The initial stocking trial, 19 years old at the time of assessment, was located in Rotoehu Forest, and incorporated genetically improved stock (with a Growth and Form factor of 13) and unimproved stock (GF3). It was planted at six levels of initial stocking from 250 to 1500 stems/ha, and thinned to 250 stems/ha. Results of the evaluation were adjusted for bias due to microsite.
There was an apparent site index differential of 1.6 m between selection extremes. This was attributed mainly to differences in initial stocking rather than to the effect of selecting taller trees. There was no significant difference (p>0.05) in mean diameter at breast height (dbh) due to selection ratio. The straightness of both the unpruned logs and the pruned butts was enhanced with increasing selection. By increasing selection ratio, total merchantable volume and pruned volume were substantially improved, owing to height differences and reduced malformation.
Because of improved quantity and quality, there was an increase in stumpage value at age 19 with increasing selection ratio, the highest selection ratio tested (6:1) being worth $4,000 (29%) more at age 19 than planting at final stockings. This difference is expected to increase to $5,900 (23% more) at age 25 and $8,200 (24% more) at age 30. GF3 genetic stock was less valuable (by $3,700/ha, or 21%) at age 19 than GF13 stock at the same 6:1 selection ratio, had a 1.8 cm smaller dbh, 1.3 m lower mean top height, 50 m3/ha less total volume, and was inferior in straightness. Although pruned volume was 13 m3/ha less, the difference was not statistically significant. GF 13 stock at a selection ratio of 1.1:1.0 was equivalent in stumpage value to GF3 stock at a 6:1 selection ratio.
At age 25, GF13 is expected to be $5,000 (18%) more valuable, and $5,800 (16%) more valuable at age 30, but this could be an under-estimate because there is some doubt as to the reliability of model projections for new breeds.