Plant trees now for economic and environmental benefits

By Bala Tikkisetty, Environment Waikato

It’s a good time to plant trees for shelter purposes at the moment as the wet winter weather means they have a better chance of surviving well the transfer from nursery to farm, and of getting established.

Shelter provides a range of benefits.

Climatic stress is a major cause of animal suffering and shelter is one of the few steps that can be taken to reduce that stress.

Animals living in their natural habitat would find their own shelter, but farmed animals may not have enough, especially if they are in a field of grass surrounded only by a wire fence.

So planting shelterbelts is a good way to reduce the negative effects of wind. Artificial windbreaks can also play a part in protecting livestock where cost allows, and rapid protection is essential.

When establishing a shelterbelt, careful consideration needs to be given to site selection and the tree species. Strategic planting is likely to be more worthwhile than blanket planting and, because of the long-term commitment, a careful decision should be made.

Also, shelterbelts help reduce evaporation of soil moisture and transpiration from the grass, and they can therefore be particularly helpful in drought or prolonged dry spells.

Strong winds speed up transpiration rates from plants and can contribute to an internal moisture deficit and slow down growth or even stop it. Movement of the grass blades against each other by the wind is in fact the most important factor in reducing grass growth. Wind can also cause actual physical damage to grasses.

A further benefit of shelter trees is that their roots hold soil together, providing significant reinforcing. Root tensile strength is important, but differs between species. The effect of trees in reducing erosion is a function of tree size, tree stocking per hectare, root tensile strength, and rate of decay after harvest.

And, besides helping with increasing farm productivity, animal welfare, erosion control and soil conservation, shelter can have a range of other benefits.

Shelter trees can be a haven for birds, give shelter for homes, buildings and stock yards, be aesthetically pleasing, and increase the tree species in an area. This is one of the largest ways of increasing biodiversity.

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.

Shelterbelts 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.

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.

Shelter is most effective when sited at right angles to the prevailing or 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.

Porosity or permeability of the shelter belt determines the wind behaviour on the leeward and to a some extent on the windward side. Practical experience has shown clearly that belts of medium porosity (about 50 per cent) produce a much more even windflow 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; turbulence occurs at a factor of about five times the shelter height. It is important that shelter filters the wind to avoid turbulence, which reduces pasture production. This is achieved by planting at pre-determined spacings and pruning.

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 for this reason that a high standard of establishment and management is required. Because of the very nature of wind patterns through the gaps it is extremely difficult to fill them in later. Any early failure must be replaced as soon as possible.

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 with some effect up to 20 times the shelter height on the leeward side and up to 5 times on the windward side, where a high degree of protection is required. Tall shelter gives the most economic protection as the area protected is directly related to the height of the windbreak

For further information call Environment Waikato on 0800 800 401.