Spruce budworm defoliation and growth loss in young balsam fir: artificial defoliation of potted trees

To simulate feeding by the spruce budworm (Choristoneurafumiferana Clem.), potted, 5-year-old balsam fir (Abiesbalsamea (L.) Mill.) trees were artificially defoliated at the peak of the sixth instar period in the first 1, 2, or 3 years of a 3-year experiment. This schedule allowed trees that were defoliated in the first 1 or 2 years to recover for 2 years and 1 year, respectively. Seven treatments were applied: 0, 33, 66, 90, or 100% of the current-year needles were manually removed, all current-year needles were clipped using scissors (clip treatment), or all current-year shoots were severed at their base (100+ treatment). The dry weights of stem axis, branch axes, and roots were measured at the end of the 3rd year, and current-year and total foliage weight, height growth, and specific volume increment were determined for each year of the experimental 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 decreased height growth. In every defoliation year, specific volume increment, height growth (manifested in the year following the defoliation), and the 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, the 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; however, there was still an inhibitory effect of all treatments on the weights of total foliage, branch axes, and roots. The growth responses found in the present investigation were compared with those observed in balsam fir trees defoliated by the spruce budworm.