Chemical signature in xylem cell wall of Salix glauca L. due to Eurois occulta L. outbreaks

Insects driven defoliations are one of the major natural disturbances in high-latitude ecosystems and are expected to increase in frequency and severity due to global climate change. Defoliations cause severe reductions in biomass and carbon investments that affect the functioning and productivity of tundra ecosystems. Recent studies have quantified the decrease in cell-wall thickness (CWT) during the outbreak and the unexpected increase in primary production the following years. Here we combine dendro-anatomical analysis with chemical imaging to investigate how the outbreaks affect carbon assimilation and vegetation productivity. Samples of Salix glauca L. featuring outbreak events of the moth Eurois occulta L. were collected at Iffiarterfik, Western Greenland. Samples were cross-dated and two pointer years in 2003 and 2010 (outbreaks) identified. These two annual rings showed a clear reduction in carbon investment such as reduction in ring-width, CWT but also colour intensity was affected, suggesting an altered biopolymer mark-up. For each outbreak event, seven growth rings were analysed (outbreak ±3years). The wider rings formed the two following years highlighted a growth release after the outbreaks. The chemical composition of the xylem cell wall material was analysed using confocal Raman imaging on cross sections of fibres, vessels, and parenchyma cells to possibly identify the chemical signatures related to insect outbreaks. Possible differences in chemical composition between cell types and between growth years were explored using chemical imaging based on cluster analysis of integrated Raman band intensities as well as by more advanced chemometric approaches.