This review attempts to cover past, present and future objectives of thinning and outline current practices with the emphasis on why and how they have evolved.
First, to point out the obvious—thinning is only one of a number of devices available in New Zealand or elsewhere to shape production forests to particular end purposes. Others include choice of sites, rates of planting, initial spacing, pruning, tree breeding, and the approach to regulating cut. In attempting to achieve whatever end use objectives are laid down none of these means of manipulating the crop operates independently of the rest.
Thus, one will not observe any uniform approach to current thinning practice in New Zealand. Sites range widely from nitrogen deficient coastal sands supporting radiata pine of fine branch and stem form to fertile pumicelands with a more malformed but swiftly growing crop. Superimposed on these physical differences there are, even State forests, widely varying management objectives from region to region and a large measure of local autonomy in how they are achieved. Moreover, except in the few areas where plantings are starting from scratch the forester inherits past planting rates, often in the form of a markedly abnormal set of age classes, the results good or bad past fashions in tending, and commitments of varying length and complexity to supply particular products to industry. He is thus commonly constrained, to a considerable degree, in his choice of management strategies. His task is usually to effect a transition from the existing forest to that which he conceives as the ideal. In so doing must meet existing commitments as efficiently as he can and create the most favourable opportunities for additional future market outlets. He seldom has sufficient mensurational data to predict with precision the quantitative and qualitative consequences of his choice of thinning and silvicultural schedules and even if the physical outcome were forecast with certainty he would still be faced with uncertainty of markets and hence of the relative return to be expected for various products, raw or processed, export or domestic, 20 years or more hence.
The inevitable consequence of uncertainty is that, when all that can be has been made explicit, there remain various choices which are essentially value judgments and for this reason alone there will be no uniformity of approach in thinning practice unless decision making is completely centralised.
Finally, despite the many constraints which he commonly faces, the forester can exert a major, lasting influence by the way he manipulates, through thinning or other practices, the younger age classes in his care.