Disaster and resurrection for Eyrewell Forest

This page will probably mostly be of interest to forestry professionals concerned with active management of planted forests. If you have the time, however, stay a while and read about a controversial forest design that was implemented to stave off repeated disastrous windthrow, subscribing to the maxim that reads ... 'People who are not prepared to heed the lessons of history must be prepared to experience them again'.

The point of this is, that if your forest is where the climate dishes out damaging winds, and if those winds come from predictable directions and there are not more than two - or maybe three wind directions for damaging winds - damage due to wind can be significantly minimised through the use of specific management practices.

Location and climate.

Eyrewell Forest is located on the Canterbury Plains of New Zealand, on the north bank of the Waimakariri River. The site is essentially flat, sloping uniformly and evenly from the west down to the east as the plains drain down to the Pacific Ocean in the east. The shallow, light loess-based soils overlay compacted gravels of greywacke rock. These gravels are hundreds of feet deep. Rainfall is a fairly evenly spread 32 inches per year.

Eyrewell Forest had its' origin in the economic depression of the 1920's and 1930's. In order to provide employment, find a use for poor quality, unwanted land, and provide a wood resource for the future, Government decided to acquire and plant this land. Although the main species was Pinus radiata, other species including Pinus ponderosa and Pinus nigra var. laricio were planted. Most of the forest of 18,500 acres was planted originally in the four years 1929 to 1931 inclusive. World War II intervened at a crucial stage for the crops, because just when the stands should have been pruned and thinned, the abled-bodied men who should have done this work were taken off to fight. Hence the stands remained mostly at the close stocking of 6 x 8 feet.

The plains are subject to strong winds from two points of the compass in particular; north-west and south-west, and these winds, the north-west föhn-type in particular, can be so strong as to inflict dreadful damage to forests. In the case of Eyrewell Forest, severe windthow damage in contiguous uniform stands of radiata pine, caused by north-west gales, led to a form of Wagners Blendersaumschlag being established, since it was considered to be probably the best silvicultural system to counteract this type of damage. However, even this method has its own problems; it is not simple for people to understand and put into practice properly - so that they tend to say that it is too difficult to bother with, it has a restrictive influence on timber sales, and it brings about new types of wind damage - which I contend are nevertheless much less bad than what existed before. Also contributing to the argument are developments in planting site preparation, seedling quality, and general silviculture which on their own do aid in conferring improved resistance to wind-throw. Opinions among forest managers in the 1980s were so divided about the benefits of Wagners Blendersaumschlag at Eyrewell Forest where it had been fully implemented, that a meeting was held to review the subject.

I present below two important papers from that review meeting.

Eyrewell Forest is probably unique in being the only medium-sized forest in the Southern Hemisphere using Wagners Blendersaumschlag as a forest-wide silvicultural system. I have put a description of the forest and some of the significant discussions about the structure of this forest on the Internet to make available some information about this interesting forest.

The map below is a freehand sketch of a slice through the South Island of New Zealand showing the approximate location of Eyrewell Forest, and for the compass-challenged, the directions of the two most damaging winds.
The size of the forest is drawn slightly larger than true scale, but the map does convey a good impression of location, orientation and shape. You can see the mountains to the west which rise to 8,000 feet and more, the green sloping plains in the lee of the mountains (in a NW wind, and that a SW wind would sweep up the flat plains with nothing much to reduce its' velocity.

A note too about the north-west wind. It comes over the Southern Alps to the west in anti-cyclonic weather conditions, which are a feature of the South Island of New Zealand. The wind arrives at the west coast loaded with moisture. As it rises up over the mountains, it loses temperature at a certain rate, say 3 deg F per 1,000 feet, until it drops its moisture which it will ( the West Coast of NZ is quite a wet place ), then it loses heat at the greater rate of, say 5 deg per 1,000 feet. All the way down the eastern slopes it gains temperature at the greater rate of 5 deg per 1,000 feet. By the time this wind reaches the forest lower down on the plains, it is hot, it is dry, and it is strong. ( I haven't checked the exact rates of change of this adiabatic lapse rate, but the rates are about right and the principle certainly is ). It not only may damage the trees in the obvious physical ways by breaking them, but it sucks moisture out of everything it touches, and pine trees suffer traumatic damage in the cambium region resulting in the formation of resin pockets. The annual rainfall of 32 inches is therefore effectively much less. The counterpart of this wind in other regions of the world are, the föhn, the chinook, the mistral.

