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New Zealand's exotic forests


H.V. HINDS is Conservator of Forests, New Zealand Forest Service. This article is based on a paper prepared for the Eighth Commonwealth Forestry Conference, Nairobi, 1962.

The development of tending practices

EXOTIC FORESTRY had its beginning in New Zealand at the turn of the last century when some far seeing administrators initiated a planting program in anticipation of the depletion of the indigenous forests. The steady increase in planting, with a crescendo in the afforestation boom of the 1920s, is history which has been well recorded. Large-scale operation resulted in bad age-class distribution and silvicultural malpractices which have embarrassed present-day management. Planting decreased during the depression of the 1930s and almost came to a standstill in the second world war.

Tending of exotics between the wars

The possibilities of tending were first raised in the 1920s. Planters of forests are usually reluctant to thin their own handiwork with any severity. In addition, the lack of markets for small wood, in the face of the still plentiful supplies of large-sized indigenous timber with a high proportion of clears, meant that any thinning would inevitably be carried out at a loss, if, indeed, the product was saleable at all. Relief labor that was available in the depression was used more for planting new acreage than for improving existing stands. In the 1930s some low pruning was done - all stems - corresponding to the brashing of European practice. The first attempts at thinning were the removal of fourth lines and the freeing of certain trees by the removal of competitors. In Southland the presence of two small mills which were willing to take small timber led to some commercial operations on orthodox lines. Thinning is notoriously a practice where one has to double the number one first thought of, and the early efforts in marking reflected the inevitable timidity of inexperienced silviculturists. Frequently only stagnating and moribund stems were removed. The usual bogey of wind-throw danger was raised. Even by those who had seen thinning practices in Europe the rate of growth of radiata pine was not appreciated. It is difficult for foresters educated in terms of "orthodox" rates of growth to realize, when looking at site II radiata pine at the age of 10 years with an average top height of 40 feet (12 meters) that in another ten years' time its top height will be 85 feet (26 meters).

In addition, in the case of radiata pine, a policy of no thinning was supported by the theory that this species "thins itself." It had been observed in old plantations that there was a wide scatter of diameters, a proportion of the trees having asserted dominance from an early stage and maintained their superiority even in dense stands.

The second world war has been blamed for the delay in tending young stands but, in view of the meager progress made in the late 1930s, this factor has probably been overestimated.

Lack of experimental treatment

While there were weighty reasons against large-scale thinning, there was a deplorable lack of the spirit of enquiry, resulting in the almost complete absence of experimental plots. A sample plot program inaugurated in the 1920s had lapsed during the depression of the early 1930s. It was not until 1943 that a plot was established in a thinned stand in the 100,000 acres of radiata pine in Kaingaroa Forest, and that only because an opportunity had arisen during the war to make a thinning to obtain wood wool. Later, some irrepressible pioneers made surreptitious trial thinnings well removed from roads and out of sight of unsympathetic eyes.

Postwar development

After the second world war a slight breeze blew through New Zealand's silviculture. A number of New Zealanders had seen European forests during service with forestry companies, and a number of officers had been trained, and some recruited, overseas. Young men, imbued with the doctrine of Craib of South Africa, or tropical foresters used to removing half the plantation of teak at the age of three years, were not prepared to accept without question the dogma of "thinning can't happen here."

By this time much progress had been made in finding out the utilization properties of radiata pine, following on the establishment of Waipa State Mill and two company-owned pulp plants about the year 1940. The chief lesson that had been learnt was its remarkably good quality; it was, as a former Director-General of Forests has said, "better than we had any right to expect." The effect of knots was clearly seen, although it was not until later that the correlation of knots with tending practice was fully appreciated. Grading rules were drawn up and with some reluctance were accepted by the timber industry. Tight knots could be .included in merchantable and dressing grades. Bark-encased knots, if small, did not disqualify from an acceptable framing grade but much knotty timber had to be classified as inferior box grade. The long clear internodes of radiata pine filled a demand for industrial uses in short lengths, leading to the acceptance of a factory grade of timber. This in turn emphasized the potential value of long clears of exotic species.

