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Management of quick-growing exotic conifers


ALTHOUGH when the first European immigrants came to New Zealand they found about 70 percent of the country covered in forest containing a wealth of excellent softwoods, much of the land first brought into pastoral use was open and treeless. The trees with which the settlers were familiar were quickly introduced to provide shelter, firewood and eventually timber. The main aim being the production of food and wool, the natural forests were rapidly cleared for agriculture and by the end of the nineteenth century there arose concern about timber supplies for the future. It had become apparent that the merchantable species in the indigenous forests grew slowly and did not regenerate readily. Attempts to propagate them artificially showed this was possible but did not overcome the difficulty of slow growth. On the other hand, several European and North American conifers had shown remarkable adaptability to local conditions, and faster growth than in their native lands. Their timber qualities were known and they were therefore favored, though not to the exclusion of trials of a wide range of other possibilities, in the creation of the new forests begun by the State in the 1890s and continued on a modest scale for some 30 years. Among those that emerged as worthy of extensive use were Douglas fir, Corsican and ponderosa pines, larch, and some species of eucalypts; but outstanding in growth was radiata pine, though not a recognized timber tree in its limited habitat in the southwest of the United States. It appears originally to have been introduced between 1850 and 1860, probably via Australia, with subsequent importations from the same source and also from its native land. By its rapid height growth and volume production, the abundance of good seed, ease of establishment and adaptability of the species, it quickly became the most popular tree for shelterbelts and small plantations throughout the country.


When the Forest Service was constituted in 1920 one of the first tasks it undertook was a review of the indigenous forest resources then remaining. The results indicated such poor prospects of their being able to meet the timber needs of the future that it was decided that the State must embark on a greatly increased program of exotic planting. About the same time, private interests saw commercial possibilities in plantations of radiata pine, which experience had shown could produce sawlogs in the phenomenally short time for a forest crop of 30 to 40 years, or even less.

Simultaneously, therefore, with a big program of state planting in the 1925-35 decade, there was also a spectacular upsurge in private planting with commercial forestry as its objective. In this development radiata pine played a major part, both in state forests (where it occupies first place by acreage) and in private forests (where it is generally the only species used). The availability at the time of suitable land in the North Island pumice region led to most of the planting effort being concentrated there. From 1936 onward planting took place on a much more modest scale, partly because the initial wave of enthusiasm was spent and partly because a series of unusually cold winters had led to widespread damage to radiata pine stands (then about five years old) from the effects of frost, followed by attacks of fungus disease (Phomopis phacidiopynis and Diplodia).

For a few years Douglas fir and Corsican, ponderosa, and lodgepole pines were favored species, but when it was seen that the fungus damage was not catastrophic, confidence in radiata pine returned. During the past few years the planting rate has been built up to about 25,000 acres (10,000 hectares) per year, well over half that area being radiata pine.


Radiata pine timber from earlier plantings reached usable sizes in some quantity about the beginning of the second world war. Prior to that it had already been put to use on a small scale. It was a new material for most of the New Zealand sawmilling industry and required different methods of conversion from those in vogue for the indigenous softwoods. The Forest Service introduced and demonstrated the suitability for the purpose of the gang-frame saws developed for sawing logs of similar character in Scandinavia.

It cannot be claimed that the timber found immediate and ready acceptance. The forests had had only limited silvicultural attention and much of the timber was marred by knots and other defects, in marked contrast to the virtually defect-free indigenous timbers to which the users were accustomed. The exigencies of war possibly forced quicker recognition of the merits of the timber than they would otherwise have received.


The 500,000 acres (200,000 hectares) or so of radiata pine in the central North Island arose as a result of a wave of enthusiasm for a species easy to establish and of rapid growth and the availability at low cost of land suitable for planting it. The full potential of the timber and the management problems the forest would present were matters for the future. After 1945, investigations to assess the capabilities of the timber were undertaken. It had quickly, been found that it could take the place of indigenous timber for rough purposes, such as packing cases and form-work for concrete, but it was only after much research and propaganda that it became recognized as an excellent general-purpose timber and as the raw material for good-quality long-fiber wood pulp. Once it started, acceptance was rapid, the easier working properties soon leading to a strong user preference. Thus were the hopes of its supporters realized and the act of faith in planting if so widely and extensively vindicated.