Explanatory note about the species and silviculture system on this site.

It is important to realise that Pinus radiata, the species of choice (because its growth rate, multipurpose timber and consequential profitability are greatly better than the next best species on such a dry site), is a strong light demander, and must be managed using clearfelling techniques. Any silviculture technique using shelterwood will fail to achieve the best potential on these sites and with this species. Also, all stands are carefully re-planted using high quality seedlings or cuttings of selected purpose-bred clones which give the forest manager some control over the result. Natural regeneration is too haphazard, variable, prolongued and susceptible to weeds and other pests, to be employed. Additionally, these days, since very good forest and stand simulation computer programs are available to forecast growth, yield and timber quality and because such forecasts depend on precise input of variables (such as clone, planting spacing), no forest owner who intends to maximise their returns from the forest will leave so much to chance.


Major review paper.

The information which follows is drawn, largely verbatim, from the in-house New Zealand Forest Service paper "Forest Structure of the Plains" by B. J. Swale (Senior Forester, NZFS, Christchurch, and M . J. Inglis (Forester, NZFS, Balmoral Forest), a report prepared for the 'Structure' group contributing to the 'Plains Forests Review' held by the New Zealand Forest Service during 14 and 15 March 1985.

Additional text has been added to help any reader not of the era understand some expressions or particular circumstances that may not be known or clearly understandable. Drawings and photographs are added to give better understanding of what is being discussed. These were not part of the original document because the participants could see for themselves what the situation was.

Another paper in the same set, dealing in detail with background to the forest strip felling and management system, and reviewing overseas literature as at 1983, is much lower down, here.

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INTRODUCTION

Wind is one of two climatic factors causing most damage to exotic forests in Canterbury. The most damaging wind in the region is from the north-west - a turbulent wind capable of flattening large areas of mature forest, and enlarging clearings within forest stands susceptible to wind damage.

There have been three major wind storm events in Canterbury in the last 40 years, all involving NW winds:

  1. In July 1945, storms with a maximum gust of 144 km/hour caused widespread damage over the Canterbury Plains. Most of the plantations damaged were 18 to 20 years old; at Balmoral Forest approximately 1,440 hectares of forest were blown over (Jolliffe, 1945),
  2. In March 1964 (over four days) a more localised storm of wind force 111 km/hour caused considerable loss of timber in Eyrewell Forest (and in many other places on the Plains). This has been described by Wendelken (1966),
  3. In August 1975, winds of maximum gusts of 170 km/hour caused severe damage to many stands circa 45 years of age, although many as young as 10 years were also badly damaged.

There are many other cases in the cited literature of less spectacular wind storm events in Canterbury.

This paper describes the development of Canterbury forests up to 1964 and reviews corrective measures taken to ensure the development of manageable stands.

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PRE-1964 SITUATION

The damage caused to plains forests by wind at Eyrewell in 1964 resulted in a re-evaluation of forest structure. During this process, a number of features of these existing forests were considered:

  1. There were rectangular extensive compartments with consequent simple logging and subsequent site preparation for re-establishment.
  2. Very little thinning had been carried out, resulting in densely stocked, unstable stands.
  3. There were relatively tall trees (many over desirable height of 15 metres).
  4. Clearfelling coupes were mostly rectangular holes cut into the crop.
  5. Clearfelling coupes were located badly in respect of windthrow initiation.
  6. Roads were at no special angle to the wind.
  7. Undisturbed soil with a severely compacted zone which limited tree root penetration to no more than 80 cm.
  8. Stand edges had had no special treatment to confer windfirmness to the stands as a whole.
  9. There was a very narrow range of age classes (see Wilson, 1978; p. 154).
These characteristics gave rise to the following problems in normal years:-
  1. Windthrow caused severe damage on the lee sides of cutovers (but little on the windward sides).
  2. Windthrow drives, once started, found no obstacle until they met the next road or felling edge - see example on p. 490 in Somerville (1980).
  3. Scattered windthrow occurred in all stands of susceptible height. The range of heights given by various authors ranges from 10 metres (Jolliffe, 1945) to 16 metres (Wendelken, 1966).
  4. Much of the forest yield normally came from salvage (of windthrown trees), (c.f. other forests in New Zealand where it does not).
  5. Trees were unstable even in moderate gales.
  6. Stem diameters were low, leading to problems in utilisation.
Wind damage resulting from a clearfelling coupe cut into part of a compartment of uniform height looked like this ...


