Overstory #129 - Windbreak Design
People, plants and animals thrive in sheltered environments. Creating a sheltered environment is an important farm management objective. The only practical way to provide shelter for broadscale agriculture and pasture is by establishing windbreaks. Farmers and other land managers increasingly recognize the value of shelter, and windbreaks are more and more a feature of rural areas throughout the world. Windbreaks have a place on almost every farm.
Key design criteria
A drive through the countryside often reveals many poorly designed and haphazardly established windbreaks. Faults are difficult to correct after planting. A little thought and care in the design of your windbreak will result in a valuable long term asset and improve a farm's productivity and profitability.
Maximising windbreak height is generally the most important windbreak design consideration.
The area protected by a windbreak is directly proportional to its height. In other words, by doubling the height of a windbreak you double the area protected. Increasing the average windbreak height in a windbreak system increases the distance between the windbreaks required to provide the same level of protection.
Windbreaks therefore should be designed and established to make sure they reach the maximum mature height possible. The most important design issue is to include at least one row of the tallest shelter trees capable of growing on the site. Also use good quality plants of species and provenances (local varieties) known to grow well in your locality. Provide good conditions for plant growth by following recommended site preparation and establishment procedures.
As well as the final mature height, other factors to consider when selecting the species in the tallest rows are their growth rate and the life span. Fast growth to the maximum height means the maximum extent of shelter is achieved earlier. Long-lived species increase the time span that this is maintained.
The length of a windbreak and the presence of gaps also greatly influences windbreak performance.
Short windbreaks tend to be very ineffective because the wind curls around each end due to turbulence. This results in a narrower area being protected than the length of the windbreak. It's best to make windbreaks as long as possible. A common rule of thumb is to make windbreaks at least 10 times as long as their mature height.
Gaps in windbreaks should be avoided if at all possible, as they decrease windbreak effectiveness. Wind speed in front of a gap is increased due to concentration and funnelling of winds.
Gaps however are sometimes necessary due to the need for gates and access through windbreaks. If this is the case, wind funnelling problems can be kept to a minimum. A small island shelterbelt just in front of the gap is one solution. In multi-row windbreaks, the gap can be angled at 45 degrees to the prevailing wind direction.
Porosity is the degree to which wind can pass through a windbreak. There is a strong relationship between windbreak porosity and the minimum wind speed produced on the lee side. Denser (or less porous) windbreaks reduce wind speed to a lower level immediately in their lee and the point with the lowest wind speed occurs is located closer to the windbreak. Very dense (< 20 % optical porosity) windbreaks are desirable where a small area of high quality shelter is required, such as livestock havens or around houses.
A significant problem with these dense windbreaks is the greater turbulence they create in their lee. Turbulence generally decreases with increased porosity. This increased turbulence is felt at ground level beyond a quiet zone of reduced turbulence which extends to about eight times the height (8H). Hence significant wind turbulence can be expected in the zone extending beyond 8H for dense windbreaks.
The vertical distribution of porosity is also important. In general, the usual aim is to have uniform porosity from ground level to the top of the windbreak. The most common mistake is to not provide low shelter down to ground level, either due to poor windbreak design or maintenance. The effect of this is funnelling of winds underneath the windbreak creating a narrow zone of increased wind speed immediately in the lee. Clearly this zone has the potential for adverse effects on crops, livestock and exposed soil. Beyond this, these windbreaks still provide an extended area of valuable shelter.
The type of trees in the windbreak greatly affects its porosity. Different species have greatly different foliage density and therefore produce windbreaks of different porosity. For example, eucalypts tend to have more open crowns whilst introduced cypresses have very dense foliage. Windbreak porosity is also reduced by decreasing the spacing between trees within rows and increasing the number of rows.
Number and spacing of rows
Single row windbreaks can provide very good shelter, providing they consist of tree species which retain foliage down to ground level. These windbreaks use the least area, but they are susceptible to problems caused by gaps and non-uniform growth. Frequently the maximum height obtained is less than for multiple row windbreaks. These problems can be overcome by increasing the number of rows at the small expense of more space used. Given the significant investment in establishing and double fencing a windbreak, compromising on the number of rows is often a false economy.
It is usually a good idea to have multiple row windbreaks. Belts with several rows give a better opportunity to use tall tree species which naturally lose their foliage near ground level (or are pruned for timber production) in conjunction with lower shrubby species. Good permeability and height can often be obtained from 2 - 4 rows with one or two rows of tall trees. Narrow windbreaks of 2 - 4 rows are generally optimal in terms of minimising the area of land removed from agriculture whilst providing maximum shelter benefits.
Other benefits can be provided by further increasing the number of rows. Rows of fast growing species can be included to provide rapid shelter which will later either die out or be removed. Wildlife habitat is usually enhanced. Stock can be moved into fenced, broad, multi-row windbreaks at critical times when maximum shelter is needed. Tree and shrub species which are more susceptible to the wind may be inappropriate in narrow windbreaks, however can be included in the protected inner rows of a broad multi-row windbreak. In very exposed climates, such as in coastal areas, a larger number of rows can assist tree survival and growth due to greater mutual support.