In this paper New Zealand's exotic softwood forests will be discussed from the point of view of their radiata pine component. Douglas fir, Corsican and other pines and other species have reached rotation age only in very limited amounts, whereas a substantial acreage of radiata pine has been clear-felled and the problems of regeneration and the second rotation have been tackled. It will be apparent that the planting boom of the 1925-35 decade produced, inevitably, very large acreages of the single species, radiata pine, of approximately the same age. At the time of planting the object was to cover the ground as quickly as possible; follow-up was perfunctory, resulting in understocked plantations. When the stage of pruning and thinning was reached (mainly during the war years) there was insufficient labor to deal With the vast program necessary. Nevertheless these plantations are providing the raw material for successful and large scale industry; they produce substantial quantities of wood, which is being put to full use; steps are being taken to improve management and, by this means, increase the proportion of higher-quality timber. The per caput usage of timber in New Zealand is very high. It is the almost universal material for house building and other light construction, and large quantities are used for decks of bridges, and for fencing and packaging.

As far as can be foreseen a large demand for sawn timber will always exist, although there is a rapidly increasing consumption of pulpwood, used to produce wall board (hard and soft), chemical pulp, mechanical pulp, kraft paper, tissues, newsprint, writings and printings, as well as particle boards. Radiata pine is the preferred species for most of these uses. The produce from the present exotic forests is deficient in timber of the quality required for the more exacting uses plywood, weatherboards, joinery, and finishing lines and one of the major current objects of management is increasing the output of the better grades of timber.

For rate of growth no species has been found which is superior to radiata pine. At 40 years of age on a good site mean top height reaches 140 feet (43 meters), mean diameter at breast height over 18 inches (45 centimeters) and volume to a 6-inch (15-centimeter) top 12,000 cubic feet per acre (840 cubic meters per hectare) from the surviving 160 or so trees in unthinned stands. Douglas fir may exceed this volume production but, when unthinned, it is distributed over more trees and in any case rotations are longer, the timber is less versatile than radiata pine, and the tree is more exacting in its site requirements.

The main management problems to be faced are, therefore, the measures required to increase production to the maximum, to improve the quality of the product, and to ensure permanent production. Incidental to these problems is the correction of the present maldistribution of age classes.


Much has been learned from experience about the site requirements of radiata pine. Though, conspicuously versatile, it does not thrive in wet situations or low-fertility podzolized clays. On shallow soils, particularly where there is a pan, it suffers windthrow. Young trees are sensitive to frost and at all stages it is subject to breakage by show. The altitudinal limits are therefore 2,000 feet (600 meters) in the North Island and 1,500 feet (450 meters) in the South Island.

In the central North Island pumice country a system of site-quality indicators based on the secondary vegetation has been developed, a useful planning aid.

FIGURE 5. - If ever a tree had a rags-to-riches story, it would be the radiata pine. Only 30 years ago the species was held in disfavor by New Zealand merchants and consumers. Today it is the most versatile species in New Zealand. (Photo: New Zealand Forest Service)

FIGURE 6. - Another view over the Kaingaroa forest, North Island, New Zealand, taken from one of the main fire look-out stations. (Photo: New Zealand Forest Service)

The species has a remarkable faculty for "asserting dominance", slower-growing stems being eliminated by the more vigorous. To obtain maximum production it is essential to carry out a series of thinnings to reap trees that would otherwise be lost by natural mortality.

Two lines of approach are used to achieve the essential improvement in the output of the higher grades of radiata pine timber, intensification of silvicultural tending and, as a long-term objective, up-grading of the quality of the stock by use of selected seed. The species shows tremendous variability which will require many years of study and observation to unravel. As interim measures for improvement of quality the forests are being searched for superior trees, using as criteria such obvious characters as rapid rate of growth, uninodal habit, light branching as nearly as possible horizontal, and absence of stem cones. Investigations of wood density and fiber length have indicated that differences in these characters hitherto revealed are not large enough for them to be significant in selection.

Seed orchards of trees selected as "superior" are being established and progeny tests are in progress. Supplies of seed from what are assessed as the highest-quality stands are inadequate to meet the very large demands, which at this stage can only be satisfied by making bulk seed collections from the best trees of the existing forests.

