More information needed for windbreaks

Press, Volume CXVI, Issue 34147, 7 May 1976, Page 6

More information needed for windbreaks

There continued to be a lack of information on suitable tree species for windbreaks on the Canterbury plains, the chief soil conservator of the North Canterbury Catchment Board, Mr R. D. Dick, said in a report to the board’s soil conservation committee this week.

His report prompted by the August storm last year was entitled “observations on the damage to trees in windbreaks, with suggestions to minimise the risk of windthrow’’. Mr Dick said that some years ago catchment authorities had drawn attention to this need and appointments had been made to the staff of the Department of Scientific and Industrial Research for this purpose. The research work carried out to date was no doubt useful in its way, but had provided little or no information to answer the practical need of catchment authorities and landowners. “The question is what are the tree species to provide the most efficient structure and pattern of windbreaks on the Canterbury plains and like sites?”' Mr Dick recommended that the Soil Conservation and Rivers Control Council be asked if it was possible to initiate investigations to help provide information on tree species and the structure and pattern of windbreaks, coupled with the aerodynamics within the windbreak system. He said that- much more quantitative data was also required on the wind flow across the Canterburyplains to site windbreaks to the best advantage. The field information could be recorded in conjunction with the current wind

energy resource survey. This information would be of value to Harewood airport and help with meteorological forecasting. Mr Dick suggested that a windbreak should be planted with the direction as near as possible to the north-south axis. This reduced the velocity of the north west wind and caused the minimum of shading by 7 the sun, the least stock fertility transference and the minimum weed problem. It should include a two or three-row belt of trees with a slower growing species on the windward side — a two tier system. It was important to plant so that the trees in one row alternated with the trees in the row on either side. The location of each tree had an affect on the air current.

At 25 years of age the row of the slower growing species would be about 60 per cent of the height of the faster growing species on the leeward side. The faster growing species at this stage would have entered the period of increased windthrow risk and the slower growing species with more stability- would give the higher growing

species some protection against the force of the wind. Some people affirmed that the roots intertwined and this gave added stability, but this was as yet quite unproven.

The establishment of the two-tier system of windbreak was the major advance in the last 25 years. Pinus ponderosa, Corsican pine, Cedrus deodara and Cupressus arizonica were used in the board's Waimakariri and Rakaia reserves in ihe 1950 s and more extensively on many of the subsidised . windbreaks on farms in North Canterbury. Today these immature windbreaks, little damaged by the storm, provided the greatest collection of two-tier windbreaks on plains land in the country. It would be regrettable if there was any 7 loss of enthusiasm for the two-tier system of windbreaks. On the medium and lighter soils of the plains, where soil erosion was most serious, two or three rows of trees should be planted at about 400 metre intervals—one row of the slower growing species which provided proportionately more of the low shelter and one or two rows of the taller growing species.

As the trees were being planted in an environment not naturally suited to tree grow’th, the selection of species seemed to be very limited and knowledge was quite limited. Largely byintuition and trial and error in the past landowners had grown a number of tree species supplied by nurserymen and these provided a source of information.

For the row on the windward side it was suggested that consideration be given in the following order of priority to: (I) Cedrus deodara, (2) C. atlantica, (3) Cupressus arizonica. Other species that were grown to , a limited extent — Cupressus benthamii, C. torulosa, western red cedar (Thuya plicata) on the more fertile soil, Eucalyptus viminalis, E delegatensis, E gunnii. lambardy poplar, a deciduous with splendid shape but subject to rust, corsican pine (P. nigra) and western yellow pine (P. ponderosa).

For the one row or two rows on the leeward side, Pinus radiata. Where there was deep soil not subject to severe drought Douglas fir was the first choice. Tall Douglas fir trees stood where neighbouring mature Pinus radiata trees were windthrown or stem snapped in the August storm. The eucalypts would stand windthrow and several species required closer

examination with a view to selection and incorporating them in windbreak, systems. The current trials on farmers’ properties with Leyland cypress and also the cross between Cuprsessus arizonica and Chamaecyparis nootkatensis. where stock had originated from research organisations, was a hopeful omen for obtaining improved trees for windbreaks.

Ihe management of the windbreak after it was established offered the opportunity to greatly reduce the risk of windthrow and damage to fences bybroken lateral branches.

The first essential was to ensure that there was adequate establishment of young trees. In a windbreak there should be not less than a 95 per cent survival rate as the soil was going to be used solely for growing trees for many years. When the slow growing species extended it lateral branches westward they needed jo be trimmed inside the protective fence to prevent the branches from growing through or over the top of the fence. This trimming should be repeated when required. If there was one row only of Pinus radiata on the leeward side it likewise required to be laterally trimmed on its eastern side within the protective fence and this needed to be repeated. The trimming should be carried out to as great a height as was practicable. On the western side of the row of pinus radiata it was important to trim the lateral branches and prvent them from over-topping and damaging the growth of the leaders of the row of slower growing species. Where two rows of Pinus radiata were grown on the leeward side, in addition to the lateral trimming previously outlined, the Pinus radiata trees should be low and high pruned at appropriate times up to a maximum of about 70 per cent of the height of the slower growing species. The intensity of prunning depended on the capacity of the row of the slower growing species to reduce the velocity of the north west wind.

The aim was to prepare as many Pinus radiata trees as was. justifiable for the next step in the management, which was to cut down the pine trees at, say, 30 to 35 years of age and sell them as merchantable wood product. These two rows of trees would be replaced with two rows of young, fast growing trees to continue the rotation. The row of slow growing species could be replaced at about 50 or 80 years.

The object was to provide a permanent windbreak pattern on arable plains land subject to wind erosion and reduce to a minimum the risk of windthrow.

The committee recommended that the boards staff, in association with the Forest Service, should draw up guide lines for follow-up advice to famers on windbreak management.