Abstract
Bark beetle infestations can cause rapid mortality in Norway spruce, yet the sequence of physiological changes leading to death is not fully understood. The aim of this research was to disentangle the earliest tree physiological signs of spruce bark beetle (Ips typographus L.) induced stress in Norway spruce (Picea abies L. Karst). Phloem girdling was used to attract bark beetles in an alpine forest and monitor tree physiological changes from the very beginning of the infestation, when symptoms are not yet visible in the crowns. Girdled–infested trees were compared with girdled–only and healthy control trees. Sap flux density, tree stem growth, and canopy optical properties were measured as indicative physiological parameters of tree stress response. Cessation of stem growth was the first stress signal after bark beetle infestation, while sap flux density declines > 40% occurred weeks later. Additionally, girdled–bark beetle–infested trees showed an anticipated decrease in sap flux density with increasing vapour pressure deficit and declines in canopy reflectance. Girdled–only trees did not show statistically significant differences in sap flux density or stem growth compared to the control, and have not yet experienced mortality since the start of the experiment. Mechanical phloem disruption alone does not cause rapid Norway spruce mortality, indicating that additional factors, such as fungal sapwood infection, might strongly contribute to bark beetle–induced death. Mortality appears to be preceded by a shift of resources from growth to defence, followed by hydraulic failure. Radial growth monitoring thus provides a valuable tool for early detection and modelling of beetle–driven spruce decline.