Shelter helps farm production and environment

By Bala Tikkisetty

The generally wet soil conditions of mid-winter mean it’s the season for planting trees for shelter.

Providing such shelter is good for farm production and the environment.

Shelter is one of the few ways of protecting stock from unfavourable weather conditions, which are one of the major causes of animal suffering. Animals living in natural habitat find their own shelter but farm animals may not have enough, especially in a field of grass surrounded by a wire fence.

An example of the benefits of shelter for stock includes recent research reports revealing that sheltering and feeding ewes before lambing has a major impact on reducing lamb losses. The reports also say that sheltering and feeding ewes two weeks before lambing has a bigger effect on lamb survival.

Growing a shelterbelt of trees is one longer-term option for reducing the adverse effects of wind on stock (artificial windbreaks can also play a part where cost allows and rapid protection is essential).

When establishing a shelterbelt, careful consideration needs to be given to site selection and tree species. An understanding of the terrain and local weather conditions is also important, along with an appreciation of the way these affect livestock behaviour. Strategic planting is likely to be more worthwhile than blanket planting and, because of the long-term commitment, a careful decision should be made over where to plant.

Besides protecting stock, shelterbelts help reduce evaporation of soil moisture and transpiration from the grass, making them particularly helpful in drought or prolonged dry spells, as we experienced in the last few seasons.

Strong winds in particular accelerate transpiration rates from grass and, if water absorption rate by the roots is lower than the transpiration rate, the plant develops an internal moisture deficit. Once this deficit reaches a certain threshold, stomatal closure occurs, the plant appears to lose its turgidity, photosynthesis is constrained, and growth is curtailed.

The most important factor here in reducing grass growth is mechanical agitation of the grass by the wind, cutting dry weight, leaf area, and height.

Wind can also cause physical damage to grasses, leading to stunting or desiccation. The threshold for physical damage in herbaceous plants in general is often stated as being about six metres per second. At higher wind speeds, grass blades knock and rub together, bend over, and frequently rotate about their longitudinal axes. Such movements may produce permanent lateral fractures and wilting of the leaf tips.

Another benefit of trees relates to the way their roots hold the soil together and prevent erosion. The ability of individual species to help prevent erosion relates to things like tree size, tree stocking per hectare, root tensile strength, and rate of root decay after tree harvest.

Shelterbelts also control the removal of top soil by the wind, when the establishment of shelter is undertaken simultaneously with other sustainable land use practices such as minimum tillage.

As well as protecting stock, grasses and soils, shelter trees can be aesthetically pleasing and:

· provide shelter for homes and buildings, stock yards

· be a haven for birds

· increase the tree species in an area.

Shelter can also screen noise and reduce odours associated with livestock operations.

The use of native plants, particularly those naturally occurring in the locality, help to preserve the local character and provide forage for bees.

Good shelter improves the microclimate of plants and soil by improving plant water relations and conserving heat and reducing physical damage.

There are four main contributing factors relating to effectiveness of shelter - orientation, permeability, length, and height.

Orientation: Shelter is most effective when sited at right angles to the eroding wind. The wind barrier should be sited directly across the most harmful wind to give maximum protection. If east-west belts are required they should include deciduous species to lessen the winter shading of pastures.

Permeability: Porosity of the shelter belt determines the wind behaviour on the leeward and, to some extent, on the windward side. Practical experience has shown clearly that shelterbelts of medium porosity (50 per cent) produce a much more even wind-flow over a much wider area. Good porosity can be achieved by correct species choice and subsequent management. When porosity is low, the wind profile is changed and turbulence occurs. It is important that shelter filters the wind to avoid turbulence. This is achieved by planting at pre-determined spacings and pruning.

Length: The longer the windbreak the better the protection. Short plantings have a disproportionate edge effect, where wind slips around the ends reducing the area of protection. Gaps in a shelterbelt cause the wind to funnel through at excessive speed. This can happen where there are missing trees or when there is a draughty space at ground level. It is very important to see that stock do not have access to planted shelter.

Height: Height of the shelter directly influences the area of wind reduction on the leeward and windward side. The greater the height the greater the area influenced. Generally, good wind shelter is provided for 15 times (and up to 20 times) the shelter height on the leeward side and up to five times on the windward side.

For further details, please call Bala Tikkisetty at Environment Waikato on 0800 800 401.