More rows in windbreaks are particularly recommended for dry regions, because growth rates are generally slow and survival rates more variable than in wetter areas.
There is some evidence that very broad windbreaks produce an aerodynamically less efficient barrier to the wind than narrower ones. This should be considered before unnecessarily including a very large number of rows in a windbreak.
Valuable timber can also be grown if one or more rows of timber producing tree species are included. In New Zealand, a system of timber producing windbreaks called 'timberbelts' has been developed and is being widely adopted in Southern Australia and elsewhere. Here a row or rows of high-pruned, timber producing species is planted alongside a row of low growing shelter species. More timber rows can be included if greater timber production is desired at the expense of less land available for crops or pastures.
Rows can be varied from 1.5 - 5 m apart to allow adequate room for the development of the trees and shrubs. In general, the fewer the rows, the closer they may be spaced.
There should be two and preferably three metres between the outside row and the fence to prevent stock browsing off too much low level foliage. Loss of low foliage results in undesirable increases in wind speed close to the windbreak due to wind funnelling. This distance can be reduced slightly with electric fencing. Leaving an adequate distance between the outside row and the fence leaves space for the trees to grow. It greatly reduces stock pressure on the fence and the likelihood that it will fail. An extra metre either side of a windbreak only occupies 0.2 ha per km of windbreak, a small price to pay for significantly enhanced windbreak performance.
Rows of low growing trees and shrubs should be located on the outside of the windbreak to prevent them being shaded out by taller species.
A common misconception in windbreak design is to aim for a sloping cross-section or profile. A sloping aerodynamic profile actually reduces windbreak effectiveness. Steep sided windbreaks create a more effective barrier to wind flow and therefore shelter a greater area. Multi-row windbreaks should if possible have their tall rows on the windward side and the lower growing rows to leeward.
If shade is desired from the windbreak, then place the tallest rows so that they cast the maximum shadow at times when shade is most needed (eg in mid afternoon).
Spacing of trees and shrubs within rows
Spacing should be sufficient for the windbreak to provide the required porosity in a reasonable amount of time. Too great a spacing leads to an excessive period of time before the gaps close between the trees. Too small a spacing can lead to insufficient porosity and results in higher establishment costs.
Tree spacing should be varied according to the types of tree being used and the site to be planted. The average spacing for medium to tall trees is 2.5 - 6 m and for small trees and large shrubs is 2 - 4 m. Compact low shrubs are commonly planted at 1.5 - 3 m spacing. In general, trees and shrubs should be more closely spaced in narrower windbreaks and can also be closer in wetter, more fertile areas.
An alternative approach is to plant trees and shrubs more closely than the final intended spacing and to thin them as they grow. This results in the faster establishment of shelter. This can be achieved in a number of ways. The simplest is to simply plant the long-term shelter species at a greater density and thin them as they develop. Another method is to alternate the planting of fast growing short-lived shrubs with slower growing long-lived species. The faster growing shrubs are thinned before they compete excessively with the long term species which will ultimately comprise the windbreak. The trees and shrubs removed can be used for firewood, fence posts or fodder.
Try to stagger the position of trees so that they are opposite spaces in adjoining rows to provide more uniform porosity, fewer gaps, faster establishment of shelter and optimal use of the space available.
Plant tree and shrub species which will provide the windbreak characteristics you require (particularly final height, growth rate and porosity).
The most important considerations when selecting species are:
- At least one row of the tallest species that will grow on the site should be included as previously described.
- Foliage density and crown shape. This varies greatly with species, so affecting windbreak porosity. For single row windbreaks, the species selected must provide adequate foliage density from treetop to ground level. Where multi-rowed windbreaks are to be established, then more flexibility in species selection is possible. Use of smaller trees and shrubs in particular can reduce problems with the loss of foliage at ground level of the tallest trees.
- Growth rate. Fast early growth is desirable as it leads to the more rapid establishment of shelter. Unfortunately it is common for rapid growing species to be short-lived. Trees which grow very fast can also be less wind firm. In these cases, a row of these species can be planted on one side of the windbreak or planted alternately with slower growing species within a windbreak row. They can then be allowed to die out or thinned when they are not required. Fast growth is also advantageous where the species are selected to provide timber fuelwood or fodder production.
- Compatibility with crops and pastures. Some species compete with adjacent crop and pasture growth to a greater extent than others. Trees and shrubs with deep, penetrating roots generally compete less for moisture than shallow spreading root systems. Some species chemically inhibit the growth of plants around them through a process called allelopathy. Conversely other trees and shrubs can have beneficial effects, for example through nitrogen fixing (particularly Allocasuarina, Casuarina, and Acacia species) and adding organic matter to soil.
- Ability to regenerate naturally under the environmental conditions of the site. Choosing these species creates perpetual self-sustaining windbreaks. Indigenous species are often able to regenerate most easily.