Attempts were made to deal with these problems, but a slowly developing market made progress difficult. Most attempts were of various forms of strip felling, either for regeneration, wind protection, or both. In general, their scale was limited and they did not succeed as Wilson (1978, p. 155) explained. In severe gales their structure failed; windthrow became more frequent as new faces were exposed in older stands.

The 1964 gales greatly accentuated the old and well-known faults, and added some relatively uncommon ones;

  1. Windthrow commonly commenced just behind stand edges (windward side) and enabled long downwind drives of windthrow to occur.
  2. Small pockets of windthrow enlarged rapidly both upwind, but predominantly, downwind.
  3. Wind damage associated with wide roads, clearfelling areas and logging tracks was greatly accentuated. No forest structure features halted the drives (there was uniform forest downwind, all equally susceptible).
  4. Roads were blocked for days and telephone lines (above ground, along roadsides), were cut.
  5. Stands over 15 metres in height suffered most damage.
Some of the gales and soil conditions giving rise to this damage were not exceptional. The 1945 wind (with wet soil) reached 144 km/hour, and the 1964 winds were only about 86 - 106 km/hour. It is important to remember that the 1964 gales were acting on over-stocked and over-mature stands for the most part.

This catalogue of problems pointed to the need for change, i.e. that forest management must take account of the effect of damaging winds.

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RE-PLANNED FOREST MANAGEMENT

Damage in the 1964 gales was so severe that major improvements in techniques were clearly called for if these forests were to remain as viable entities. (Senior management had asked whether or not, considering the nature and persistence of the windthrow and other problems, it would be better to quit the forests). The major emphasis was in restructuring the layout of these forests.

Keith Prior (1959, p. 67) was probably the first to recommend progressive clearfelling in strips as the optimum forest structure for the Plains forests. John Wendelken (1966), however, was the first to publish detailed proposals and these were taken up by Harold Wilson in the working plan for Eyrewell, 1966-71.

This plan specified that the strips should be 10 metres wide, laid out as at present (see diagram below). Under pressure from the then Conservator of Forests (C.E.O. of the region), Mr M. J. Conway, the plan also specified that trials should be initiated of:-

  1. Progressive clearfelling in blocks.
  2. Strips planted with their long axes in line with the N W wind.
  3. Strips aligned at right angles to the N W wind, but with varying height intervals and widths.

These trials were not proceeded with because:-

  1. Research by Papesch (pers. com.) on wind turbulence gave clear indications of the superiority of 120-metre wide strips over blocks.
  2. (The fact that elsewhere) only one small usage of strips parallel with the wind (in a small forest in Denmark) (was known world-wide), and significant planning problems, precluded use of this system. (The use of strips at right angles to the wind was fairly well known, by comparison).
  3. The normal (operational) variation in timing (of felling and replanting, from the ideal timing) would give the variation in height sought, and some variation had already occurred.

In respect of wind, the major step taken was to alter forest structure away from the simple, haphazard (they mean 'regular', but disregarding the wind direction) arrangement of rectangular compartments, as a result of these decisions.

There was good evidence from elsewhere in the world (Troup, 1952; Smith, 1946; Gratowski, 1956; and others) that the system known as 'Progressive clearfelling in strips', with the strips organised into cutting sections, was clearly the optimum system.

The forest climate here has one outstanding charcteristic favouring such a system: damaging winds come from two directions only, with one direction clearly predominant, and the two at ninety degrees to each other.