Mortality associated with Sirex

The postwar, silvicultural awakening coincided with a heavy mortality associated with the wood wasp Sirex noctilio; it is thought that the outbreak was triggered off by the abnormal drought of 1946. The result was a heavy loss in radiata pine in stands where crown competition had become severe, sometimes from the age of 10 years but strikingly evident in the 20-year-old stands dating from the boom years. Mortality took the form of a low thinning, including suppressed sub dominants and many of the large proportion of malformed stems common in the exotic forests of New Zealand's pumice country. The superior survival of the vigorous dominants was a witness for the provision of silvicultural growing space, and the large quantity of dead wood left standing or fallen in the forest emphasized the loss of increment that might have been put on the trees left in a properly thinned stand.

In forests of high site index at Matahina which were being worked for pulp, the drop in outturn was appreciable.


Age of stands worked, in years

Outturn cu. ft per acre (m3 per ha)





































But where there was little prospect of immediate utilization operations, the mortality amounted to a beneficial thinning, and in some localities remarkable increments in trees left were recorded. Losses in other areas with more temperate climates were of a lesser degree, suggesting a varied tempo of competition possibly capable of reflection in thinning schedules.

These events, and fears of a more serious outbreak in the future, did much to convince the sceptics; the "radiata pine thins itself" theory lost ground, even though it was apparent that it was partially correct. The conviction was further strengthened by the report of an overseas pathologist called in to advise on forest hygiene.


In the immediate postwar years tending could do no more than try to overtake arrears of work in the prewar stands. After a few years it was possible for the Forest Service to implement a policy of giving priority to work in postwar plantings, with the aim of obtaining healthy, well-spaced stands which would yield much higher quality timber than that produced in the previous 40 years.

Lack of markets again reduced large-scale utilization in the older stands. Prewar radiata pine in the state forests was not suitable for thinning because of heavy losses in stems and a predicted short rotation. The thinning of dense Corsican pine for sawlogs only resulted in heavy losses, but Douglas fir could be profitably thinned because of a high yield per acre and better stump age rates. A roundwood outlet for larch was a profitable line, but limited treating facilities caused slow progress and poor sawlog prices often meant a net thinning loss.

Demand for firewood has never been sufficient to have any effect on thinning. The pit-prop market is not extensive and has had to be shared with Nothofagus spp. according to proximity of local supplies.

In spite of the absence of outlets for small produce, the desire to obtain increment on selected stems, and the fact that thinned or sparsely stocked stands are in a much better condition to hold until advantage can be taken of potential markets, led to the acceptance by the Forest Service of at least one thinning without extraction. But other forest owners, with stricter financial inhibitions, were unwilling to outlay money on tending without visible return.

Recent forecasts of timber supply and demand for the rest of this century have once more stressed the urgent necessity for quality as well as quantity. At present the lowest grade of timber (box) accounts for 40 to 50 percent of the yield from the exotic forests. This grade is now disposed of with difficulty, even though its proportion of total sawn-timber consumption falls to 23 percent owing to the effect of indigenous supplies, which provide a high proportion of high-quality grades. In the year 2000, when the indigenous timber supply will have dwindled to a comparative trickle, it has been estimated that there will be an excess of 169 million board feet (765,570 cubic meters) of box grade and a deficiency of 168 million board feet (761,240 cubic meters) of finishing grades. To upgrade some 169 million board feet from box to finishing, for harvesting in the year 2000, involves pruning final-crop trees in state forests to 18 feet (5.5 meters) and half of them to 36 feet (11 meters). These figures are subject to market developments involving composite timber or other substitutes, but they do indicate the importance of pruning in present-day management plans.

The Forest Service is accordingly committed to a vigorous pruning policy. Priority is given to pines over Douglas fir as clears are of less importance in a predominantly structural timber, but some fir is pruned with the plywood market in view.

