Impact of old foliage removal, simulating defoliation by the balsam fir sawfly, on balsam fir tree growth and photosynthesis of current-year shoots

Abstract

To investigate the effect of defoliation by the balsam fir sawfly ( Neodiprion abietis) on current-year growth and photosynthesis, all 1-year-old and older foliage were removed by manual clipping before the start of the growing season from balsam fir seedlings cultivated at two levels of fertilization. Measurements carried out late in the same growing season showed that both defoliation and withholding fertilizer reduced current-year foliar dry weight, shoot length, needle size, and stem xylem radial width, as well as total foliar dry weight, and root dry weight. Defoliation increased the projected area to weight ratio, nitrogen concentration, net photosynthetic rate per unit of dry weight, and ratio of photosynthesis to internal CO 2 concentration in current-year needles. In contrast, withholding fertilizer did not alter the projected area to weight ratio and decreased the nitrogen concentration, net photosynthetic rate per unit of dry weight, and ratio of photosynthesis to internal CO 2 concentration. Both defoliation and withholding fertilizer also decreased the number of current-year needles produced in the growing season following treatment. The results indicate that (1) the reduction in total photosynthesizing biomass caused by removing old foliage is much greater than the biomass of old foliage removed due to decreased production of current-year foliage, and (2) old foliage removal stimulates net photosynthesis in current-year shoots, probably by improving foliar nitrogen relations. Removing old foliage reduced the total photosynthesizing biomass more, and stimulated net photosynthesis of remaining foliage less, than the removal of current-year foliage measured in earlier studies, which helps explain why tree recovery is slower following defoliation by the balsam fir sawfly than after spruce budworm ( Choristoneura fumiferana) attack.