The new system was designed and installed with the following features:-

  1. A new roading system was laid out, eliminating many roads suseptible to blocking by windthrow.
  2. Stands were laid out to suit progressive clearfelling in strips, with four strips each 120 metres wide in each cutting section, aligned at 90 degrees to the north west wind.
  3. A new compartment layout was required to suit the new strip direction and roading system.
  4. Soil and subsoil gravels were loosened by ripping to 45 cm depth, with a high proportion deep-ripped to a depth of 1.2 metres, to improve tree root penetration.
  5. Relatively heavy thinnings and low stockings were adopted, based on merchantable yield data from the first crop (Wilson stated in the 1966 Eyrewell Working Plan that there were about 300 - 370 merchantable stems/ha in the old crop).
  6. Adoption of a maximum rotation of 32 years to minimise windthrow hazard.

Thus, in re-planning the management of Eyrewell Forest, over-riding consideration was given to minimisng the effect of wind by altering road aligment, forest layout and structure, as well as establishment and silvicultural techniques.

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ADVANTAGES OF RE-PLANNED FORESTS

The advantages are seem as being due primarily to either a progressive strip felling profile, or other changes made as a consequence of adopting the system.

3.1 Advantages claimed for a strip system are:-

  1. Fresh pockets of windthrow in newly exposed mature stand edges will seldom develop because they are never exposed to the NW wind. This is a consequence of felling into the NW wind. This progressive felling sequence could be retained with a strip system as a present, or with a progressive felling of blocks.
  2. The younger shorter trees which are less prone to windthrow, help deflect wind up and over the older taller trees in their lee.
  3. Stands are presented in the best way to the worst wind (NW), and should present a good aspect to the next most damaging wind (SW), despite risks of long windthrow drives.
  4. Extensive lines of windthrow from the NW should be longer be possible.
  5. Tracks for silviculture treatment must be at right angles to the NW, and therefore should not initiate windthrow, in the manner that tracks at other directions almost certainly will.
  6. The whole forest is being developed in a structure that should educe the likeihood of wind damage. However, should damage occur, it is likely to be less serious due to a spreading of the age classes (throughout the forest).

3.2 Advantages due to the rest of the actions taken, which would be advantageous under either a block or strip system:-

  1. Roads will never be blocked again as badly by windthrown trees.
  2. Re-oriented roads can less frequently direct wind into the flanks of stands and thus initiate windthrow drives at the points of high air pressure.
  3. Any one age class can be dispersed over the whole forest, therefore a catastrophe in one limited area will not destroy all of one age class.
  4. Shorter, fatter, more stable trees result from the wider spacings and shorter rotations.

PERCEIVED DISADVANTAGES OF THE STRIP SYSTEM

In evaluating the disadvantages of the strip system a separation has been made into two categories, namely those inherent in the system itself, and those practical difficulties in managing the system as it now stands.

As Eyrewell has been in the system for 16 years or half a rotation (c.f. 6 years for Balmoral Forest), the discussion on the disadvantages of the strip system concentrates on the former forest.

4.1 Disadvantages inherent in the progressive felling strip system.

  1. Somerville (1981) has reported that during gales, excessive downdraught may develop in the lee of older stands and damage adjacent younger stands.
  2. Serious growth suppression occurs in the first planted row in the lee of the oldest strip, and the less obvious growth reduction of the next adjacent row will have an effect on the total merchantable volume in the narrow Eyrewell strips.
  3. At the time of clearfelling the 32 year old strip, the sudden exposure to turbulent winds will not only affect the 8 year old stand downwind, but also the lee of the 24 year old stand upwind of the clearfelled strip.
  4. In a study of wind damage associated with the exposed edges of closed canopy stands, Somerville (1980) concluded that abrupt increases in stand height of at least five metres caused concentrated damage in the first 100-200 m of the stand down wind from this edge. He suggests that "this pattern of damage calls into question the safety of laying out strips with abrupt changes in height confronting a potentially hazardous wind, viz, the 'strip system' in Canterbury". The profile of the strip system is shown in the diagram below, and confirms Somerville's fears. Height profile of a cutting series.
  5. Forest staff have to follow the forest stand pattern carefully and logically when establishing, or clearfelling and re-establishing the forest. Mistakes are expensive to correct.
  6. The large number of small stands that are a result of the strip system complicate all management operations. The strip system requires significant increases in the time and expenditure in forests which, due to their flat topography, should be simple to manage and administer.
  7. Strips designed to combat NW winds may be susceptible to winds from other directions.
  8. Should windthrow occur in patches, re-establishment would be complicated, particularly if the damage occurs over two or moe age classes.
  9. At Eyrewell Forest, where the system is being established from bare ground, there is a loss of productive area of one-sixteenth of the forest productivity. Compounding site preparation and weed control costs, in addition to administration costs, are further expenses due to the fallowing of land under the strip system.
  10. Market constraints of the strip system may be a problem, being dependent on markets for smallwood being available at a set time. For example, past history with Canterbury Timber Products ( medium density fibreboard plant, the major consumer of industrial smallwood in Canterbury ) has revealed significant harvesting and marketing problems. These constraints involve interaction with timber supply from other forests.

