THINNING AND NITROGEN-FERTILIZATION IN A GRAND FIR STAND INFESTED WITH WESTERN SPRUCE BUDWORM .2. TREE GROWTH-RESPONSE

The effects of thinning and nitrogen fertilization on tree growth in a grand fir (Abies grandis [Dougl.] Forbes) stand infested with western spruce budworm (Choristoneura occidentalis Freeman) were evaluated over 5 years by a replicated split-plot experiment. Fertilization treatments resulted in significantly reduced defoliation and significantly heavier biomass of shoots and foliage for the last 3 years of the study. There were, however, 30% fewer buds on fertilized grand fir midcrown branches at the end of the study. Height growth of fertilized grand fir midcrown branches at the end of the study. Height growth of fertilized trees was significantly greater than unfertilized trees 3 to 5 years after treatment and was almost double that of controls. Radial growth measured at breast height and base of live crown was significantly greater for fertilized trees 3 to 5 years after treatment; 5 years after treatment, radial growth of thinned trees also was significantly greater than the controls. Trees thinned and fertilized had almost double the growth of controls 2 to 5 years after treatment. Fertilized trees apparently produced fewer buds m-2 of foliage but more foliage per shoot than budworm larvae could destroy, and this resulted in significantly increased tree growth. The results of the study have implications for using fertilization as a management option for some budworm outbreaks.