The effect of top pruning, girdling, and gibberellin A4/7 application on the production and distribution of pollen and seed cones in Sitka spruce

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Abstract
In intact grafted Piceasitchensis (Bong.) Carr., which had about 10 whorls of main branches, the seed cones were largely confined to the upper 4 whorls; pollen cones were broadly distributed throughout the tree, though they were most numerous in the same region as the seed cones. Girdling plus gibberellin A4/7 application to the stem did not greatly alter the distribution of seed cones, but the zone of densest pollen cones occurred lower down. When the treatments were applied directly to branches in the lower crown, very few seed cones were produced on the treated branches but the number of pollen cones was enhanced. The lower crown of intact trees thus appears to be a zone that does not normally produce seed cones. However, when trees were top pruned (decapitated at whorl 6) and treated at the stem, numerous seed cones were produced on whorls previously in the nonproducing zone in the lower crown. Top-pruned trees became bushier and, in a good flowering year, 3 years after pruning, they produced as many seed cones as intact trees. Top pruning thus greatly improved the accessibility of seed cones and appears to be a practical management technique for Sitka spruce breeding programmes and seed orchards.