Insect defoliation impacts forest productivity worldwide, highlighting the relevance of plant-insect interactions. The larch budmoth (Zeiraphera griseana Hübner) is one of the most extensively studied defoliators, where numerous tree-ring based analyses on its host (Larix decidua Mill.) have aided in identifying outbreak dynamics over the past millennia. Yet, outbreaks have been widely absent after the early 1980s and little is known about the in situ tree-physiological responses and the allocation of carbon resources during and after defoliation. In summer 2018, we tracked an on-going larch budmoth outbreak in a well-studied larch forest in the Swiss Alps. We performed bi-weekly monitoring on an affected and unaffected site using a unique combination of xylogenesis observations, measurements of non-structural carbohydrates (NSC), isotopic analysis of needle assimilates, and ground-based and remoted sensed leaf trait observations. The budmoth induced a defoliation that lasted 40 days and could be detected by satellite observations. Soluble sugars significantly decreased in needles and stem phloem of the defoliated trees, while starch levels remained stable in the stem and root xylem compared to the control. Carbon and oxygen isotope ratios in needle assimilates indicated that neither photosynthetic assimilation rates nor stomatal conductance were different between sites before, during and after the outbreak. Defoliated trees ceased cell wall thickening 17 days earlier than unaffected trees, showing the earliest halt of ring formation recorded from 2007 till 2013 and causing significant thinner cell walls, particularly in the latewood. No significant differences were found for cell enlargement rates and ring width. Our study revealed that an outbreak causes a downregulation of cell wall thickening first, while no starch is mobilized or leaf physiology is adjusted to compensate for the reduced carbon source due to defoliation. Our observations suggest that affected larch trees prioritize leaf recovery and carbon storage over wood biomass development.

Keywords: Zeiraphera griseana; Cell wall thickness; European Alps; Insect outbreak; Non-structural carbohydrates (NSC); Stable Isotopes; Tree rings; Wood formation.