Wood formation in Abies balsamea seedlings subjected to artificial defoliation.

We determined the cambial sensitivity and quantified the anatomical differences in xylem of Abies balsamea (L.) Mill. seedlings subjected to artificial defoliation to simulate spruce budworm feeding. Defoliation was performed by removing two-thirds of needles of all current-year shoots for up to four consecutive growth cycles to account for inter- and intra-annual xylem formation. In Experiment 1, xylem development was studied from May to October 2005 in seedlings defoliated at the end of June. In Experiment 2, anatomical features of the xylem were measured along the tree rings formed in 2005 and 2006 during the four cycles of growth and defoliation. Control and defoliated seedlings showed similar patterns of cambial activity and timing of xylem differentiation, although fewer enlarging cells were observed in August to September in defoliated seedlings. Tree-ring widths were similar in control and defoliated seedlings, with thinner rings produced in the greenhouse in winter. No effect of defoliation on cell lumen area was observed, and effects on radial cell diameter and wall thickness were found only occasionally. The results indicate that the A. balsamea seedlings produced all the resources required to maintain stem growth during the four cycles of defoliation.