Evolution of pruning practice

Pruning practice evolved in the postwar years. It was considered unnecessary to treat all the trees of a stand, and efforts were made to prune earlier, with the aim of the minimum 6-inch (15-centimeter) knotty core advocated by Craib. Generally, low-pruning plans aimed at removing branches to 8 to 10 feet (2.4-3 meters) as soon as the stands allowed easy access, followed by high pruning of radiata pine to 16 to 18 feet (4.9-5.5 meters) at 8 to 10 years; other species were treated at corresponding heights. There was, however, such a backlog of work from the prewar plantations that much pruning was done at a far later age than was desirable, and in many cases it was not accompanied by a thinning. Progress in the full postwar program was restricted by shortage of labor. Ideas on the necessary compromise between rate of growth and tree form varied locally - in many areas some trees were pruned which later fell behind their competitors. It was in this period that piecework methods were adopted, again after some reluctance. The result was a much greater speed in covering the ground, although not always with any great saving per acre.

In the state exotic forests of 456,000 acres (182,400 hectares) an average of 5,870 acres (2,375 hectares) per year was low pruned and 3,560 acres (1,440 hectares) high pruned in the 10 years 1946 to 1956.

Evolution of thinning practice

In the absence of markets for small material it was generally considered that the best that could be expected for radiata pine was a usable thinning at the age of 20 to 25 years after one thinning had been carried out at about the time of first intense canopy competition; these first thinning would follow closely upon high pruning. On the pumice lands a regime was adopted aimed at keeping stands below levels indicated by natural mortality. The latter figures are only approximately applicable as they may be influenced abnormally by mortality associated with Sirex, which in turn is likely to be ruled by climate; e.g., mortality in the period 1956-61 has not followed that of the same age-classes in the period 1948-53. The regime adopted was:


Age, in years




5 ft



Thin to 1,000 stems per acre 1 (natural regeneration only)

35-45 ft



Thin to 240 s.p.a.

90 ft



Thin to 80 s.p.a.

11 acre= 0.4 hectare

Outside the pumice region, lower mortalities led to the first thinning about the age of ten years being more conservative, to 250 to 350 or even 400 stems per acre. Stands in Southland in the 20 to 30 years age-class were logged to 250 to 300 stems per acre.

First thinnings without extraction were carried out in the early years by axe; chain saws came into use later. The operation leaves a fearsome amount of slash on the ground and makes subsequent tending difficult.

Regeneration of early clear felling was generally denser than plantation espacement and it was found that a first (slasher) thinning at about the age of three years was desirable to reduce the stocking to about 1,000 stems per acre.

The results of permanent sample plots established after the second world war had an appreciable effect on thinning evolution, not only in indicating desirable grades but also in providing demonstration stands (for which data were available on the site) and thus helping to sell the idea of thinning to remaining sceptics. Experimental treatments were largely influenced by new ideas on relative spacing which were first applied to radiata pine to compare the degree of crowding of unthinned stands grown from various initial espacements. By using the relative spacing system it was possible to compare New Zealand practice with that advocated overseas for South Africa and South Australia, and to arrive at spacing-percent figures which represented limiting densities when stands should be thinned under light, medium, and heavy thinning regimes. A detailed system was evolved for maintaining uniform growing conditions during the life of a stand. The permanent-sample-plot program was accordingly worked on this prescription. At the same time, plots laid out in areas that had been given routine thinning provided useful additional data. The relative-spacing conception was also found useful in training.

In the 10 years 1946 to 1956 an average of 3,236 acres (1,309 hectares) was thinned in state forests each year.

Mill studies

In the middle 1950s New Zealand foresters began to be conscious of the intimate relation between timber quality and silvicultural tending - a realization that was fostered by a close association with the Forest Research Institute of a section dealing with grading and other related aspects of the timber industry. Mill studies were initiated to obtain more concrete evidence of the effect of past pruning and thinning, and to provide more information on which forecasts of future production of clear timber could be based. A study of the southern pines, which had been treated according to postwar ideas, revealed that, although with some species results had been disappointing, with Pinus patula pruning had already paid for itself, with a gain in value of the butt log of 7s. 4d. (U.S. $ 1.00) as against a pruning cost (without interest) of 9d. (10 cents). High pruning of P. patula and P. elliottii to 32 feet (9.7 meters) was advocated.

