To evaluate selective butt log (6 m) pruning 14 treatments (most combinations of 1-, 2-, 3- and 4-lift pruning removing a nominal 20, 35, 50 and 60% of the green crown at each lift) were applied in unthinned (initial spacing 2.4 x 1.8 m) radiata pine stands. Selected trees were dominants in all treatments, except one. In application, the nominal pruning intensities were substantially exceeded.
The size of the knotty core decreased with an increase in both the severity and the frequency of pruning. For a given severity, pruning to 6 m in three, instead of two, lifts decreased the knotty core by 2.5 cm. For a given pruning lift the size of the knotty core was related to the tree height at the time of pruning. An increase of 1 m in tree height was equivalent to an increase of 1.5 cm in the diameter of the knotty core-indicating the importance of timely pruning.
The size of the largest branch and total basal area of branches removed decreased with an increase in both the intensity and frequency of pruning.
Loss of dominance became important if over 40% of the length of the green crown was pruned. For those schedules considered most practical only 25-50% of the pruned dominants remained dominant. At these pruning intensities the proportion of good form stems increased.
Some loss of height increment occurred in all pruning treatments. Losses increased with the increase in both pruning severity and frequency: basal area increment trends were similar but more marked. For the more practical pruning schedules height and basal area increments were respectively, 11-25%, and 38-57% lower than the controls. In the absence of thinning, restoration of normal increments was observed only in the least severe pruning treatments. Measurements of form at the end of the trial failed to reveal any trends between treatments.
Adventitious ("epicormic") shoots became more prevalent when either pruning severity or frequency increased. For the practical schedules at least 45% of stems can be expected to produce some adventitious shoots. Thinning of pruned stems favoured their development.
Twenty percent of pruned codominants later became dominants.
The major management implication is that since small knotty cores must be a prime pruning objective and since this necessitates intensive pruning in three lifts, heavy thinning at the time of pruning is desirable, in order to minimise losses in dominance and stem diameter growth.
The suggested early tending regime to achieve a mean knotty core of 12.5 cm
At height 5.0 m prune 4-500 stems/ha to 2.0-2.5 m (and thin out all unpruned stems at each pruning lift);
At height 7.5 m prune 300 stems/ha to 4.3 m;
At height 10.0 m prune 200 stems/ha to 6.0 m.