High Country Forestry

Douglas-fir or Oregon pine is one of the world's best known timber trees. As a commercial timber species it is by far the best prospect for moister areas of the high country.

Douglas-fir is a native of Pacific coastal regions of North America. The species occupies a huge range of environments from humid coastal to dry interior timberline sites.

Douglas-fir is an evergreen tree with a conical shape, reaching to over 50 m in height and 200 cm in diameter in NZ. The main branches are mostly clustered in whorls, with the lower branches of large, open-grown trees characteristically downswept. The bark is thick, corky and deeply furrowed. Needles are 1.5 to 3 cm long, pointing forward, more or less parted on top of the shoot, and fragrant when crushed. Cones are 5 to 10 cm long, oval-ovoid in shape and hang from near the ends of branches. Cones from true fir, Abies spp. sit upright on branches.

Douglas-fir was first recorded in NZ in 1859. It has been used in New Zealand plantation since the early 1900s and is the second most important species after radiata pine in terms of the area planted and timber yields. While smallish plantings have been made throughout the South Island the largest early plantings of Douglas-fir were in the central North Island.

A series of national provenance trials were established in the 1950s and 60s. These determined that the best origins for New Zealand conditions originate from the western coast of North America between northern California and Washington states. New Zealand now has second-generation seed stands descended from these more vigorous coastal provinces. In the 1990s further seed source and progeny trials have been established with the aim of further improving growth performance, particularly stem form. These, and other recent trials looking at tree nutrition and silviculture, have been carried out under the auspices of the Douglas-fir Research Cooperative.

The use of Douglas-fir in New Zealand forestry declined in the mid 60s after the fungal disease Swiss needle blight (Phaeocrytopus gaumanii) established in the North Island. Interest and enthusiasm for Douglas-fir is now keenest in the South Island where growing conditions are more favourable, and wherever Swiss needle blight has not had a significant impact on stand health and productivity.

The majority of new Douglas-fir plantings are now in the southern half of the South Island. Here there are large areas where Douglas-fir has distinct advantages as the primary commercial species due to its good growth and tolerance of winter climatic extremes.

Douglas-fir grows vigorously on moist sites with moderately fertile, free-draining soils. It enjoys cool conditions and low humidity providing rainfall is adequate. Below 1000 mm rainfall, growth is best on shadier faces less exposed to drying northwest winds. These faces are often least value for grazing.

The main limitation to siting is out of season frost which can damage and kill new foliage. Seedling vulnerability to frosting requires siting on slopes with good air drainage. Douglas-fir must not be planted in frost hollows or cold air ponding sites.

The ability of Douglas-fir to withstand extremes of wind, snow, and low winter air temperature, together with its low silvicultural requirements, are major advantages in the high country.

The growth potential of Douglas-fir is largely determined by moisture availability, particularly rainfall. At higher rainfalls its productivity is exceptional and, over the long term, is on a par with top radiata growth rates. Mean annual growth rates of more than 35 m3/ha/yr have been recorded from fully stocked mature high country stands on sites receiving 1300 mm+ rainfall. On top sites, biological productivity peaks at between 60 and 90 years stand age. Douglas-fir is a slow starter compared to radiata and longer rotations are required if its growth potential is to be realised. The figure below shows the wood yield of fully stocked, mature stands (greater than 50 year old) relative to site rainfall in the Canterbury high country.

Douglas-fir seedlings should be purchased from select seedlots. In the past Ashley (compartment 12) and Eyrewell seed stand have been most frequently used, but these source can give poor form, especially on the best growth sites. Until trials have determined the best of more recently bred, selected and imported origins, the Tramway or Beaumont (and possibly Fort Bragg) origins are the recommended seed sources for the cooler areas of the South Island.

For practical purposes the most important consideration with a new planting is to establish just sufficient density of seedlings to achieve the required branch size control whilst allowing maximum growth of individual trees. Initial stocking also has the single largest effect on the cost of establishment, and subsequent tending costs. The minimum initial stockings for high country sites suggested may need adjustment (higher or lower) for local growing conditions.

The export value of Douglas-fir timber is very sensitive to grade. The usual objective in management of Douglas-fir is to produce good structural grade timber that meets internationally recognised quality standards. The North American F5 grade structural timber with a maximum knot size of 38 mm is the goal. Small branch (knot) size is achieved by maintaining sufficient stocking for inter-tree competition to suppress branch diameter growth.

