Shelterbelts for control of wind erosion

Most Australian soils are old, have low levels of nutrients and have very thin topsoils. These topsoils take a very long time to form and can be eroded by even slight winds.

Shelterbelts can provide a relatively cheap and long-term option for reducing wind erosion on farms. Shelterbelts are used to decrease windspeed within the sheltered zone and reduce the erosion of fertile soils on agricultural land. This can increase the productivity of agricultural land by protecting this valuable topsoil and nutrients.

Impacts of wind erosion on crops

Nutrients lost due to soil erosion by wind must be purchased and replaced to avoid productivity losses.

Wind erosion can have a direct impact on crops in several ways, including:

• exposing plant roots
• burying plants under moving soils
• blowing plants out of the ground.

Nutrients in topsoils

Most soil nutrients are held in the organic matter of plants and in the top layer of soils. This top layer of soils often contains fine silt or clay particles that hold valuable nutrients.

These fine particles of soil and nutrients are the first to move during a wind event. The fine nature of these particles means that, once they've been dislodged, they can be carried by wind over large distances.

Protecting soils with vegetation

Soils are most vulnerable:

• if they have been heavily grazed or cultivated
• during dry or drought conditions.

Dry conditions result in low vegetative cover because of slow growth and overstocking.

Constructing a shelterbelt and re-establishing vegetation can protect soils from erosion in a number of ways:

• The leaves and vegetative matter within and dropped by vegetation intercept raindrops and prevent the pounding of soil and dislodgment of soil particles.
• The litter dropped by vegetation increases the organic matter within soils, increasing the soil's ability to absorb and hold water and to reduce run-off.
• The roots of plants hold the soil in place.

Shelterbelts to reduce the impact of wind erosion

The quantity of soil carried in wind increases in proportion to the wind velocity cubed. So a modest reduction in wind speed can result in a significant reduction in soil erosion. Reducing wind speed by half can reduce the rate of erosion to one-eighth.

Narrow or open belts can reduce wind speed for a distance many times the height of the belt. Even wind approaching belts on angles that are almost parallel to the belt can decrease wind speed. Although the area over which shelter is provided will be reduced.

Shelterbelt design for reductions in wind erosion

The location, density, height and length of a shelterbelt will determine its effectiveness in reducing wind erosion.

Shelterbelts provide the highest level of protection when they are located at right angles to erosive winds. Good protection from wind erosion can be maintained for up to 30H of a shelterbelt if wind is approaching the belt at right angles. Therefore consideration should be given to the direction of winds protection is required from when planning a shelterbelt network.

Although maximum protection from erosive winds is achieved by placing belts at right angles to the wind direction, soil erosion can still be substantially reduced if wind is coming from other directions. However, the area protected by a belt will be reduced during events of higher wind speeds.

Dense shelterbelts provide a high level of protection but over a reduced area when compared to less dense belts. Moderately dense belts (~40% density) will provide a considerable reduction in wind speed to a distance of at least 20H. Dense belts generally provide good shelter to a distance of at least 15H.

To control wind erosion over a large area the height of a belt needs to be maximised. Therefore it is wise to incorporate at least one row of tall species within the belt.

It is important to incorporate shrubs into a belt where an even density is required from the ground to the top of the belt. It is also advisable to fence out a belt to prevent stock grazing on the lower limbs of vegetation and creating gaps. If gaps occur within a belt, wind will tunnel through the gaps at an accelerated rate. This can result in blowouts near the gaps and also be detrimental to stock and crops.

Wind speed accelerates around the ends of shelterbelts. By increasing the length of the belts or joining them to other belts this impact is reduced. Belts can be joined where they can form a right angle for protection from a range of wind directions as illustrated in Figure 1. Stock should not be able to graze below or at the ends of a shelterbelt as these areas are very susceptible to erosion. The more effective a belt is in reducing wind erosion, the more eroded the sites at the end of the belt can be, unless the ground surface is protected.

References

Bird, P.R. (1998) 'Tree windbreaks and shelter benefits to pastures in temperate grazing systems'. Agroforestry Systems 41: 35-54.

Bird, P.R., Bicknell, D., Bulman, P.A., Burke, S.J.A., Leys, J.F., Parker, J.N., van der Sommen, F.J. and Voller, P. (1992). 'The role of shelter in Australia for protecting soils, plants and livestock'. Agroforestry Systems 18: 59- 86.

Brandle, J.R.,Hodges, L. and Wight, B. (2000) 'Windbreak Practices'. North American Agroforestry: An Integrated Science and Practice 4: 79-118.

Burke, S. (1998) Windbreaks. Inkata Press, Sydney.

Cremer,K.W. (1990) Trees for Rural Australia. Inkata Press, Melbourne and Sydney.