A technique was developed of studying branch occlusion and pruning results by splitting nodes from felled trees; it gave much cheaper and quicker results than those of mill-conversion studies. Further work on Corsican pine and on radiata pine emphasised the extreme degrading importance of encased knots, and, in the case of radiata pine, of stem cone holes. Natural pruning over short rotations in New Zealand cannot be compared with that in the initially dense, frequently thinned, long-rotation stands of Europe, and it was established that dead branches should be avoided if at all possible, either by pruning or by thinning to retain a green crown far down the stem.

Poison thinning

The use of poison in the thinning of young stands where produce could not be extracted was developed as a cheap method which enabled ground to be covered quickly and which left it in a much more accessible condition than if stems had been felled.

Salvage thinnings of old stands

Although radiata pine of 30 years was considered to be either beyond the age of adequate thinning or of low priority, it was found that thinning of other species was not ineffective and in some cases profitable. Alternatively, as they were too extensive to be clear felled in a short period, some kind of thinning was essential if they were to be retained without further stagnation. Data from these belated operations will sound strange to foresters brought up in the "light and often" text-book prescriptions catering for intensive markets. At Kaingaroa Forest 45-year-old Corsican pine originally planted at 4 × 4 feet (1.2 × 1.2 meters) has been given a first thinning to about 240 stems per acre with an extracted yield of 7,500 cubic feet per acre (525 cubic meters per hectare); these stands show a response to thinning. Douglas fir of 35 years old has yielded profitable thinnings where 4,500 to 5,000 cubic feet per acre (315 to 360 cubic meters per hectare) has been obtained, leaving a stand of 190 stems per acre. Fifty-year-old larch with 400 to 500 stems per acre has been given a first thinning to 120 stems per acre to yield 5,000 to 6,000 cubic feet (350 to 420 cubic meters per hectare) of roundwood and saw timber; stands of this age do not show much recovery of crown or increment.

Influence of pulp mills

The installation of three pulp mills in the pumice region of the North Island has been a powerful stimulus towards thinning. Thinnings of radiata pine are used in all mills and the Tasman mill takes in addition several million cubic feet per year of Corsican and ponderosa pines and Douglas fir.

Influence of roundwood markets

A spectacular feature of the 1950s has been the development of markets for preservative-treated roundwood from the exotic forests. Ten years ago the only round produce treated was larch and Douglas fir from the State monopolies of those species. Pines were under suspicion owing to fear that the node formation caused insufficient strength. This has been found to be unjustified, and good posts of pine, and, in the last few years, poles also, have been brought on to the market. At present the use of the otherwise very suitable Corsican pine is restricted owing to sapstaining taking place before drying. In a pastoral country in an advanced state of electrification this market is of great importance; the demand for posts is of the order of 750,000 per year. But even this substantial outlet can easily be saturated by the great wealth of roundwood that is available, and it appears that thinning to waste will not be eliminated, especially in the less accessible stands. The use of homegrown exotics has resulted in the reduction of imports of eucalypt posts and poles; and it has eliminated the humiliating sight of New Zealand, with hundreds of thousands of acres crying out for thinning, importing split hardwood posts from southeast Asia. Locally, concrete posts remain competitors, but in many areas wood is gaining ground by being lighter and less brittle.

The effect on profitable tending has been a major one. In 1960-61 round produce was extracted from 20 state forests as against 11 forests in 1959-60. Stump ages from post extraction result in a profit at 8 to 10 years instead of waiting until 20 years or more for a sawlog operation which will probably only be marginal. Thinnings to waste are depressing to the keen and tidy forester, and roundwood extraction has been a fillip to the interest and morale of forest staff.

Present pruning practice

In 1961 the trend is toward even earlier and more frequent pruning with the objects of still further restricting the knotty core and of reducing or eliminating any dead sections in the pruned branches. This may entail pruning to 20 feet in two, possibly three, stages.

It is now thought desirable to do first-stage pruning on a higher proportion of stems than formerly, to facilitate access and piece work. Pruners of the Porter type, once extensively used and then discarded in favour of saws, have come back into favor for the early pruning of 5- to 7-year-old trees; hand secateurs and saws of the hacksaw type are also used from the ground. For higher work the curved saw, with five teeth to the inch (2.5 centimeters), and fitted on a pole, is still the main pruning tool. Low pruning takes 8 to 12 man hours per acre; high pruning is of the order of 16 hours. In 1960-61, 8,300 acres (3,350 hectares) were low pruned and 7,860 acres (3,200 hectares) were high pruned.

