The effect of silviculture on genetic gain in growth of Pinus radiata at one-third rotation

A trial planted on six sites representing five growth modelling regions and a range of site qualities in New Zealand was used to examine the interaction of silviculture and genetic improvement in Pinus radiata D. Don. Four seed lots with differing genetic potential for growth were grown under four stocking treatments from 100 to 600 stems/ha, all with a thinning ratio of 5:2. In addition, two treatments were planted at 500 stems/ha, one pruned and one unpruned. Among sites, differences were large for mean top height (8.1-13.9 m), basal area (7.9-21.5 m2/ha), and volume (25-108 m3/ha) at 8 years from planting. Differences among silvicultural treatments were large for basal area (with a range of 4.3-18.8 m2/ha over sites) and volume (17-86 m3/ha) and smaller but statistically significant for mean top height (10.1-11.6 m). Differences among seed lots were statistically significant for all traits but larger for basal area (12.1-13.6 m2/ha) and volume (51-59 m3/ha) than for height (10.7-11.1 m). Genetically different seed lots performed similarly relative to each other over the range of sites and silvicultural treatment, with a slight trend for greater differences among seed lots in the faster growing silvicultural treatments. However, differences in tree growth were many times greater among sites and silvicultural treatment than among genetically different seed lots.