Cephalcia arvensis defoliation on Norway spruce: a case study analysis by tree-rings and stable isotopes approach

This paper focuses on carbon and oxygen stable isotopes in conjunction with tree-ring chronologies to investigate the shortand long-term effects of Cephalcia arvensis defoliation on Picea abies. The authors found massive growth loss and significantly different carbon and oxygen stable isotope patterns associated with insect feeding; while carbon isotope values increased, oxygen isotope values decreased in the defoliated trees. Depletion of δ18O reached its peak of –22.8‰ (1989), coinciding with the year of highest growth loss. The values in defoliated trees were close to those in control trees (1990), 2 years before complete growth recovery (1992). The depletion of δ18O started 1 year ahead of growth reduction, whereas 18O enrichment started 1 year before growth recovery. The authors hypothesized that a period of severe drought in the outbreak area before the insect attack may have caused the trees to mobilise reserve starch, which made trees more susceptible to Cephalcia attack as a result of increased soluble sugars and aminoacids concomitant with the direct effect of high temperature and dry weather on the insect populations. Moreover, the carbon and oxygen isotope patterns could be explained by both an increase in photosynthetic rate and a resort to starch reserves following insect feeding