Shelter Belts

Background

This datasheet explains the principles of shelter and the things that should be taken into account when designing shelter on a property. Talk to you local nurserymen for more information of which tree species suit your area of Otago.


Advantages of Shelter

Properly designed wind barriers offer many benefits to agriculture and horticulture. As well as limiting stock losses during lambing, calving and shearing, good farm shelter can dramatically increase crop yields and pasture production. For plants such as cut-flowers, a lack of effective wind shelter will result in crop losses every year.

Good farm shelter can help reduce soil erosion, improve both the capital value and resale desirability value of the land, produce firewood, provide pollen for honey production, have a possible source of timber production, allow a pleasant working environment and generally makes for an attractive environment.


Disadvantages of Shelter

Even well established shelter belts can have some disadvantages. These include occupying land potentially productive land, reducing visibility, taking time to maintain, reducing drying effects of the wind on very wet blocks and increasing pest and disease problems. Nevertheless, with proper planning, good shelter on 5% of a farm will result in extra production on the remaining 95%.

Careful choice of species, correct establishment procedures and good maintenance will eliminate many potential problems.

Some species of trees for shelter require a heavy maintenance programme simply because they grow rapidly. Once the primary shelter is established subsequent belts can be of easy-care species.


Which Direction Should the Shelter Run?

Likely locations of shelter belts can be marked on the property plan. The wind barrier should be sited directly across the most harmful wind to give maximum protection. The direction that this wind comes from varies depending on where the block of land is. In general most of the damaging winds in Otago come form either the north-west or south. Therefore two belts one running north-south and the other east-west may be needed to protect a block of land.

If east-west belts are required they should be planted with deciduous species to lessen the winter shading of pastures. On rolling country contours can affect the direction and strength of the wind -the problem areas are through saddles and at the tops of slopes. Aim to augment on areas of natural shelter. Sharp changes in the direction of continuous belts should be avoided for they obstruct normal farm management activities and more importantly they make the trees more vulnerable to storm damage.


Principles of Shelter

A large amount of research and mathematical equations can be used to describe the passage of wind through and over a windbreak. However, to put it simply, shelter belts must be designed as a wind filter not as a complete wind barrier. Ideally, aim for 40-60% permeability to slow the wind down.

Wind hitting a solid barrier moves up and over the top of the obstruction creating an area of low pressure on the lee side of the obstruction. Wind then eddies back, goes into a reverse flow, potentially dumping down in this zone. Any plants and/or stock in this area can be badly affected.

How far out the zone of calmer air extends on the lee side of a shelter is a direct function of the height of the shelter. As rule of thumb, the zone should extend about 5 times the total height of the shelterbelt. After this distance the wind speed picks up and the sheltering effects are reduced. After approximately 15x the height all sheltering affects are lost. Keep this rule in mind when deciding how far apart to place shelterbelts.

Lack of continuity of shelter is one of the major faults of shelter belts established by our predecessors. All species have a limited life and must be replaced before they become over-mature. To obtain continuity it is preferable to have two species of trees with differing lengths of rotation, e.g., a fast growing and a slow growing species. Continuity can also be achieved by planting new belts to take over the role of shelter 10-15 years before a mature belt comes down. Landowners need to know the life expectancy of their existing trees and plan for future requirements. Use a denser slow growing species on the windward side with the faster growing trees on the leeward side. In addition, the double planting of the two different species can be used to give a more effective filtering type shelter in its upper canopy. When mature, the fast growing leeward row can be felled and replanted.


Establishment and Management

Pasture growth must be controlled around newly planted trees as this competes for nutrients and water. 100% establishment of the planted trees is required for successful shelter. Chemical sprays can be used, either before or after planting. Seek advice for the best products.


Fencing off Shelter Belts

The trees need to be fenced off prior to planting. The recommended minimum distance from non-electric fence to a row of trees is two metres. A typical spacing distance between rows of trees is around three metres. This will mean that 2 row shelter belts need to have a seven metre gap between each protecting fence. These spacings ensure that livestock damage does not occur, and it affords adequate room for tree development. The gap between trees within a row depends a little on the species selected. As a rule a 2m gap is adequate.


Ripping Prior to Planting

Improved establishment can be obtained on most soils by winged ripping the tree lines prior to planting. Ripping should be undertaken in summer (when soils are dry) to give maximum effect. This timing also allows the soil to settle back and give more suitable planting conditions. Ripping should be done with a winged ripper as this gives a maximum shatter zone. It is the most cost-effective operation you can do to ensure good establishment.

It is most important to use only top quality tree planting stock. And to plant correctly to avoid root strangulation and later toppling, i.e. deep enough, upright stem, and top soil well compacted around the seedling.


Nutrient Requirements

A fertiliser such as DAP (50gm/tree) should be used at planting to maximise tree growth. Once the belts are established fertilisers are not needed. When spreading fertiliser after planting do not place it directly next to the trunks of the trees. This will not encourage root growth out into the soil and the trees can become unstable with small roots and large tops.


Young Tree Training

Tree training or pruning regimes depend on the tree species chosen. As a basic rule for Leylandii and Macrocarpa, prune side growths off (especially on Leylandii, Leightons Green). You will most likely need to prune the lee-side before the side into the wind. Take the tip out of the tree in the second year and then allow it to grow up. This will give a good side span growth to the other trees in the row. Failure to do so will disadvantage the root anchoring ability of the tree, especially in windy conditions. It also wastes headland space. Talk to your shelter professional for detailed advice on your specific site.


Planting Around Ponds

Plantings should always be made to shelter the pond from the prevailing wind. Note: wild duck generally fly into the pond against the wind. Tall trees growing on the wrong margin of the pond not only obstruct entry but also exclude sun.

With the exception of one or two overhanging willows or native Carex species, all plantings should be kept at least 6m back from the pond edges. A sunny, sheltered loafing margin is most important.

Tall growing trees such as poplar or some species of eucalypts interspersed with native toe-toe and flax are a most attractive combination, exclude draught and create good nesting cover.

In the past the traditional practice has been to fence off trees around a pond and allow stock access for drinking water. With a change to more sustainable farming systems and more care for waterways, it may be better to completely fence ponds. Water is then pumped from this reservoir and the stock watered from troughs.


Planting Stream Banks

The importance of protecting stream margins from erosion, water resource, fisheries, wildlife and aesthetic point of view should not be underestimated. On-site stock water supply will be important in dry weather. Certainly clean down-stream water supply for rural, urban and industrial users is becoming increasingly important. Do not plant too close to stream channels or ditches which restrict root run. Come away three metres or more for establishing trees.

Soil

Very few of the farmed soil types in Otago have fertility limitations to growing trees. Limitations arise mostly due to excessive wetness or dryness. Problem soils can be peatís, swampy soils, soils with impeded drainage, soils with hard pans preventing root establishment, and deep gravels formed on old river beds.

The limitations of these problem soil types must be recognised and catered for by properly preparing the site before planting. Then select the species most tolerant of the problem in question.