration. Soil and air temperature are higher and water vapor concentration is higher. Windbreaks reduce plant transpiration by reducing windspeed. 
- Windbreaks reduce crop water stress by reducing soil water loss. 
- Reduce water use for irrigation.
- Reduce wind blown soil erosion
- Shelter animals from hot summer wind or cold winter wind
- Reduce animal stress
- Reduce animal mortality, particularly young (e.g. calves)
- Reduce animal feed requirements (animals will burn more calories if they are in colder conditions)
- Keep snow drifts out of feed lots
- Reduce spread of wind-borne pathogens.
- Apiaries benefit from windbreaks. The bees will not tend to be active during windy weather (also if it is too cold or wet). Workers will not forage as much, and consequently the efficiency of pollination will decrease.
- Windbreaks increase pollination efficiency by bees (see above) and other insect pollinators. In a temperate climate, fruit trees (e.g. peach, plum) blossom early in the year. At this time weather conditions may be poor, and the trees can suffer significantly reduced yields due to poor pollination. In the UK, there tend to be cold Easterly winds at blossom time, so windbreaks for orchards and forest gardens may be best orientated against the prevailing easterly wind at this time of year.
- Buildings in windy areas can reduce heating costs up to 30% by placement of a sheltering windbreak.
- Carbon sequestration.
- Windbreaks can reduce gaseous ammonia by approximately 50% downwind of poultry farms. Up to a threshold, plants also benefit from aborbing ammonia as a source of nitrogen, improving growth. Over the threshold, too much ammonia causes plant tissue necrosis, reduced growth and increased sensitivity to frost damage.
- Reduce dust
- Reduce noise
- Provide habitat for wildlife. Windbreaks may offer a travel corridor for wildlife between neighbouring areas of habitat. This improves the resilience of a species to large scale habitat loss. However, depending on the interaction between the wildlife and the land use of the sheltered area, this may be possitive (e.g. habitat for predatory birds and insects which prey on insect pests); or be considered negative (see below).
- New equipment and skills need to be adopted by farmers that previously may have only held experience in livestock or field crops.
- Requires more management than arable farming with no windbreaks.
- Requires space which could otherwise be used for other purposes, e.g. annual crop production. Modern machinery in industrial agriculture has lead to increases field sizes to increase efficiency.
- May block views.
- Takes time for the trees to grow to full height and therefore the impact of the windbreak will not be fully apparent for several years, and consequently the return on investment is gradual.
- Windbreaks may act as habitat for pests e.g. insects and weeds.
In agroforestry science, 7 structural elements are said to influence windbreak effectiveness, namely: Height, density, orientation, length, width, continuity/uniformity and cross-sectional shape. There are a few other issues to dconsider such as frost pockets and rain shadow.
The height (H) is hugely important in the size of the area of protection. Wind speed is reduced for an area upwind equivalent to 2-5 times the windbreak height (2-5H), for an area downwind equivalent to up to 30H.
The density (porosity) is the ratio of the solid portion of the windbreak relative to the total area. Less dense barriers will let more wind through, more dense barriers will let less wind through. The density is strongly influential on maximum windspeed reduction. Density is closely related to width.
Windbreaks work best when they are orientated perpendicular to the prevailing wind. However the direction of wind changes, and so it may be beneficial to orientate windbreaks along 2 axes ("legs"). Some advise that a forest garden site should be bounded by windbreaks in all directions.
The advised ratio of length to height should be over 10:1. This reduces influence of end-turbulence and allows best efficiency in terms of area of protection.
In terms of wildlife habitat, it is known that the wider the windbreak, the more diverse the wildlife ecosystem it can support.
Existing fences and property borders may dictate location of windbreaks. Existing fences can be used to train shrubs against. Windbreaks located along the tops of ridges provide greater area of protection compared to windbreaks planted along low points such as streams. On the other hand, windbreaks located near water are more beneficial for wildlife (see: "Riparian buffer systems"W).
