Analysis of growth and yield fkom three - Kaingaroa thummg experiments

This paper examines net basal arealha and mean dbhob development in three replicated radiata pine thinning experiments in Kaingaroa Forest. Stockings represented range from 200 to 1300 stemslha. The analysis here is focussed, however, on lower stockings (200,300 and 4001 ha), primarily because of the interest of forest managers in the reliability of yield forecasts for low stocking regimes. By age 24 years in the oldest of the three trials, it was evident that whereas basal area trends for stockings of 300lha and above were similar and logically related one to the other, the 200lha stockings had a noticeably lower growth trajectory path. The same consistent trends were demonstrated in the younger trials. Mean dbhob of the top 200 in stockings of 200, 300 and 4001ha were not as different as might have been expected. Pruning schedules imposed on two of the trials are difficult to analyse ade- quately, because of inadequate replication. It is important to continue measurement in these experiments for at least another ten years. It is now over 20 years ago that Fenton and Sutton (1968) pro- posed a tending regime for Pinus radiata in New Zealand, which, at the time, represented a radical departure from the then conventional silviculture. In essence, the regime recom- mended heavy early thinning to waste, with pruning in three lifts, to shorten rotations to 25-26 years, and to concentrate growth on the pruned butt log so as to produce sawlogs with average dbhob estimated to be around 58 cm. In a later contri- bution, Sutton (1976) gave estimates of 63.7 m2/ha of basal area and a mean diameter of 64.5 cm at age 26, achievable in stands thinned to 198 stemstha when top-height is 10.7 m for a site index of about 29 m. At the time, the amount of long-term mensurational data available to support these yield estimates was scant. Accor- dingly, between 1969 and 1973 Messrs J.W. Shirley and D.A. Elliott, then of the New Zealand Forest Service, established four well-replicated thinning experiments in different com- partments of Kaingaroa State Forest. These trials are now between 19 and 24 years old with up to 18 annual remeasure- ments; they provide a rare opportunity to study growth devel- opment of various thinning regimes, obtained from extensive and reliable data. What we have done, and now report here, is an analysis of basal area and dbhob development in three of those trials. The comparisons were of considerable interest to us and are likely to be also to forest managers today; emphasis here is placed on presentation of the major results to date, without full details of statistical analyses.