With radiata pine, extension of pruning from 20 feet to 36 feet (6 to 11 meters) is considered to be economically attractive and the practice is now firmly established in a number of forests. In 1960-61, 1,300 acres (526 hectares) were covered at a cost of between 2s. 3d. and 3s. (34 to 43 U.S. cents) per tree for the section above 18 to 20 feet (5.4 to 6 meters). Longhandled saws have been used for some of the work but operation over 24 feet (7.3 meters) from the ground is difficult, and the method has lost ground to that of using sectional ladders.

Some pruning schedules are shown in the section below on thinning.

Present thinning practice

Radiata pine is not only inherently diverse but it varies in height growth and stocking according to site. Indices (top height at 20 years) may range from over 100 feet (30 meters) in the north of the North Island to 55 to 60 feet (16.5 to 18 meters) on the Canterbury plains. Habit and coarseness of branching may also vary with the site. Douglas fir and Corsican pine produce much more uniform stands.

Tending practice has varied locally in conformity with the characteristics described above, and also according to the supply of labor, markets - both present and potential - and the amount of tending backlog to be made up. The result has been that so far no blanket prescriptions for the whole country have been considered desirable. Now that a greater use of contract work and some winter relief labor have enabled most of the arrears to be overtaken, more uniformity may be expected.

Except in Canterbury and in some parts of Southland, wind throw has not been serious, even in salvage thinnings in old stands.

Radiata pine

Radiata pine in Auckland on gumland and sand dune soils tend to be light crowned with comparatively moderate diameter growth. Whereas the first attribute allows tight spacing, the second demands the opposite if sizable timber is to be produced on short rotations. The following thinning regime has recently been adopted where roundwood products are salable:

Age, in years

Top height


Relative spacing before thinning2






Thin to 400-500 s.p.a.





Thin to 250-300 s.p.a.





Thin to 125-250 s.p.a.





Thin to 75 s.p.a.


In the pumice region of Rotorua former practice in state forests has undergone some modification, largely because of the possibility of pulpwood sales, but also from a consideration of tree form produced by the former regime. Coarse branching has indicated that stands should be more closely spaced in youth. Where pulpwood is likely to be extractable, the following prescriptions aim to satisfy both requirements:

Age, in years

Top height


Relative spacing before thinning2






Thin to 350-400 s.p.a.




Prune to 10 ft (3 m)





Prune 100-150 s.p.a. to 20 ft (6 m)





Prune 40-80 s.p.a. to 36 ft (11 m) min.




Thin to 180 s.p.a.





Thin to 80 s.p.a.


2 Relative spacing = 100S/H, where S = average distance between trees, and H = mean top height.
Assuming a triangular spacing, relative spacing = 22,430/
((H/N), where N = number of stems (Beekhuis 1954).
s.p.a. = stems per acre.

The 1953 thinning regime will be retained on the more inaccessible country.
On present plans it will be necessary to retain some prewar radiata pine (previously unthinned) for a further 20 to 25 years. Treatment now at an age of some 30 years is considered feasible and a program to thin to 40 to 70 stems per acre has recently been introduced.

In other state forest areas, outlets for pulpwood do not exist at present.

Some flexibility in working-plan prescriptions has been found necessary as roundwood markets are still in course of development. In these areas the details of thinning regimes are still very much in course of evolution, as influenced by markets but still following the basic conception of a first thinning to 250 to 350 stems per acre at top heights ranging from 35 to 50 feet (10.5 to 15 meters). In Ngaumu state forest, in a timber-hungry district, sawlogs have been sold from a 10 to 12-year-old stand, but elsewhere it appears unlikely that they can be sold other than at the second thinning of stands with a top height of 75 feet (22.8 meters) and upward.

