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Radiata Wood Properties
Cell Structure
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Radiata Pine
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A New Strategy For Growth
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Since the large scale planting of trees began in New Zealand in the late 1920s, radiata pine has become the most popular species and it now accounts for almost 90% of plantation forests. It is important to note that the trees planted today are of much better quality as a result of intensive genetic improvement with selection for growth and form and, more recently, wood properties. Tree breeders from Scion initiated the genetic improvement of radiata pine, with increased industry involvement over the last two decades. The success of the genetic improvement, with substantial genetic gain and subsequent increase in economic value of radiata pine products, has been recognised internationally.

Radiata pine shares many of the usual physical properties of Northern Hemisphere softwood conifer species. It is a relatively tall tree with a round, straight stem and small branches along its length. The stem can be easily cut into logs for transport and processing.

Cell Structure

The actual structure of the wood is an important consideration for processing. Like most other conifer species, radiata wood is made up of millions of tracheids, fibre cells with relatively thick walls providing the strength to hold the crown of the tree high up in the light. The cell walls of tracheids are made up of long chain-like carbon molecules known as cellulose, reinforced and joined by other compounds called lignin.

The length and diameter of the individual cells varies, with cells formed in late summer (late wood) having thicker walls and smaller diameter than those formed in the spring (early wood). Late wood makes up the denser, darker rings visible in cross sections of the stem.

The hardness of the cell wall, the variation in wood density across the rings and across the cross section of the tree, the presence or absence of resins and oils are factors which determine the uses which may be made of the wood produced by different tree species.

Radiata pine wood is made up of cells that are relatively uniform in terms of density and strength across the stem, although those nearest the centre are definitely shorter and weaker. This central zone or corewood is therefore usually not permitted in timber sawn for uses where strength is important. Wood from the outside of the stem has longer and stronger cells so is well suited to structural timber and pulps where greater strength is important (eg newsprint, wrapping and packaging papers and paper sacks for cement, etc). The wood also has lower resin/lignin content, an important factor for pulping. These same properties are also important for other fibre products such as MDF.

The natural durability of wood is influenced by its chemical composition. Radiata pine, with its low chemical levels has non-durable wood, but because the pores linking cells are relatively open, chemicals to increase durability quickly pass through the wood. The same features also mean that wood density is lower (i.e., the wood is lighter) - a real advantage for building/construction purposes. But its bending and strength properties are still good, equal to most other softwood species used for similar purposes.

Other features of radiata pine associated with its relatively uniform cell structure and even density are:

•     good finish when cut and sanded
•     easy to nail but still holds nails well
•     paint and other coatings dry to a good finish
•     little 'resin bleeding' once dried.

These and other properties of radiata pine rate well alongside the important commercial softwood species from other parts of the world.

Because of its suitability for a variety of end-uses, radiata pine has become one of the most widely planted softwood species, particularly in the Southern Hemisphere where an estimated 4.5 million hectares have now been established.

The importance of radiata pine has encouraged scientists to put further effort into improving its properties and increasing its versatility. Innovations such as wood hardening seek to overcome perceived performance weaknesses. Forest Research is also involved in tree breeding programmes aimed at increasing wood density and strength.

Click to Enlarge
Radiata pine has low chemical levels and that reduces the durability of the wood - unless 'treated'.

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