Shelterbelts and Shade Trees

Trees add aesthetic appeal to the farm landscape, but their advantages go much further with increases in productivity and better animal welfare when shelter is provided. Southerly spring storms occur annually, killing newly born lambs, and while well planned shelters will not completely solve the problem, they will go a long way to giving stock protection from the effects of cold driving rain. Animal productivity is reduced in cold windy conditions as energy is used to maintain body temperature, instead of producing milk, meat and wool. The other extreme is when animals seek shade in the hottest part of the day.

Shade provided by single large trees or a shelterbelt, can reduce the air temperature by 5-10 degrees. Providing shade against solar radiation and high temperatures is vital.

Initial planning and good establishment management is required to obtain the maximum long term benefit. Take into account prevailing adverse weather conditions, the noonday sun angle and stock movement from paddock to paddock. Gateways and non-continuous shelter can create wind funnels.

Irrigators, especially centre pivot, have their own special challenges. A farm plan is a useful tool for planning and recording shelter plantings.

Every farming region has different reasons for shelter and every property is different in its requirements. Shelterbelts can be planted as a barrier, as a wind filter, or a combination of both.

A barrier type shelter is normally evergreen using conifers like Pinus radiata, Cedrus or Leyland cypress, and Phormium (flax) and Pittosporums where height restrictions apply. The advantage of this type of shelter is that during adverse weather, animals can shelter close to the trees; the disadvantage is that tumbling
air is created in the lea of the shelter giving less benefit downwind .

A semi-permeable shelterbelt can reduce wind speed by 20-50% over a distance 10-20 times its height. Moisture losses due to drying winds are reduced, resulting in greater crop and grass growth.

Deciduous versus evergreen is an important consideration. Tall evergreen shade creates long winter shadows, pugging in paddocks and dangerous ice on roads. Deciduous has the disadvantage of leaf fall resulting in a more open draughty shelter at ground level. Regular hedge trimming of some deciduous species such as Beech cause the leaves to hold on throughout the winter until new buds emerge in the spring.

A good compromise is to use deciduous to create the more open upper wind porosity, while using a native evergreen to provide stock shelter in the lower third.

New Zealand is fortunate to have a useful range of native scrubs and small trees suitable for shelter. Cortaderia, Phormium, Pittosporum, Dodonaea and Griselinia along with taller Sophora, Plagianthus and Manuka can be planted together or to compliment taller growing exotics.

BASIC PRINCIPLES OF SHELTERBELTS

POROSITY - Ideally 50%, to slow the wind, not stop it altogether.
HEIGHT - Preferably as tall as the situation allows. Deciduous trees can provide excellent
shade and shelter, with little shading in winter.
SPECIES - A mix of deciduous trees and native evergreens has the benefit of the ideal
50% porosity.
SPACING - Single row shelter 1.2 to 1.5m apart. Multiple rows and timber belts 1.8 to 2.5m.

Multi row mix of native and deciduous trees

Single row shelterbelts tend to be used where land use or values restrict wider multi-row shelters. Horticulture and dairying are examples where Italian alders ( Alnus cordata) or Poplar and evergreens such as Pinus radiata and Leyland Cypress are trimmed into formal narrow shelterbelts. However almost any species can be trimmed into a shelterbelt, care needs to be taken to form prune early and not cut beyond live needles in conifers.

The sheltered environment of a mixed species shelterbelt or woodlot creates a corridor for migrating wildlife, valuable habitat for beneficial insects, birds and forage for bees.

Species choice is dependant on local soil and climatic conditions. Local advice and experience from tree growers with successful established shelter is very valuable as conditions vary from region to region. Macrocarpa, Radiata, Leyland cypress and Cedrus deodara are traditional faster growing conifers.

Deciduous species such as Poplar 'Crows Nest' and Italian alder, along with a wide range of other species such as Beech, Hornbeam and Oaks offer diversity in leaf shape and autumn colour.

Because windbreaks have a long term impact on the landscape, design and layouts that link with other farm plantings, such as woodlots and riparian areas fit more comfortable into the farm scape. By using natural features, terraces and valleys, shelterbelts will compliment and appear more connected to the landscape.

Potential timber species can be planted with the intention of being pruned to produce a quality saw log, whilst habitat species form the understorey. Alternatively, every second tree can be pruned with the unpruned trees lower branches growing to fill the gaps. Pinus radiata and Leyland cypress are species which prune well under these regimes.

Fast growing species such as Pinus radiata and Poplars can be used to create quick initial shelter with a slow growing longer term species, either interplanted or in an adjoining row.

Careful management is important to allow for the timely removal of the initial shelter species before it suppresses the slower growing shelter.