Silviculture

After establishment, the two main operations that are commonly performed on the crop are thinning and pruning. Thinning is done for the same purpose as a gardener thins carrots – good diameter growth requires adequate space per plant.

More trees are planted than are needed at harvest for at least four reasons: to ensure that the site is fully occupied (some trees will die and there might otherwise be gaps); to provide some selection so that the best trees can be chosen; to suppress understorey vegetation such as gorse to enable easier access for pruning gangs; and finally to reduce the size of lower branches for easier pruning, or if unpruned, for improved wood quality.

There are also many reasons for pruning, but the most common is to produce clearwood, ie knot-free timber. Pruning is expensive but many growers consider the cost to be justified by the high premiums expected for clearwood.

The operation is done with hand-held loppers and jacksaws (for larger branches); aluminium ladders are used for higher lifts. Most growers prune the stem to 5.5-6.0 metres so as to make one valuable pruned butt-log per tree. (Incidentally, a stem grows by expanding outwards not by stretching – the pruned height will be the same twenty years later).

The timing of pruning and thinning is critical. If the pruning is done too early, there will be too many expensive pruning lifts; too late, and the operation will not yield much clearwood.

If thinning is done too early, the debris will impede access for pruning as well as failing to achieve the previously described benefits of a high initial stocking; too late and the growth of the final-crop trees will have been seriously retarded – besides, it can be costly and difficult to thin out very large trees, and the resultant stand might be very unstable in New Zealand’s windy climate.