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.
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.
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:
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.
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:
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;
This catalogue of problems pointed to the need for change, i.e. that forest management must take account of the effect of damaging winds.
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:-
These trials were not proceeded with because:-
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:-
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.
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:-
3.2 Advantages due to the rest of the actions taken, which would be advantageous under either a block or 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.
After 16 years experience in Canterbury, the following difficulties have become apparent:-
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.
The future direction of planning effort will be addressed separately for each 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.
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.
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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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.
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
1.1 General methods of study.The following equipment and sites have been used:
1.2 Wind behavioural patterns.
Clearly the speed of wind in gusts causes most damage in many storms. The frequency of gusts in relation to the natural frequency of oscillation of trees (which varies by species and silviculture) is important. Many New Zealand regions experience wind gusts exceeding 165 km/hour.
Turbulence is generated as a result of a shearing velocity gradient in a viscous fluid ( eg air ) and occurs in boundary regions ( eg a forest canopy ). Turbulence consists of elements of fluid rotating vertically or horizontally; elements may have diameters of 3 to 10 metres near the ground or forest canopy, but have maximum diameters of from 1,200 to 32,000 metres on a large scale.
1.3.1 Aerodynamics of trees.
Pinus radiata, the main forest tree of the plains, because of its crown shape, inherently produces a high canopy roughness and therefore produces high shear stresses and turbulence.
1.3.2 Aerodynamics of stands.
There have been several investigations into the mean wind loading of stands in wind.
Reproduced below is a report on probably the earliest of these.
The text from Smith 1946, reads:-
"Woelfle (1936-7) also conducted some very interesting wind tunnel experiments to determine the frictional resistance of different types of stand contours (i.e. configuration of the top of the crown canopy). the model forests were composed of spruce twigs attached upright to a (sic) fixed bases.
Figure 21 illustrates the cross-sections of Woelfle's models and shows the total resistance which each model offered to the wind.
Figure 21 illustrates the cross-sections of Woelfle's models and shows the total resistance which each model offered to the wind.
(Here insert big gif, Woelfle 1937 from Smith 1946)
Stand borders, through vertical constriction of the wind streamlines, increase wind velocity by 25 to 60 percent in the border region, with associated gustiness and turbulence. In the zone up to ten times the tree height, six-fold increases in wind velocity have been recorded.
Progressive clearfelling in strips is noted as providing the optimal streamlining for groups of stands, as well as limiting wind effects by causing gradual changes in canopy stress and changes in turbulence scale.
1.3.3 Cd, Coefficient of drag, or drag factor, of trees.
Trees, unlike rigid objects, deform under a steady wind. As a result, the drag force of trees varies directly with wind speed and not with the square of wind speed as for rigid objects. Drag of trees also varies directly with tree weight, but at different rates on different sites.
Groups of trees have higher mean Cd than solitary trees.
1.3.4 Cd Drag factor of stands.
Various investigators, using several methods, have found that
(a) and extremely rough surface
(b) not solid
(c) moving, with large gaps.
These factors create turbulence.
Turbulence results in the transfer of energy from wind to trees, and vice-versa, and in the process some of the energy is converted to heat. There are three distinct forms of aerodynamic roughness associated with the progressive clearfelling strip system.
1.4.1 Stability; or resistance of trees to uprooting.
The decreasing stability of stands with increasing age has been analysed thus:
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:
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.
4.1 Nursery treatment.
From the root development and stability point of view, seedlings grown from seed sown in situ are superior (at least with radiata pine) and it is both desirable and feasible that nurseries produce seedlings which can redevelop, after planting, the natural root growth pattern of strong taproot and first-order lateral roots with even distribution.
Planting technique, coupled with the qualities of the tree seedlings and other establishment factors, has a major and long-lasting effect on sapling and tree stability. There have been particular improvements in planting technique on the plains forests since 1979, and although stands established earlier exhibit weaknesses attributable to faulty planting and seedlings, current plantings should be much more stable.
4.3 Site preparation.
Ripping the plains forest soils with rock rippers has given very significant benefits to seedling survival and to sapling stability. In some cases however, the potential is not realised, due to aspects of seedling and planting quality, until about age 10 - 12 years.
Somerville (1979), testing 111/2 year-old radiata pine trees at Eyrewell Forest found the following indications of resistance to uprooting, in very dry conditions (optimum for stability).
These resistance trends are the very opposite to those found by Brummer (1976) who, three years earlier had analysed the effect of a gale of up to 170 km/hour on the same stands.
The deep-ripping treatments will be especially beneficial in countering the effects of gales when the soils are wet and therefore weak.
There is no clear evidence from the plains forests on the benefit for stability of pruning or not pruning. However, it is clear from work elsewhere that pruning, coupled with carefully controlled heavy thinning of windward stand borders, is beneficial through letting wind relieve the zone of turbulence behind otherwise firm borders.
4.5 Thinning, including relative spacing.
The influence of thinning on stability of stands has been debated over at least four centuries. Only relatively recently however, have adequate measures of thinning intensitybeen available for use and even they are not widely used.
Heavy thinning ecourages stability. It also leads to windthrow, and the tendancy is for more windthrow the older the stand is. But there are optima - too heavy thinning in young stands leads to instability as well. Stands regain stability after thinning.
One useful measure of thinning is relative spacing S/H (square spacing/height) and a useful measure of the response of an individual tree to changing space is the relative height H/D (height/diameter at breast height).
Empirically determined stability-limiting values of these parameters have not been well disseminated in New Zealand, unfortunately.
Papesch, (pers. comm.) found, from wind-tunnel dynamic studies, that the energy absorbtion of trees is particularly high as S/H exceeds 0.4 and that the optimum value is about S/H = 0.33 . Results from other researchers support the finding.
In this context it must be noted that the proposed thinning schedules for plains forests (Inglis 1983, this review) give S/H values which peak (age 10 - 11 years) at about 0.57 (when the trees are only just utilising the stability of the soil ripping). reach 0.4 a full 41/2 to 5 years later, and reach 0.33 only about 7 years later. (See Table 1).
Papesch (pers. comm.) showed that for radiata pine aged 10, 19, 29 and 39 years on unripped Eyrewell Forest sites, resistance to uprooting varied with stem volume, regardless of spacing. Larger trees had more grip.
This grip peaks at about age 20, but remains high for 15 more years. (See graph.)
Faber (1975) writing about Douglas fir, showed that H/D values of 40 were much more stable than values of 70.
4.6 Exposure as a factor influencing tree development.
Excessive wind lowers rates of photosynthesis and damages plant tissues so that growth rates slow.
Conifers and eucalypts react to the swaying caused by moderate winds by increasing their stem diameter growth. Conifers react to this swaying also by increasing the diameter growth of the lee-side propping roots. Mechanical pulling stimulated root growth, in experiments.
4.7 Tree behaviour - trees as beams.
The stem of a tree simulates a beam of uniform resistance to bending stress all along its length. Its shape is paraboloid. Tree stem sway period and damping ratios accord with beam physics laws. Interaction of wind gust frequency and sway periodically is the cause of much stem break. Stand stocking influences all these factors and other related factors.
5.1 Effects of stand boundary shape and topography on the susceptibility of stands to wind damage.
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