Effective shelterbelt design
Shelterbelts provide many benefits to farms, including:
- protection of crops, livestock and the home
- reduction of soil erosion
- salinity control
- improved biodiversity.
What you want the shelterbelt to do determines the type of shelterbelt to plant. The success and efficiency of your shelterbelt will depend how well you've considered the design and location when planning. Shelterbelts can have a detrimental impact on farm productivity if they are not appropriately designed.
How shelterbelts work
Shelterbelts are vegetative barriers that are designed to reduce wind speed and provide sheltered areas on the leeward (the side away from the wind) and windward (the side toward the wind) sides of the shelterbelt.
As wind approaches the belt, some goes around the end of the belt, some goes through the belt and most goes over the top of the belt.
Air pressure builds up on the windward side and decreases on the leeward side. It is this difference in pressure that drives the shelter effect and determines how much reduction in wind speed occurs and how much turbulence is created.
The amount of air pressure difference is determined by the structure of the shelterbelt. The more dense the shelter, the greater the difference in air pressure.
Choosing the right location
The location of shelterbelts will determine their level of effectiveness. Think about the location of a shelterbelt that will provide the maximum benefit for:
The direction of prevailing and other winds and the location of stock and crops that require protection are major deciding factors on the orientation of shelterbelts. Shelterbelts should be placed perpendicular to problem winds.
No single orientation of a shelterbelt will provide protection from all winds. Therefore several belt orientations will provide greater shelter.
A consideration for the siting of shelterbelts is the ability to move livestock into sheltered areas during adverse conditions. It can be difficult to move stock in a direction that opposes the wind direction. You should weigh this against potential reduction in protection if the belt is not perpendicular to the problem winds.
During summer, shelterbelts can protect pasture and crops from moisture losses by reducing the impact of hot drying winds. Shelterbelts can also reduce erosion by wind during summer months when soils can be bare. Ideally, belts should form a grid using north-south and east-west orientations. This will provide shade for stock at different times of the day and protection from winds coming from all directions.
Alternatively, cornered windbreaks provide protection from winds that come from a range of directions. Therefore a windbreak established in a right angled corner protects a larger area from a range of wind directions.
North-south orientated windbreaks will prevent permanent shading of pastures and crops as they will receive sunlight at different times of the day.
Incorporating remnant vegetation will provide a cost-effective belt and one that enhances biodiversity.
Shelterbelts can generally be established at the mid-slopes on the contour of a hill or on the crest of a hill. Gullies on a hill are generally protected. Steeper and higher areas of the hill are generally more exposed to wind. Planting on the ridge of a hill provides extra height which in turn increases the area sheltered. Hilltop plantings can protect areas that are prone to erosion and strong winds.
Plantings on the contour of a hill can trap air and create a localised frost zone unless precautions are taken. Establishing gaps within the belt or allowing the air to drain out at one end can reduce this effect. Alternatively reducing the density of the belt on a contour can reduce the effect also.
Designing an effective shelterbelt
When designing a windbreak, consider the following elements:
- number of rows
- the species to be used.
The height determines the distance of the protected area, so it's important to maximise the height of the shelterbelt.
Use the tallest suitable shelter species in at least one row of the belt to increase the effective windbreak area. The species you use for the taller row can be something that's fast growing, to achieve maximum height quickly.
Longer windbreaks are more effective than short ones.
Wind has a tendency to flow around the ends of a shelterbelt and increase turbulence at that point. So, to reduce overall turbulence, create a shelterbelt that's as long as possible.
For maximum efficiency the uninterrupted length of the windbreak should be at least 10 times its height.
Avoid creating gaps in your shelterbelt, as they can make it less effective. It's possible for gaps to actually increase wind speed because the wind accelerates as it funnels through (this is often called wind tunneling).
When a gaps in a break is necessary, such as for gateways, you can add a small strip of island shelter in front of the gap (figure 1) or stagger rows to create an angled gap (figure 2). This should prevent possible wind tunneling.
Density of plants
Density is the proportion of solid material, such as foliage and branches within a windbreak. Using species with dense foliage, having more rows or spacing plants closer together can increase windbreak density.
Your design should usually aim for a medium density of around 40 percent to 60 per cent.
A very high-density shelterbelt will provide a high level of shelter but over a shorter distance. They can also result in more wind turbulence because very dense windbreaks force wind to be pulled down on the leeward side.
With a medium density, more air passes through the shelterbelt, reducing wind turbulence and extending the down-wind protected area.
Your shelterbelt should have an even density from the ground level to the top. If the belt doesn't provide ground-level shelter, wind will tunnel through the gaps at the bottom. To achieve even density, use shrubs and ground cover species as well as taller species.
