Tree-by-tree model of the bark beetle crisis under climate change[Modèle de la crise scolyte, arbre par arbre, sous changement climatique]

Between 2018 and 2021, an unprecedented crisis led to the destruction of 360 million cubic meters of spruce trees by the European spruce bark beetle in Central Europe, with France also being severely affected (Grand Est, Bourgogne-Franche-Comté, Auvergne-Rhône-Alpes, Normandy). Climate change, through an increase in droughts and heat waves, acts as a dual factor by weakening the trees' defenses and accelerating the bark beetles' life cycle. This study presents an agent-based model developed in NetLogo that simulates these dynamics at the individual level (each tree and each bark beetle) on a spatial grid. The model incorporates local climate projections from 1850 to 2100 and links tree growth and vigor to phenology and the dynamics of cooperative insect attacks. The results show that individual rules allow for the emergence of complex large-scale dynamics; the model reproduces the spatial spread of bark beetle outbreaks from tree to tree. Furthermore, without calibration, the model successfully simulates a population peak that coincides temporally with the actual crisis observed in Europe. Designed as an interdisciplinary bridge between ecology, economics, and decision support (as part of the X-RISKS project), this model is currently being enhanced to incorporate a "manager" agent that simulates human decision-making and a detailed water balance.