Managing plantations involves thinning and pruning, depending on whether the trees are being grown for high-value saw logs, or as lower-quality logs.
With a harvest age of 25–35 years, and final tree numbers of around 300 stems per hectare, two thirds of the trees planted are usually cut down during the early stages of the growing cycle to make more room for the others. The felled trees are either left on the ground to rot, or sometimes harvested as posts, poles or pulpwood.
About five years after planting, the best trees are usually pruned up to a height of 4 metres. At around seven years, the trees may be pruned up to 6.5 metres. At around nine years, when the trees are 16 metres high, all the unpruned trees are felled to allow the remainder room to grow. This system is designed to produce a substantial yield of valuable knot-free wood in the lower part of the trunk.
Some forest management systems aim to grow a high yield of logs with small knots – suitable for structural timber such as house frames, roof trusses and poles. In this case the trees aren’t pruned, and they are grown closer together (450 stems per hectare) to limit branch growth and keep the knots small.
Types of log
No matter what kind of management system is followed, the forest will yield a range of log types. For example, large pruned logs are used for clear timber and veneer, large knotty logs for structural timber, and small logs from the top of the trees for wood chips, wood pulp, fibreboard and particleboard.
Logging is done by teams who:
- fell the trees using chainsaws or mechanical shears
- pick up and move the logs using crawler tractors, wheeled skidders or overhead cable haulers
- cut the branches off the logs
- use cranes to load the logs onto trucks for transport to log depots, mills or ports for export.
The logs are carefully sorted into categories according to their intended use and value.
The amount of wood a hectare can produce depends on the rotation (lifespan) of the trees, how many are grown to maturity, and the productivity of the site, which is related to soil depth and fertility, and climate. Under very good conditions, such as a deep pumice soil and mild climate, a hectare of trees may produce 840 cubic metres of wood when harvested at 28 years. This would give a mean annual growth increment of 30 cubic metres per hectare per year; the national average is around 23.