Shelter Trees For Economic And Environmental Gains
Planting a shelterbelt is an option for some livestock farmers to reduce the adverse effects of inclement weather. And this is the time to plan for planting shelter.
Whether it is heat or cold, physiological stresses impact on productivity by the indirect effects of reduced grazing periods and therefore reduced feed intake and production.
Traditionally it’s been thought shelterbelts help to reduce evaporation of soil moisture and transpiration from the plants. Live shelter is particularly helpful in drought or prolonged dry spells.
In particular, strong winds enhance transpiration rates, 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, the plant appears to lose its turgidity, photosynthesis is constrained and growth is curtailed.
Agitation of the grass by the wind is the more important factor in reducing grass growth. The reduced growth rates are reflected in reduced dry weight, leaf area and height, but leaf area is much more sensitive to wind than growth of whole plant weight, which supports the concept that leaf cell expansion is specifically limited.
Shelterbelts control the removal of topsoil 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.
Roots hold the 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.
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.
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 important, along with an appreciation of the interaction with 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.
Shelter is most effective when sited at the 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.
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 belts of medium porosity (50%) 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 5 times the shelter height.
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.
Height of the shelter directly influences the area of wind reduction on the leeward and windward side. Tall shelter gives the most economic protection as the area protected is directly related to the height of the windbreak.
- Bala Tikkisetty is a sustainable agriculture advisor (technical) at Waikato Regional Council. Contact him on 0800 800 401 or email email@example.com.