Tracing the influence of larch-bud-moth insect outbreaks and weather conditions on larch tree-ring growth in Engadine (Switzerland)

Tree-ring analysis of insect-defoliated trees has so far been used for detecting past insect outbreaks only. We hypothesize that the impact of the larch-bud-moth (LBM) Zeiraphera dinian outbreaks on the growth of larch Larix decidua in the Engadine (Switzerland) is closely coupled to the spatial development of the outbreak and the ecological characteristics of the respective sites and weather conditions. We tested this hypothesis by reviewing data sets available in the literature and by analysing original data. We monitored LBM population densities and the needle phenology, growth and defoliation of larch over 28 years, i.e. over four outbreak cycles. In addition, information on defoliation patterns covering six earlier outbreaks over 50 years was matched with tree-ring information. Tree-ring chronologies of 18 larch stands were analysed with regard to abrupt growth changes and latewood events. Defoliation induces an immediate reduction in latewood, followed by a reduction in needle length and a significant decrease in radial growth in the subsequent year. We have called this tree-ring pattern the "larch-bud-moth syndrome". A careful analysis of the various parameters of the LBM syndrome for two specific population cycles enabled us to define different interaction patterns between weather conditions and tree growth. These can then be included in climate change models to help disentangle the impact of insect defoliation from that of adverse climatic conditions.