Rows of plants
Shelterbelts with multiple windbreak rows are less susceptible to the impacts of gaps and non-uniform growth. They're also more likely to achieve a greater overall height. You should tailor the number of rows in your shelterbelt design to your objective.
An effective windbreak design often consists of 2 to 4 rows, using taller species that provide the benefits of a tall belt combined with shrub species that provide shelter lower down. These types of belt can provide significant benefits while not requiring large areas of land to be removed from direct productivity purposes.
Including a row of fast growing species can provide quicker benefits and also protect species that are slower to establish. This row can later be removed if desired.
Belts with 1 or 2 rows can be effective and economical if they are well designed and use appropriate species. The effectiveness of these belts depends on high survival rates of plants. They may be significantly less effective if the form of the species varies significantly.
Single row species can be effective if they're established using a species that has a uniform foliage density from ground level to the top of the belt.
Increasing the number of rows can provide other benefits, even if they don't offer more protection of land. Some of these are:
- more biodiversity
- a haven for stock in extreme weather conditions (though stock will damage the belt and should usually be kept out)
- timber production (timberbelts consist of a row or rows of timber species combined with lower growing shrub species).
Plant location and angles
A common misconception is that a sloping cross-section profile enhances windbreak effectiveness. Steep-sided belts shelter a larger area because they can provide a greater height barrier and a lower density on the windward side. But sloping profiles on the windward side can actually reduce the distance over which protection is provided.
Aim to place taller species in the centre of a belt with lower growing species on each side. This design is better several reasons:
- Large tree branches are less likely to fall and damage fences.
- Smaller trees and shrubs won't be shaded out by the taller species.
- It creates a more habitable environment for wildlife.
Spacing between plants
When deciding the spacing between plants, consider the time it will take for the plants to reach the desired density level and the size of the species selected. Keep the following tips in mind:
- Space rows between 2 to 4 metres apart to allow the plants to grow relatively unrestricted (medium to tall trees are usually spaced 3 to 4 metres apart, large shrubs can be spaced between 2.5 to 4 metres, and smaller growing shrubs are generally placed 1.5 to 2.5 metres apart)
- In belts with fewer rows, place plants closer together to increase density and provide protection more quickly.
- Stagger trees in alternate rows to improve density and reduce gaps.
- Leave at least 2 metres between the first row of plants and the fence to prevent stock from grazing on the plants.
The species you select for your shelterbelt should provide the height, growth rate and density characteristics suitable for the belt's purpose.
When selecting plant species, consider the following points:
- Locally native species generally have higher survival and establishment rates.
- Locally native species provide valuable habitat for local wildlife species.
- Species that will grow tall on the site should be used for one or more rows. Noting the height and health of particular tree species in the area can identify these species.
- Species with an appropriate foliage density that complements the height and density of other selected species to obtain even and suitable density should also be used.
- The growth rate of species should be taken into consideration. Where the effects of shelterbelts are required quickly, you can use fast-growing species.
- The use of species that regenerate naturally on the site may be useful where this is desirable.
- Having too many different species can reduce the uniformity of the shelterbelt. Generally people use 1 species per row or species with similar or compatible growth forms.
You might choose to use species that provide timber for firewood fence posts or commercial uses may be desired. But using a shelterbelt for timber production may require more specific management practices.
Fodder species can be used in a shelterbelt to provide a food source. These plants can be grazed directly by stock or cut and provided to stock. Removing fodder from the belt can compromise its ability to provide shelter.
Preparing the site
Thorough site preparation will help the shelterbelt establish and reach an ideal height. In the longer term, there is less work involved in the establishment of a belt on a site that's been well prepared.
Thorough weed control and ripping is vital to site preparation.
Undertake weed removal well before planting or direct seeding to allow moisture to be held within the soil.
On some sites, deep ripping of the site will also improve the water availability for tubestock plantings and should also be undertaken in advance. Ripping also promotes deep, strong root growth.
Mounding the site may be an important preparation technique for sites prone to waterlogging or cracking.
Young plants in a shelterbelt are vulnerable to damage by grazing animals, including:
- pest animals
- native animals
- stock animals.
Young trees are susceptible to grazing by rabbits and hares, while native species such as kangaroos and cockatoos may also damage young plants. Stock can damage fences to graze on plants.
Make sure you place rabbit-proof netting around the site to protect it from reinvasion after pest animal control. Also construct stock-proof fencing around the site before you plant.
This information was developed in collaboration with Dr James Brandle (USA).