Combining empirical and mechanistic understanding of spruce bark beetle outbreak dynamics in the LPJ-GUESS (v4.1, r13130) vegetation model

Abstract. For evaluating the forests’ performance in a future with changing climate for different management alternatives, dynamic vegetation models are important tools. One of the functions in such models that has a big influence on the results is tree mortality. Bark beetles are important for the pattern of mortality in forest, especially for needle leaved forest in the temperate and boreal zones. The European spruce bark beetle (SBB, Ips typographus) has in the most recent years replaced wind as the most important disturbance agent in European forests. Historically, SBB damage is typically triggered by wind storms as they create breeding material with no defences to overcome for the beetles. Drought can contribute to increased damage and prolonged outbreaks by lowering the defence of the trees, but has been the main driver of some of the European forest damage in the last decade. In this study we implemented a SBB damage module in a dynamic vegetation model (LPJ-GUESS) that includes representation of wind damage and forest management. The module was calibrated against observations of storm and SBB damage in Sweden, Switzerland, Austria and France. An index of the SBB population size that changed over time driven by phenology, drought, storm felled spruce trees and density of the beetle population, was used to scale modelled damage. The model was able to catch the start and duration of outbreaks triggered by storm damage reasonably well but there was a large variability that partly can be related to salvage logging of storm felled forest and sanitary cutting of infested trees. The model showed increased damage in most recent years with warm and dry conditions, although below the level reported, which may suggest that the drought stress response of spruce in LPJ-GUESS is underestimated. The new model forms a basis to explore vulnerability of European forests to spruce bark beetle infestations.