Defoliation of Sitka spruce by the European spruce sawfly, Gilpinia hercyniae (Hartig):: a retrospective analysis using the needle trace method

1 During 1968-74, an outbreak of the European spruce sawfly Gilpinia hercyniae, defoliated many stands of Sitka spruce in commercial forests in mid-Wales. The needle trace method was used to determine retrospectively the temporal pattern and intensity of defoliation in a stand of Sitka spruce in Hafren Forest that had been damaged severely at the time of the sawfly outbreak. 2 An initial calibration experiment, designed to test the reliability of the needle trace method when applied to Sitka spruce, indicated that artificially induced defoliation of up to 75% was detected by the technique with an accuracy of +/-7%. Higher rates of defoliation were underestimated by up to 17%. 3 For the main sample of trees, the needle trace method demonstrated that retention of needle sets (average needle retention) was reduced by up to 33-38% over a 10-year period coincident with the G. hercyniae outbreak and the years immediately afterward when the canopy was recovering. 4 Analysis of needle loss within separate needle cohorts and age-classes revealed that 1-year-old needles were the most severely defoliated. The density of 1-year-old needles was reduced by 51-78% in 1970-73. 5 Defoliation at the time of the G. hercyniae outbreak was associated with reductions in annual height, radial, and volume increments of 24-49%, 30-59% and 32-56%, respectively. Radial and volume increments suffered their greatest reductions I year later than height increments. 6 The study demonstrates that the needle trace method can be applied successfully to Sitka spruce to quantify defoliation caused by an insect outbreak occurring many years previously, and that the technique can provide data on needle loss that is valuable for interpreting reductions in tree growth.