Characteristically the branches of radiata pine are persistent even when killed off by side shade. If the branches are not pruned off when green they give rise to loose knots; so pruning practice has been developed with the object of keeping pace with the green crown as the tree grows in height and restricting the knotty core to 6-inch (15-centimeter) diameter. A common regime is a first pruning 0 to 8 feet (0 to 240 centimeters) when the average height of the stand is 20 to 25 feet (6 to 7.5 meters). To reduce cost this treatment is often given only to the 250 best stems per acre (675 per hectare) but, in the interests of fire protection, sometimes all trees are pruned to this stage either in peripheral strips or over whole compartments.

The next pruning, 8 to 18 or 20 feet (2.4 to 5.4 or 6 meters), takes place when the trees are about 40 feet (12 meters) high. The aim then is to prune 150 to 180 stems per acre (375 to 450 stems per hectare) and to carry out a thinning, usually without extraction, to reduce the stocking to 250 to 300 stems per acre (625 to 750 stems per hectare), removing defective and suppressed trees to favor the trees selected for pruning. This is intended to produce a butt log with a high proportion of clear wood. In recent years pruning has been carried higher - up to 36 feet (10.8 meters) - on the 80 to 100 best stems when 70 feet (21 meters) or so high; but it is not yet proved that this operation will achieve the results desired because the diameter increment after pruning may be insufficient to produce usable clear wood economically.

Utilization considerations affect thinning regimes subsequent to the first thinning. Where there is a market for pulpwood the first marketable thinnings can be taken earlier and more thinnings are possible than where pulpwood is not salable. In some forests fencing material is sold from early thinnings. Otherwise the first utilization thinning is for sawlogs at 20 to 25 years of age. The broad objectives are to minimize losses by suppression of trees that would otherwise be usable and to give the best stems maximum freedom. For steep country, where extraction of thinnings may be uneconomic, consideration is being given to clear-felling on short "pulpwood" rotations, a practice which has hitherto had limited application.

Rotations for radiata pine crops are still somewhat flexible, but for most forests between about 30 and 40 years is considered appropriate. In utilizing the very large even-aged blocks planted in the central North Island in 1925-35, it is inevitable that some stands will be held to age 60-70 years. Fresh ideas about appropriate rotations, particularly for the higher site qualities, may develop. Technical difficulties arising from a greater proportion of heartwood may outweigh the advantages of larger logs.


Establishment and tending techniques have now achieved a reasonable degree of uniformity and efficiency. The methods used cannot be claimed to have more than local application but this paper would not be complete without a brief description of them. Most of the country allocated for forestry is marginal or submarginal for agriculture owing to soil (e.g., sand dunes) or topography, and is covered in secondary growth of varied density. It is usually cleared by burning - in two or more stages where the cover is heavy. Crushing of scrub or line-cutting is done in certain circumstances. Normal practice is to plant one-year-old nursery stock at 650 to 700 per acre (1,425 to 1,750 per hectare); in a few cases this is increased to 1,000 to 1,200 (2,500 to 3,000). On hill country planting is done by hand but where practicable machine planters are used.

The pruning described earlier is done with curved saws mounted on handles of appropriate length to about the 25-foot (7.5-meter) stage. Thereafter the operator must use ladders.


RADIATA PINE (unthinned)


Site quality I:

Site quality II:

Site quality III:

Range of height


Range of height


Range of height




cu. ft/acre




cu. ft/acre




cu. ft/acre





7 200




6 150




5 200





11 000




9 800




8 600





12 900




11 500




9 900





13 600




11 900




10 300


DOUGLAS FIR (unthinned)


One site quality

Range of height


Planted: 6 x 6 ft (1.8 x 1.8 m)

Planted 8 ft x 8 ft (2.4 x 2.4 m)



cu. ft/acre


cu. ft/acre





3 000


2 800





9 400


8 600





12 500


11 600





14 500

1 015

13 600





16 100

1 126

15 300

1 070




17 300

1 210

16 500

1 155

CORSICAN PINE (unthinned)


Site quality I

Range of height


Planted: 6 x 6 ft (1.8 x 1.8 m)
Planted: 4 x 4 ft (1.2 x 1.2 m)

Planted 8 ft x 8 ft (2.4 x 2.4 m)



cu. ft/acre


cu. ft/acre





3 300


3 100





7 500


6 450





10 600


9 300





12 650


11 400





13 900


12 650





14 700

1 029

13 500


Unproductive thinning has for some years been done by poisoning unwanted trees with arsenic or ammonium sulfamate, but felling with light power saws is now being revealed as almost as cheap.

Experience with natural regeneration after clear-felling shows varying success.