4.2 Practical Difficulties in Managing the Strip System.

After 16 years experience in Canterbury, the following difficulties have become apparent:-

  1. Administration as a whole has not come to grips well enough with the needs of these forests. This is highlighted by delays in tending, loss of Forest Service logging, and in general al lack of understanding, both on stations and by Conservancy ( the regional head office ), of the special requirements of the strip system.
  2. In the past, strips have not been set up properly; 'eyeball' layout and out-of-sequence strips contribute heavily to current problems.
  3. A good deal of the potential of the system has been jeopardised by the use of too shallow ripping, sub-optimal planting techniques, and sub-optimal seedling preparation and handling techniques. These have resulted in inadequate root systems with a higher incidence of toppling and windthrow than might have been expected. Current techniques, however, are much improved.
  4. Delays in siliculture tending have a significant impact on subsequent forest structure and windfirmness. Thinnings to waste and utilisation thinnnings have been delayed. Possible benefits of maximised thinning yield in no way compensate for the jeopardised crop stability and loss in volume growth of the main crop. Pruning efforts have occasionally had their potential benefits decreased by similar delays.
  5. By not thinning the catch crops, the loss of stand permeability ( to NW wind ) may further endanger the strip system. (The expression 'catch crop' in this context means a short rotation crop (16 - 24 years) planted in the cutting series started on bare ground, so as to obtain some yield from the 3rd and 4th strip sites until the year comes for them to be planted in their proper sequential time. )
EVALUATING 1960's AND 1970's RE-PLANNING EFFORT.

The discussion to date has covered the need for change, for management to allow for the damaging effects of wind. Steps taken in the 1960's have been outlined, and their benefits listed. In addition, those disadvantages inherent in such a structure have been detailed. Practical difficulties currently encountered in managing an imperfectly established and maintained system are also stated. These factors all point to a need to re-evaluate the forest structure and management practices appropriate to the acknowledged damaging effects of wind on the Canterbury Plains.

While the establishment of the strip system is virtually complete at Eyrewell, for Balmoral (with only 6 years on the system), a single years' planting of fallow strips could eliminate the strip system. It is also worthwhile to note the reasons stated for the two-fold difference in strip width between Balmoral and Eyrewell.

The width of the strips at Eyrewell Forest has been chosen to suit windrowing efficiency for site preparation and also clearfelling ( it is the minimum workable width for felling 28 to 30 m trees ). It is the optimum for wind protection. Wind turbulence studies by Papesch have shown that a strip width of 120 m results in a degree of turbulence over each cutting section which is likely to cause least damage from NW winds.

The reason for adopting 250 m wide strips at Balmoral appears to be that damaging winds occur over a wider arc and the trees have more lean; strip width has accordingly not been regarded as quite so significant (Wilson, 1978, p. 178).

The effectiveness of the strip system, however, has not been proved, at the half-way stage of the 32-year rotation. Some doubts exist concerning its effectiveness, but management problems and associated higher costs have arisen in implementing the system. The advantages of improved establishment and tending techniques may well be sufficient to confer stand stability whether managed under a strip system or a block system.