Plants vary in their response to frost. Frost reduces availability of soil moisture for plant roots to replace water loss by transpiration. Generally speaking deeper rooted plants are less vulnerable to frost damage compared to shallow rooted plants, because deeper roots extend below the frost line. Sometimes newly planted or shallow rooted plants can be lifted up slightly by expansion of water in soil as it freezes. Cell sap expands when frozen and may cause plant cell walls to be destroyed. Repeated freezing and rapid thawing can be particularly damaging to roots. New, spring growth is vulnerable to late frosts, e.g. by damaing blossom of fruit trees such as peach, preventing successful pollination, and therefore decreasing yield. Tree bark can be split open by freezing and thawing on the equator facing side of the trunk (see frost crackW). Black frost (also termed "killing frost") refers to the necrosis of plant tissues due to frost damage.
Sometimes what appear to be more sheletered sites can be at higher risk of hard frosts. As air temperature drops during night or cold weather, it becomes less dense and therefore sinks. Cold air will therefore tend to flow down slopes and accumulate in the lowerest point, e.g. the bottom of a valley or a hollow. Barriers such as banks, hedges, walls and fences will influence the movement of cold air. To visualize how cold air behaves, imagine pouring thick fluid over the landscape. If a barrier exists which prevents the cold air from flowing away, a frost pocket is formed. In frost pockets, frost will be more severe and persist for longer in the day.
Therefore it is potentially advantageous to design gaps in the windbreak to allow cold air to flow out of an area. Barriers can also be positioned to intercept cold air and divert it away from a location. To achieve this, the windbreak could be sited slightly off contour. A small gap can also be left in the windbreak to allow cold air to flow away, but this should not create a wind tunnel effect, e.g. by facing away from the prevailing wind direction. Another option is to thin the trees to allow the cold air to flow through, although reducing the effectiveness of the windbreak.
The rain shadow effect is where there is less rainfall immediately downwind of a barrier such as a windbreak (relative to prevailing wind direction). This effect is far more pronounced with impermeable windbreaks, and the area of relative dryness is also proportional to the height of the windbreak.
Use trees or shrubs that will reach the desired height when mature. This elimates any need for trimming or cutting.
Choice of species should also strongly take into account suitability of local conditions (e.g. soil type, pH, fertility, water availability, hardiness zone, etc.). Use of multiple different species in a windbreak increases the ability of the windbreak to function in a variety of different weather conditions, and be more resistant to disease and pests, which will tend to cause much greater problems in any monoculture.
Deciduous trees provide a less dense windbreak during winter, which is potentially when windy weather is more likely.
Non-native species are sometimes used in windbreaks, e.g. because of their fast growth or tolerance of local conditions. However, this may harm local ecosystems if the species become invasive.
The speed of growth of trees used in windbreaks is important. The faster the rate of growth, the more quickly the windbreak will become established and provide the desired benefits. Generally speaking however, faster growing trees have brittle wood, and reach useful lifespan earlier (e.g. 20 years), are more prone to insect damage and disease.
Lists of selected tree and shrub species suitable for use as windbreaks in the UK climate are given in appendix 2 of "Creating a Forest Garden." Plants for a Future have several detailed lists on a range of options for UK climate. Advised species for Natrona County, Wyoming are given in this booklet by the Natrona County Conservation District.
Windbreaks can offer wood. One example used a system of 3 windbreaks of 2 rows of trees each. One row from one of the windbreaks was harvested each year on a 6 year rotation, without completely removing the shelter function.
Large Scale Examples
- The Great Plains Shelterbelt, USA.W This program was a response to the 1930s "Dust Bowl" of the High Plains. The climate is semi-arid grassland with infrequent wet years and intervening periods of drought and frequent windy conditions. The US Government gave land away free to settlers. A period of higher precipitation and lack of understanding about the ecosystem lead to settlers implementing european style farming techniques. Mechanization was also taking place at the time. Inappropriate cultivation of vast areas of land combined with years of drought and wind caused large scale soil erosion and dust storms. Many thousands of people were displaced from the region as farms became buried beneath dust. Windbreaks were used to prevent wind blown soil erosion. By 1942, 30,233 shelterbelts had been planted, which contained 220 million trees and stretched for 18,600 square miles (48,000 km2), in areas ranging from North Dakota to Texas.
- In China, "The Great Plains Project", "The Great Green Wall", and the "Three Norths Shelter Project" are all examples of very large scale windbreak projects.
- Met Office learning materials: Wind.
- Met Office learning materials: Global Circulation Patterns.
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