As long as branch size is kept within limits, no premium has been paid for pruned logs. However, low pruning to 3 m is an option for stock access and fire control. Trials are currently under way to determine the most desirable pruning regimes, should pruning ever become more cost effective.

Two broad regime options exist for Douglas-fir, namely production thinning(s) and late clearfelling or waste thinning and early clearfelling.

The waste thin regime will generate the main income from clearfelling at a younger age but the yields of higher quality F5 grade timber, and the total income, will be less.

The production thinning regime delays the date of first significant income but yields more high quality timber, and provides a higher sustainable income from the forest. This regime can only be undertaken on sites of more gentle topography, and care has to be exercised not to create conditions for windthrow of remaining crop trees. Shelter along the windward edge of the forest and at strategic intervals within the forest should be maintained.

Production Thin Regime Objective: Maximum yield F5 export grade logs and maximum sustainable income.		Waste Thin Regime Objective: Shorter rotation, F5 grade logs and earlier cash flow.
Year	Activity	Year	Activity
0	Plant 1480 stems/ha¹	0	Plant 1480 stems/ha¹
28-33	First production thin² 450-550 stems/ha	12-15	waste thin 500-600 stems/ha²
40-50	Second production thin³ Remove dominant trees or clearfell	35+	Clearfell
55+	Repeat production thinning at 10-15 yr intervals or clearfell.
¹ Initial stocking depends mostly on rainfall - see Table above. ² First production thin as soon as small sawlogs are marketable ³ 2^nd and subsequent production thinnings are top down thinnings where 30-40% of largest stems are harvested.		¹ Initial stocking depends mostly on rainfall - see table above. ² Wide spaced agroforestry regime with Douglas-fir is not recommended as there is no control of branch size.

Douglas-fir has an excellent reputation as a general purpose structural timber in both the domestic and export arenas - it is already well known and accepted in the world's major timber markets. This gives it a key marketing advantage, as all future timber grown in New Zealand will be for export.

Huge shortfalls in the supply of Douglas-fir to traditional markets are already occurring because of overcutting of Pacific North-West forests of USA and increasing conservation pressure to preserve habitat for wildlife such as the spotted owl. Less than 18% of the original old crop Douglas-fir resource remains to be harvested.

Future supplies of North American Douglas-fir will be dominated by lower quality timber from second crop "regrowth" forests. The New Zealand plantation-grown Douglas-fir is similar in quality to North American second crop timber, which enhances the relative marketability of New Zealand Douglas-fir on world markets.

Exports of unprocessed New Zealand Douglas-fir logs to the Asian markets have shown the high value of Douglas-fir. Wharf gate prices paid for unpruned structural grade Douglas-fir logs have often been approximately double the prices paid for pruned radiata clearwood logs.

Douglas-fir plantations do not require as much tending as pine and are not as sensitive as pine to timing of thinning operations. Even at high stockings, volume increment of mature Douglas-fir will continue to increase. This can be attractive to farmers with large areas of plantations (cf., radiata pine, which is expensive to thin and prune early in its rotation), and those not wanting to be tied to strict tending and harvesting schedules. (sentence omitted)

Ecological sustainability is an important consideration in land management. Longer rotation multiple thinning regimes avoid the disturbance of clearfelling, while still providing high yields of quality timber. Regrowth of native shrubs, as well as regeneration of the second crop, can be encouraged in a selection logged forest.

The main disease of concern is Swiss needle blight. In the central North Island and on moist low altitude South Island sites the disease can have a severe impact on forest health, and stand productivity may be reduced by up to 25%. Swiss needle blight has been recorded at Hanmer, Fairlie, and Naseby but at this time most high country Douglas-fir stands are still uninfested. Maintaining vigorous growth and good stand health is the best protection for existing stands. Attention to good siting where there is adequate moisture and free draining soils, and avoidance of overstocking stress are important. For new plantings care should be exercised that seedlings are from disease-free areas.

Douglas-fir is capable of regenerating vigorously under favourable conditions, in the absence of a closed canopy vegetation cover or significant grazing pressure. The species is more shade tolerant than the pines and, unlike them, is capable of invading under native forest canopy gaps. The control or eradication of unwanted seedling spread should be considered and planned for around any new forest.

Rainfall (mm)	Spacing (m)	Stems/ha
600–700 700–800 800–900 900–1000 1000+	3.5 × 3.5 3.0 × 3.0 2.7 × 2.7 2.6 × 2.6 2.5 × 2.5	800 (approx) 1100 1380 1480 1600