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FUTURE DIRECTION FOR FOREST MANAGEMENT

The future direction of planning effort will be addressed separately for each forest.

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Eyrewell Forest.

At Eyrewell, re-evaluating forest structure is not so simple. Virtually the whole forest has been established under the system, some correctly, other areas not as planned. Preliminary investigations show sizeable costs associated with retaining fallow strips. One option currently receiving most attention is that of converting areas established since 1979 to blocks by planting the fallow strips. This poses problems with currently established strips.

Over much of the forest, there is no real option but to retain the strip system. However, further investigation will ensue to determine whether or not recent fallow strips be planted, those occasioning small blocks throughout the forest, and on the practicality and costs, of converting one of Eyrewell's three blocks, probably the western block, to a block system.

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Balmoral Forest.

The strip system has been laid out over only a minor area of the forest, beginning in 1977. At 250 m wide strips where no tending has occurred, with strips installed as clearfelled areas became available for re-establishment, the system has not been properly implemented. If thought necessary, it would be a relatively simple matter to convert Balmoral Forest; a single year's effort would plant the fallow strips, thus returning the forest to a block system. In this way, Balmoral Forest could serve as a ( experimental-speak ) control against a continuing strip system at Eyrewell. At Balmoral, there is a lesser cost if the forest returns to a block system and it is subsequently proved that the strip system is more successful, than if it were to continue to be converted to the strip system and have the strip system proved no more satisfactory.

As the strip system is unproven, and as little opportunity cost is incurred in abandoning the system, this option ( of converting to a block system ) has been recommended for the majority of Balmoral Forest. Such a proposal obviously suggests a management judgement that establishment and tending practices will be sufficient to confer stand stability in all but exceptional gales. Again this is unproven, and is questioned in some quarters.

In Medbury Block, where the system was first established, and where the remaining old crop will be clearfelled this decade, it is proposed to establish the strip system correctly. This not only involves a well-planned layout, but reassessing strip width, and ensuring that once embarked on the system, it is adhered to correctly.

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ACKNOWLEDGEMENTS

This account is an unholy coalition of opposing views tied together at the 11th hour, with considerable strengthening by non-members of the group, to reflect the confusion of unresolved factors, yet hopefully suggesting some options available for forest management. Members of the group were N C Clifton (Principal Forester, Christchurch), M J Inglis (Forester, Balmoral Forest), F D Neither (Forest Ranger, Eyrewell Forest), D J Powell (Forest Ranger, Balmoral Forest), R R Robson, (formerly Senior Forest Ranger, Eyrewell Forest, until January 1983), and B J Swale (Senior Forester, Christchurch).

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REFERENCES

  1. Clifton, N. C., 1983; Plains Forests Review - Siliviculture Group. The Old Crop. N. Z. Forest Service unpublished report. (4 pp).
  2. Gratowski, H. J., 1956. Windthrow around staggered settings in old-growth Douglas fir. For. Sci., (2): 60 - 74.
  3. Jolliffe, W. H. 1945. Wind damage in Canterbury. N. Z. Journal of Forestry, 5(2): 154-155.
  4. Prior, K. W. 1959. Wind damage to exotic forests in Canterbury. N. Z. Journal of Forestry, 8(1): 56 - 68.
  5. Smith, D. M. 1946. Storm Damage in New England forests. Thesis, Yale School of Forestry.
  6. Somerville, Alan. 1980. Wind stability: forest layout and silviculture. N. Z. Journal of Forest Science. 10(3): 476 - 501.
  7. Somerville, Alan. 1981. Wind damage profiles in a Pinus radiata stand. N. Z. Journal of Forest Science. 11(2): 43 - 65.
  8. Troup, R. 1952. Silviculture Systems. (2nd edition). Oxford, Clarendon Press.
  9. Wendelken, W. J., 1966. Eyrewell Forest: a search for stable mamagement. N. Z. Journal of Forestry., 11(1). 43 - 65.
  10. Wilson, H. H. 1978. Management of Canterbury State Exotic Forests. N. Z. Forest Service unpublished report (in 'Workshop on Canterbury Forests', 1978; NZFS, Wellington).

