Evidences of cyclic Eurois occulta outbreaks in West Greenland based on shrub-ring anatomy and remote sensing

Insect outbreaks are among the major disturbances in high latitude ecosystems. Such outbreaks cause severe canopy defoliation and reduce the vegetation biomass and C investments, with potential consequences for species composition, functioning and productivity of tundra ecosystems. Outbreaks are expected to increase in severity and frequency in the future due to climate changes. Despite their importance, up to now only few studies tried a retrospective reconstruction of past outbreaks, and none has investigated their effect on shrub anatomical structure, e.g., cell wall thickness. In this research, we use a dendro-anatomical approach combined with remotely sensed data to assess and reconstruct past outbreaks of the moth Eurois occulta in West Greenland. We additionally quantify changes in annual growth and C investment for the host species Salix glauca L. We analysed Salix glauca L. samples collected along the Nuuk fjord (7 sites, 136 samples) to identify outbreak events and quantify inter-annual variation in conduit diameters and wall thickness. Time series of Landsat images were used to detect NDVI deviations caused by reductions in the photosynthetic activity in the area. Wood samples were successfully crossdated and 7 chronologies were established spanning more than 50 years. We clearly identified three distinctive pointer years of reduced annual growth (1997, 2003 and 2010), where wood-anatomical traits showed either a significant reduction in cell-wall thickness or no variation in vessel size. This implies that under defoliations Salix glauca L. undergoes an adjustment in the xylem traits aimed to maintain the hydraulic structure but with a detrimental effect on fiber cell walls. This multi-proxy approach allowed us to distinguish between abiotic (climate) and biotic (the moth) drivers of narrow ring formation.