In the central North Island, on the pumice soils, felling in summer is usually followed by plentiful regeneration, the seed from the felled trees falling onto ground disturbed by the logging operations. When felling is done in winter the seed is not shed from the cones till the following spring or summer, when it remains exposed on the surface and is eaten by birds and rodents, or competing vegetation smothers the seedlings. Aerial seeding, using seed coated with bird repellent and rodenticide, and planting have both been successfully used to restock areas where natural regeneration is inadequate.

On soils other than pumice and in different climatic regions experience with regeneration is limited but supplementary or complete replanting has been found necessary in many places because climatic and biotic factors, separately or together, prevent the achievement of adequate stocking by natural regeneration alone. Damage to regeneration by frost can be reduced by leaving an overwood of 10 to 15 trees per acre (25 to 37 per hectare); clear-felling in strips may be equally effective on frosty sites.

Where natural regeneration is successful the stocking is often embarrassingly dense. The trend is to reduce it to a manageable 800 to 1,000 stems per acre (2,000 to 2,500 per hectare) as soon as possible, e.g., at age 3 to 5 years, and follow up with a pruning and thinning regime much the same as is applied to planted areas.

Second rotation radiata pine has in a few places shown signs of nutrient deficiency; generally, though, second rotation crops are, so far, at least as vigorous as, and possibly more vigorous than, the first.


Yield tables based on data from unthinned stands are used, adjusted for stocking, to make the estimates of volume of growing stock from which the allowable cut is determined. Experience has shown that logging waste is about 5 percent so that to provide a specified volume, this allowance must be made. Yield regulation has hitherto been on volume only.

Data to compile yield tables is being collected from thinned stands and as pressure on the forests, particularly those making supplies to large industrial enterprises, increases, closer attention must be paid to accurate inventories to realize their full potential.

The figures in Table 1, extracted from the yield tables in current use in Kaingaroa forest, are of interest (volumes in cubic feet per acre to 4-inch top, inside bark).


The historical background already given explains how, in the central North Island and elsewhere, there have been created large exotic forests of virtually the same age dominated by the one species, radiata pine.

Until recently they have received little silvicultural tending, and the older radiata pine not yet pruned or thinned is past the stage to benefit from such treatment.

The vulnerability of such forests to severe climatic damage, particularly windthrow, and to devastating epidemics is fully appreciated. Except for an alarming upsurge in the population of Sirex noctilio, an introduced insect which caused heavy and widely dispersed casualties in the early 1950s, damage by insects and fungi has not been extensive, though there has been more than one "scare." Potentially dangerous insects and diseases are present in the forests. It is thought to be important to reduce susceptibility to epidemics by breaking the continuity of the uniform age-class blocks. The opportunity for this is provided only when large-scale utilization begins, and can be effected by a system of "staggered settings" in each cutting series. This will have the result of scattering relatively small blocks of each age class through the forest. The size of each setting is still a matter for trial. Where the topography is easy enough for logging to be done by tractor, settings of 80 to 100 acres (31 to 40 hectares) each have been worked; where it is necessary to use haulers for logging a setting of about 120 acres (49 hectares) has been usual. Opinion is veering toward larger settings, making the compartment of about 250 acres (100 hectares) the normal unit. An interval of three to, preferably, five years is to elapse before contiguous units are logged.

In the forests now being planted annual programs are much smaller than they were in 1925-35, so age classes are better spread; from the utilization viewpoint it is necessary that the forests contain a large proportion of radiata pine and that the blocks of this species are not unduly fragmented. Except where there is heavy gorse and in sand country, sites suitable for radiata pine are also generally suitable for Douglas fir, and this species can usually be used to break up continuous blocks of radiata pine. Ideal proportions and arrangements, if they exist, have yet to be determined.

It is a matter of a compromise between incompatibles. If a forest is to support an industry it must be of adequate size for the industry to be on a profitable scale and it must yield a raw material of reasonable uniformity. There is therefore no alternative to planting large areas of one or a few species, concentrated rather than dispersed. Risks of epidemic damage must be accepted. The silvicultural attention warranted depends on the use to which the timber is to be put. If high quality is essential, the acreage planted must be within the limits of the resources available for tending.

New Zealand experience with exotic species also provides another lesson. Radiata pine has a limited natural range but is noteworthy for the extreme variation in tree type. The most suitable for each set of conditions must be worked out locally. With species of wider natural distribution provenance trials are an essential preliminary to using them for a large program.

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