Radiata pine and Douglas fir are the two main plantation species grown in New Zealand.
Radiata Pine
The wood properties of radiata pine are highly variable. Wood processors need to match the wood resource against the end product requirements, in both the solidwood and pulp and paper industries
Wood Density:
This varies from region to region and is affected by the environment, tree age and log position, management practices and genetics.
Density is the most important measure of wood quality. It is closely related to the strength, stiffness, hardness, drying, machine-ability and finishing properties of solid timber. It is also connected to fibre length, which influences the timber's suitability for pulping and other types of processing.
Density and fibre length are linked to mean annual temperature and tend to decrease with increasing altitude and latitude. Wood density in New Zealand falls into three regional groups: high density (Northland, Auckland); medium density (central North Island); and low density (Wellington, Westland, Canterbury, Otago and Southland).
Wood density varies by site and within trees. The two most important variables are the position of the wood within the tree and the tree age. Wood density increases across the tree's growth rings from the corewood to the outer-wood.
The relative proportions of corewood and outer-wood in each of the log height classes influences the wood densities of the butt, second and top logs within trees. In butt logs only the centre of the tree will contain corewood. The upper logs will contain a much higher proportion and the top sections will be made up exclusively of this type of wood.
Branching:
The number and positioning of branches influences the distribution of knots in sawn timber. The lengths of clear timber available from between the branch whorls is another important measure. These branching characteristics determine the value of visually graded timber and are strongly influenced by the environment.
Branch size is related in part to genetics and environment but is mainly influenced by silvicultural practices. In general terms, the number of branch whorls decreases and the lengths of clearwood between the whorls increase from the north of New Zealand to the south.
Stem Cones:
These typically first appear on radiata pine around the six-metre height. They are usually located just below the branch whorls, but may also appear elsewhere. As the tree grows and the cones are pushed outwards, the cone stalk leaves behind a hole about 15 millimetres in diameter. This results in a hole in the sawn timber, causing downgrading of the pieces and a shortening of clear timber between whorls. Stem cones occur on radiata pine throughout the country.
Resin Pockets:
These are usually lens-shaped accumulations of liquid or solidified resin. They occur in radiata pine throughout the country, but particularly on the Canterbury Plains, and thought to be caused by environmental stresses. Other areas of high frequency are the Awatere Valley (Marlborough), coastal Otago in the South Island and the Waitemata and Wairarapa districts in the North Island. Distribution and size of the pockets within and between trees, and between forests, are unpredictable, except that they tend to be concentrated within the first six to eight metres of the tree. Their effect is to reduce clearwood and clear cuttings yields.
Douglas Fir
In contrast to radiata pine, which is essentially a 'sapwood' species, Douglas fir is considered a 'heartwood' species. It has developed a reputation as a good, highly textured structural product.
Douglas fir has more uniform properties within trees than radiata pine and does not have a well defined corewood zone. Longitudinal shrinkage near the pith, spiral grain and compression wood are virtually absent in the species.
Wood Density:
This is a very important property in Douglas fir, as it is used mainly for structural applications where stiffness and strength are vital. The within-ring variability is high, with the earlywood commonly having a density of 200 kilograms per cubic metre and corresponding latewood more than 800 kilograms per cubic metre. After a small initial density decrease over the first five to 10 rings from the pith, the density gradually increases with the age of the tree. Wood density decreases with the height up the tree.
In contrast to radiata pine, existing stands of Douglas fir show no clear-cut regional variation related to environmental factors. Variations in wood density are more likely to be attributable to genetic variation in seed sources.
Branching:
Douglas fir does not produce distinct whorls of branches. While this is an advantage for structural timber, it all but precludes the recovery of clearwood through techniques such as random width sawing and finger-jointing.
Resin Pockets:
They rarely occur in this species.
Other Softwoods
Macrocarpa and Cupressus lusitanica are low- to-medium-density softwoods that look and work like kauri. The heart-woods of both are golden-brown and have a speckled appearance that is rare in timber. Density (12% moisture content) is 485 kilograms per cubic metre, which lies between low and medium density and is slightly less than radiata pine.
Cypress timber does not vary much in density between trees or within a single tree. In contrast to radiata pine, there is only a slight increase in density with distance from the pith. Cypress timber is moderately stiff and strong - almost on a par with New Zealand-grown Douglas fir, so it is suitable as a framing timber. It is also an attractive furniture timber.
Corsican pine is a medium-density softwood, with similar wood density and stiffness to radiata pine. The sawn timber is easily distinguished from radiata, with closer growth rings, a smaller and pinker heartwood, smaller pith and smaller resinous knots. Generally it does not suffer to the same extent as radiata pine from a low-density, high-shrinkage, spirally grained zone of wood surrounding the pith. It is a good framing timber, but because of colour variation and higher resin content is regarded as inferior to radiata pine for appearance grades.
Hardwoods
Of the more than 550 species of eucalyptus native to Australia, only about a dozen have been seriously considered for timber production in New Zealand. After establishing the appropriate species, the aim should be to leave the trees until they reach a diameter of at least 75 centimetres. The larger the logs when sawn, the less trouble there is with growth stresses. Quarter sawing is also recommended for the ash-type eucalypts, such as Eucalyptus fastigata, E. obliqua and E. regnans.