Douglas fir

There is little chance of any utilization of Douglas fir before the sawlog stage is reached. Experience with older stands has shown that it has excellent powers of recovery and that espacement in early life has a marked effect on tree form and branch size. In addition it has been shown that the timber should desirably be grown not faster than six rings per inch. Accordingly there is little reason to make an initial thinning before the age of 25 to 30 years (top height 65 to 80 feet [19.7 to 24 meters]) when it is planned to reduce stems to 200 to 250 stems per acre. A light thinning at 15 to 20 years would be beneficial in the removal of wolf trees, but the cost may not justify it. Chemicals cannot be used for this purpose as Douglas fir is the most resistant to poison of all the conifers grown in New Zealand. Pruning to 18 feet (5.5 meters) should be completed before a top height of 36 feet (10.9 meters) is reached.

Corsican pine

The tending of Corsican pine gives rise to problems which have not yet been finally solved. Difficulties in drying have precluded large-scale use for posts and poles. Timber from dense stands has too high a proportion of bark-encased knots. The fastest rate of growth that can be expected is not more than six rings per inch (2.5 centimeters) so that as a pruning proposition Corsican pine falls a long way behind radiata pine. Pruning of a single bottom log appears justified, but thinning to keep crowns to near the pruned height is likely to produce knotty timber in which the size of the knots may outweigh the advantage that they are inter grown. Tentative proposals for post-1940 plantings in site I (top height 42 feet [14 meters] at 20 years) are:

Age, in years

Top height







Prune 400 s.p.a. to 6-8 ft (1.8-2.4 m)




Prune 350 s.p.a. to 12-14 ft (3.6-4.2 m)




Prune 150 s.p.a. to 20 ft (6 m)



Thin to 300 s.p.a. without extraction




Thin to 220 s.p.a.




Thin to 120 s.p.a.

Southern pines

Stands of southern pines were among the first to receive early pruning and thinning at or before the age of 10 years. The results have been disappointing although it is likely that poor genetical origins have been partly responsible. Pinus patula has shown good diameter growth under a heavy thinning regime but has become coarse above 20 feet (6.1 meters). Pinus taeda has produced large persistent bark pockets at the sites of pruning; Pinus eliottii is more satisfactory in this respect. It appears that these species will require treatment similar to that of radiata pine when out of reach of a pulp mill, i.e., a thinning to waste or for roundwood at about 10 years, and the first thinning to remove sawlogs at 20-25 years. The following is a sample schedule:

Age in years

Top height







Prune 30 to 50 percent of stems



to 10 ft (3 m)




Prune 150 s.p.a. to 18 ft (5.4 m)



Thin to 250-350 s.p.a. P. taeda



Thin to 300-350 s.p.a. P. patula




Prune P. patula to 32 ft (9.7 m




Thin to 120 to 150 s.p.a.

Future trends

As markets develop and the demand for exotic timber grows in relation to indigenous supplies, more frequent thinnings may be expected. One major pulp company aims at thinning at six-year intervals. There will be a closer approach to maximum yields by the elimination of deadwood in the forest. A recent investigation on Douglas fir suggests that yields in terms of mean annual increments may be higher than previously expected; further light on such points may be obtained from results from permanent sample plots as they grow older. It is probable that more pulp mills outside the pumice region, where all three existing mills are now centered, will stimulate optimum thinning and maximum production. Further development and decentralization of roundwood markets seems assured.

Much investigational work requires to be done, both by forest-products research workers and by silvicultural economists. Thinning and pruning regimes require careful assessment to correlate optimum production with economic efficiency. More accurate data are required on stem diameter, branch size, timber quality, and consequent value of treated stands. Few figures are available so far on the survival of pruned stems of different crown classes. Closer attention to costing is essential, and much work remains to be done on the comparative efficiency and improvement of pruning tools.

Evolution of thinning and pruning practice in New Zealand has necessarily had to proceed at a much higher tempo than in the slower-growing forests of Europe with their established outlets for produce and a background of several centuries' experience. Market developments have been more dynamic. Management problems resulting from boom afforestation have necessitated a good deal of opportunism and an ad hoc approach to tending. They have emphasized more strongly than ever that the forester cannot be concerned only with the growing and tending of trees; he must be thoroughly conversant with their utilization potential and the marketing of their end-products.

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