Modelling Dynamic Predisposition to Ips typographus

The proposed study is based on the hypothesis that Norway spruce is increasingly susceptible to mass attack by the Eurasian spruce bark beetle, Ips typographus, beyond a critical threshold of tree water stress. Actual water shortage instantly acts upon tree disposition to bark beetle attack, implicating changes at the level of tree physiology. Looking at the interactions between soil, tree and herbivore (bark beetle) species, the aim of the project is to identify “attack thresholds” subject to “water deficits” accumulated at study plots, and specific tree physiological parameters indicating water stress of potential host trees. Rain-out shelters are established at the experimental site situated in the Forest Demonstration Centre of BOKU in order to artificially induce severe and moderate tree water deficiency. Investigations during the 3-year project period involve the examination of microclimatic, soil, hydrological, tree physiological and biometric parameters, as well as entomological experiments on stressed and non-manipulated control trees. The results of field and laboratory analyses are incorporated into a “Water deficit model” based on the hydrological model BROOK90, which describes soil-water-plant interactions and is suited to determine stress thresholds and model water stress on a stand scale. Main objective of the project is to develop a dynamic predisposition assessment system (DPAS) accounting for spatial and temporal variability of tree and stand susceptibility to bark beetle infestation. A unique, dynamic monitoring system is to be set-up for day-to-day assessment of attack probabilities subject to water supply. The quality of proposed research lies in the availability and advancement of already well-established models and monitoring systems (Brook90, PAS, PHENIPS) and in the direct practical applicability of project outputs for pest risk assessment in forest management. DPAS shall allow for an improved assessment of tree and stand susceptibility to outbreaks of the Eurasian spruce bark beetle by considering seasonal and spatial variations in predisposition, especially due to actual occurring host tree stressors (drought). The newly developed monitoring instrument shall serve for an evaluation of actual stand susceptibility as affected by short-term water deficits, modelled in a dynamic way similar to the accumulation of day-degrees for predicting bark beetle development (PHENIPS) and for short-term prospective analyses, e.g. on the basis of weather forecasts. We expect improved understanding of landscape scale outbreak dynamics and more reliable evaluation of risk by incorporating retrospective analyses and drawing of model scenarios for stand susceptibility subject to changes of external factors (e.g. due to climate change) and stand internal processes (e.g. aging processes, changes in stand management). There is high potential for the implementation of an advanced, dynamic predisposition assessment system given the increasing incidence of elevated summer temperatures and short-term water deficiencies of Norway spruce dominated forests.