- The lifespan of tree species. Selecting long-lived species obviously maximises the useful life of the windbreak. Try to choose species which can live together harmoniously for a long time. By selecting species of roughly equivalent lifespan, impaired windbreak performance as plants gradually die out can be avoided. Be careful of the quickest growing species as these are often short-lived or prone to disease. The benefit of quick shelter may be gained at the expense of a much shorter period of effective shelter. Alternatively, closely planted nurse rows of rapid growing, short-lived shrubs or trees may be worth considering to provide early shelter for slower growing species, especially on exposed sites. The nurse rows can be removed or be allowed to die out as the windbreak matures.
- Number of species in each row. Try not to include too many, particularly for windbreaks with only a few rows, as this tends to decrease uniformity in height and permeability. Often one species per row is best. There are some exceptions to this rule, such as the alternate planting of slow and fast growing species within a row described above or the alternate planting of tall pruned timber species of tree with slow growing, shrubby shelter species within the same row.
- Fire resistance or ability to regenerate is desirable in fire-prone areas. This can save much in re-establishment costs. For example, many eucalypts reshoot from branches and trunks after being burnt. Other species respond to being burnt by fire through massive germination of seed accumulated in the soil. This is common with Acacia species. The result is rapid regrowth of the windbreak at no cost.
- Flammability of foliage. Tree and shrub species of low flammability can protect assets against the heat of fires when used as radiation shields. These should be located well away from buildings as they will still burn under extreme fire conditions.
- Landscape and wildlife. Select species which blend in with existing vegetation and landscapes. Species can also be selected to provide food and shelter for wildlife. Indigenous species (ie. those naturally occurring in your locality) usually offer the best value for providing wildlife habitat. They are part of the local environment and local wildlife are well adapted to them.
- Timber and tree crops. Species can be selected so that timber and tree crops are produced directly from the windbreak. Timber can be grown in farm windbreaks without compromising shelter benefits. Firewood, posts and poles can be provided for on-farm use. These products, together with sawlogs and pulpwood, can be produced for sale to supplement and diversify farm income. To grow useable timber, care must be taken with windbreak design, and management, as well as species selection. Trees in windbreaks can also be selected to provide other valuable tree crops such as seed, nuts, honey and broombush.
- Coppicing ability. Trees which coppice (or re-sprout from cut stumps) reduce re-establishment costs after the trees are harvested for timber or fodder. Coppicing species can greatly assist with windbreak re-establishment and renovation.
- Fodder. Low windbreaks can consist entirely of fodder species of shrubs (for example, tagasaste, saltbush or Acacia saligna). Alternatively a fodder row or rows can be included in a more conventional windbreak. If these are located close to the fenceline, the shrubs can be simply lopped and thrown over the fence to stock.
Most importantly, the windbreak species must be able to grow well on your site. Two rules of thumb can be applied here. Look around your district and see what species are healthy and being successfully used in windbreaks. Secondly, plant species indigenous to your locality. They are well adapted to the local environmental conditions, having evolved there over thousands of years. Seek advice from an experienced treegrower, nurseryman or government advisory officer about the windbreak species that have performed well in your area.
Windbreaks are capable of providing a multitude of benefits to the landholder and the wider community. Give careful consideration to what functions a windbreak is to perform then design and plan windbreak establishment accordingly.
Bird, P.R. 1991. "The role of trees in protecting soils, plants and animals in Victoria". In: Papers of The Role of Trees in Sustainable Agriculture, A National Conference. Rural Industry Research and Development Corporation, Canberra, pp 77-87.
Burke, S.J.A. 1991. "Effect of shelterbelts on crop yields at Rutherglen". In: Papers of The Role of Trees in Sustainable Agriculture, A National Conference. Rural Industry Research and Development Corporation, Canberra, p89-99.
Raine, J.K. and Stevenson, D.C. 1977. Wind protection by model fences in a simulated atmospheric boundary layer. J. Indust. Aerodyn. 2: 159-180.
This article was adapted with the kind permission of the author and publisher from:
Burke, S. 1998. Windbreaks. Butterworth-Heinemann, Woburn, Massachusetts, USA.
This comprehensive and well-illustrated book covers the siting, design, species selection, establishment, and maintenance of multipurpose windbreaks. It is available from the publisher at http://books.elsevier.com/.
About the author
After graduating with an honors degree in Forest Science from the University of Melbourne, Steven Burke worked for seven years providing forestry extension services to landholders in Victoria, Australia. Following his strong interest in windbreaks and other agroforestry systems, he undertook landmark graduate research into crop responses to windbreaks in South-Eastern Australia which resulted in a Masters Degree in Agricultural Science. He has also managed statewide programs in community forestry and community-based revegetation whilst working for the Non-Government Organisation Greening Australia, and managed pest plant and animal management programs for the Victorian Department of Natural Resources and Environment. He is currently managing invasive species and vegetation management programs for King County in Washington State, USA. Steven can be contacted at Burke_s@hotmail.com.
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