The quality of plantation-grown trees is influenced by genetic selection, silvicultural practice, site selection and rotation age.
Over the last 60 years there have been many changes in forestry practices which have resulted in greater control over the type of wood produced.
New Zealand pine forests are established with genetically selected stock and managed to provide a predictable, premium quality log resource for a wide range of world markets. Ideal growing conditions and appropriate management permit the harvesting of large logs (up to 80 cm diameter) on rotations of approximately 30 years. The logs are typically healthy, containing no decay, internal splits, or growth stresses.
Plantation grown New Zealand pine is sometimes referred to as a "sapwood" tree because of the relatively small proportion of heartwood. At 30 years of age, 80% of the tree volume is sapwood, with a fresh moisture content of about 150%, measured as percentage of 'oven dry' wood weight. This results in an average weight of about 1 tonne/m3. If the logs are left too long without protection before processing they are prone to infection from bluestain fungi.
As with other softwoods such as Douglas fir, wood properties of New Zealand pine are influenced by geographic location and tree age. Basic wood density of the mature wood zone is, therefore, variable but averages between 400 and 420 kg/m3 at rotation age.
Within the tree there are defined quality zones which need to be recognised during processing. Juvenile wood (or corewood) is typical of the inner 10 growth rings and it can have an impact on stability. In addition to lower density, juvenile wood has wider growth rings, shorter wood cells, higher longitudinal shrinkage and increased spiral grain. Surrounding the juvenile zone, wood properties are more "mature", i.e. higher wood density, narrower growth rings, and straighter grain.
New Zealand pine does not shed its branches when grown under the regimes of wide initial spacing, early thinning, and the 30 year rotation now common practice in New Zealand. However trees can be artificially pruned and the "knotty core" restricted to a small cylinder, around which defect free "clearwood" is produced. Pruning is restricted to the butt log with variable heights of between 4 and 8 metres.
NZ Pine User Guide, courtesy of NZ Pine Manufacturers Association