Résumé :
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Defoliation of 3-year-old Sitka spruce (Picea sitchensis (Bona.) Carr.) by green spruce aphid (Elatobium abietinum (Walker)), in a field experiment in Hafren forest, Wales, resulted in significantly shorter shoots being produced in the upper canopy in 1994, the year of infestation, and in 1995. The dry weight (DW) of individual needles produced by defoliated trees was also reduced, by up to 19%, but whereas in 1994 the DW of the new needles was reduced only on the most heavily infested branches, in 1995 needle DW decreased throughout the canopy. In 1996, needles produced by previously defoliated trees were 10.4% heavier, on average, than needles produced by undefoliated trees. The total numbers of new needles produced by defoliated and undefoliated trees in 1994 and 1995 were similar, but defoliated trees produced 10% fewer needles in 1996. Consequently, and as a result of the changes in individual needle DW, the total DW of current-year needles on defoliated trees was 11% less than that on undefoliated trees in 1994, 20% less in 1995, and about the same in 1996. Stem volume increment of individual trees was related to the total DW of current-year needles (r(2) = 0.43-0.52, p
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