SPRUCE BUDWORM DEFOLIATION AND GROWTH LOSS IN YOUNG BALSAM FIR - ARTIFICIAL DEFOLIATION OF POTTED TREES

To simulate feeding by Choristoneura fumiferana, 5-yr-old Abies balsamea were artificially defoliated at the peak of the 6th instar period. Needle removal decreased growth throughout the tree, the growth loss increasing with increasing intensity and frequency of defoliation. In the 1st year of defoliation, all treatments reduced specific volume increment, whereas only the 100+ treatment (all current-year shoots served at the base) decreased height growth. In every defoliation year, specific volume increment, height growth and final weights of stem axis, branch axes, and roots were generally related curvilinearly to total foliage weight. The 100% and 100+ treatments induced the sprouting of axillary and nodal buds that remained dormant in undefoliated trees. Retaining the defoliated shoot axes (100% treatment), compared with removing them (100+ treatment), increased specific volume increment. Retaining the needle base (clip treatment) prevented the apex necrosis that occurred in some shoots subjected to the 100% treatment. In trees allowed to recover, specific volume increment increased in the 1st year, degree of recovery increasing with decreasing intensity and frequency of prior defoliation treatment. After 2 recovery years, specific volume increment and height growth were not affected by any previous defoliation treatment, and current-year foliage weight and stem axis weight were decreased only by the 100+ treatment, but there was still an inhibitory effect of all treatments on the weights of total foliage, branch axes and roots. -from Authors