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Major review paper.

WIND AND FORESTS - A REVIEW, WITH SPECIAL REFERENCE
TO THE CHOICE OF SILVICULTURAL SYSTEM
FOR THE STATE PINE FORESTS OF THE CANTERBURY PLAINS.

B J Swale

ABSTRACT

This paper is prepared for a review of the management of Canterbury Plains State forests, held in Christchurch in March 1983. The dry windy climate and flat sites on these plains restrict much forestry to systems of uniform forest with clearfelling. The dominant species is Pinus radiata, although Pinus muricata is suggested as an alternative because its wood develops significantly fewer resin pockets on such sites.

The stability to windthrow and windbreak of these forests is the main topic. The turbulent and gusty nature of wind is discussed and explained, and related to tree development and such factors as soil, root fungi, weeds, nursery treatment, planting technique, planting site preparation, pruning, thinning and clearfelling, and stand structure.

The interactions of forest with wind, the effect of topography, stand edges, stand structure and spacing are reviewed. Critical height, hazard classification and development of priority ratings are discussed. The significance of the predictability of direction of winds in Canterbury is emphasised.

It is recommended that the system of progressive clearfelling in strips be retained and extended, and that heavy thinning regimes developed using the N Z Forest Service suite of computer programmes called Silvicultural Stand Model ( SILMOD ) be modified to use heavier relative stockings which wind research indicates should place the forest at more acceptable and markedly reduced hazard from wind.

Enhanced levels of research, training and evaluation of wind in forests and into wind damage are recommended.

INTRODUCTION

This paper is a precis of a review of over 70 papers. The intent is to bring together the latest knowledge of wind and forests with the best of the older knowledge. In the interests of brevity, few citations are made and detailed examples kept to a minimum. These are to be given in the fuller version, with all references.

A. SPECIES CHOICE

Light-demanding species are most suited to the canterbury plains sites, and three genera have been most successful; Pinus, Eucalyptus and Pseudotsuga. Pinus radiata has clear advantages over most other pines, although the nearly equivalent growth of Pinus muricata and its significantly lower production of timber-degrading resin pockets warrants investigation. Performance of Eucalyptus species in a forest situation on these sites is insufficiently well understood currently for recommendation to be made. Pseudotsuga is insufficiently tested in the State plains forests to be recommended, but performs well in other plains forests.

B. CHOICE OF SILVICULTURAL SYSTEM

Suitable systems appear to be:

1. Uniform stands, with generally rectangular, large coupes.
   1.1 arranged in order of progressive felling into the wind
   1.2 not arranged in any particular order in respect of wind.

2. Uniform stands with relatively narrow coupes
   2.1 arranged in order of progressive felling into the wind
   2.2 arranged in other orders or directions.

3. Non-uniform stands with either mixed species or mixed ages or both. They can be created and managed as for the options given for (1) and (2) above.

C. FACTORS TO BE CONSIDERED IN THE CHOICE OF A SILVICULTURAL SYSTEM

1. Wind

  • 1.3 Aerodynamics.

    2. Soil

    Much concern exists overseas on problems associated with sodden soils and impeded drainage. Mostly the plains sites are free of this type of problem, but the presence of a compacted layer at about 45 cm can lead to similar problems in wet weather. The compacted layer also results in discontinuous root development and resultant tree instability. Soil ripping with rock tines ameliorates both problems. Soil particle size affects stability as the following graph illustrates:

    .

    Angle of maximum resistance, from Fraser, 1964.
    .

    (Fig. 4 from Fraser 1964]

    3. Fungi and other plants

    The Canterbury Plains so far have no records of root-rotting fungi, but in moister forests, Armellariella root rot is a significant problem in New Zealand.

    Ih the younger stages grass and herbs, and at the sapling stage of growth as well gorse and broom (woody weeds) can severely restrict tree root development, with consequent loss of tree stability.

    4. Trees: factors influencing their development.

    Some stand parameters at Eyrewell forest.
    .

    Mean bending moment/stem volume ratio, versus age in years, from Papesch, 1980.
    .

    [ Fig 19 from Papesch.]

    5. Interactions of forests with wind.

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